Re: After many hours all outbound connections get stuck in SYN_SENT

["James Nichols" <jamesnichols3@gmail.com>]

On 12/19/07, Eric Dumazet <dada1@cosmosbay.com> wrote:
> James Nichols a écrit :
> >> So you see outgoing SYN packets, but no SYN replies coming from the remote
> >> peer ?  (you mention ACKS, but the first packet received from the remote
> >>  peer should be a SYN+ACK),
> >
> > Right, I meant to say SYN+ACK.  I don't see them coming back.
>
> So... Really unlikely a linux problem, but ...
>


I don't know how you can be so sure.  Turning tcp_sack off instantly
resovles the problem and all connections are succesful.  I can't
imagine even the most far-fetched scenario where a router or every
single remote endpoints would suddenly stop causing the problem just
by removing a single TCP option.


> > I can take these captures and take a look at the results.
> > Unfortunately, I don't think I'll be able to make the captures
> > available to the general public.
>
> I dont understand, why dont you change IPs to mask them with 192.168.X.Y, or
> just ME, and peer1, peer2, peer...

I will see if I can do that, but it's major pain with 2000 hosts.
Plus, there is application data in the packets that I can't allow into
the public domain.  I really don't think I can pull it off... I
literally would have to go through our legal department.

>
> Random ideas :
>
> 1) Is your server behind a NET router or something ?

What's a NET router?  I am behind a Cisco router and a firewall, but
these network components have completely been replaced/rebuilt several
times in the 4+ years that we've had this problem.  I've looked at the
logs there and neither are doing anything other than passing the
traffic along.

> 2) Are you sure you are not using connection tracking, and hit a limit on it ?

I'm using ip_conntrack, but the limit I have for max entries is 65K.
The most I've seen in there are a couple thousand- that was one of the
first things I monitored very closely.