LKBEN10954: Howto quickly debug an ssh connection on a commandline

Symptom

The SSH connection does not work or seems to work strange

Cause

This can have lots of causes, most of them are configuration issues

Solution

Try connection with verbose mode active. You will be able to see all the actions taken by the protocol.

ssh -v -X -l username remotehost

The following output is an example:

debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.123.3 [192.168.123.3] port 22.
debug1: Connection established.
debug1: identity file /home/wpeeters/.ssh/identity type -1
debug1: identity file /home/wpeeters/.ssh/id_rsa type -1
debug1: identity file /home/wpeeters/.ssh/id_dsa type -1
debug1: Remote protocol version 1.99, remote software version OpenSSH_4.1
debug1: match: OpenSSH_4.1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.2p1 Debian-7ubuntu3.1
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host '192.168.123.3' is known and matches the RSA host key.
debug1: Found key in /home/wpeeters/.ssh/known_hosts:3
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Trying private key: /home/wpeeters/.ssh/identity
debug1: Trying private key: /home/wpeeters/.ssh/id_rsa
debug1: Trying private key: /home/wpeeters/.ssh/id_dsa
debug1: Next authentication method: keyboard-interactive
Password:
debug1: Authentication succeeded (keyboard-interactive).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Sending environment.
debug1: Sending env LANG = de_DE.UTF-8
Last login: Fri Oct 26 18:22:21 2007 from 192.168.123.22

This connection has X11 forwarding active. You can see it in the output by searchgin "Requesting X11 forwarding with authentication spoofing". 

Disclaimer:

About the Author

Author: Wim Peeters - Keskon GmbH & Co. KG

Wim Peeters is electronics engineer with an additional master in IT and over 30 years of experience, including time spent in support, development, consulting, training and database administration. Wim has worked with SQL Server since version 6.5. He has developed in C/C++, Java and C# on Windows and Linux. He writes knowledge base articles to solve IT problems and publishes them on the Lubby Knowledge Platform.