Linux – SSH to server with xargs

linuxscriptingsshxargs

I have a pretty easy problem I'm trying to resolve here. I'm constantly running the (host) command on domains, to get their IPs, then (host) again on those IPs to get their PTRs, then I'm SSHing to that server represented in the PTR:

[root@box ~]$ host DomainIWant.com
DomainIWant.com has address 123.123.123.123

[root@box ~]$ host 123.123.123.123
123.123.123.123.in-addr.arpa domain name pointer vps2010.DomainIWantHosts.com

[root@box ~]$ ssh vps2010.DomainIWantHosts.com

Simple enough right? It's just tedious doing this over and over, so as with everything Linux I want to speed it up by automating it:

[root@box ~]$ host DomainIWant.com | awk '{print $4}' | xargs host | awk '{print $5}' | xargs ssh -tt

The problem that I'm having is when I call ssh via xargs, I do get SSH'd into the remote server but with the error [tcgetattr: Invalid argument]. I'm sitting at the remote server's shell but when I try to run any command it just hangs and eventually I have to Ctrl-C to break out of it.

If I don't tack on the (ssh -tt) then I get a [Pseudo-terminal will not be allocated because stdin is not a terminal] error. This seems like such a simply problem so I'm hoping someone more familiar with ssh via xargs can let me know if it's even possible.

Best Answer

If you're using a shell that supports backticks- or $()-style command substitution (most shells do), then in your case, you can avoid using xargs entirely, like this:

ssh $(host -t PTR $(host -t A DomainIWant.com | awk '{print $4}') | awk '{print $5}')

(I added the -t flags to the host commands, to ensure they only emit 1 line of output.)

Related Solutions

How to Remove Strict RSA Key Checking in SSH and Understand the Issues

Please don't delete the entire known_hosts file as recommended by some people, this totally voids the point of the warning. It's a security feature to warn you that a man in the middle attack may have happened.

I suggest you identify why it thinks something has changed, most likely an SSH upgrade altered the encryption keys due to a possible security hole. You can then purge that specific line from your known_hosts file:

sed -i 377d ~/.ssh/known_hosts

This deletes line 377 as shown after the colon in the warning:

/home/emerson/.ssh/known_hosts:377

Alternatively you can remove the relevant key by doing the following

ssh-keygen -R 127.0.0.1 (obviously replace with the server's IP)

Please DO NOT purge the entire file and ensure this is actually the machine you want to be connecting to prior to purging the specific key.

Ssh – How to automate SSH login with password

Don't use a password. Generate a passphrase-less SSH key and push it to your VM.

If you already have an SSH key, you can skip this step… Just hit Enter for the key and both passphrases:

$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/username/.ssh/id_rsa.
Your public key has been saved in /home/username/.ssh/id_rsa.pub.

Copy your keys to the target server:

$ ssh-copy-id id@server
id@server's password:

Now try logging into the machine, with ssh 'id@server', and check-in:

.ssh/authorized_keys

Note: If you don't have .ssh dir and authorized_keys file, you need to create it first

to make sure we haven’t added extra keys that you weren’t expecting.

Finally, check to log in…

$ ssh id@server

id@server:~$

You may also want to look into using ssh-agent if you want to try keeping your keys protected with a passphrase.

Best Answer

Related Solutions

How to Remove Strict RSA Key Checking in SSH and Understand the Issues

Ssh – How to automate SSH login with password

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