ssh -X remote does not work due to problem with xauth

Thu May 10 18:30:08 UTC 2018

Hello,

I'm facing a problem with running from FreeBSD 'ssh -X remote host'. The
remotre server is a recent SLES Linux system. This thread started in
freebsd-questions@ and I now move it to here as it seems more a problem
of the Xorg server than a ssh problem of the base system.

When I launch 'ssh -X remote host' to get X11 forwarded to my workstation
this does not work, i.e. does not produce a DISPLAY env on the remote
host and says:

$ ssh -X apitzm at srap16dxr1
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Last login: Wed May  9 12:37:17 2018 from 10.49.8.33
apitzm at srap16dxr1:~>

I digged into it with -vv options for ssh and trussing the client. It seems it's
our end causing the problem:

$ uname -a
FreeBSD r314251-amd64 12.0-CURRENT FreeBSD 12.0-CURRENT #0 r314251: Sat Feb 25 17:14:40 CET 2017
root at r303343-amd64:/usr/obj/usr/src/sys/GENERIC  amd64

$ truss -s128 -f -o ssh.tr ssh -X -vv apitzm at srap16dxr1

$ grep xauth ssh.tr
 2505: stat("/usr/local/bin/xauth",{ mode=-rwxr-xr-x ,inode=15268663,size=41056,blksize=32768 }) = 0 (0x0)
 2505: write(2,"debug2: client_x11_get_proto: /usr/local/bin/xauth -f /tmp/ssh-adSkh4btDHWr/xauthfile generate :0 MIT-MAGIC-COOKIE-1 untrusted timeout 1260 2>/dev/null\r\n",153) = 153 (0x99)
 2506: execve("/usr/local/bin/xauth",0x80145a260,0x80145a408) = 0 (0x0)
 2506: stat("/tmp/ssh-adSkh4btDHWr/xauthfile-c",0x7fffffffdb80) ERR#2 'No such file or directory'
 2506: openat(AT_FDCWD,"/tmp/ssh-adSkh4btDHWr/xauthfile-c",O_WRONLY|O_CREAT|O_EXCL,0600) = 3 (0x3)
 2506: pathconf("/tmp/ssh-adSkh4btDHWr/xauthfile-c",_PC_LINK_MAX) = 32767 (0x7fff)
 2506: link("/tmp/ssh-adSkh4btDHWr/xauthfile-c","/tmp/ssh-adSkh4btDHWr/xauthfile-l") = 0 (0x0)
 2506: access("/tmp/ssh-adSkh4btDHWr/xauthfile",F_OK) ERR#2 'No such file or directory'
 2506: open("/tmp/ssh-adSkh4btDHWr/xauthfile",O_RDONLY,0666) ERR#2 'No such file or directory'
 2506: access("/tmp/ssh-adSkh4btDHWr/xauthfile",F_OK) ERR#2 'No such file or directory'
 2506: write(2,"/usr/local/bin/xauth:  file /tmp/ssh-adSkh4btDHWr/xauthfile does not exist\n",75) = 75 (0x4b)
 2506: access("/tmp/kde-guru/xauth-1001-_0",R_OK) = 0 (0x0)
 2506: open("/tmp/kde-guru/xauth-1001-_0",O_RDONLY,0666) = 4 (0x4)
 2506: write(2,"/usr/local/bin/xauth: (argv):1:  ",33) = 33 (0x21)
 2506: lstat("/tmp/ssh-adSkh4btDHWr/xauthfile-c",{ mode=-rw------- ,inode=24961,size=0,blksize=32768 }) = 0 (0x0)
 2506: unlink("/tmp/ssh-adSkh4btDHWr/xauthfile-c") = 0 (0x0)
 2506: lstat("/tmp/ssh-adSkh4btDHWr/xauthfile-l",{ mode=-rw------- ,inode=24961,size=0,blksize=32768 }) = 0 (0x0)
 2506: unlink("/tmp/ssh-adSkh4btDHWr/xauthfile-l") = 0 (0x0)
 2505: unlink("/tmp/ssh-adSkh4btDHWr/xauthfile") ERR#2 'No such file or directory'
 2505: write(2,"Warning: untrusted X11 forwarding setup failed: xauth key data not generated\r\n",78) = 78 (0x4e)

$ grep Sec ssh.tr
 2505: read(3,"\0\0\^A\^D\v\^_\0\0\0h\0\0\0\^Secdsa-sha2-nistp256\0\0\0\bnistp256\0\0\0A\^D\^Qa<x\^U\M-jW\M-Hy/\M-:\M-b\^Qc\M-\^\^SG\M^ZT|\M-ck7\M-H\M-s\M-gKNi\n\M^Ex\M-;D.\M-w\M-'\^O\^T\^Uls\M^Y\M-D\M-5{\M-6\M-3d\M-U\M-P\M-Ck\M-*7\a\^U\^W\M-^\M^Il\M-n\M-,\0\0\0 M\M-G"...,8192) = 364 (0x16c)
 2506: write(2,"couldn't query Security extension on display ":0"\n",50) = 50 (0x32)

One can simulate the xauth call locally and without redirecting STDERR and
will see the same error:

$ /usr/local/bin/xauth -f /tmp/xauthfile generate :0 MIT-MAGIC-COOKIE-1 untrusted timeout 1260
/usr/local/bin/xauth:  file /tmp/xauthfile does not exist
/usr/local/bin/xauth: (argv):1:  couldn't query Security extension on display ":0"

This, and the fact that my colleagues (coming from Ubuntu workstations)
do not face the problem, let me strongly think in a LOCAL problem, maybe
of the Xorg server which is xorg-7.7_2 from the ports.

I straced as well the remote sshd while I ssh'ed into the server. On the remote side there
is no xauth called:

$ grep exec sshd.tr
1369  execve("/usr/sbin/sshd", ["/usr/sbin/sshd", "-D", "-R"], [/* 3 vars */] <unfinished ...>
1369  <... execve resumed> )            = 0
1373  execve("/bin/bash", ["-bash"], [/* 12 vars */]) = 0
1375  execve("/usr/bin/readlink", ["readlink", "/proc/1373/exe"], [/* 15 vars */]) = 0
1377  execve("/usr/bin/tty", ["tty"], [/* 15 vars */]) = 0
1379  execve("/bin/uname", ["/bin/uname", "-n"], [/* 16 vars */]) = 0
1381  execve("/bin/uname", ["/bin/uname", "-m"], [/* 16 vars */]) = 0
1383  execve("/usr/bin/manpath", ["/usr/bin/manpath", "-q"], [/* 32 vars */]) = 0
1385  execve("/usr/bin/ls", ["ls", "/etc/bash_completion.d"], [/* 38 vars */]) = 0
1387  execve("/usr/bin/tty", ["tty"], [/* 43 vars */]) = 0
1391  execve("/usr/bin/sed", ["sed", "-r", "s@/*:|([^\\\\]):@\\1\\n at g;H;x;s@/\\n@"...], [/* 48 vars */]) = 0
1395  execve("/usr/bin/sed", ["sed", "-r", "s@/*:|([^\\\\]):@\\1\\n at g;H;x;s@/\\n@"...], [/* 49 vars */]) = 0
1397  execve("/usr/bin/dircolors", ["/usr/bin/dircolors", "-b", "/etc/DIR_COLORS"], [/* 53 vars */]) = 0
1399  execve("/usr/bin/tput", ["/usr/bin/tput", "hs"], [/* 55 vars */]) = 0
1401  execve("/usr/bin/tput", ["/usr/bin/tput", "tsl"], [/* 55 vars */]) = 0
1403  execve("/usr/bin/tput", ["/usr/bin/tput", "fsl"], [/* 55 vars */]) = 0
1405  execve("/usr/bin/tput", ["tput", "sc"], [/* 55 vars */]) = 0
1407  execve("/usr/bin/tput", ["tput", "rc"], [/* 55 vars */]) = 0
1410  execve("/usr/bin/tty", ["tty"], [/* 55 vars */]) = 0

$ grep xauth sshd.tr
$ 

The problem is LOCAL on FreeBSD.

Any ideas to nail this down?

Thanks

	matthias

-- 
Matthias Apitz, ✉ guru at unixarea.de, http://www.unixarea.de/ +49-176-38902045
Public GnuPG key: http://www.unixarea.de/key.pub
May, 9: Спаси́бо освободители! Thank you very much, Russian liberators!
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