P-662H/HW-D Series User’s Guide
Chapter 11 Firewall Configuration
199
You should make any changes to the threshold values before you continue configuring
firewall rules.
11.10.2 Half-Open Sessions
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. 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.
The ZyXEL Device 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 ZyXEL Device starts deleting half-open sessions as required to accommodate new
connection requests. The ZyXEL Device 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
ZyXEL Device starts deleting half-open sessions as required to accommodate new connection
requests. The ZyXEL Device 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.
11.10.2.1 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.
Whenever the number of half-open sessions with the same destination host address rises above
a threshold (
TCP Maximum Incomplete
), the ZyXEL Device starts deleting half-open
sessions according to one of the following methods:
• If the
Blocking Time
timeout is 0 (the default), then the ZyXEL Device deletes the oldest
existing half-open session for the host for every new connection request to the host. This
ensures that the number of half-open sessions to a given host will never exceed the
threshold.
• If the
Blocking Time
timeout is greater than 0, then the ZyXEL Device blocks all new
connection requests to the host giving the server time to handle the present connections.
The ZyXEL Device continues to block all new connection requests until the
Blocking
Time
expires.
Summary of Contents for 802.11g ADSL 2+ 4-Port Security Gateway HW-D Series
Page 2: ......
Page 10: ...P 662H HW D Series User s Guide 10 Customer Support ...
Page 24: ...P 662H HW D Series User s Guide 24 Table of Contents ...
Page 32: ...P 662H HW D Series User s Guide 32 List of Figures ...
Page 38: ...P 662H HW D Series User s Guide 38 List of Tables ...
Page 64: ...P 662H HW D Series User s Guide 64 Chapter 2 Introducing the Web Configurator ...
Page 84: ...P 662H HW D Series User s Guide 84 Chapter 4 Bandwidth Management Wizard ...
Page 108: ...P 662H HW D Series User s Guide 108 Chapter 5 WAN Setup ...
Page 122: ...P 662H HW D Series User s Guide 122 Chapter 6 LAN Setup ...
Page 156: ...P 662H HW D Series User s Guide 156 Chapter 8 DMZ ...
Page 202: ...P 662H HW D Series User s Guide 202 Chapter 11 Firewall Configuration ...
Page 210: ...P 662H HW D Series User s Guide 210 Chapter 12 Anti Virus Packet Scan ...
Page 214: ...P 662H HW D Series User s Guide 214 Chapter 13 Content Filtering ...
Page 232: ...P 662H HW D Series User s Guide 232 Chapter 14 Content Access Control ...
Page 238: ...P 662H HW D Series User s Guide 238 Chapter 15 Introduction to IPSec ...
Page 273: ...P 662H HW D Series User s Guide Chapter 17 Certificates 273 Figure 144 My Certificate Details ...
Page 292: ...P 662H HW D Series User s Guide 292 Chapter 18 Static Route ...
Page 304: ...P 662H HW D Series User s Guide 304 Chapter 19 Bandwidth Management ...
Page 308: ...P 662H HW D Series User s Guide 308 Chapter 20 Dynamic DNS Setup ...
Page 332: ...P 662H HW D Series User s Guide 332 Chapter 22 Universal Plug and Play UPnP ...
Page 338: ...P 662H HW D Series User s Guide 338 Chapter 23 System ...
Page 344: ...P 662H HW D Series User s Guide 344 Chapter 24 Logs ...
Page 350: ...P 662H HW D Series User s Guide 350 Chapter 25 Tools ...
Page 364: ...P 662H HW D Series User s Guide 364 Chapter 27 Troubleshooting ...
Page 368: ...P 662H HW D Series User s Guide 368 Product Specifications ...
Page 372: ...P 662H HW D Series User s Guide 372 Appendix C Wall mounting Instructions ...
Page 408: ...P 662H HW D Series User s Guide 408 Appendix F Wireless LANs ...
Page 420: ...P 662H HW D Series User s Guide 420 Appendix H Command Interpreter ...
Page 436: ...P 662H HW D Series User s Guide 436 Appendix L NetBIOS Filter Commands ...
Page 462: ...P 662H HW D Series User s Guide 462 Appendix M Internal SPTGEN ...
Page 484: ...P 662H HW D Series User s Guide 484 Appendix P Triangle Route ...