Chapter 9 Firewall Configuration
P-793H User’s Guide
146
9.10.2 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.
9.10.2.1 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.
9.10.3 Configuring Firewall Thresholds
The ZyXEL Device also sends alerts whenever
TCP Maximum Incomplete
is exceeded. The
global values specified for the threshold and timeout apply to all TCP connections.
Click
Firewall
, and
Threshold
to bring up the next screen.
Summary of Contents for G.SHDSL.bis 4-port Security Gateway P-793H
Page 2: ......
Page 7: ...Safety Warnings P 793H User s Guide 7 This product is recyclable Dispose of it properly ...
Page 8: ...Safety Warnings P 793H User s Guide 8 ...
Page 30: ...List of Figures P 793H User s Guide 30 ...
Page 36: ...List of Tables P 793H User s Guide 36 ...
Page 38: ...38 ...
Page 68: ...Chapter 4 Point to 2 point Configuration P 793H User s Guide 68 ...
Page 70: ...70 ...
Page 114: ...Chapter 7 Network Address Translation NAT Screens P 793H User s Guide 114 ...
Page 116: ...116 ...
Page 152: ...Chapter 10 Content Filtering P 793H User s Guide 152 ...
Page 180: ...Chapter 12 Static Route P 793H User s Guide 180 ...
Page 194: ...Chapter 14 Dynamic DNS Setup P 793H User s Guide 194 ...
Page 216: ...Chapter 16 Universal Plug and Play UPnP P 793H User s Guide 216 ...
Page 217: ...217 PART IV Maintenance System 219 Logs 225 Tools 229 Diagnostic 235 ...
Page 218: ...218 ...
Page 224: ...Chapter 17 System P 793H User s Guide 224 ...
Page 234: ...Chapter 19 Tools P 793H User s Guide 234 ...
Page 238: ...238 ...
Page 244: ...Chapter 21 Introducing the SMT P 793H User s Guide 244 ...
Page 248: ...Chapter 22 General Setup P 793H User s Guide 248 ...
Page 256: ...Chapter 23 WAN Setup P 793H User s Guide 256 ...
Page 262: ...Chapter 24 LAN Setup P 793H User s Guide 262 ...
Page 278: ...Chapter 27 Static Route Setup P 793H User s Guide 278 ...
Page 308: ...Chapter 30 Filter Configuration P 793H User s Guide 308 ...
Page 310: ...Chapter 31 SNMP Configuration P 793H User s Guide 310 ...
Page 312: ...Chapter 32 System Password P 793H User s Guide 312 ...
Page 322: ...Chapter 33 System Information Diagnosis P 793H User s Guide 322 ...
Page 352: ...Chapter 37 Schedule Setup P 793H User s Guide 352 ...
Page 360: ...360 ...
Page 366: ...Appendix B Wall mounting Instructions P 793H User s Guide 366 ...
Page 400: ...Appendix F IP Address Assignment Conflicts P 793H User s Guide 400 ...
Page 404: ...Appendix G Common Services P 793H User s Guide 404 ...
Page 432: ...Appendix K Legal Information P 793H User s Guide 432 ...
Page 443: ...Index P 793H User s Guide 443 ...
Page 444: ...Index P 793H User s Guide 444 ...