02.08.10, 22:10, "Mitchell Erblich" <erblichs@earthlink.net>:

quoted text
> 
>  On Aug 2, 2010, at 9:11 AM, Franchoze Eric wrote:
>  
>  > 
>  > 
>  > 02.08.10, 12:17, "Florian Westphal" :
>  > 
>  >> Franchoze Eric  wrote:
>  >>> Just sirious why do we need printk each 1 second (60*HZ) about possible syn-flood? It really floods dmesg. Is there something dengerous? I have suggestion to turn off printk about sending tcp cookie each 1 second.
>  >> 
>  >> It is handled exactly like other printks in the networking path,
>  >> e.g. receipt of tcp wscale == 15.
>  >> 
>  >> Why does this need special treatment?
>  >> 
>  > 
>  > For now I see "possible SYN flooding on port %d. Sending cookies.\n" message each second on my server. I know that there are a lot of SYNs and I know that kernel sends cookie. Why do I need so mach printk?
>  > So I suggested add new value to /proc/sys/net/ipv4/tcp_syncookies, which will enable cookie but this printk will be turned off.
>  
>  
>  Once print per sec is a very good GENERIC informative msg to an admin that 
>  this system either has some  very small config'd or default values
>  (normally set up as a percentage of memory or set sock option and/or .. )
>  and/or that for some reason that a large number of SYNs are being rec'vd
>  and/or that a number of connections are being un/intentionally being
>  retried and/or dropped
>

There is no much settings to tune syn requests.
tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow
  
As for me, than I have about 3000 clients which do a little bit less then 3000 SYNs for nginx port.
I'm ok with sending syn cookies to clients. Also it's not possible to turn syncs off with setting
bigger value to tcp_max_syn_backlog and application works well so I would simple remove 
this messages from dmesg.

If I limit syncs with iptables it starts to drop needed packets. So it's no solution. That's why I think that we need turn off 
printk without turning off syn cookies.  

quoted text
>  Remember each printk may only be a small fraction of the number of SYNs
>  rcv'd and this fraction COULD depend on the Mb/Gb of the intf(s) or more
>  likely some type of  avg of summation of the number of network paths
>  involved.
>  
>  Mitchell Erblich
>  
>  
>  
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