So I ran sshd on the target machine in a debugger: # gdb /usr/sbin/sshd GNU gdb (GDB) 7.0.1-debian Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i486-linux-gnu". For bug reporting instructions, please see: ... warning: The current binary is a PIE (Position Independent Executable), which GDB does NOT currently support. Most debugger features will fail if used in this session. Reading symbols from /usr/sbin/sshd...(no debugging symbols found)...done. (gdb) set args -dddd -p22222 (gdb) run Starting program: /usr/sbin/sshd -dddd -p22222 debug2: load_server_config: filename /etc/ssh/sshd_config debug2: load_server_config: done config len = 637 debug2: parse_server_config: config /etc/ssh/sshd_config len 637 debug3: /etc/ssh/sshd_config:5 setting Port 22 debug3: /etc/ssh/sshd_config:9 setting Protocol 2 debug3: /etc/ssh/sshd_config:11 setting HostKey /etc/ssh/ssh_host_key debug3: /etc/ssh/sshd_config:13 setting HostKey /etc/ssh/ssh_host_rsa_key debug3: /etc/ssh/sshd_config:14 setting HostKey /etc/ssh/ssh_host_dsa_key debug3: /etc/ssh/sshd_config:16 setting UsePrivilegeSeparation yes debug3: /etc/ssh/sshd_config:19 setting KeyRegenerationInterval 3600 debug3: /etc/ssh/sshd_config:20 setting ServerKeyBits 768 debug3: /etc/ssh/sshd_config:23 setting SyslogFacility AUTH debug3: /etc/ssh/sshd_config:24 setting LogLevel INFO debug3: /etc/ssh/sshd_config:27 setting LoginGraceTime 600 debug3: /etc/ssh/sshd_config:28 setting PermitRootLogin yes debug3: /etc/ssh/sshd_config:29 setting StrictModes yes debug3: /etc/ssh/sshd_config:31 setting RSAAuthentication yes debug3: /etc/ssh/sshd_config:32 setting PubkeyAuthentication yes debug3: /etc/ssh/sshd_config:36 setting IgnoreRhosts yes debug3: /etc/ssh/sshd_config:38 setting RhostsRSAAuthentication no debug3: /etc/ssh/sshd_config:40 setting HostbasedAuthentication no debug3: /etc/ssh/sshd_config:45 setting PermitEmptyPasswords no debug3: /etc/ssh/sshd_config:51 setting PasswordAuthentication yes debug3: /etc/ssh/sshd_config:62 setting X11Forwarding yes debug3: /etc/ssh/sshd_config:63 setting X11DisplayOffset 10 debug3: /etc/ssh/sshd_config:64 setting PrintMotd no debug3: /etc/ssh/sshd_config:65 setting PrintLastLog yes debug3: /etc/ssh/sshd_config:66 setting TCPKeepAlive yes debug3: /etc/ssh/sshd_config:72 setting Subsystem sftp /usr/lib/openssh/sftp-server debug3: /etc/ssh/sshd_config:74 setting UsePAM yes debug1: sshd version OpenSSH_5.5p1 Debian-6 debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-1024 debug1: Checking blacklist file /etc/ssh/blacklist.RSA-1024 debug1: private host key: #0 type 0 RSA1 debug3: Not a RSA1 key file /etc/ssh/ssh_host_rsa_key. debug1: read PEM private key done: type RSA debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-1024 debug1: Checking blacklist file /etc/ssh/blacklist.RSA-1024 debug1: private host key: #1 type 1 RSA debug3: Not a RSA1 key file /etc/ssh/ssh_host_dsa_key. debug1: read PEM private key done: type DSA debug1: Checking blacklist file /usr/share/ssh/blacklist.DSA-1024 debug1: Checking blacklist file /etc/ssh/blacklist.DSA-1024 debug1: private host key: #2 type 2 DSA debug1: rexec_argv[0]='/usr/sbin/sshd' debug1: rexec_argv[1]='-dddd' debug1: rexec_argv[2]='-p22222' debug3: oom_adjust_setup Set /proc/self/oom_adj from 0 to -17 debug2: fd 3 setting O_NONBLOCK debug1: Bind to port 22222 on 0.0.0.0. Server listening on 0.0.0.0 port 22222. debug2: fd 4 setting O_NONBLOCK debug3: sock_set_v6only: set socket 4 IPV6_V6ONLY debug1: Bind to port 22222 on ::. Server listening on :: port 22222. debug3: fd 5 is not O_NONBLOCK debug1: Server will not fork when running in debugging mode. debug3: send_rexec_state: entering fd = 8 config len 637 debug3: ssh_msg_send: type 0 debug3: send_rexec_state: done debug1: rexec start in 5 out 5 newsock 5 pipe -1 sock 8 Executing new program: /usr/sbin/sshd warning: The current binary is a PIE (Position Independent Executable), which GDB does NOT currently support. Most debugger features will fail if used in this session. debug1: inetd sockets after dupping: 3, 3 Connection from 172.28.0.5 port 58160 Did not receive identification string from 172.28.0.5 Program exited with code 0377. So when logging in, the sshd process is just exiting.