ZyWALL 5 User’s Guide
182
Chapter 10 Firewall Screens
10.10.1 Threshold Values
Tune these parameters when something is not working and after you have checked the firewall
counters. These default values should work fine for normal small offices with ADSL
bandwidth. Factors influencing choices for threshold values are:
1
The maximum number of opened sessions.
2
The minimum capacity of server backlog in your LAN network.
3
The CPU power of servers in your LAN network.
4
Network bandwidth.
5
Type of traffic for certain servers.
If your network is slower than average for any of these factors (especially if you have servers
that are slow or handle many tasks and are often busy), then the default values should be
reduced.
You should make any changes to the threshold values before you continue configuring
firewall rules.
10.10.2 Half-Open Sessions
For TCP, half-open means that the session has not reached the established state-the TCP three-
way handshake has not yet been completed (see
). For UDP, half-open
means that the firewall has detected no return traffic. An unusually high number of half-open
sessions (either an absolute number or measured as the arrival rate) could indicate that a
Denial of Service attack is occurring.
The ZyWALL measures both the total number of existing half-open sessions and the rate of
session establishment attempts. Both TCP and UDP half-open sessions are counted in the total
number and rate measurements. Measurements are made once a minute.
When the number of existing half-open sessions rises above a threshold (
max-incomplete
high
), the ZyWALL starts deleting half-open sessions as required to accommodate new
connection requests. The ZyWALL continues to delete half-open requests as necessary, until
the number of existing half-open sessions drops below another threshold (
max-incomplete
low
).
When the rate of new connection attempts rises above a threshold (
one-minute high
), the
ZyWALL starts deleting half-open sessions as required to accommodate new connection
requests. The ZyWALL continues to delete half-open sessions as necessary, until the rate of
new connection attempts drops below another threshold (
one-minute low
). The rate is the
number of new attempts detected in the last one-minute sample period.
10.10.2.1 TCP Maximum Incomplete and Blocking Time
An unusually high number of half-open sessions with the same destination host address could
indicate that a Denial of Service attack is being launched against the host.
Summary of Contents for ZyXEL ZyWALL 5
Page 1: ...ZyWALL 5 Internet Security Appliance User s Guide Version 3 64 3 2005 ...
Page 2: ......
Page 26: ...ZyWALL 5 User s Guide 24 ...
Page 44: ...ZyWALL 5 User s Guide 42 ...
Page 86: ...ZyWALL 5 User s Guide 84 Chapter 3 Wizard Setup ...
Page 96: ...ZyWALL 5 User s Guide 94 Chapter 4 LAN Screens ...
Page 102: ...ZyWALL 5 User s Guide 100 Chapter 5 Bridge Screens ...
Page 130: ...ZyWALL 5 User s Guide 128 Chapter 7 WAN Screens Figure 45 WAN PPPoE Encapsulation ...
Page 138: ...ZyWALL 5 User s Guide 136 Chapter 7 WAN Screens Figure 50 Dial Backup Setup ...
Page 144: ...ZyWALL 5 User s Guide 142 Chapter 7 WAN Screens ...
Page 152: ...ZyWALL 5 User s Guide 150 Chapter 8 DMZ Screens ...
Page 179: ...ZyWALL 5 User s Guide Chapter 10 Firewall Screens 177 Figure 73 My Service Rule Configuration ...
Page 220: ...ZyWALL 5 User s Guide 218 Chapter 13 Introduction to IPSec ...
Page 238: ...ZyWALL 5 User s Guide 236 Chapter 14 VPN Screens Figure 110 VPN Rules IKE Network Policy Edit ...
Page 261: ...ZyWALL 5 User s Guide Chapter 15 Certificates 259 Figure 122 My Certificate Details ...
Page 280: ...ZyWALL 5 User s Guide 278 Chapter 16 Authentication Server Figure 133 Local User Database ...
Page 317: ...ZyWALL 5 User s Guide Chapter 19 Bandwidth Management 315 ...
Page 318: ...ZyWALL 5 User s Guide 316 Chapter 19 Bandwidth Management ...
Page 352: ...ZyWALL 5 User s Guide 350 Chapter 21 Remote Management ...
Page 366: ...ZyWALL 5 User s Guide 364 Chapter 23 Logs Screens Figure 188 Log Settings ...
Page 389: ...ZyWALL 5 User s Guide Chapter 24 Maintenance 387 Figure 210 Restart Screen ...
Page 390: ...ZyWALL 5 User s Guide 388 Chapter 24 Maintenance ...
Page 416: ...ZyWALL 5 User s Guide 414 Chapter 27 WAN and Dial Backup Setup ...
Page 432: ...ZyWALL 5 User s Guide 430 Chapter 30 DMZ Setup ...
Page 492: ...ZyWALL 5 User s Guide 490 Chapter 37 System Information Diagnosis ...
Page 508: ...ZyWALL 5 User s Guide 506 Chapter 38 Firmware and Configuration File Maintenance ...
Page 520: ...ZyWALL 5 User s Guide 518 Chapter 40 Remote Management ...
Page 536: ...ZyWALL 5 User s Guide 534 Chapter 42 Troubleshooting Figure 343 Java Sun ...
Page 564: ...ZyWALL 5 User s Guide 562 Appendix C IP Subnetting ...
Page 570: ...ZyWALL 5 User s Guide 568 Appendix E PPTP ...
Page 571: ...ZyWALL 5 User s Guide 569 ...
Page 572: ...ZyWALL 5 User s Guide 570 ...
Page 590: ...ZyWALL 5 User s Guide 588 Appendix G Triangle Route ...
Page 620: ...ZyWALL 5 User s Guide 618 Appendix J Importing Certificates ...
Page 622: ...ZyWALL 5 User s Guide 620 Appendix K Command Interpreter ...
Page 628: ...ZyWALL 5 User s Guide 626 Appendix L Firewall Commands ...
Page 632: ...ZyWALL 5 User s Guide 630 Appendix M NetBIOS Filter Commands ...
Page 638: ...ZyWALL 5 User s Guide 636 Appendix O Brute Force Password Guessing Protection ...
Page 660: ...ZyWALL 5 User s Guide 658 Appendix Q Log Descriptions ...