There are many guides that show you how to set-up your SSH client for password-less login using public-private key certificates. If you have different clients, you may have several different private keys. How can you manage them?
It was pointed out that ssh-agent and PuTTY’s Pagent can also be used to manage multiple private keys.
SSH has a per-user configuration file called ‘~/.ssh/config’ that it can use to select your private keys based on the remote user name and remote host by using wildcards. Let’s check out my ‘config’ file:
IdentityFile ~/.ssh/ids/%h/%r/id_rsa IdentityFile ~/.ssh/ids/%h/%r/id_dsa IdentityFile ~/.ssh/ids/%h/id_rsa IdentityFile ~/.ssh/ids/%h/id_dsa IdentityFile ~/.ssh/id_rsa IdentityFile ~/.ssh/id_dsa
The percent-h and percent-r take the host and the remote user from your SSH user and hostname arguments. Consider this example command:
$ ssh remote_user@remote_hostname.example.com
From the example command, the SSH client would use the wildcards to seek the correct key to use:
~/.ssh/ids/remote_hostname.example.com/remote_user/
This means that if you had two private keys that you used to access two different servers, you would arrange them as follows. The first one is arranged as follows:
$ ls -l ~/.ssh/ids/remote.example.com/remote_user/ total 16 -rw------- 1 kelvin staff 668 Mar 24 20:09 id_dsa -rw-r--r-- 1 kelvin staff 610 Mar 24 20:09 id_dsa.pub $ ssh remote_user@remote.example.com [remote_user@remote ~]$ |
Our second example uses a simple hostname. If a remote user is not required, you can just use the hostname:
$ ls -l ~/.ssh/ids/webby.example.org/ total 16 -rw------- 1 kelvin staff 668 Mar 24 20:09 id_rsa -rw-r--r-- 1 kelvin staff 610 Mar 24 20:09 id_rsa.pub $ ssh webby.example.org [webby ~]$ |
For sure, these are totally contrived examples, but you can watch the cascade yourself by adding the verbosity flag(s) to your SSH client session (this one is my client’s WebFaction account):
Trinity:.ssh kelvin$ ssh -v user@user.webfactional.com OpenSSH_5.2p1, OpenSSL 0.9.7l 28 Sep 2006 debug1: Reading configuration data /Users/kelvin/.ssh/config debug1: Reading configuration data /etc/ssh_config debug1: Connecting to user.webfactional.com [192.168.0.254] port 22. debug1: Connection established. debug1: identity file /Users/kelvin/.ssh/ids/user.webfactional.com/user/id_rsa type -1 debug1: identity file /Users/kelvin/.ssh/ids/user.webfactional.com/user/id_dsa type 2 debug1: identity file /Users/kelvin/.ssh/ids/user.webfactional.com/id_rsa type -1 debug1: identity file /Users/kelvin/.ssh/ids/user.webfactional.com/id_dsa type -1 debug1: identity file /Users/kelvin/.ssh/id_rsa type 1 debug1: identity file /Users/kelvin/.ssh/id_dsa type -1 debug1: Remote protocol version 2.0, remote software version OpenSSH_4.3 debug1: match: OpenSSH_4.3 pat OpenSSH_4* debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_5.2 debug1: SSH2_MSG_KEXINIT sent debug1: SSH2_MSG_KEXINIT received debug1: kex: server->client aes128-ctr hmac-md5 none debug1: kex: client->server aes128-ctr 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 'user.webfactional.com' is known and matches the RSA host key. debug1: Found key in /Users/kelvin/.ssh/known_hosts:41 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,password debug1: Next authentication method: publickey debug1: Trying private key: /Users/kelvin/.ssh/ids/user.webfactional.com/user/id_rsa debug1: Offering public key: /Users/kelvin/.ssh/ids/user.webfactional.com/user/id_dsa debug1: Server accepts key: pkalg ssh-dss blen 433 debug1: read PEM private key done: type DSA debug1: Authentication succeeded (publickey). debug1: channel 0: new [client-session] debug1: Entering interactive session. Last login: Thu Mar 31 22:31:08 2015 from 192.168.0.200 [user@web ~]$