Segment failed SYNCOOKIE?

Abdullah Ibn Hamad Al-Marri almarrie at
Mon May 28 17:36:08 UTC 2007

On 5/28/07, Andre Oppermann <andre at> wrote:
> Abdullah Ibn Hamad Al-Marri wrote:
> > On 5/26/07, Steve Kargl <sgk at> wrote:
> >
> >> Anyone have ideas on how to cure
> >>
> >> May 25 16:20:03 node13 kernel: TCP: []:53815 to
> >> []:50992 tcpflags 0x11<FIN,ACK>; syncache_expand:
> >> Segment failed SYNCOOKIE authentication
> >>
> >> The hardware and kernel on and
> >> are identical.
> >>
> >> --
> >> Steve
> >
> > 7.0-CURRENT FreeBSD 7.0-CURRENT #0: Sat May 26 04:25:29 GMT 2007
> >
> > I got the same problem and my sever paniced today.
> Please provide the panic message and if available a backtrace for the
> panic.  We have to track down the exact cause of it (which may not
> necessarily be the syncache).
> > TCP: []:54686 to [IP removed for security reasons]:59999
> > tcpflags 0x18<PUSH,ACK>; syncache_expand: Segment failed SYNCOOKIE
> > authentication
> Logging of TCP segment validation failure has recently been enabled
> to aid debugging of TCP (interoperability) issues.
> This particular message means that a SYN was received on a listen
> socket but no matching syncache entry was found.  The second test
> for a syncookie also failed.  Normally this means a spoofed packet
> or port scan is hitting your machine.  To make this certain you should
> answer a couple of questions:  a) What daemon is running on your port
> 59999?  b) Do you know [] and does it have any business
> in contacting your daemon on 59999?
> I agree that the log message should be made more clear to avoid
> unnecessary confusion.  Nothing is broken and syncache is doing its
> job just fine.
> --
> Andre

Hello Andre,

Thanks for looking into this issue.

The server IP isn't known by anyone, just me and my friend, and yes I
know which is his IP in a Linux box which runs distro

I run sshd in 59999, and we were both connected to it, then it died.

This is a server, so I removed the debug options to not slow it down.

If you think port scan could crash 7.0-CURRENT, Can you run nmap and
test it 7.0-CURRENT?

Do you think disabeling syncache would prevent my box against the same
panic again?


-Abdullah Ibn Hamad Al-Marri
Arab Portal

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