Break ssh command from within a script and save debug log to a file-CodePudding

I am trying to write a bash script that runs ssh command with debug (ssh -vvv) against a specified host/hosts. I am not trying to login to the server, but rather just trying to see what all kex, mac and ciphers does the server offer.

I did create a script to find the kex algos after negotiation using the -G option.

#!/bin/bash
for f in `cat servers.txt`;
do echo "### $f ###";
        echo -e "kexalgorithms"
        result=$(ssh -G $f uname -a | grep kexalgorithms)
        echo $result;
done

However, I now realize that this output with depend on the ssh config on the local machine and will not give me the exact kex offered by the remote server.

If I do a ssh -vvv <host> it would give this info in the line

debug2: peer server KEXINIT proposal

debug2: KEX algorithms: curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ssh-ed25519 debug2: ciphers

But, if run ssh -vvv from the script it will be stuck at username/password prompt.

Is there any way that I can save the standard output till the password prompt to a file (probably with the host name as the filename) and break the script run at a password prompt?

Thanks in advance

CodePudding user response：

Get all supported algorithms for key exchange from remote ssh-server with nmap:

nmap --script ssh2-enum-algos -p 22 YOUR-SSH-SERVER \
  | awk -v a='kex_algorithms:' '$2==a{getline; while( $0~/^\| {7}/ ){ print $2; getline }}'

Output (e.g.):

curve25519-sha256
[email protected]
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
diffie-hellman-group-exchange-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group14-sha256

CodePudding user response：

You can use ssh -o BatchMode=yes. It'll exit when the remote side asks for a password.

According to man ssh_config:

BatchMode

If set to yes, user interaction such as password prompts and host key confirmation requests will be disabled. This option is useful in scripts and other batch jobs where no user is present to interact with ssh(1). The argument must be yes or no (the default).