How do you fix the issue with a SSH server which freezes after authenticating?
$ ssh root@xxx.xxx.xxx.xxx
The authenticity of host ‘xx.xx.xxx.xxx (xx.xx.xxx.xxx)’ can’t be established.
RSA key fingerprint is replaced for security.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added ‘xx.xx.xxx.xxx’ (RSA) to the list of known hosts.
Password:
PTY allocation request failed on channel 0
In the above scenario, you can see that I logged in but I’m not able to get into the console.
Once you’re in debug more, you will see lots of messages elaborating the SSH access steps in detail. These messages might point you to the root cause or the stage at which you’re being locked.
In my case, I got freezed after authenticating myself.
Resolution is just to make pts and restart sshd
mkdir /dev/pts
mount /dev/pts
/etc/init.d/ssh restart
and permanent solution is just instaling udev via:
apt-get install udev
Your aim
You want to use Linux and OpenSSH to automize your tasks. Therefore you need an automatic login from host A / user a to Host B / user b. You don’t want to enter any passwords, because you want to call ssh from a within a shell script.
How to do it
First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:
a@A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa):
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A
Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):
a@A:~> ssh b@B mkdir -p .ssh
b@B's password:
Finally append a’s new public key to b@B:.ssh/authorized_keys and enter b’s password one last time:
a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys'
b@B's password:
From now on you can log into B as b from A as a without password:
a@A:~> ssh b@B hostname
B
A note from one of our readers: Depending on your version of SSH you might also have to do the following changes:
- Put the public key in .ssh/authorized_keys2
- Change the permissions of .ssh to 700
- Change the permissions of .ssh/authorized_keys2 to 640