Hello!

On Mon, Aug 08, 2011 at 02:01:55PM -0400, Marki555 wrote:

> Thanks, I wasn't sure whether kernel or nginx replies to the SYN
> packets. Now the question is how can I check for the network problems.
> SYN_RECV is also when kernel has replied with SYN+ACK, but I waiting for
> a final ACK? But SYN_RECV is also when user-space is unable to accept()
> the new connections so fast. How can I distinguish between these 2
> reasons?

Again: SYN_RECV connections are sitting in kernel waiting for ACK
to happen. Only ESTABLISHED connections are passed to userland.

SYN_RECV connections may appear on Linux as a result of listen
queue being overflowed (as Linux just drops ACKs by default if
there isn't enough room in listen queue), but this is *not* a
listen queue, these connections can't be acepted. This is rather
artifact of the way how Linux handles listen queue overflows.

> But anyway, why should that prevent legitimate normal connections from
> being accepted? When I try to access nginx' server-status, I am waiting
> for many seconds...

First of all, you may want to actually check how many connections
are sitting in listen queue. Under FreeBSD use

netstat -Lan

to find queue lengths. Under Linux 2.6.18+ it should be possible
to examine listen queue with

ss -nlt

For older Linux versions check

netstat -ntp

and count connections in ESTABLISHED state without associated
process.

Maxim Dounin

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