Re: SYN attack, with FIN flag set

[Eric Dumazet <eric.dumazet@gmail.com>]

Le samedi 03 décembre 2011 à 00:29 +0200, Denys Fedoryshchenko a écrit :
> Hi
> 
>  Recently i started to get SYN attacks, and managed them.
>  syncookies didn't helped, here is "perf report" info:
>  -     26.89%        swapper  [kernel.kallsyms]     [k] _raw_spin_lock
>     - _raw_spin_lock
>        - 94.97% tcp_v4_rcv
>             ip_local_deliver_finish
>             ip_local_deliver
>             ip_rcv_finish
>             ip_rcv
>             __netif_receive_skb
>             process_backlog
>             net_rx_action
>             __do_softirq
>             call_softirq
>             do_softirq
>           + irq_exit
> 
>  But then i got attack that made server to choke and bypassed "--syn" 
>  rule, and i was surprised, that stack are handling invalid combination 
>  of flags, SYN+FIN.
>  Is it valid behaviour?
> 
>  in tcp_input.c, tcp_rcv_state_process(), it just does check for rst (to 
>  discard), but maybe packet with fin set should be discarded too?

I believe netfilter tcp conntrack considers SYN|FIN as INVALID

Yes, we could drop SYN|FIN messages, but what prevents attacker to just
use SYN messages to attack your machine ?