telnet problem

Alex Dyas adyas at twowaytv.co.uk
Tue Oct 30 14:23:44 GMT 2001


tcpdump seems to indicate that the trigger for the lockup is a handshake
type signal from the remote machine:

16:05:14.251640 fbsd_machine.4360 > solaris_machine.telnet: . ack 4 win 17520 (DF) [tos 0x10]
16:05:14.351688 fbsd_machine.4360 > solaris_machine.telnet: P 3:5(2) ack 4 win 17520 (DF) [tos 0x10]
16:05:14.352186 solaris_machine.telnet > fbsd_machine.4360: P 4:6(2) ack 5 win 8760 (DF)
16:05:14.456755 fbsd_machine.4360 > solaris_machine.telnet: . ack 6 win 17520 (DF) [tos 0x10]
16:05:14.457095 solaris_machine.telnet > fbsd_machine.4360: P 6:74(68) ack 5 win 8760 (DF)
16:05:14.551646 fbsd_machine.4360 > solaris_machine.telnet: . ack 74 win 17520 (DF) [tos 0x10]
16:05:30.370151 solaris_machine.telnet > fbsd_machine.4360: . ack 5 win 0				<- trigger
16:05:30.370203 fbsd_machine.4360 > solaris_machine.telnet: . ack 74 win 17520 (DF) [tos 0x10]
16:05:41.811973 fbsd_machine.4360 > solaris_machine.telnet: . 5:6(1) ack 74 win 17520 (DF) [tos 0x10]
16:05:41.853928 solaris_machine.telnet > fbsd_machine.4360: . ack 6 win 8760 (DF)
16:05:41.853977 fbsd_machine.4360 > solaris_machine.telnet: P 6:7(1) ack 74 win 17520 (DF) [tos 0x10]

the solaris box sends the "handshake" (<-trigger), the fbsd machine
responds, at which point the lock up occurs.

tcpdump of a similar session between a windows box and the solaris machine
shows the same "handshake" going on but without the lockup.

also, there is no evidence of any blocking in the firewall logs.

alex..

On Tue, 30 Oct 2001, Paul Civati wrote:

> Alex Dyas <adyas at twowaytv.co.uk> wrote:
>
> > i'm having problems with fbsd and telnetting through a firewall.
> > basically what happens is this - i telnet from my own machine
> > (r2d2,FreeBSD 4.2R) in subnet A across a firewall to any one of
> > 10 solaris machines in subnet B.  all is fine until i leave the
> > telnet session for more than about 15 seconds. if i then try to
> > do anything there is a ~8 second delay before it responds to key
> > presses, after which all is well again.
>
> This sounds very much like your firewall doing something to
> the TCP connections passing through it, some will drop "idle"
> connections after a certain time, but that doesn't sound the
> case here.
>
> > windows and linux machines in subnet A doing the same thing
> > don't suffer from the same delays.
>
> Not sure why this would only effect FreeBSD, I think you'll
> have to run tcpdump and see exactly what's happening during
> that ~8 second delay.
>
> -Paul-
>
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