[Cialug] X11 applications over SSH very slow to start

Thu Mar 14 12:16:38 CDT 2013

Sorry for the noise, but i got this figured out about 5 minutes after 
sending my call for help.

I did a few comparisions with ssh -vvv to a server where everything 
works as expected. Turns out the problem is IPv6. Specifically, that 
IPv6 is enabled in the kernel, but dropped by iptables. If i turn off 
iptables, X11 over SSH works. If i remove "::1 localhost" in /etc/hosts, 
it works. I haven't tried disabling IPv6 in the kernel, but presumably 
that will also work. I'm not sure why SSH was trying to do anything over 
IPv6 when i was making my connection over IPv4, but now that i know the 
issue is IPv6-related it will be easy enough to fix permanently.

Again, sorry for the noise.

On 2013-03-14 at 12:04:38, Daniel A. Ramaley wrote:
>I have a Debian Testing server that i'm SSH'ing to. The box is on a
>LAN with gigabit ethernet, so the network is nice and fast. SSH login
>is very fast. X11 applications, once started, perform very fast. The
>problem is that X11 applications are *very* slow to start. By "very"
>i mean from when i run a command there will be a 5 to 15 *minute*
>delay before the application appears. Once started, however,
>everything is nice and fast as one would expect on a gigabit LAN.
>
>I tried setting "UseDNS no" in sshd_config on the server and
>restarting; no effect.
>
>I tried SSH'ing with "-vvv". Here's the debug output i get when
>running an X11 program (emacs, but i don't think that matters since
>all X11 applications seem to be affected):
>
>
>debug1: client_input_channel_open: ctype x11 rchan 3 win 65536 max
>16384 debug1: client_request_x11: request from 127.0.0.1 42410
>debug2: fd 7 setting O_NONBLOCK
>debug3: fd 7 is O_NONBLOCK
>debug1: channel 1: new [x11]
>debug1: confirm x11
>debug2: channel 1: rcvd eof
>debug2: channel 1: output open -> drain
>debug2: channel 1: obuf empty
>debug2: channel 1: close_write
>debug2: channel 1: output drain -> closed
>debug1: channel 1: FORCE input drain
>debug2: channel 1: ibuf empty
>debug2: channel 1: send eof
>debug2: channel 1: input drain -> closed
>debug2: channel 1: send close
>debug3: channel 1: will not send data after close
>debug2: channel 1: rcvd close
>debug3: channel 1: will not send data after close
>debug2: channel 1: is dead
>debug2: channel 1: garbage collecting
>debug1: channel 1: free: x11, nchannels 2
>debug3: channel 1: status: The following connections are open:
>  #0 client-session (t4 r0 i0/0 o0/0 fd 4/5 cc -1)
>  #1 x11 (t4 r3 i3/0 o3/0 fd 7/7 cc -1)
>
>debug1: client_input_channel_open: ctype x11 rchan 3 win 65536 max
>16384 debug1: client_request_x11: request from 127.0.0.1 42412
>debug2: fd 7 setting O_NONBLOCK
>debug3: fd 7 is O_NONBLOCK
>debug1: channel 1: new [x11]
>debug1: confirm x11
>debug2: channel 1: rcvd eof
>debug2: channel 1: output open -> drain
>debug2: channel 1: obuf empty
>debug2: channel 1: close_write
>debug2: channel 1: output drain -> closed
>debug1: channel 1: FORCE input drain
>debug2: channel 1: ibuf empty
>debug2: channel 1: send eof
>debug2: channel 1: input drain -> closed
>debug2: channel 1: send close
>debug3: channel 1: will not send data after close
>debug2: channel 1: rcvd close
>debug3: channel 1: will not send data after close
>debug2: channel 1: is dead
>debug2: channel 1: garbage collecting
>debug1: channel 1: free: x11, nchannels 2
>debug3: channel 1: status: The following connections are open:
>  #0 client-session (t4 r0 i0/0 o0/0 fd 4/5 cc -1)
>  #1 x11 (t4 r3 i3/0 o3/0 fd 7/7 cc -1)
>
>debug1: client_input_channel_open: ctype x11 rchan 3 win 65536 max
>16384 debug1: client_request_x11: request from 127.0.0.1 42414
>debug2: fd 7 setting O_NONBLOCK
>debug3: fd 7 is O_NONBLOCK
>debug1: channel 1: new [x11]
>debug1: confirm x11
>debug1: client_input_channel_open: ctype x11 rchan 4 win 65536 max
>16384 debug1: client_request_x11: request from 127.0.0.1 42416
>debug2: fd 8 setting O_NONBLOCK
>debug3: fd 8 is O_NONBLOCK
>debug1: channel 2: new [x11]
>debug1: confirm x11
>debug2: channel 2: rcvd eof
>debug2: channel 2: output open -> drain
>debug2: channel 2: obuf empty
>debug2: channel 2: close_write
>debug2: channel 2: output drain -> closed
>debug1: channel 2: FORCE input drain
>debug2: channel 2: ibuf empty
>debug2: channel 2: send eof
>debug2: channel 2: input drain -> closed
>debug2: channel 2: send close
>debug3: channel 2: will not send data after close
>debug2: channel 2: rcvd close
>debug3: channel 2: will not send data after close
>debug2: channel 2: is dead
>debug2: channel 2: garbage collecting
>debug1: channel 2: free: x11, nchannels 3
>debug3: channel 2: status: The following connections are open:
>  #0 client-session (t4 r0 i0/0 o0/0 fd 4/5 cc -1)
>  #1 x11 (t4 r3 i0/0 o0/0 fd 7/7 cc -1)
>  #2 x11 (t4 r4 i3/0 o3/0 fd 8/8 cc -1)
>
>
>After about 6 or 7 minutes and all of those debug lines, then emacs
>finally opened. Any ideas?
>
>__
>Daniel A. Ramaley
>Network Engineer 2
>
>Dial Center 112, Drake University
>2407 Carpenter Ave / Des Moines IA 50311 USA
>Tel: +1 515 271-4540
>Fax: +1 515 271-1938
>E-mail: daniel.ramaley at drake.edu
>_______________________________________________
>Cialug mailing list
>Cialug at cialug.org
>http://cialug.org/mailman/listinfo/cialug
__
Daniel A. Ramaley
Network Engineer 2

Dial Center 112, Drake University
2407 Carpenter Ave / Des Moines IA 50311 USA
Tel: +1 515 271-4540
Fax: +1 515 271-1938
E-mail: daniel.ramaley at drake.edu