Weird "ignoring syn" problem

Adam McDougall mcdouga9 at
Tue Jun 12 18:20:48 UTC 2007

On Tue, Jun 12, 2007 at 10:19:49AM -0400, Bill Moran wrote:

  This one has got me pretty befuddled.
  We're seeing some really odd behaviour with FreeBSD ignoring SYN packets.
  I've been trying to diagnose this for a couple of weeks now, and my current
  guess is that there's something wrong with the em driver.  Here's a narrowed
  down list of what I've ruled out:
  *) I've done my best to eliminate other network components as the problem.
     My theory at this point is that it can't possibly be any other network
     hardware, based on the tcpdump show below.
  *) The problem occurred on both FreeBSD 6.1 and FreeBSD 6.2-p3.
  *) The problem does not appear to be tied to CPU usage -- the CPU is nearly
     idle when the problem occurs.
  *) I can now reproduce it pretty easily, so I'll know when it's fixed.
  *) The system exhibiting the problem is running 15 jails, but they are
     idle 95% of the time.  The problem initially occurred inside one of
     the jails, but I just recreated it outside the jail (on the host) and
     it's _easier_ to reproduce outside the jail.
  *) The problem occurred with both GENERIC, and the SMP kernel (this is a
     dual-CPU, hyperthreaded system)
  *) I've tested and the behavior occurs both with a dynamically generated
     file (from PHP) or from a static file.
  The nature of the beast is that we've got a SOAP application running under
  Apache and PHP.  This application is subject to many requests in rapid
  succession, such that load can be simulated by the following loop:
  while true; do fetch; done
  The problem is that occasionally, the Apache server machine just ignores
  SYN packets.  Take the following tcpdump output for example:
  13:34:17.312296 IP > S 2645061726:2645061726(0) win 65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690201156 0,sackOK,eol>
  13:34:20.312398 IP > S 2645061726:2645061726(0) win 65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690204156 0,sackOK,eol>
  13:34:23.512626 IP > S 2645061726:2645061726(0) win 65535 <mss 1380,nop,wscale 1,nop,nop,timestamp 2690207356 0,sackOK,eol>
  This is the _only_ traffic on port 80 during the test.  It looks like the
  kernel has ignored the initial syn packet and two duplicates.  I've seen it
  take as long as 45 seconds to establish a connection, and this causes
  ugly performance problems, as well as frequent timeouts on the client end.
  The only clue I've found so far is this output from netstat -s.

Does the Apache server have a firewall of any sort?  (Could be making unexpected
decisions there, even not part of a fw rule)

Try net.inet.ip.portrange.randomized=0 on the client?  (If this is the problem,
we would probably see a reused port if you had a tcpdump of a few minutes
if started after waiting for several minutes of "silence")

Are both systems on the same subnet?  If not, can/have you tried that?

Can you show tcpdump output using -e on the requests that aren't answered
as well as an example that IS answered?  (I have seen routers mess up the MAC
addresses for the source and destination and if I kept staring at layer 3
data all day I might never have seen the problem)

Better yet, can you post files containing tcpdump output using -w of an entire
session that ideally contains failed attempts that eventually work?  That way
people could look at a broader picture and perhaps pick up on something subtle.
Its worth comparing a SYN that works, directly with a SYN that doesn't work.

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