Listen queue overflow: N already in queue awaiting acceptance

Andre Oppermann andre at
Thu Jul 11 15:43:16 UTC 2013

On 11.07.2013 17:04, Andriy Gapon wrote:
> on 11/07/2013 17:28 Andre Oppermann said the following:
>> Andriy for example would never have found out about this problem other
>> than receiving vague user complaints about aborted connection attempts.
>> Maybe after spending many hours searching for the cause he may have
>> interfered from endless scrolling in Wireshark that something wasn't
>> right and blame syncache first.  Only later it would emerge that he's
>> either receiving too many connections or his application is too slow
>> dealing with incoming connections.
> That's true, but OTOH there are many interesting network conditions like
> excessive packet loss that we don't shout about.  The stats are quietly gathered
> and can be examined with netstat.  If a system is properly monitored then such
> counters are graphed and can trigger alarms.  If the system just misbehaves then
> an administrator can use netstat for inspection.
> Spamming logs in the case of e.g. DDoS attack is not very helpful, IMO.

I agree with that.

I try to make the system behavior more transparent so that even "hidden" problems
can be detected easily.  This includes adding more of them, like excessive packet
loss.  This makes FreeBSD a more friendly platform for sysadmins whereas previously
people may have quietly move on to some other OS due to such unspecific complications.

Most of the TCP related debugging it is protected by net.inet.tcp.log_debug.  In this
case it's more complicated because the socket code where this happens is protocol
agnostic and I can't bond it with TCP.

I'm currently looking into a) applying a rate limiter to the message (as suggested
by Luigi); and b) add a per-socket accept queue overflow statistic that is visible
via netstat.  I'll post patches for testing when done.


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