[Cialug] X11 applications over SSH very slow to start

Barry Von Ahsen barry at vonahsen.com
Thu Mar 14 12:54:45 CDT 2013 

is ssh still binding to IPv6 though?  on my wheezy box, I netstat shows

tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
tcp6       0      0 :::22                   :::*                    LISTEN     

even though I've never configured anything for IPv6.  I'm sure it snuck in during some random upgrade.  I understand why distros want to support IPv6, but not why they don't disable it, then let those who 1) want it and 2) know how to configure it override that  /rant


-barry


On Mar 14, 2013, at 12:16 PM, Daniel A. Ramaley wrote:

> 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
> _______________________________________________
> Cialug mailing list
> Cialug at cialug.org
> http://cialug.org/mailman/listinfo/cialug

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