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Syncookies: Difference between revisions
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* "SYN cookies 'do not allow to use TCP extensions' such as large windows. Reality: SYN cookies don't hurt TCP extensions. A connection saved by SYN cookies can't use large windows; but the same is true without SYN cookies, because the connection would have been destroyed." | * "SYN cookies 'do not allow to use TCP extensions' such as large windows. Reality: SYN cookies don't hurt TCP extensions. A connection saved by SYN cookies can't use large windows; but the same is true without SYN cookies, because the connection would have been destroyed." | ||
** This is only true for machines expected to suffer SYNflood attacks. | ** This is only true for machines expected to suffer SYNflood attacks. | ||
** The [[TCP|usefulness of TCP Large Window Extensions]] means I disable SYNcookies on internal machines | ** The [[TCP|usefulness of TCP Large Window Extensions]] and [[TCP|SACK]] means I disable SYNcookies on internal machines | ||
*** Linux 2.6.26 added support for encoding some options into timestamps (see [http://lwn.net/Articles/277146/ this LWN article]). | *** Linux 2.6.26 added support for encoding some options into timestamps (see [http://lwn.net/Articles/277146/ this LWN article]). | ||
Revision as of 01:54, 11 June 2009
DJB's page: http://cr.yp.to/syncookies.html
Issues with DJB's Writeup
- "SYN cookies 'do not allow to use TCP extensions' such as large windows. Reality: SYN cookies don't hurt TCP extensions. A connection saved by SYN cookies can't use large windows; but the same is true without SYN cookies, because the connection would have been destroyed."
- This is only true for machines expected to suffer SYNflood attacks.
- The usefulness of TCP Large Window Extensions and SACK means I disable SYNcookies on internal machines
- Linux 2.6.26 added support for encoding some options into timestamps (see this LWN article).
Other Issues
- Only eight distinct MSS values can be chosen, due to only three bits for MSS in the 32 bits of a SYNcookie
Other Mitigations for SYNfloods
- SYNproxying by a powerful intermediary
