linux-kernel - Re: After many hours all outbound connections get stuck in SYN

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Date:	Wed, 19 Dec 2007 19:03:27 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	James Nichols <jamesnichols3@...il.com>
CC:	Jan Engelhardt <jengelh@...putergmbh.de>,
	linux-kernel@...r.kernel.org,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: After many hours all outbound connections get stuck in SYN_SENT

James Nichols a écrit :
> On 12/19/07, Eric Dumazet <dada1@...mosbay.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.

I still dont understand.

"tcpdump -p -n -s 1600 -c 10000" doesnt reveal User data at all.

Without any exact data from you, I am afraid nobody can help.

> 
>> 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.

Typo error, I meant NAT. Most routers doing NAT have some limits, timers, hacks...

> 
>> 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.

Now please try without conn tracking module. I saw many failures in the past
that were trigered by conntrack.

Do you have some firewall rules, using some netfilter modules like hashlimit ?


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