Connection timed out while connecting to upstream, syn packet volume is high

Lukas Tribus luky-37 at
Mon Dec 12 19:15:26 UTC 2011

Are you sure this isn't something network related, like packet loss/reordering/duplication? What nginx version are you running?

> To: nginx at
> Subject: Connection timed out while connecting to upstream,	syn packet volume is high
> From: nginx-forum at
> Date: Mon, 12 Dec 2011 13:24:57 -0500
> I've been troubleshooting this issue on and off the last couple days and
> have not been able to eliminate this error from nginx:
> 2011/12/12 13:12:32 [error] 27810#0: *36209658 connect() failed (110:
> Connection timed out) while connecting to upstream
> Our setup is a single machine running nginx that serves as reverse proxy
> to two separate apache servers.  The setup is handling perhaps a few
> dozen requests per second.  The boxes themselves are almost idle, no
> issue with load in that respect.
> The upstream apache servers are not getting hit hard and are no where
> near their maxclient settings.
> This error was happening every second or two on the nginx side until I
> set "net.ipv4.tcp_syncookies = 1" on the apache machines (not sure why
> it wasn't turned out by default).  After making that change the
> frequency dropped to 1 or 2 connection time outs per minute.  Similarly,
> leaving syncookies off but increasing "net.ipv4.tcp_max_syn_backlog"
> alleviated the issue as well, though didn't stop it entirely.
> If I use tcdump to count syn packets I see 8x the volume to these
> nginx-fronted apache machines that I see for other datacenters handling
> far more load (hundreds of concurrent connections) that are behind
> traditional hardware load balancers.
> Is there a reason nginx would be generating so many syn packets?  Or,
> anyone have recommended settings for alleviating it as much as possible
> at least?
> Any help is greatly appreciated.
> Posted at Nginx Forum:,219955,219955#msg-219955
> _______________________________________________
> nginx mailing list
> nginx at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the nginx mailing list