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show}forall showpage}if end}def end}bd end %%EndResource /SVDoc save def %%EndProlog %%BeginSetup Win35Dict begin ErrHandler statusdict begin 0 setjobtimeout end statusdict begin statusdict /jobname (Microsoft Word - FWALLS.DOC) put end /oldDictCnt countdictstack def {}stopped { countdictstack oldDictCnt lt { Win35Dict begin } {1 1 countdictstack oldDictCnt sub {pop end } for } ifelse } if /oldDictCnt countdictstack def {letter }stopped { countdictstack oldDictCnt lt { Win35Dict begin } {1 1 countdictstack oldDictCnt sub {pop end } for } ifelse } if [{ } /exec load currenttransfer /exec load] cvx settransfer /setresolution where { pop 300 300 setresolution } if %%EndSetup %%Page: 1 1 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 75 75 0 0 0 69 /Times-Bold /font25 ANSIFont font 0 0 0 fC 787 334 826 (Thinking About Firewalls) 826 SB 32 0 0 58 58 0 0 0 54 /Times-Bold /font25 ANSIFont font 977 446 446 (Marcus J. Ranum) 446 SB 32 0 0 58 58 0 0 0 52 /Times-Italic /font27 ANSIFont font 1049 540 301 (mjr@tis.com) 301 SB 32 0 0 58 58 0 0 0 53 /Times-Roman /font28 ANSIFont font 807 633 785 (Trusted Information Systems, Inc.) 785 SB 953 727 493 (Glenwood, Maryland) 493 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 888 185 (Abstract) 185 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 946 1464 (Generally, he who occupies the field of battle first and awaits his enemy is at ease. ) 1464 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1795 933 51 (\276) 51 SB 1794 933 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1845 946 156 ( Sun Tzu) 156 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 1046 1417 (Many companies connect to the Internet, guarded by "firewalls" designed to prevent) 1417 SB 300 1096 1683 (unauthorized access to their private networks. Despite this general goal, firewalls span a continuum) 1683 SB 300 1146 1647 (between ease of use and security. This paper describes some of the considerations and tradeoffs in) 1647 SB 300 1196 1707 (designing firewalls. A vocabulary for firewalls and their components is offered, to provide a common) 1707 SB 300 1246 375 (ground for discussion.) 375 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 1346 353 (Why a Firewall?) 353 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 1405 1725 (Against those skilled in the attack, an enemy does not know where to defend. Against the experts in) 1725 SB 300 1455 914 (defence, the enemy does not know where to attack. ) 914 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1215 1442 51 (\276) 51 SB 1214 1442 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1265 1455 156 ( Sun Tzu) 156 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 1555 1522 (The rationale for installing a firewall is almost always to protect a private network against) 1522 SB 300 1605 1677 (intrusion. In most cases, the purpose of the firewall is to prevent unauthorized users from accessing) 1677 SB 300 1655 1723 (computing resources on a private network, and often to prevent unnoticed and unauthorized export of) 1723 SB 300 1705 1736 (proprietary information. In some cases export of information is not considered important, but for many) 1736 SB 300 1755 1794 (corporations that are connecting this is a major though possibly unreasoning concern. Many organizations) 1794 SB 300 1805 1695 (will want simply to address the problem by not connecting to the Internet at all. This solution can be) 1695 SB 300 1855 1589 (difficult to implement. If the private network is loosely administered or decentralized, a single) 1589 SB 300 1905 1464 (enterprising individual with a high speed dialup modem can quickly effect an Internet ) 1464 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1764 1914 67 (SLIP) 67 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1831 1905 195 ( connection) 195 SB 300 1955 958 (which can compromise the security of an entire network.) 958 SB 480 2055 1158 (Often it is safe to say that a firewall needs to be put in place for the ") 1158 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1638 2064 69 (CYA) 69 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1707 2055 17 (") 17 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1724 2051 17 (1) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1741 2055 345 ( factor. Even though) 345 SB 300 2105 1386 (an employee could compromise proprietary information by carrying it offsite on a ) 1386 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1686 2114 68 (DAT) 68 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1754 2105 318 ( or floppy disk, the) 318 SB 300 2155 1243 (Internet represents a tangible threat, populated with dangerous "vandals.") 1243 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1543 2151 17 (2) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1560 2155 448 ( It could very easily cost a) 448 SB 300 2205 1771 (network manager his job if a break-in occurs via this route, even if the damage is no more extensive than) 1771 SB 300 2255 1679 (could have been inflicted over a dialup line or by a disgruntled employee. Generally, for a would-be) 1679 SB 300 2305 1716 (Internet site, the technical difficulties of implementing a firewall are greatly outweighed by the public) 1716 SB 300 2355 1774 (relations problems of "selling" upper management on the idea. In summary, because Internet services are) 1774 SB 300 2405 1505 (so highly visible, they are much more likely to require official oversight and justification.) 1505 SB 32 0 0 42 42 0 1 0 38 /Times-Roman /font28 ANSIFont font 300 2683 220 ( ) 220 SB 520 2683 220 ( ) 220 SB 740 2683 165 ( ) 165 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 300 2729 17 (1) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 317 2733 17 (") 17 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 334 2733 311 (Cover Your Assets) 311 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 645 2733 514 (" - this is a family publication.) 514 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 300 2779 17 (2) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 317 2783 1781 (The Vandals were a collection of tribes of roughneck barbarians who sacked Rome in 455 and looted it of) 1781 SB 300 2833 1628 (all its portable wealth. Some use the term "hackers" to describe Internet snoopers, but "vandals,") 1628 SB 300 2883 708 ("crackers," or "jerks" is more appropriate.) 708 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 2 2 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 0 0 0 fC 300 233 356 (Design Decisions) 356 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 292 499 (Examine your environment ) 499 SB 32 0 0 50 50 0 0 0 51 /Symbol font 830 279 51 (\276) 51 SB 829 279 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 880 292 346 ( Miyamoto Musashi) 346 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 392 1578 (In configuring a firewall, the major design decisions with respect to security are often already) 1578 SB 300 442 1792 (dictated by corporate or organizational policy; specifically, a decision must be made as to whether security) 1792 SB 300 492 1709 (is more important than ease-of-use, or vice versa. There are two basic approaches that summarize the) 1709 SB 300 542 139 (conflict:) 139 SB 32 0 0 42 42 0 0 0 42 /Symbol font gs 768 3195 0 0 CB 749 588 19 (\267) 19 SB gr 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 768 592 882 ( That which is not expressly permitted is prohibited.) 882 SB 32 0 0 42 42 0 0 0 42 /Symbol font gs 768 3195 0 0 CB 749 638 19 (\267) 19 SB gr 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 768 641 11 ( ) 11 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 779 642 871 (That which is not expressly prohibited is permitted.) 871 SB 480 742 1573 (The importance of this distinction cannot be overemphasized. In the former case, the firewall) 1573 SB 300 792 1736 (must be designed to block everything, and services must be enabled on a case-by-case basis only after a) 1736 SB 300 842 1788 (careful assessment of need and risk. This tends to impact users directly, and they may see the firewall as a) 1788 SB 300 892 1741 (hindrance. In the second case, the systems administrator is placed in a reactive mode, having to predict) 1741 SB 300 942 1751 (what kinds of actions the user population might take that would weaken the security of the firewall, and) 1751 SB 300 992 1734 (preparing defenses against them. This essentially pits the firewall administrator against the users in an) 1734 SB 300 1042 1719 (endless arms race that can become quite fierce. A user can generally compromise the security of their) 1719 SB 300 1092 1777 (login if they try or aren't aware of reasonable security precautions. If the user has an open access login on) 1777 SB 300 1142 1763 (the firewall system itself, a serious security breach can result. The presence of user logins on the firewall) 1763 SB 300 1192 1775 (system tends to magnify the problem of maintaining the system's integrity. A second important statement) 1775 SB 300 1242 1793 (of policy is implicit in the "that which is not expressly permitted is prohibited" stance. This stance is more) 1793 SB 300 1292 1162 ("fail safe," since it accepts that the administrator is ignorant of what ) 1162 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1462 1301 61 (TCP) 61 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1523 1292 566 ( ports are safe, or what holes may) 566 SB 300 1342 1795 (exist in the manufacturer's kernel or applications. Since many vendors are slow to publicise security holes,) 1795 SB 300 1392 1758 (this is clearly a more conservative approach. It is an admission of the fact that what you don't know can) 1758 SB 300 1442 152 (hurt you.) 152 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 1542 347 (Levels of Threat) 347 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 1601 1561 (If ignorant both of your enemy and yourself, you are certain in every battle to be in peril ) 1561 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1892 1588 51 (\276) 51 SB 1891 1588 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1942 1601 156 ( Sun Tzu) 156 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 1701 1606 (There are several ways in which a firewall can fail or be compromised. While none of them are) 1606 SB 300 1751 1760 (good, some are decidedly worse than others. Since the purpose of many firewalls is to block access, it's a) 1760 SB 300 1801 1730 (clear failure if someone finds a loophole through it which permits them to probe systems in the private) 1730 SB 300 1851 1748 (network. An even more severe situation would result if someone managed to break into the firewall and) 1748 SB 300 1901 1729 (reconfigure it such that the entire private network is reachable by anyone. For the sake of terminology,) 1729 SB 300 1951 1695 (this type of attack will be referred to as "destroying" a firewall, as opposed to a mere "break-in." It is) 1695 SB 300 2001 1737 (extremely difficult to quantify the damage that might result from a firewall's destruction. An important) 1737 SB 300 2051 1759 (measure of how well a firewall resists threat is the information it gathers to help determine the course of) 1759 SB 300 2101 1699 (an attack. The absolute worst thing that could happen is for a firewall to be completely compromised) 1699 SB 300 2151 1795 (without any trace of how the attack took place. The best thing that can happen is for a firewall to detect an) 1795 SB 300 2201 1731 (attack, and inform the administrator politely that it is undergoing attack, but that the attack is going to) 1731 SB 300 2251 67 (fail.) 67 SB 480 2351 1617 (One way to view the result of a firewall being compromised is to look at things in terms of what) 1617 SB 300 2401 1790 (can be roughly termed as "zones of risk." In the case of a network that is directly connected to the Internet) 1790 SB 300 2451 1674 (without any firewall, the entire network is subject to attack. This does not imply that the network is) 1674 SB 300 2501 1792 (vulnerable to attack, but in a situation where an entire network is within reach of an untrusted network, it) 1792 SB 300 2551 1736 (is necessary to ensure the security of every single host on that network. Practical experience shows that) 1736 SB 300 2601 541 (this is difficult, since tools like ) 541 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 841 2601 103 (rlogin) 103 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 944 2601 1145 ( that permit user-customizable access control are often exploited by) 1145 SB 300 2651 1738 (vandals to gain access to multiple hosts, in a form of "island hopping" attack. In the case of any typical) 1738 SB 300 2701 1793 (firewall, the zone of risk is often reduced to the firewall itself, or a selected subset of hosts on the network,) 1793 SB 300 2751 1785 (significantly reducing the network manager's concerns with respect to direct attack. If a firewall is broken) 1785 SB 300 2801 1688 (into, the zone of risk often expands again, to include the entire protected network. A vandal gaining) 1688 SB 300 2851 1775 (access to a login on the firewall can begin an island hopping attack into the private network, using it as a) 1775 SB 2079 3033 21 (2) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 3 3 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 0 0 0 fC 300 233 1779 (base. In this situation, there is still some hope, since the vandal may leave traces on the firewall, and may) 1779 SB 300 283 1677 (be detected. If the firewall is completely destroyed the private network can undergo attack from any) 1677 SB 300 333 1450 (external system and reconstructing the course of an attack becomes nearly impossible.) 1450 SB 480 433 1540 (In general, firewalls can be viewed in terms of reducing the zone of risk to a single point of) 1540 SB 300 483 1690 (failure. In a sense, this seems like a bad idea, since it amounts to putting all of one's eggs in a single) 1690 SB 300 533 1773 (basket, but practical experience implies that at any given time, for a network of non-trivial size, there are) 1773 SB 300 583 1771 (at least a few hosts that are vulnerable to break-in by even an unskilled attacker. Many corporations have) 1771 SB 300 633 1745 (formal host security policies that are designed to address these weaknesses, but it is sheer foolishness to) 1745 SB 300 683 1694 (assume that publishing policies will suffice. A firewall enhances host security by funneling attackers) 1694 SB 300 733 1719 (through a narrow gap where there's a chance of catching or detecting them first. The well-constructed) 1719 SB 300 783 1604 (medieval castle had multiple walls and interlocking defense points for exactly the same reason.) 1604 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 883 700 (Firewalls and Their Components) 700 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 942 1410 (There may be a hundred combat postures, but there is only one purpose: to win. ) 1410 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1741 929 51 (\276) 51 SB 1740 929 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1791 942 242 ( Heiho Kaden) 242 SB 300 992 67 (Sho) 67 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 1092 1592 (In discussing firewalls there is often confusion of terminology since firewalls all differ slightly) 1592 SB 300 1142 1020 (in implementation if not in purpose. Various discussions on ) 1020 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1320 1151 126 (USENET) 126 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1446 1142 587 ( indicate that the term "firewall" is) 587 SB 300 1192 1704 (used to describe just about any inter-network security scheme. For the sake of simplifying discussion,) 1704 SB 300 1242 1026 (some terminology is proposed, to provide a common ground:) 1026 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1316 313 (Screening Router) 313 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 763 1317 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 774 1304 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 824 1317 1206 ( A screening router is a basic component of most firewalls. A screening) 1206 SB 450 1367 1531 (router can be a commercial router or a host-based router with some kind of packet filtering) 1531 SB 450 1417 1639 (capability. Typical screening routers have the ability to block traffic between networks or specific) 1639 SB 450 1467 1504 (hosts, on an IP port level. Some firewalls consist of nothing more than a screening router) 1504 SB 450 1517 734 (between a private network and the Internet.) 734 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1591 219 (Bastion host) 219 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 669 1592 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 680 1579 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 730 1592 1344 ( Bastions are the highly fortified parts of a medieval castle; points that overlook) 1344 SB 450 1642 1629 (critical areas of defense, usually having stronger walls, room for extra troops, and the occasional) 1629 SB 450 1692 1589 (useful tub of boiling hot oil for discouraging attackers. A bastion host is a system identified by) 1589 SB 450 1742 1604 (the firewall administrator as a critical strong point in the network's security. Generally, bastion) 1604 SB 450 1792 1628 (hosts will have some degree of extra attention paid to their security, may undergo regular audits,) 1628 SB 450 1842 559 (and may have modified software.) 559 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1916 406 (Dual Homed Gateway ) 406 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 856 1917 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 867 1904 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 917 1917 1071 ( Some firewalls are implemented without a screening router, by) 1071 SB 450 1967 1298 (placing a system on both the private network and the Internet, and disabling ) 1298 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1748 1976 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1847 1967 209 ( forwarding.) 209 SB 450 2017 1648 (Hosts on the private network can communicate with the gateway, as can hosts on the Internet, but) 1648 SB 450 2067 1621 (direct traffic between the networks is blocked. A dual homed gateway is, by definition, a bastion) 1621 SB 450 2117 81 (host.) 81 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 300 2217 726 (Figure 1: A typical Dual Homed Gateway) 726 SB 0 lc 0 lj 0 0 0 pC 6 4 SP 192 192 192 fC /fm 256 def 173 53 1118 2537 B 1 F S n 1 lc 1 lj 1119 2537 M -18 17 -139 0 -17 -17 174 0 4 PP 1 F S n 1121 2534 M 169 0 1 PP S n 0 0 0 fC 1126 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1128 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1131 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1133 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1136 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1138 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1141 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1144 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1146 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1149 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1151 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1154 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1156 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1159 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1162 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1164 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1167 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1169 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1172 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1174 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1177 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1180 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1182 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1185 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1187 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1190 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1193 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1195 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1198 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1200 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1203 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1205 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1208 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1211 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1213 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1216 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1218 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1221 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1223 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1226 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1229 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1231 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1234 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1236 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1239 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1241 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1244 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1247 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1249 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1252 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1254 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1257 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1259 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1262 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1265 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1267 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1270 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1272 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1275 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1278 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1280 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1283 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1285 2577 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1288 2577 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 0 lc 0 lj 6 4 SP 192 192 192 fC 35 27 1123 2541 B 1 F S n 50 27 1159 2541 B 1 F S n 53 27 1210 2541 B 1 F S n 22 27 1265 2541 B 1 F S n 0 0 0 fC 8 12 1274 2544 B 1 F S n 192 192 192 fC 7 1 1275 2548 B 1 F S n 2 0 1267 2546 B 1 F S n 2 0 1267 2553 B 1 F S n 128 128 128 fC 16 9 1176 2545 B 1 F S n 0 0 0 fC 16 4 1176 2550 B 1 F S n 44 3 1162 2549 B 1 F n 6 4 SP 0 128 0 fC 3 1 1161 2545 B 1 F S n 0 0 0 fC 4 4 1199 2558 B 1 F S n 1126 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1128 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1130 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1132 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1134 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1136 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1138 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1140 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1142 2561 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1144 2561 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1145 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1148 2561 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1149 2561 M 0 -6 -2 0 0 6 2 0 4 PP 1 F n 1151 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1153 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1155 2561 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1 lc 1 lj 6 4 SP 1123 2560 M 82 0 0 5 11 0 0 -4 73 0 5 PP S n 0 lc 0 lj 192 192 192 fC 272 6 1066 2618 B 1 F S n 1 lc 1 lj 1066 2618 M -19 31 -239 0 -15 -31 273 0 4 PP 1 F S n 1088 2590 M -12 -24 256 0 -14 24 3 PP S n 128 128 128 fC 1098 2590 M 2 -4 -6 0 -2 4 6 0 4 PP 1 F S n 1118 2590 M 1 -4 -32 0 -2 4 33 0 4 PP 1 F S n 1162 2590 M 2 -4 -33 0 0 4 31 0 4 PP 1 F S n 1200 2590 M 1 -4 -33 0 0 4 32 0 4 PP 1 F S n 1240 2590 M 0 -4 -28 0 0 4 28 0 4 PP 1 F S n 1275 2593 M -2 -3 -34 0 3 3 33 0 4 PP 1 F S n 1107 2598 M 106 0 1 PP S n 1111 2602 M 108 0 1 PP S n 1112 2606 M 94 0 1 PP S n 1114 2609 M 13 0 1 PP S n 1092 2600 M 11 0 1 PP S n 1090 2604 M 10 0 1 PP S n 1087 2607 M 19 0 1 PP S n 1131 2609 M 64 0 1 PP S n 1218 2598 M 15 0 1 PP S n 1222 2602 M 12 0 1 PP S n 1214 2606 M 20 0 1 PP S n 1198 2609 M 10 0 1 PP S n 1211 2610 M 22 0 1 PP S n 1239 2600 M 30 0 1 PP S n 1243 2604 M 26 0 1 PP S n 1244 2610 M 27 0 1 PP S n 1279 2600 M 29 0 1 PP S n 1276 2604 M 24 0 1 PP S n 1279 2607 M 22 0 1 PP S n 1278 2610 M 28 0 1 PP S n 1304 2604 M 8 0 1 PP S n 1307 2608 M 7 0 1 PP S n 192 192 192 fC 1146 2525 M -7 9 -105 0 -7 -9 119 0 4 PP 1 F S n 0 lc 0 lj 118 7 1146 2525 B 1 F S n 1 lc 1 lj 1174 2518 M -4 0 -4 -1 -4 0 -4 1 -3 0 -3 0 -2 1 -3 0 -3 0 -2 1 -2 0 -2 1 -1 1 -2 0 -1 1 -1 0 0 1 0 10 63 0 0 -10 20 PP 0 0 0 -1 -1 -1 -1 0 -1 0 -3 -2 -1 0 -3 -1 -3 0 -3 0 -2 -1 -4 0 12 PP 1 F S n 1132 2403 1278 2510 13 13 RR 1 F S n 128 128 128 fC 1148 2415 1262 2499 10 10 RR 1 F S n 0 128 0 fC 1155 2420 1255 2494 9 9 RR 1 F S n 0 lc 0 lj 3 0 1255 2505 B 1 F S n 1 lc 1 lj 6 5 SP 255 255 255 fC 154 155 542 2553 E 1 F S n 32 0 0 47 47 0 0 0 42 /Times-Roman /font28 ANSIFont font 0 0 0 fC gs 1709 508 345 2270 CB 445 2526 169 ( Internet) 169 SB gr 255 255 255 fC 154 155 1856 2553 E 1 F S n 0 0 0 fC gs 1709 508 345 2270 CB 1787 2498 144 ( Private) 144 SB gr gs 1709 508 345 2270 CB 1787 2555 165 (Network) 165 SB gr 6 9 SP 724 2540 M 364 0 1 PP S n 1311 2540 M 364 0 1 PP S n 1227 2341 M 83 85 -83 57 83 56 -55 57 55 57 -55 57 6 PP S n gs 1709 508 345 2270 CB 864 2725 688 (No traffic directly between networks) 688 SB gr gs 1709 508 345 2270 CB 1088 2271 242 (Bastion Host) 242 SB gr 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 2079 3033 21 (3) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Roman %%Page: 4 4 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 0 0 0 fC 480 331 428 (Screened Host Gateway) 428 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 908 332 22 ( ) 22 SB 32 0 0 50 50 0 0 0 51 /Symbol font 930 319 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 980 332 1066 ( Possibly the most common firewall configuration is a screened) 1066 SB 300 382 1796 (host gateway. This is implemented using a screening router and a bastion host. Usually, the bastion host is) 1796 SB 300 432 1795 (on the private network, and the screening router is configured such that the bastion host is the only system) 1795 SB 300 482 1706 (on the private network that is reachable from the Internet. Often the screening router is configured to) 1706 SB 300 532 1598 (block traffic to the bastion host on specific ports, permitting only a small number of services to) 1598 SB 300 582 358 (communicate with it.) 358 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 300 682 754 (Figure 2: A typical Screened Host Gateway) 754 SB 1 lc 1 lj 0 0 0 pC 6 4 SP 255 255 255 fC /fm 256 def 147 156 473 1024 E 1 F S n 32 0 0 47 47 0 0 0 42 /Times-Roman /font28 ANSIFont font 0 0 0 fC gs 1833 662 283 729 CB 380 997 169 ( Internet) 169 SB gr 255 255 255 fC 214 227 1642 1009 E 1 F S n 0 0 0 fC gs 1833 662 283 729 CB 1569 948 144 ( Private) 144 SB gr gs 1833 662 283 729 CB 1569 1031 165 (Network) 165 SB gr 63 63 63 fC 974 1035 982 1038 4 1 RR 1 F n 1119 1035 1128 1038 4 1 RR 1 F n 6 4 SP 192 192 192 fC 971 985 M -161 0 24 31 113 0 24 -31 4 PP 1 F S n 0 lc 0 lj 159 50 971 986 B 1 F S n 159 159 159 fC 154 44 974 989 B 1 F S n 95 95 95 fC 9 15 978 995 B 1 F S n 0 128 0 fC 5 4 978 1022 B 1 F S n 192 192 192 fC 95 35 1028 993 B 1 F S n 128 128 128 fC 33 4 1088 1018 B 1 F n 95 95 95 fC 1101 1017 M -16 -1 0 -4 16 5 3 PP 1 F n 63 63 63 fC 4 4 1117 1018 B 1 F n 6 4 SP 32 3 1088 1018 B S n 159 159 159 fC /fm 256 def 1121 995 M 0 -21 2 3 0 15 -2 3 4 PP 1 F n 1030 995 M 0 -21 -2 3 0 15 2 3 4 PP 1 F n 95 95 95 fC 1030 995 M -92 0 3 -3 87 0 2 3 4 PP 1 F n 128 128 128 fC 1030 1016 M -91 0 2 3 87 0 2 -3 4 PP 1 F n 63 63 63 fC 1032 998 M -87 0 0 -15 87 0 0 15 4 PP 1 F n 6 4 SP 90 18 1030 996 B S n 95 95 95 fC /fm 256 def 7 9 1070 1018 B 1 F S n 128 0 0 fC 1 3 1043 1024 B 1 F n 255 0 0 fC 1 3 1049 1024 B 1 F n 63 63 63 fC 0 3 1030 1024 B 1 F n 0 3 1032 1024 B 1 F n 0 3 1033 1024 B 1 F n 1 3 1034 1024 B 1 F n 0 3 1036 1024 B 1 F n 0 3 1038 1024 B 1 F n 1 3 1058 1024 B 1 F n 0 3 1060 1024 B 1 F n 0 3 1061 1024 B 1 F n 0 3 1063 1024 B 1 F n 0 3 1065 1024 B 1 F n 0 3 1066 1024 B 1 F n 0 3 1068 1024 B 1 F n 0 3 1079 1024 B 1 F n 0 3 1080 1024 B 1 F n 0 3 1082 1024 B 1 F n 0 3 1084 1024 B 1 F n 0 3 1085 1024 B 1 F n 0 3 1087 1024 B 1 F n -1 3 1089 1024 B 1 F n 0 3 1090 1024 B 1 F n 1 3 1091 1024 B 1 F n 0 3 1093 1024 B 1 F n 0 3 1095 1024 B 1 F n 0 3 1097 1024 B 1 F n 0 3 1098 1024 B 1 F n 0 3 1099 1024 B 1 F n 0 3 1101 1024 B 1 F n 0 3 1103 1024 B 1 F n 0 3 1104 1024 B 1 F n 0 3 1106 1024 B 1 F n -1 3 1108 1024 B 1 F n 0 3 1109 1024 B 1 F n 0 3 1111 1024 B 1 F n 0 3 1112 1024 B 1 F n 0 3 1114 1024 B 1 F n 0 3 1116 1024 B 1 F n 0 3 1117 1024 B 1 F n 0 3 1119 1024 B 1 F n -1 3 1121 1024 B 1 F n 1 3 1039 1024 B 1 F n 0 3 1041 1024 B 1 F n 0 3 1046 1024 B 1 F n 0 3 1047 1024 B 1 F n 0 3 1052 1024 B 1 F n 1 3 1053 1024 B 1 F n 0 3 1055 1024 B 1 F n 0 3 1057 1024 B 1 F n 6 4 SP 192 192 192 fC 166 52 1404 1235 B 1 F S n 1 lc 1 lj 1404 1235 M -18 17 -133 0 -16 -17 167 0 4 PP 1 F S n 1407 1232 M 162 0 1 PP S n 0 0 0 fC 1410 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1413 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1416 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1418 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1421 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1423 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1425 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1428 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1430 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1433 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1435 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1438 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1440 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1442 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1445 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1447 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1450 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1452 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1455 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1457 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1460 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1463 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1465 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1468 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1470 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1473 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1475 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1478 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1480 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1482 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1485 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1487 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1490 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1492 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1494 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1497 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1499 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1502 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1504 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1507 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1510 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1512 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1515 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1517 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1520 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1522 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1524 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1527 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1529 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1532 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1534 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1537 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1539 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1541 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1544 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1546 1275 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1549 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1552 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1554 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1557 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1559 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1562 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1564 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1567 1275 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 0 lc 0 lj 6 4 SP 192 192 192 fC 34 27 1408 1239 B 1 F S n 47 27 1443 1239 B 1 F S n 52 27 1491 1239 B 1 F S n 21 27 1545 1239 B 1 F S n 0 0 0 fC 7 12 1554 1242 B 1 F S n 192 192 192 fC 5 1 1555 1246 B 1 F S n 2 0 1547 1244 B 1 F S n 2 0 1547 1251 B 1 F S n 128 128 128 fC 16 9 1459 1243 B 1 F S n 0 0 0 fC 16 4 1459 1248 B 1 F S n 42 3 1446 1247 B 1 F n 6 4 SP 0 128 0 fC 3 1 1444 1243 B 1 F S n 0 0 0 fC 4 4 1481 1256 B 1 F S n 1411 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1413 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1414 1259 M 0 -6 -2 0 0 6 2 0 4 PP 1 F n 1417 1259 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1418 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1421 1259 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1422 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1424 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1426 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1428 1259 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1430 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1432 1259 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1433 1259 M 0 -6 -2 0 0 6 2 0 4 PP 1 F n 1435 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1437 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1439 1259 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1 lc 1 lj 6 4 SP 1408 1258 M 78 0 0 5 11 0 0 -4 70 0 5 PP S n 0 lc 0 lj 192 192 192 fC 262 6 1353 1316 B 1 F S n 1 lc 1 lj 1353 1316 M -19 31 -229 0 -15 -31 263 0 4 PP 1 F S n 1375 1288 M -11 -24 246 0 -14 24 3 PP S n 128 128 128 fC 1384 1288 M 2 -4 -6 0 -2 4 6 0 4 PP 1 F S n 1403 1288 M 1 -4 -31 0 -2 4 32 0 4 PP 1 F S n 1445 1288 M 1 -4 -31 0 0 4 30 0 4 PP 1 F S n 1482 1288 M 1 -4 -32 0 0 4 31 0 4 PP 1 F S n 1520 1288 M 0 -4 -28 0 1 4 27 0 4 PP 1 F S n 1554 1290 M -2 -4 -33 0 3 4 32 0 4 PP 1 F S n 1393 1296 M 101 0 1 PP S n 1396 1300 M 104 0 1 PP S n 1398 1303 M 90 0 1 PP S n 1400 1307 M 12 0 1 PP S n 1378 1298 M 11 0 1 PP S n 1376 1302 M 10 0 1 PP S n 1374 1305 M 17 0 1 PP S n 1416 1307 M 61 0 1 PP S n 1499 1296 M 15 0 1 PP S n 1503 1300 M 12 0 1 PP S n 1496 1303 M 19 0 1 PP S n 1480 1307 M 9 0 1 PP S n 1492 1308 M 21 0 1 PP S n 1520 1298 M 28 0 1 PP S n 1524 1302 M 25 0 1 PP S n 1524 1308 M 26 0 1 PP S n 1558 1298 M 28 0 1 PP S n 1555 1302 M 23 0 1 PP S n 1558 1305 M 21 0 1 PP S n 1557 1308 M 26 0 1 PP S n 1582 1302 M 8 0 1 PP S n 1585 1306 M 7 0 1 PP S n 192 192 192 fC 1430 1222 M -7 8 -101 0 -7 -8 115 0 4 PP 1 F S n 0 lc 0 lj 113 7 1430 1223 B 1 F S n 1 lc 1 lj 1457 1215 M -4 0 -3 0 -5 0 -3 0 -3 0 -3 0 -3 1 -2 0 -3 0 -2 1 -2 0 -1 1 -2 1 -2 0 -1 1 0 0 0 2 0 9 60 0 0 -9 20 PP 0 -1 0 -1 -1 -1 -1 0 -1 0 -3 -2 -1 0 -3 -1 -3 0 -2 0 -2 -1 -4 0 12 PP 1 F S n 1416 1101 1557 1208 13 13 RR 1 F S n 128 128 128 fC 1433 1113 1541 1196 10 10 RR 1 F S n 0 128 0 fC 1439 1117 1535 1192 9 9 RR 1 F S n 0 lc 0 lj 3 0 1535 1203 B 1 F S n 0 0 0 fC gs 1833 662 283 729 CB 1347 1338 242 (Bastion Host) 242 SB gr 1 lc 1 lj 128 128 128 pC 6 9 SP 649 1124 M 349 114 376 0 2 PP S n gs 1833 662 283 729 CB 649 1139 324 (Traffic Permitted) 324 SB gr 649 926 M 1209 0 1 PP S n gs 1833 662 283 729 CB 649 855 297 (Traffic Blocked) 297 SB gr 0 0 0 pC 1213 897 M -81 57 1 PP S n 1132 897 M 81 57 1 PP S n 0 lc 0 lj 6 4 SP 192 192 192 fC 166 52 1860 1008 B 1 F S n 1 lc 1 lj 1860 1008 M -18 17 -134 0 -16 -17 168 0 4 PP 1 F S n 1863 1005 M 163 0 1 PP S n 0 0 0 fC 1867 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1869 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1872 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1874 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1877 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1879 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1882 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1884 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1887 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1890 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1892 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1895 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1897 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1899 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1902 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1904 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1907 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1909 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1912 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1914 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1917 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1919 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1921 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1924 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1926 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1929 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1931 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1934 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1937 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1939 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1942 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1944 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1947 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1949 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1951 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1954 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1956 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1959 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1961 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1963 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1966 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1968 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1971 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1973 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 1976 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1979 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1981 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1984 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1986 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1989 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1991 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1994 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1996 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 1998 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 2001 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2003 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 2006 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2008 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2011 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2013 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2016 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2018 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 2020 1048 M 0 -11 -2 0 0 11 2 0 4 PP 1 F n 2023 1048 M 0 -11 -1 0 0 11 1 0 4 PP 1 F n 0 lc 0 lj 6 4 SP 192 192 192 fC 33 27 1865 1012 B 1 F S n 48 27 1899 1012 B 1 F S n 52 27 1948 1012 B 1 F S n 22 27 2001 1012 B 1 F S n 0 0 0 fC 7 12 2010 1015 B 1 F S n 192 192 192 fC 6 1 2011 1019 B 1 F S n 2 0 2003 1017 B 1 F S n 2 0 2003 1024 B 1 F S n 128 128 128 fC 15 9 1916 1016 B 1 F S n 0 0 0 fC 15 5 1916 1020 B 1 F S n 42 3 1902 1020 B 1 F n 6 4 SP 0 128 0 fC 3 1 1901 1016 B 1 F S n 0 0 0 fC 4 4 1937 1029 B 1 F S n 1868 1032 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1869 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1871 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1873 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1875 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1877 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1879 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1881 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1883 1032 M 0 -6 0 0 0 6 0 0 4 PP 1 F n 1884 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1886 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1888 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1890 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1892 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1894 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1896 1032 M 0 -6 -1 0 0 6 1 0 4 PP 1 F n 1 lc 1 lj 6 4 SP 1865 1031 M 79 0 0 4 10 0 0 -4 70 0 5 PP S n 0 lc 0 lj 192 192 192 fC 261 7 1810 1088 B 1 F S n 1 lc 1 lj 1809 1089 M -20 31 -228 0 -16 -31 264 0 4 PP 1 F S n 1831 1061 M -12 -24 247 0 -14 24 3 PP S n 128 128 128 fC 1841 1060 M 2 -5 -6 0 -1 5 5 0 4 PP 1 F S n 1860 1060 M 1 -5 -31 0 -1 5 31 0 4 PP 1 F S n 1902 1060 M 1 -5 -30 0 0 5 29 0 4 PP 1 F S n 1938 1060 M 0 -5 -32 0 0 5 32 0 4 PP 1 F S n 1977 1060 M 0 -5 -27 0 0 5 27 0 4 PP 1 F S n 2011 1063 M -2 -4 -33 0 3 4 32 0 4 PP 1 F S n 1849 1069 M 102 0 1 PP S n 1853 1073 M 104 0 1 PP S n 1854 1076 M 91 0 1 PP S n 1857 1080 M 12 0 1 PP S n 1835 1071 M 10 0 1 PP S n 1833 1075 M 10 0 1 PP S n 1830 1078 M 18 0 1 PP S n 1872 1080 M 62 0 1 PP S n 1956 1069 M 15 0 1 PP S n 1960 1073 M 11 0 1 PP S n 1952 1076 M 19 0 1 PP S n 1937 1080 M 9 0 1 PP S n 1949 1081 M 21 0 1 PP S n 1976 1071 M 29 0 1 PP S n 1980 1075 M 25 0 1 PP S n 1981 1081 M 25 0 1 PP S n 2015 1071 M 27 0 1 PP S n 2012 1075 M 22 0 1 PP S n 2015 1078 M 21 0 1 PP S n 2014 1081 M 26 0 1 PP S n 2039 1075 M 7 0 1 PP S n 2042 1079 M 6 0 1 PP S n 192 192 192 fC 1887 995 M -7 8 -101 0 -6 -8 114 0 4 PP 1 F S n 0 lc 0 lj 113 6 1887 996 B 1 F S n 1 lc 1 lj 1914 988 M -3 0 -4 0 -5 0 -3 0 -3 0 -3 0 -2 1 -3 0 -2 0 -3 1 -2 1 -1 0 -2 1 -1 0 -1 1 -1 0 0 2 0 9 60 0 0 -9 20 PP 0 -1 0 -1 -1 -1 -1 0 -1 0 -2 -1 -2 -1 -3 -1 -2 0 -3 0 -2 -1 -4 0 12 PP 1 F S n 1873 874 2014 981 13 13 RR 1 F S n 128 128 128 fC 1889 886 1998 969 10 10 RR 1 F S n 0 128 0 fC 1895 890 1992 965 9 9 RR 1 F S n 0 lc 0 lj 3 0 1992 976 B 1 F S n 0 0 0 fC gs 1833 662 283 729 CB 1831 1111 225 (Other Hosts) 225 SB gr gs 1833 662 283 729 CB 917 1054 327 (Screening Router) 327 SB gr 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1516 299 (Screened Subnet) 299 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 749 1517 22 ( ) 22 SB 32 0 0 50 50 0 0 0 51 /Symbol font 771 1504 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 821 1517 1188 ( In some firewall configurations, an isolated subnet is created, situated) 1188 SB 450 1567 1624 (between the Internet and the private network. Typically, this network is isolated using screening) 1624 SB 450 1617 1498 (routers, which may implement varying levels of filtering. Generally, a screened subnet is) 1498 SB 450 1667 1511 (configured such that both the Internet and the private network have access to hosts on the) 1511 SB 450 1717 1503 (screened subnet, but traffic across the screened subnet is blocked. Some configurations of) 1503 SB 450 1767 1600 (screened subnets will have a bastion host on the screened network, either to support interactive) 1600 SB 450 1817 801 (terminal sessions or application level gateways.) 801 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 300 1917 632 (Figure 3: A typical Screened Subnet) 632 SB 1 lc 1 lj 255 255 255 fC 133 130 435 2194 E 1 F S n 32 0 0 39 39 0 0 0 34 /Times-Roman /font28 ANSIFont font 0 0 0 fC gs 1835 631 260 1964 CB 352 2173 140 ( Internet) 140 SB gr 255 255 255 fC 193 188 1711 2206 E 1 F S n 0 0 0 fC gs 1835 631 260 1964 CB 1646 2156 119 ( Private) 119 SB gr gs 1835 631 260 1964 CB 1646 2225 135 (Network) 135 SB gr 0 lc 0 lj 192 192 192 fC 150 43 1860 2205 B 1 F S n 1 lc 1 lj 1860 2205 M -16 14 -121 0 -14 -14 151 0 4 PP 1 F S n 1863 2202 M 146 0 1 PP S n 0 0 0 fC 1866 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1868 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1871 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1873 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1875 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1877 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1879 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1882 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1884 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1886 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1888 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1891 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1893 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1895 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1898 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1900 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1902 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1904 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1907 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1909 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1911 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1913 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1915 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1918 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1920 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1922 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1924 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1927 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1929 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1931 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1934 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1936 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1938 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1940 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1942 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1945 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1947 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1949 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1951 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1953 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1956 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1958 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1960 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1962 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1965 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1967 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1969 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1971 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1974 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1976 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1978 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1981 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1983 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1985 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1987 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1989 2238 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 1992 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1994 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1996 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 1998 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 2001 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 2003 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 2005 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 2007 2238 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 0 lc 0 lj 6 4 SP 192 192 192 fC 30 21 1864 2209 B 1 F S n 42 21 1896 2209 B 1 F S n 46 21 1940 2209 B 1 F S n 18 21 1988 2209 B 1 F S n 0 0 0 fC 6 9 1996 2211 B 1 F S n 192 192 192 fC 4 0 1997 2215 B 1 F S n 1 -1 1990 2213 B 1 F S n 1 -1 1990 2219 B 1 F S n 128 128 128 fC 14 6 1910 2212 B 1 F S n 0 0 0 fC 14 2 1910 2216 B 1 F S n 38 2 1898 2215 B 1 F n 6 4 SP 0 128 0 fC 2 0 1897 2212 B 1 F S n 0 0 0 fC 3 2 1930 2223 B 1 F S n 1867 2225 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 1868 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1870 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1872 2225 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 1873 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1875 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1877 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1878 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1880 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1882 2225 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 1883 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1885 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1887 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1888 2225 M 0 -5 -2 0 0 5 2 0 4 PP 1 F n 1891 2225 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 1892 2225 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1 lc 1 lj 6 4 SP 1864 2224 M 71 0 0 3 10 0 0 -3 63 0 5 PP S n 0 lc 0 lj 192 192 192 fC 235 5 1815 2272 B 1 F S n 1 lc 1 lj 1814 2272 M -17 26 -207 0 -14 -26 238 0 4 PP 1 F S n 1834 2249 M -10 -20 223 0 -13 20 3 PP S n 128 128 128 fC 1842 2249 M 2 -4 -6 0 -2 4 6 0 4 PP 1 F S n 1860 2249 M 1 -3 -28 0 -1 3 28 0 4 PP 1 F S n 1898 2249 M 1 -4 -27 0 0 4 26 0 4 PP 1 F S n 1931 2249 M 1 -4 -29 0 0 4 28 0 4 PP 1 F S n 1965 2249 M 0 -4 -25 0 0 4 25 0 4 PP 1 F S n 1996 2251 M -2 -3 -30 0 3 3 29 0 4 PP 1 F S n 1850 2256 M 92 0 1 PP S n 1854 2259 M 93 0 1 PP S n 1855 2262 M 82 0 1 PP S n 1857 2265 M 11 0 1 PP S n 1837 2257 M 9 0 1 PP S n 1835 2260 M 9 0 1 PP S n 1833 2263 M 16 0 1 PP S n 1871 2265 M 55 0 1 PP S n 1947 2255 M 13 0 1 PP S n 1950 2259 M 10 0 1 PP S n 1943 2262 M 17 0 1 PP S n 1929 2265 M 9 0 1 PP S n 1940 2265 M 19 0 1 PP S n 1965 2257 M 26 0 1 PP S n 1968 2261 M 23 0 1 PP S n 1969 2265 M 23 0 1 PP S n 2000 2257 M 25 0 1 PP S n 1997 2260 M 21 0 1 PP S n 2000 2263 M 19 0 1 PP S n 1999 2266 M 24 0 1 PP S n 2022 2260 M 6 0 1 PP S n 2024 2264 M 6 0 1 PP S n 192 192 192 fC 1884 2194 M -7 7 -91 0 -6 -7 104 0 4 PP 1 F S n 0 lc 0 lj 102 5 1884 2195 B 1 F S n 1 lc 1 lj 1908 2188 M -4 0 -3 0 -4 0 -3 0 -3 0 -2 0 -3 1 -2 0 -2 0 -2 1 -2 0 -1 1 -2 0 -1 1 -1 0 -1 1 0 1 0 7 55 0 0 -7 20 PP 0 -1 -1 0 0 -1 -1 0 -1 -1 -2 -1 -2 0 -2 -1 -3 0 -2 0 -2 -1 -3 0 12 PP 1 F S n 1872 2093 1998 2182 12 11 RR 1 F S n 128 128 128 fC 1886 2103 1984 2172 9 8 RR 1 F S n 0 128 0 fC 1892 2107 1979 2168 8 7 RR 1 F S n 0 lc 0 lj 2 -1 1979 2179 B 1 F S n 0 0 0 fC gs 1835 631 260 1964 CB 1105 2339 266 (Traffic Permitted) 266 SB gr gs 1835 631 260 1964 CB 1105 2031 244 (Traffic Blocked) 244 SB gr gs 1835 631 260 1964 CB 1833 2291 186 (Other Hosts) 186 SB gr 1 lc 1 lj 255 255 255 fC 133 129 897 2265 E 1 F S n 0 0 0 fC gs 1835 631 260 1964 CB 813 2220 153 ( Screened) 153 SB gr gs 1835 631 260 1964 CB 813 2268 119 ( Subnet) 119 SB gr 63 63 63 fC 1229 2227 1236 2229 3 1 RR 1 F n 1360 2227 1368 2229 4 1 RR 1 F n 6 4 SP 192 192 192 fC 1226 2185 M -146 0 22 25 103 0 21 -25 4 PP 1 F S n 0 lc 0 lj 143 40 1227 2187 B 1 F S n 159 159 159 fC 139 36 1229 2189 B 1 F S n 95 95 95 fC 8 12 1232 2194 B 1 F S n 0 128 0 fC 4 2 1232 2217 B 1 F S n 192 192 192 fC 85 29 1278 2192 B 1 F S n 128 128 128 fC 30 3 1331 2213 B 1 F n 95 95 95 fC 1343 2213 M -15 0 0 -4 15 4 3 PP 1 F n 63 63 63 fC 3 3 1358 2213 B 1 F n 6 4 SP 29 2 1332 2214 B S n 159 159 159 fC /fm 256 def 1362 2194 M 0 -17 2 2 0 13 -2 2 4 PP 1 F n 1279 2194 M 0 -18 -2 3 0 13 2 2 4 PP 1 F n 95 95 95 fC 1279 2194 M -83 0 2 -2 79 0 2 2 4 PP 1 F n 128 128 128 fC 1279 2211 M -83 0 2 2 79 0 2 -2 4 PP 1 F n 63 63 63 fC 1281 2196 M -79 0 0 -13 79 0 0 13 4 PP 1 F n 6 4 SP 81 15 1280 2195 B S n 95 95 95 fC /fm 256 def 5 7 1316 2213 B 1 F S n 128 0 0 fC 1 1 1291 2219 B 1 F n 255 0 0 fC 1 1 1296 2219 B 1 F n 63 63 63 fC 0 1 1279 2219 B 1 F n 0 1 1281 2219 B 1 F n 0 1 1282 2219 B 1 F n 0 1 1283 2219 B 1 F n 0 1 1285 2219 B 1 F n 0 1 1286 2219 B 1 F n 0 1 1305 2219 B 1 F n 0 1 1307 2219 B 1 F n 0 1 1308 2219 B 1 F n 0 1 1309 2219 B 1 F n 0 1 1311 2219 B 1 F n 0 1 1312 2219 B 1 F n 0 1 1314 2219 B 1 F n -1 1 1324 2219 B 1 F n 0 1 1325 2219 B 1 F n 0 1 1326 2219 B 1 F n 0 1 1328 2219 B 1 F n 0 1 1329 2219 B 1 F n 0 1 1331 2219 B 1 F n 0 1 1332 2219 B 1 F n 0 1 1333 2219 B 1 F n 0 1 1335 2219 B 1 F n 0 1 1336 2219 B 1 F n 0 1 1338 2219 B 1 F n 0 1 1339 2219 B 1 F n 0 1 1341 2219 B 1 F n 0 1 1342 2219 B 1 F n 0 1 1343 2219 B 1 F n 0 1 1345 2219 B 1 F n 0 1 1347 2219 B 1 F n 0 1 1348 2219 B 1 F n -1 1 1350 2219 B 1 F n 0 1 1351 2219 B 1 F n 0 1 1352 2219 B 1 F n 0 1 1354 2219 B 1 F n 0 1 1355 2219 B 1 F n 0 1 1357 2219 B 1 F n 0 1 1358 2219 B 1 F n 0 1 1360 2219 B 1 F n 0 1 1361 2219 B 1 F n 0 1 1288 2219 B 1 F n 0 1 1289 2219 B 1 F n 0 1 1293 2219 B 1 F n 0 1 1295 2219 B 1 F n 0 1 1299 2219 B 1 F n 0 1 1300 2219 B 1 F n 0 1 1302 2219 B 1 F n 0 1 1304 2219 B 1 F n 0 0 0 fC gs 1835 631 260 1964 CB 765 2552 201 (Bastion Host) 201 SB gr 6 4 SP 192 192 192 fC 150 43 791 2465 B 1 F S n 1 lc 1 lj 791 2465 M -17 14 -121 0 -14 -14 152 0 4 PP 1 F S n 794 2462 M 147 0 1 PP S n 0 0 0 fC 797 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 799 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 802 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 804 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 807 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 809 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 811 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 813 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 815 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 818 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 820 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 822 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 824 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 826 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 829 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 831 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 833 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 835 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 838 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 840 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 842 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 845 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 847 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 849 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 851 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 854 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 856 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 858 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 860 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 862 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 865 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 867 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 869 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 871 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 873 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 876 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 878 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 880 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 882 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 884 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 887 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 889 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 892 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 894 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 896 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 898 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 900 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 903 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 905 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 907 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 909 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 912 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 914 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 916 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 918 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 920 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 923 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 925 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 928 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 930 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 932 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 934 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 936 2498 M 0 -10 -2 0 0 10 2 0 4 PP 1 F n 939 2498 M 0 -10 -1 0 0 10 1 0 4 PP 1 F n 0 lc 0 lj 6 4 SP 192 192 192 fC 30 21 795 2469 B 1 F S n 42 21 827 2469 B 1 F S n 46 21 871 2469 B 1 F S n 19 21 919 2469 B 1 F S n 0 0 0 fC 6 10 927 2471 B 1 F S n 192 192 192 fC 5 0 928 2475 B 1 F S n 1 -1 921 2473 B 1 F S n 1 -1 921 2479 B 1 F S n 128 128 128 fC 14 7 841 2472 B 1 F S n 0 0 0 fC 14 3 841 2476 B 1 F S n 38 2 829 2475 B 1 F n 6 4 SP 0 128 0 fC 2 0 828 2472 B 1 F S n 0 0 0 fC 3 3 861 2483 B 1 F S n 798 2485 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 799 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 801 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 803 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 805 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 807 2485 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 808 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 810 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 812 2485 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 813 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 815 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 817 2485 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 818 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 820 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 822 2485 M 0 -5 0 0 0 5 0 0 4 PP 1 F n 823 2485 M 0 -5 -1 0 0 5 1 0 4 PP 1 F n 1 lc 1 lj 6 4 SP 795 2484 M 71 0 0 4 9 0 0 -3 64 0 5 PP S n 0 lc 0 lj 192 192 192 fC 236 5 746 2532 B 1 F S n 1 lc 1 lj 745 2532 M -18 26 -207 0 -13 -26 238 0 4 PP 1 F S n 765 2509 M -10 -20 223 0 -13 20 3 PP S n 128 128 128 fC 774 2509 M 2 -4 -5 0 -2 4 5 0 4 PP 1 F S n 791 2509 M 1 -3 -28 0 -1 3 28 0 4 PP 1 F S n 829 2509 M 1 -4 -28 0 0 4 27 0 4 PP 1 F S n 862 2509 M 1 -4 -29 0 0 4 28 0 4 PP 1 F S n 897 2509 M 0 -4 -24 0 0 4 24 0 4 PP 1 F S n 928 2511 M -1 -3 -30 0 3 3 28 0 4 PP 1 F S n 781 2516 M 92 0 1 PP S n 785 2519 M 93 0 1 PP S n 786 2522 M 82 0 1 PP S n 788 2525 M 11 0 1 PP S n 768 2517 M 10 0 1 PP S n 767 2521 M 9 0 1 PP S n 764 2523 M 16 0 1 PP S n 802 2525 M 56 0 1 PP S n 878 2516 M 13 0 1 PP S n 881 2519 M 11 0 1 PP S n 874 2522 M 18 0 1 PP S n 860 2525 M 9 0 1 PP S n 871 2526 M 20 0 1 PP S n 896 2517 M 26 0 1 PP S n 900 2521 M 22 0 1 PP S n 900 2526 M 23 0 1 PP S n 931 2517 M 25 0 1 PP S n 929 2521 M 20 0 1 PP S n 931 2523 M 19 0 1 PP S n 930 2526 M 24 0 1 PP S n 953 2521 M 6 0 1 PP S n 955 2525 M 6 0 1 PP S n 192 192 192 fC 815 2455 M -7 8 -91 0 -6 -8 104 0 4 PP 1 F S n 0 lc 0 lj 101 5 816 2455 B 1 F S n 1 lc 1 lj 839 2449 M -3 0 -3 -1 -4 0 -3 1 -3 0 -2 0 -3 0 -2 1 -2 0 -3 0 -2 1 -1 0 -1 1 -2 0 -1 1 0 0 0 1 0 8 55 0 0 -8 20 PP 0 0 -1 -1 0 -1 -1 0 -1 0 -3 -1 -1 -1 -3 0 -2 0 -2 -1 -2 0 -4 0 12 PP 1 F S n 803 2354 930 2442 12 11 RR 1 F S n 128 128 128 fC 818 2363 915 2432 9 9 RR 1 F S n 0 128 0 fC 823 2367 910 2428 8 8 RR 1 F S n 0 lc 0 lj 2 -1 910 2439 B 1 F S n 1 lc 1 lj 128 128 128 pC 6 8 SP 570 2113 M 292 0 73 0 510 0 73 0 4 PP S n 1056 2302 M 437 0 1 PP S n 595 2207 M 145 47 1 PP S n 0 0 0 fC gs 1835 631 260 1964 CB 1178 2244 272 (Screening Router) 272 SB gr 0 0 0 pC 1348 2089 M -73 47 1 PP S n 1275 2089 M 73 47 1 PP S n 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 2720 488 (Application Level Gateway) 488 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 938 2721 88 ( \(or ") 88 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 1026 2720 261 (proxy gateway) 261 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1287 2721 53 ("\) ) 53 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1340 2708 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1390 2721 629 ( Much of the software on the Internet) 629 SB 450 2771 825 (works in a store-and-forward mode; mailers and ) 825 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1275 2780 126 (USENET) 126 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1401 2771 600 ( news collect input, examine it, and) 600 SB 450 2821 1638 (forward it. Application level gateways are service-specific forwarders or reflectors, which usually) 1638 SB 450 2871 1612 (operate in user mode rather than at a protocol level. Generally, these forwarding services, when) 1612 SB 2079 3033 21 (4) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Roman %%Page: 5 5 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 0 0 0 fC 450 233 1320 (running on a firewall, are important to the security of the whole. The famous ) 1320 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1770 233 152 (sendmail) 152 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1922 233 170 ( hole that) 170 SB 450 283 1598 (was exploited by the Morris Internet worm is one example of the kinds of security problems an) 1598 SB 450 333 1600 (application level gateway can present. Other application level gateways are interactive, such as) 1600 SB 450 383 63 (the ) 63 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 513 383 11 ( ) 11 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 524 391 56 (FTP) 56 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 580 383 95 ( and ) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 675 383 95 (telnet) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 770 383 1326 ( gateways run on the Digital Equipment Corporation firewalls. In general, the) 1326 SB 450 433 1585 (term "application level gateway" will be used to describe some kind of forwarding service that) 1585 SB 450 483 1579 (runs across a firewall, and is a potential security concern. In general, crucial application level) 1579 SB 450 533 791 (gateways are run on some kind of bastion host.) 791 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 607 328 (Hybrid Gateways ) 328 SB 32 0 0 50 50 0 0 0 51 /Symbol font 778 595 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 828 608 1235 ( Hybrid gateways are the "something else" category in this list. Examples) 1235 SB 450 658 1624 (of such systems might be hosts connected to the Internet, but accessible only through serial lines) 1624 SB 450 708 1533 (connected to an ethernet terminal server on the private network. Such gateways might take) 1533 SB 450 758 1640 (advantage of multiple protocols, or tunneling one protocol over another. Routers might maintain) 1640 SB 450 808 630 (and monitor the complete state of all ) 630 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1080 817 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1179 808 847 ( connections, or somehow examine traffic to try to) 847 SB 450 858 572 (detect and prevent an attack. The ) 572 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1022 867 90 (AT&T) 90 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1112 858 310 ( corporate firewall) 310 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 1422 866 43 ([1]) 43 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1465 858 628 ( is a hybrid gateway combined with a) 628 SB 450 908 213 (bastion host.) 213 SB 480 1008 1528 (Taking the components described above, we can accurately describe most of the forms that) 1528 SB 300 1058 1781 (firewalls take, and can make some general statements about the kinds of security problems each approach) 1781 SB 300 1108 1682 (presents. Assuming that a firewall fulfills its basic purpose of helping protect the network, it is still) 1682 SB 300 1158 987 (important to examine each type of firewall with respect to:) 987 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1232 284 (Damage control) 284 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 734 1233 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 745 1220 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 795 1233 1220 ( If the firewall is compromised, to what kinds of threats does it leave the) 1220 SB 450 1283 1561 (private network open? If destroyed, to what kinds of threats does it leave the private network) 1561 SB 450 1333 99 (open?) 99 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1407 233 (Zones of risk) 233 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 683 1408 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 694 1395 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 744 1408 1350 ( How large is the zone of risk during normal operation? A measure of this is the) 1350 SB 450 1458 1210 (number of hosts or routers that can be probed from the outside network.) 1210 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1532 237 (Failure mode) 237 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 687 1533 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 698 1520 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 748 1533 1225 ( If the firewall is broken into, how easy is this to detect? If the firewall is) 1225 SB 450 1583 1590 (destroyed, how easy is this to detect? In a post mortem, how much information is retained that) 1590 SB 450 1633 591 (can be used to diagnose the attack?) 591 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1707 197 (Ease of use) 197 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 647 1708 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 658 1695 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 708 1708 804 ( How much of an inconvenience is the firewall?) 804 SB 32 0 0 42 42 0 0 0 39 /Times-Bold /font25 ANSIFont font 450 1782 119 (Stance) 119 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 569 1783 11 ( ) 11 SB 32 0 0 50 50 0 0 0 51 /Symbol font 580 1770 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 630 1783 1469 ( Is the basic design philosophy of the firewall "That which is not expressly permitted is) 1469 SB 450 1833 1233 (prohibited" or is it "That which is not expressly prohibited is permitted?") 1233 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 1933 724 (Firewalls using Screening Routers) 724 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 1992 1628 (In large scale strategy, when the enemy embarks on an attack, if you make a show of strongly) 1628 SB 300 2042 919 (suppressing his technique, he will change his mind. ) 919 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1220 2029 51 (\276) 51 SB 1219 2029 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1270 2042 346 ( Miyamoto Musashi) 346 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 2142 1566 (Many networks are firewalled using only a screening router between the private network and) 1566 SB 300 2192 1756 (the Internet. This type of firewall is different from a screened host gateway in that usually there is direct) 1756 SB 300 2242 1673 (communication permitted between multiple hosts on the private network, and multiple hosts on the) 1673 SB 300 2292 1726 (Internet. The zone of risk is equal to the number of hosts on the private networks, and the number and) 1726 SB 300 2342 1746 (type of services to which the screening router permits traffic. For each service provided via peer-to-peer) 1746 SB 300 2392 1762 (connection the size of the zone of risk increases sharply. Eventually it is impossible to quantify. Damage) 1762 SB 300 2442 1783 (control is difficult as well since the network administrator would need to regularly examine every host for) 1783 SB 300 2492 1773 (traces of a break-in. If there is no regular audit one must hope to stumble on a clue such as a mismatched) 1773 SB 300 2542 428 (system accounting record) 428 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 728 2550 43 ([2]) 43 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 771 2542 11 (.) 11 SB 480 2642 1480 (In the case of total destruction of the firewall, it tends to be very hard to trace or even to) 1480 SB 300 2692 1689 (discover. If a commercial router \(which does not maintain logging records\) is used, and the router's) 1689 SB 300 2742 1788 (administrative password is compromised, the entire private network can be laid open to attack very easily.) 1788 SB 300 2792 1781 (Cases are known where commercial routers have been configured with erroneous screening rules, or have) 1781 SB 300 2842 1772 (come up in some pass-through mode because of hardware or operator error. Generally, this configuration) 1772 SB 2079 3033 21 (5) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 6 6 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 0 0 0 fC 300 233 1743 (is a case of "That which is not expressly prohibited is permitted" as the ingenious user can fairly easily) 1743 SB 300 283 1752 (piggyback protocols to achieve a higher level of access than the administrator expects or wants. Given a) 1752 SB 300 333 1769 (collaborator on an external host, it is left as an exercise to the reader to implement a remote login stream) 1769 SB 300 383 751 (protocol over Domain Name Service packets) 751 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1051 379 17 (3) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1068 383 11 (.) 11 SB 480 483 1605 (Screening routers are not the most secure solution, but they are popular since they permit fairly) 1605 SB 300 533 1713 (free Internet access from any point within the private network. Many consultants and network service) 1713 SB 300 583 1699 (providers offer screening routers in a "firewall" configuration. It is uncertain if the various trade-offs) 1699 SB 300 633 1749 (involved are clear to the customer; the author would not recommend use of a screening router to protect) 1749 SB 300 683 1641 (sensitive information or trade secrets, since screening routers are very permeable from the inside.) 1641 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 783 491 (Dual Homed Gateways) 491 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 842 1604 (It was Hannibal's principle to keep his forces united and to garrison but one single fortress ) 1604 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1935 829 51 (\276) 51 SB 1934 829 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 300 892 359 (Emperor Napoleon I) 359 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 992 1504 (An often used and easy to implement firewall is the dual homed gateway. Since it doesn't) 1504 SB 300 1042 143 (forward ) 143 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 443 1051 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 542 1042 1533 ( traffic, it acts as a complete block between the Internet and the private network. Its ease of) 1533 SB 300 1092 1743 (use is determined by how the systems manager chooses to set up access; either by providing application) 1743 SB 300 1142 309 (gateways such as ) 309 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 609 1142 95 (telnet) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 704 1142 1377 ( forwarders or by giving users logins on the gateway host. If the former approach) 1377 SB 300 1192 1746 (is taken, the stance of the firewall is clearly "That which is not expressly permitted is prohibited"; users) 1746 SB 300 1242 1761 (can only access Internet services for which there is an application gateway. If users are permitted logins,) 1761 SB 300 1292 1773 (then, in the opinion of the author, the firewall's security is seriously weakened. During normal operation,) 1773 SB 300 1342 1777 (the only zone of risk is the gateway host itself, since it is the only host that is reachable from the Internet.) 1777 SB 300 1392 1726 (If there are user logins on the gateway host, and one of the users chooses a weak password or has their) 1726 SB 300 1442 1800 (account otherwise compromised, the zone of risk expands to encompass the entire private network. From a) 1800 SB 300 1492 1784 (standpoint of damage control, the administrator may be able to track the progress of an intruder, based on) 1784 SB 300 1542 1764 (the access patterns of the compromised login, but a skillful vandal can make this quite difficult. If a dual) 1764 SB 300 1592 1798 (homed gateway is configured without direct user access, damage control can be somewhat easier, since the) 1798 SB 300 1642 1682 (very fact that someone has logged in to the gateway host becomes a noteworthy security event. Dual) 1682 SB 300 1692 1774 (homed gateways have an advantage over screening routers from the standpoint that their system software) 1774 SB 300 1742 1715 (is often easier to adapt to maintain system logs, hard copy logs, or remote logs. This can make a post-) 1715 SB 300 1792 1709 (mortem easier for the gateway host itself, but may or may not help the network administrator identify) 1709 SB 300 1842 1654 (what other hosts on the private network may have been compromised in an island-hopping attack.) 1654 SB 480 1942 1557 (Attacking a dual homed gateway leaves the attacker a fairly large array of options. Since the) 1557 SB 300 1992 1788 (attacker has what amounts to local network access if a login can be obtained, all the usual attacks that can) 1788 SB 300 2042 741 (be made over a local network are available. ) 741 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1041 2051 60 (NFS) 60 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1101 2042 650 (-mounted file systems, weaknesses in ) 650 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1751 2042 113 (.rhosts) 113 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1864 2042 93 ( files,) 93 SB 300 2092 1677 (automatic software distribution systems, network backup programs and administrative shell scripts ) 1677 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1977 2079 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 2027 2092 54 ( all) 54 SB 300 2142 1755 (may provide a toehold on systems on the internal network. Once a toehold is secured, it then provides a) 1755 SB 300 2192 1674 (base from which to launch attacks back at the gateway itself. The weakest aspect of the dual homed) 1674 SB 300 2242 1696 (gateway is its failure mode. If the firewalll is destroyed it is possible that a skillful attacker might re-) 1696 SB 300 2292 1351 (enable routing and throw the entire private network open to attack. In the usual ) 1351 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1651 2301 81 (UNIX) 81 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1732 2292 317 (-based dual homed) 317 SB 300 2342 159 (gateway, ) 159 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 459 2351 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 558 2342 1092 ( routing is often disabled by modifying a kernel variable named ) 1092 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1650 2342 220 (ipforwarding) 220 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1870 2342 189 (; if systems) 189 SB 300 2392 1774 (privileges can be obtained or stolen on the gateway, this variable can be changed. Perhaps this seems far-) 1774 SB 300 2442 1752 (fetched, but unless great care is paid to monitoring the software revision levels and configuration on the) 1752 SB 300 2492 1755 (gateway host, it is not improbable that a vandal with a copy of the release notes for the operating system) 1755 SB 300 2542 810 (version and a login can compromise the system.) 810 SB 32 0 0 42 42 0 1 0 38 /Times-Roman /font28 ANSIFont font 300 2733 220 ( ) 220 SB 520 2733 220 ( ) 220 SB 740 2733 165 ( ) 165 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 300 2779 17 (3) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 317 2783 1017 (The author has implemented just such a scheme, tunnelling ) 1017 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1334 2792 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1433 2783 597 ( traffic through a firewall. Running) 597 SB 300 2833 84 (over ) 84 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 384 2842 38 (T1) 38 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 422 2833 1388 ( lines interactive session response is quite good and users have complete access to ) 1388 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1810 2833 95 (telnet) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1905 2833 22 (, ) 22 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1927 2833 103 (rlogin) 103 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 2030 2833 11 (,) 11 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 300 2891 56 (FTP) 56 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 356 2883 149 (, etc. As ) 149 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 505 2892 54 (PPP) 54 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 559 2883 1201 ( \(Point to Point Protocol\) becomes more widespread, so will this threat.) 1201 SB 2079 3033 21 (6) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 7 7 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 0 0 0 fC 300 233 527 (Screened Host Gateways) 527 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 292 1751 (Ground to which access is constricted, where the way out is tortuous, and where a small enemy force) 1751 SB 300 342 816 (can strike my larger one is called "encircled." ) 816 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1117 329 93 ( \276 ) 93 SB 1116 329 93 ( \276 ) 93 SB 32 0 0 50 50 0 0 0 46 /Times-BoldItalic /font26 ANSIFont font 1209 334 175 (Sun Tzu) 175 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 442 1401 ( Several articles have described screened host gateways, and how to construct them) 1401 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 1881 450 68 ([3,4]) 68 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1949 442 11 (.) 11 SB 300 492 1774 (Generally, the screened host gateway is very secure, while remaining fairly easy to implement. Typically,) 1774 SB 300 542 1758 (a bastion host is configured on the private network, with a screening router between the Internet and the) 1758 SB 300 592 1737 (private network, which only permits Internet access to the bastion host. Since the bastion host is on the) 1737 SB 300 642 1673 (private network, connectivity for local users is very good, and problems presented by exotic routing) 1673 SB 300 692 1757 (configurations do not present themselves. If the private network is, as many are, a virtual extended local) 1757 SB 300 742 1699 (area network \(e.g.: no subnets or routing\) the screened host gateway will work without requiring any) 1699 SB 300 792 1762 (changes to the local network, as long as the local network is using a legitimately assigned set of network) 1762 SB 300 842 1763 (addresses. The zone of risk of a screened host gateway is restricted to the bastion host, and the screening) 1763 SB 300 892 1780 (router, and the security stance of the screened host gateway is determined by the software running on that) 1780 SB 300 942 1705 (system. If an attacker gains login access to the bastion host, there is a fairly wide range of options for) 1705 SB 300 992 1664 (attacking the rest of the private network. In many ways, this approach is similar to the dual homed) 1664 SB 300 1042 1777 (gateway, sharing similar failure modes and design considerations with respect to the software running on) 1777 SB 300 1092 276 (the bastion host.) 276 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 1192 376 (Screened Subnets) 376 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 1251 1375 (When the enemy gets in an inconvenient position, do not let him look around. ) 1375 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1706 1238 51 (\276) 51 SB 1705 1238 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1756 1251 187 ( Miyamoto) 187 SB 300 1301 148 (Musashi) 148 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 1401 1558 (A screened subnet is usually configured with a bastion host as the sole point of access on the) 1558 SB 300 1451 1749 (subnet. The zone of risk is small, consisting of that bastion host or hosts, and any screening routers that) 1749 SB 300 1501 1761 (make up the connections between the screened subnet, the Internet, and the private network. The ease of) 1761 SB 300 1551 1791 (use and basic stance of the screened subnet will vary, but generally a screened subnet is appealing only for) 1791 SB 300 1601 1778 (firewalls that are taking advantage of routing to reinforce the existing screening. This approach forces all) 1778 SB 300 1651 1779 (services through the firewall to be provided by application gateways, and places the stance strongly in the) 1779 SB 300 1701 1058 ("That which is not expressly permitted is prohibited" category.) 1058 SB 480 1801 1619 (If a screened subnet based firewall with inter-network routing blocked is attacked with an intent) 1619 SB 300 1851 1666 (to destroy it, the attacker must reconfigure the routing on three networks, without disconnecting or) 1666 SB 300 1901 1796 (locking himself out, and without the routing changes being noticed. No doubt this is possible, but it can be) 1796 SB 300 1951 1757 (made very difficult by disabling network access to the screening routers, or by configuring the screening) 1757 SB 300 2001 1737 (routers to only permit access from specific hosts on the private network. In this case, an attacker would) 1737 SB 300 2051 1784 (need to break into the bastion host, then into one of the hosts on the private network, and then back out to) 1784 SB 300 2101 348 (the screening router ) 348 SB 32 0 0 50 50 0 0 0 51 /Symbol font 648 2088 50 (\276) 50 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 698 2101 934 ( and would have to do it without setting off any alarms.) 934 SB 480 2201 1582 (Another advantage of screened subnets is that they can be put in place in such a way that they) 1582 SB 300 2251 1795 (hide any accidents of history that may linger on the private network. Many sites that would like to connect) 1795 SB 300 2301 1748 (to the Internet are daunted by the prospect of re-addressing and re-subnetting existing networks. With a) 1748 SB 300 2351 1734 (screened subnet with blocked inter-network routing, a private network can be connected to the Internet) 1734 SB 300 2401 1759 (and changed gradually to new subnet and network addresses. In fact, this approach has been observed to) 1759 SB 300 2451 1717 (significantly accelerate the adoption of new network addresses on loosely controlled private networks.) 1717 SB 300 2501 1737 (Users will be more receptive to changing their host addresses if they can realize the benefits of Internet) 1737 SB 300 2551 1779 (connectivity thereby, since hosts that are not correctly addressed cannot use the firewall properly. In most) 1779 SB 300 2601 1791 (other respects, the screened subnet is very much dependent on the suite of software running on the bastion) 1791 SB 300 2651 1779 (host. Screening a whole subnet provides functionality similar to the dual homed gateway or screened host) 1779 SB 300 2701 1778 (gateway; it differs primarily in the extra level of complexity in routing and configuration of the screening) 1778 SB 300 2751 128 (routers.) 128 SB 2079 3033 21 (7) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 8 8 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 0 0 0 fC 300 233 376 (Hybrid Gateways) 376 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 292 1705 (There is in guerrilla warfare no such thing as a decisive battle; there is nothing comparable to the) 1705 SB 300 342 982 (fixed, passive defense that is a feature of orthodox war. ) 982 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1283 329 51 (\276) 51 SB 1282 329 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1333 342 454 ( Mao on guerrilla warfare) 454 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 442 1552 ("Security through obscurity" is not sufficient in and of itself, but there is no question that an) 1552 SB 300 492 1727 (unusual configuration, or one that is hard to understand, is likely to give an attacker pause, or to make) 1727 SB 300 542 1747 (them more likely to reveal themselves in the process of trying to figure out what they are facing. On the) 1747 SB 300 592 1707 (other hand there is a real advantage to having a security configuration that is easy to understand, and) 1707 SB 300 642 1751 (therefore easier to evaluate and maintain. Since the hybrid gateway is mentioned here in the category of) 1751 SB 300 692 1744 ("something else" no attempt will be made to describe the indescribable. Some hypothetical hybrids may) 1744 SB 300 742 1464 (serve to show how hybrid gateways might differ from and be similar to the other types.) 1464 SB 480 842 1608 (Let us postulate a hybrid gateway that consists of a box sitting on the Internet, which is capable) 1608 SB 300 892 1285 (of routing traffic, but also maintains a complete notion of the state of every ) 1285 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1585 901 61 (TCP) 61 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1646 892 393 ( connection, how much) 393 SB 300 942 1746 (data has gone across it, where it originated, and its destination. Presumably, connections can be filtered) 1746 SB 300 992 1759 (based on arbitrarily precise rules, such as: "permit traffic between host A on the private network and all) 1759 SB 300 1042 732 (hosts on network B on the Internet via the ) 732 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1032 1042 95 (telnet) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1127 1042 890 ( service if and only if the connection originated from) 890 SB 300 1092 1722 (host A between the hours of 9:00 am and 5:00 pm and log the traffic." This sounds terrific, providing) 1722 SB 300 1142 1667 (arbitrary control with great ease of use, but some problems simply refuse to go away. Consider that) 1667 SB 300 1192 1657 (someone wishing to circumvent the firewall, who broke into the private network via an unguarded) 1657 SB 300 1242 1371 (modem, might very easily set up a service engine that was piggybacked over the ) 1371 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1671 1242 95 (telnet) 95 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1766 1242 227 ( port. This is) 227 SB 300 1292 673 (actually a fairly easy firewall to destroy.) 673 SB 480 1392 1595 (Another hybrid gateway might take advantage of various forms of protocol tunneling. Suppose) 1595 SB 300 1442 1634 (the requirement is to connect to the Internet with very tight restrictions, but that a high degree of) 1634 SB 300 1492 1798 (connectivity is required between the private network and an external network that is somewhat trusted \(for) 1798 SB 300 1542 363 (example, a corporate ) 363 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 663 1551 79 (R&D ) 79 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 742 1542 592 (department needs to be able to run ) 592 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1334 1551 23 (X) 23 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1357 1542 721 (-windows applications on a supercomputer) 721 SB 300 1592 1754 (at another facility\). The usual archetypal gateways discussed here could provide general purpose e-mail) 1754 SB 300 1642 1606 (connectivity, but for secure point-to-point communications, an encrypted point-to-point virtual ) 1606 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1906 1651 99 (TCP/IP) 99 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 300 1692 1673 (connection might be set up with the remote system, after users had authenticated themselves with a) 1673 SB 300 1742 1760 (cryptographic smart card. This would be extremely secure, and might be made fairly easy to use, but has) 1760 SB 300 1792 1590 (the disadvantage that the protocol driver needs to be added to every system that wants to share) 1590 SB 300 1842 1739 (communication. It is hard to make any guesses about the failure mode of such a system, but the zone of) 1739 SB 300 1892 1728 (risk is neatly limited to all the hosts which are running the tunneling protocol driver, and to which the) 1728 SB 300 1942 1747 (individual user has smart card access. Some of this might be implemented in hardware or in the routers) 1747 SB 300 1992 185 (themselves) 185 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 485 1988 17 (4) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 502 1992 1560 (. In the future, it is likely that the rapid growth of the Internet will fuel more development in) 1560 SB 300 2042 1683 (this area, and we will see various hybrid gateways arise. The basic issues surrounding configuring a) 1683 SB 300 2092 1114 (firewall will probably remain the same as the ones discussed here.) 1114 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 2192 577 (Other firewall-related tools) 577 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 2251 1557 (Subtle and insubstantial, the expert leaves no trace; divinely mysterious, he is inaudible. ) 1557 SB 32 0 0 50 50 0 0 0 51 /Symbol font 1888 2238 51 (\276) 51 SB 1887 2238 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 1938 2251 156 ( Sun Tzu) 156 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 2351 1465 (There is active research and development on tools to aggressively seek out and identify) 1465 SB 300 2401 1647 (weaknesses in an entire network, or to detect the patterns that might indicate when an attack is in) 1647 SB 300 2451 739 (progress. These tools range from the simple) 739 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 1039 2459 8 ( ) 8 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1047 2451 149 (checklist) 149 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 1196 2459 43 ([5]) 43 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1239 2451 741 ( to complex "expert systems" with inference) 741 SB 300 2501 1797 (engines and elaborate rule bases. Many firewalls today run software that is designed to go forth and gather) 1797 SB 300 2551 1563 (information relating to possible attacks and their origins, often using and abusing tools like ) 1563 SB 32 0 0 42 42 0 0 0 38 /Times-Italic /font27 ANSIFont font 1863 2551 101 (finger) 101 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1964 2551 73 ( and) 73 SB 32 0 0 33 33 0 0 0 30 /Times-Italic /font27 ANSIFont font 300 2609 161 (SNMP[6,7].) 161 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 461 2601 1573 ( Unless true artificial intelligence is developed, however, these tools cannot guard against an) 1573 SB 300 2651 1779 (unknown form of attack, since they cannot possibly match the creativity of a network vandal. While often) 1779 SB 32 0 0 42 42 0 1 0 38 /Times-Roman /font28 ANSIFont font 300 2733 220 ( ) 220 SB 520 2733 220 ( ) 220 SB 740 2733 165 ( ) 165 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 300 2779 17 (4) 17 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 317 2783 1675 (Routers performing point-to-point encryption for specified networks are beginning to appear on the) 1675 SB 300 2833 1728 (market. Until recently, the only demand for such hardware was for government applications, and most) 1728 SB 300 2883 688 (solutions employed Type I cryptography.) 688 SB 2079 3033 21 (8) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%Page: 9 9 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 0 0 0 fC 300 233 1795 (billed as being "proactive" they are in fact reactive, and generally will serve only to catch systems crackers) 1795 SB 300 283 1793 (armed with last year's bag of tricks. Catching the small fry is still worth doing, but it is likely that they are) 1793 SB 300 333 1735 (less of a threat than the fellow who is so eager to break into your network that he is doing research and) 1735 SB 300 383 824 (development in new system cracking techniques.) 824 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 483 725 (No Conclusions, but Observations) 725 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 330 542 1663 (Those skilled in war cultivate the Tao and preserve the laws and are therefore able to formulate) 1663 SB 300 592 347 (victorious policies. ) 347 SB 32 0 0 50 50 0 0 0 51 /Symbol font 648 579 51 (\276) 51 SB 647 579 51 (\276) 51 SB 32 0 0 42 42 0 0 0 38 /Times-BoldItalic /font26 ANSIFont font 698 592 156 ( Sun Tzu) 156 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 480 692 1612 (It is the privilege of a writer to use the last section of a publication to state his opinions and call) 1612 SB 300 742 1658 (them "conclusions." In dealing with firewalls, it is simply not reasonable to say that any particular) 1658 SB 300 792 1793 (approach is best, since there are so many factors that determine what the best firewall for a given situation) 1793 SB 300 842 1727 (may be. Cost, corporate policy, existing network technology, staffing, and intra-organizational politics) 1727 SB 300 892 1146 (may all easily outweigh the technical considerations presented here.) 1146 SB 480 992 1546 (There are a few observations worth making about firewalls at a very general level. Firstly, a) 1546 SB 300 1042 1790 (firewall is a leverage-increasing device from a network management point of view. Rather than looking at) 1790 SB 300 1092 1777 (it as "all eggs in one basket," it can also be viewed as a trustworthy basket, and a single point from which) 1777 SB 300 1142 1766 (a very important security system can be controlled. The size of the zone of risk is crucial to the design; if) 1766 SB 300 1192 1735 (it is small, security can be maintained and controlled easily but if security is compromised, the damage) 1735 SB 300 1242 1744 (can be more severe. The ideal would be to have such strong host-based security that a firewall would be) 1744 SB 300 1292 1635 (redundant. Systems administration costs and a hard dose of reality prevents this ideal from being) 1635 SB 300 1342 185 (obtainable.) 185 SB 480 1442 1522 (A second important aspect of firewall building is that it is not something to undertake in a) 1522 SB 300 1492 1715 (vacuum. Many sites are connected with a simple firewall consisting of a screening router and nothing) 1715 SB 300 1542 1763 (more because someone told them that it was "secure enough." In setting up a firewall one must trade off) 1763 SB 300 1592 1794 (time and money, security, and risk. "Secure enough" depends largely on what you risk losing. Finally, it is) 1794 SB 300 1642 1696 (important when approaching implementing a firewall to avoid the urge to start from scratch. System) 1696 SB 300 1692 1791 (security is a lot like pregnancy; one is seldom only broken into a little bit, and it only takes a little mistake) 1791 SB 300 1742 1793 (or a moment of inattention to find oneself in a delicate position. Leaning on the experiences of others, and) 1793 SB 300 1792 1730 (learning from their mistakes and successes is very important. Setting up a firewall is definitely an area) 1730 SB 300 1842 1737 (where having a wide background in experience to draw upon is important. The vandals on the network) 1737 SB 300 1892 1594 (have a wide background in experience to draw upon as well, and a firewall administrator must) 1594 SB 300 1942 1770 (communicate with others, and must keep up to date on other firewall-related happenings on the network.) 1770 SB 300 1992 1791 (Static defenses do not work unless they keep up with emerging tricks of the trade, or one's firewall may be) 1791 SB 300 2042 389 (the next Maginot Line.) 389 SB 480 2142 1586 (The purpose of this paper is not to discourage companies from connecting to the Internet. The) 1586 SB 300 2192 1789 (Internet is an incredibly valuable resource, one which will in the coming years completely change the way) 1789 SB 300 2242 1764 (people work and communicate on a global level. The benefits of connection far outweigh the costs, but it) 1764 SB 300 2292 1702 (is wise to reduce the costs and potential costs as much as possible, by being aware of the dangers and) 1702 SB 300 2342 1794 (being as protected as is necessary. The purpose of this paper is not to sow "fear, uncertainty and doubt" in) 1794 SB 300 2392 1734 (order to sell any particular firewall or software. If the author has succeeded to any degree in describing) 1734 SB 300 2442 1729 (some of the considerations and tradeoffs in setting up a firewall, this paper has been a success, and the) 1729 SB 300 2492 1138 (reader is left to their own devices to determine how best to proceed.) 1138 SB 32 0 0 50 50 0 0 0 46 /Times-Bold /font25 ANSIFont font 300 2592 230 (References) 230 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 360 2676 1071 ([1] Bill Cheswick, "The Design of a Secure Internet Gateway," ) 1071 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1431 2685 119 (USENIX) 119 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1550 2676 539 ( proceedings. Available for FTP) 539 SB 300 2726 941 (from research.att.com: /dist/secure_internet_gateway.ps) 941 SB 360 2801 592 ([2] Cliff Stoll, "The Cuckoo's Egg") 592 SB 2079 3033 21 (9) 21 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Roman %%Page: 10 10 %%PageResources: (atend) SS 0 0 25 23 798 1100 300 SM 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 0 0 0 fC 360 233 1326 ([3] Smoot Carl-Mitchell and John Quarterman, "Building Internet Firewalls," ) 1326 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1686 242 81 (UNIX) 81 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1767 233 290 ( World, February) 290 SB 300 283 84 (1992) 84 SB 360 358 885 ([4] Simson Garfinkel and Gene Spafford, "Practical ) 885 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 1245 367 81 (UNIX) 81 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1326 358 686 ( Security," O'Reilly and Associates, June) 686 SB 300 408 84 (1991) 84 SB 360 483 304 ([5] Dan Farmer, ") 304 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 664 492 82 (COPS) 82 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 746 483 242 ( and Robbers, ) 242 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 988 492 85 (UN*X) 85 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 1073 483 1024 ( System Security," Internet software. Available for FTP from) 1024 SB 300 533 458 (cert.sei.cmu.edu: /pub/cops) 458 SB 360 608 1656 ([6] Bill Cheswick, "An Evening with Berferd in which a cracker is Lured, Endured, and Studied,") 1656 SB 32 0 0 33 33 0 0 0 29 /Times-Roman /font28 ANSIFont font 300 667 119 (USENIX) 119 SB 32 0 0 42 42 0 0 0 38 /Times-Roman /font28 ANSIFont font 419 658 1169 ( proceedings, Jan 20, 1990 Available for FTP from research.att.com:) 1169 SB 300 708 548 (/dist/internet_security/berferd.ps) 548 SB 360 783 1550 ([7] Marcus J. Ranum, "An Internet Firewall," proceedings of World Conference on Systems) 1550 SB 300 833 1759 (Management and Security, 1992 Available for FTP from decuac.dec.com: /pub/docs/firewall/firewall.ps) 1759 SB 2058 3033 42 (10) 42 SB 1 #C EJ RS %%PageTrailer %%PageResources: font Times-Roman %%Trailer SVDoc restore end %%Pages: 10 % TrueType font name key: % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT310000 = % MSTT31c1bd = 0857DTimes New RomanF0000004b000002bc0000 % MSTT31c1ca = 0857DTimes New RomanF0000003a000002bc0000 % MSTT31c1d7 = 0857DTimes New RomanF0000003a000001900001 % MSTT31c1e4 = 0857DTimes New RomanF0000003a000001900000 % MSTT31c1f1 = 0857DTimes New RomanF00000032000002bc0000 % MSTT31c1fe = 0857DTimes New RomanF0000002a000002bc0001 % MSTT31c20b = 0857DTimes New RomanF0000002a000001900000 % MSTT31c218 = 0857DTimes New RomanF00000021000001900000 % MSTT31c225 = 0857DTimes New RomanF0000002a000001900001 % MSTT31c232 = 0857DTimes New RomanF0000002a000002bc0000 % MSTT31c23f = 0857DTimes New RomanF0000002f000001900000 % MSTT31c24c = 0857DTimes New RomanF00000019000001900000 % MSTT31c259 = 0857DTimes New RomanF00000021000001900001 % MSTT31c266 = 0857DTimes New RomanF00000019000002bc0001 % MSTT31c273 = 0857DTimes New RomanF00000027000001900000 % MSTT31c280 = 0857DTimes New RomanF00000015000002bc0001 % MSTT31c28d = 0857DTimes New RomanF00000032000002bc0001 %%DocumentSuppliedResources: procset Win35Dict 3 1 %%DocumentNeededResources: font Symbol %%+ font Times-Bold %%+ font Times-BoldItalic %%+ font Times-Italic %%+ font Times-Roman %%EOF