Sunday, August 28, 2011

Linux Server SSH

To continue with my Ubuntu Linux server odyssey, I got SSH working this afternoon for access from my LAN. As always, heed the warnings, these posts are without a warranty of any kind, not least because I am a total amateur. They exist because it's easy to have Google to "index my brain" or rather, what was in my brain at a certain time.

The objective is to enable remote login for my Ubuntu machine running with a local ip address of We begin by setting a stronger password for my account on Ubuntu under: System > Preferences > About Me (weirdly, it's not under Passwords and Encryption Keys ..).

On the client (OS X Snow Leopard Server), I already have some key pairs from a previous test

> ls ~/.ssh
authorized_keys	id_dsa	known_hosts

but they are DSA keys, which the notes I'm going to follow deprecate. We'll make a short 1024 byte bit RSA pair for now:

ssh-keygen -b 1024 -t rsa

> cat 
ssh-rsa AAAAB..5Eoec=

Not sure what the pseudo ip address or "comcast" thing is about.

I also take this opportunity to set a passphrase for the public key (and of course I make a note of it). This will be useful (it says) because we can set up the server to require both a password to gain access to the public key, as well as the corresponding private key before login is allowed.

[UPDATE: I'm still working my way through this, but I suspect that the previous statement isn't correct. The passphrase is used to protect the value of the private key. So, in this setup OS X will decrypt the private key using the passphrase, and the stored value of the private key in id_rsa is encrypted. I'll try to figure all this out soon. ]

The easiest way to get my new public key onto the server is to use SSH (with password, before we disable it). The next question is, do we need to install an SSH server? It appears yes.

sudo apt-get install openssh-server

There is a pre-existing config file /etc/ssh/ssh_config but after the previous command there are more including sshd_config. I make sure to save a copy of this file before I modify it. Now try:

ssh te@

ssh: connect to host port 22: Connection refused

OK, so we need to modify /etc/ssh/sshd_config. Port22 is already uncommented. Now uncomment:

PermitRootLogin no
ChallengeResponseAuthentication yes
PasswordAuthentication yes   # we'll set it to no eventually

On the server:

sudo /etc/init.d/ssh restart

On the client:

> ssh te@
The authenticity of host ' (' can't be established.
RSA key fingerprint is d1: .. :83.
Are you sure you want to continue connecting (yes/no)? 

Check the fingerprint on the the server:

ssh-keygen -l -f /etc/ssh/

2048 d1: .. :83 /etc/ssh/ (RSA)

Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '' (RSA) to the list of known hosts.
Connection closed by

> ssh te@
te@'s password: 
Welcome to Ubuntu 11.04 (GNU/Linux 2.6.38-11-generic x86_64)

Now to change to not using our password. We need to copy our RSA public key to the server in some secure way. According to the sshd_config file, the "authorized keys file" is
%h/.ssh/authorized_keys. That's in my home directory. I do:

> scp ~/.ssh/ te@
te@'s password:                                    100%  260     0.3KB/s   00:00    

te@VB:~$ logout
Connection to closed.

> ssh te@
Identity added: /Users/telliott_admin/.ssh/id_rsa (/Users/telliott_admin/.ssh/id_rsa)
Welcome to Ubuntu 11.04 (GNU/Linux 2.6.38-11-generic x86_64)

* Documentation:

Last login: Sun Aug 28 14:35:49 2011 from osxserver.local

Now, finally, be sure to turn off password authentication for the ssh server: PasswordAuthentication no. At first, I'm not being prompted for a password to retrieve my public key on the server.. just getting automatically logged in by:

ssh te@

I forgot to do:

ChallengeResponsePasswords yes

sudo /etc/init.d/ssh restart

Restart and I get the challenge.. Although it is apparently cached sometimes. Looks like it works.

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