Previously Bernd Eckenfels wrote: > Because the kernel is turning them on automatcally if (and only if) the > kernel recognices a SYN Attack ("possible SYN flooding on port %d. Sending > cookies."). That's not true. From Documentation/networking/ip-sysctl.txt: tcp_syncookies - BOOLEAN Only valid when the kernel was compiled with CONFIG_SYNCOOKIES Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common 'syn flood attack' Default: FALSE Note, that syncookies is fallback facility. It MUST NOT be used to help highly loaded servers to stand against legal connection rate. If you see synflood warnings in your logs, but investigation shows that they occur because of overload with legal connections, you should tune another parameters until this warning disappear. See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow. syncookies seriously violate TCP protocol, do not allow to use TCP extensions, can result in serious degradation of some services (f.e. SMTP relaying), visible not by you, but your clients and relays, contacting you. While you see synflood warnings in logs not being really flooded, your server is seriously misconfigured. Wichert. -- ________________________________________________________________ / Generally uninteresting signature - ignore at your convenience \ | wichert@liacs.nl http://www.liacs.nl/~wichert/ | | 1024D/2FA3BC2D 576E 100B 518D 2F16 36B0 2805 3CB8 9250 2FA3 BC2D |
Attachment:
pgp1WMeDQqaAc.pgp
Description: PGP signature