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SubjectRe: Strange ssh hang with kernel 2.6.9
Ariel Garcia wrote:
> Hi,
> after i upgraded to 2.6.9 (from 2.6.8-rc4), I started having a similar
> problem to the one somebody else reported on Oct. 27 (thread "SSH and
> 2.6.9"), namely my ssh client hanging soon after doing some cat to
> screen...
> Actually, dissabling CONFIG_LEGACY_PTYS, or SOFTWARE_SUSPEND does not
> solve the problem for me.
> Also, the problem happens only when i do a ssh to my desktop running also
> 2.6.9 (or at least only in that case it happens fast enough so i can
> trigger it simply by opening the mc or cat'ing something to screen).
> Appended is the strace of the client Andrew requested, details follow.
> Client is OpenSSH_3.8.1p1 Debian-8.sarge.3, OpenSSL 0.9.7d 17 Mar 2004
> on Debian testing/unstable and i have a plain old /dev , no udev or devfs.
> Symptoms:
> - they happen even with a very slow output:
> while true; do
> echo -n "a"
> sleep 1
> done
> This went ok for the first ~40 chars, then blocked and printed a block of
> ~170chars toghether, this happened twice, then it blocked forever after
> printing a couple of short 8 char long lines (ie, "aaaaaaaa\n")
> - If i kill the forked sshd receiving my connection at the desktop, the
> client at the laptop stays "blocked" (no reaction to keys, escape commands)
> and it exists after a while with
> "Read from remote host <desktop>: Connection reset by peer"
> - I can also directly kill the ssh client with a plain kill (kill -15),
> and it is not in any strange state, just S+ in ps. However, it does not
> answer to the escape commands (~., ~B, ~*)
> - Actually, i tested also while running a kernel compile, and with that
> load ssh did not hang as fast as without load.
> - tested from Xwin, (konsole, TERM=xterm) but also from a text console,
> TERM=linux, same results
> - scp also hangs with "0% 0.0KB/s stalled"
> - putting the daemon in debugging mode (ssh -ddd) does not help, i get no
> additional debugging output after the connection is made, but in that case
> the client responds to ~. after the "hang". If i let it stay "hanged"
> however, the daemon sends keepalives, it prints
> debug2: channel 0: request
> and the connection stays indefinitely.
> - I tested several configurations, between the following machines:
> laptop: 2.6.9 (with or w/o swsuspend and LEGACY_PTYS, plain /dev),
> Debian, OpenSSH_3.8.1p1
> desktop: 2.6.9 (CONFIG_LEGACY_PTYS=y, plain /dev), Debian, OpenSSH_3.8.1p1
> H: 2.4.27+skas3 (plain /dev), Debian, OpenSSH_3.8.1p1
> U: 2.4.20, RedHat 7.3, OpenSSH_3.1p1
> all work ok except the first one:
> laptop -> desktop: hangs
> laptop -> laptop: OK
> laptop -> {H,U} -> desktop -> desktop: OK
> laptop -> {H,U} -> desktop -> laptop: OK
> desktop -> {H,U}: OK (day long sessions completely fine)
> Debugging output:
> - I made a "strace -f -o ssh-strace.txt ssh <desktop>"
> Immediately after log in i run:
> for i in `seq 1 200` ; do echo -n =; done
> the client hanged after 171 chars (but this is not always so, once it
> hanged before giving me the prompt, other time after much more chars),
> then i pressed <enter> once, then i killed the process after a while. You
> can see all this in the attached log (ssh-strace1.txt). (remark that the 3
> "write(5, "==="....) lines near the end of the log add up to exactly 171
> chars)
> - i also attach strace logs made for both the client and the server at the
> same time,
> strace -f -r -o sshd-desktop-strace.txt sshd -D -e -p 24
> strace -f -r -o ssh-laptop-strace.txt ssh artemisa -p 24
> This time i run
> cat /etc/services
> to hang the client.
> Hope it helps, feel free to request some other test/strace.
> [[ Please, CC directly to me in replys, thanks! ]]
> Sorry, the attachments make the mail bigger than 100K, or are simply not
> accepted by majordomo... Please, find the files here:

I tried this between two 2.6.9-ac4 machines and did not see it. I don't
know if that's in any way helpful or not :-(

-bill davidsen (
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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