Project:Infrastructure/SSH Key Guide

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This mini-guide explains on how to create and use ssh-keys, especially for use on cvs.gentoo.org.

Key Handling

Your SSH keypair authenticates you to Gentoo Infrastructure. Properly handling these keys is vital to keeping our machines safe. Please try to follow these guidelines:

  • Place your private keys only on machines you trust. This means only you have root on these machines and they are not shared with other users.
  • Do not trust Gentoo Infrastructure. Do not place copies of your keys on Gentoo machines (like dev.gentoo.org.) You may forward your SSH agent through Gentoo managed machines if they are configured to allow users to agent forward (more on forwarding later.)
  • Encrypt your keys with a strong passphrase. If you have trouble making a passphrase try emerge pwgen; pwgen -sB 25
  • Do not access Gentoo infrastructure from untrusted machines such as business kiosks at hotels, internet cafes, or machines at computer conferences. Many of these machines are infected with malware.
  • If you believe your keys were compromised, contact infrastructure immediately. You can do this via #gentoo-infra on irc.freenode.net or by emailing incidents@gentoo.org.
  • Official hostkey fingerprints for Gentoo Infrastructure servers are available on the server specifications page.

Creating the SSH keys

First of all, be physically logged on to your own computer. Make sure that no-one will see you typing stuff in, since we are going to type in passphrases and such. So get your pepperspray and fight all untrusted entities until you are home alone.

Now we are going to create our ssh keys, RSA keys to be exact. The key should be at least 2048 bits in length, but 4096 bits is recommended. Log onto your computer as the user that you are going to be using when you want to access cvs.gentoo.org. Then issue ssh-keygen -t rsa -b 4096 :

CodeCreating SSH keys

$ ssh-keygen -t rsa -b 4096
Generating public/private rsa key pair.
Enter file in which to save the key (/home/temp/.ssh/id_rsa): ## (Press enter)
Created directory '/home/temp/.ssh'.
Enter passphrase (empty for no passphrase): ## (Enter your passphrase)
Enter same passphrase again: ## (Enter your passphrase again)
Your identification has been saved in /home/temp/.ssh/id_rsa.
Your public key has been saved in /home/temp/.ssh/id_rsa.pub.
The key fingerprint is:
85:35:81:a0:87:56:78:a2:da:53:6c:63:32:d1:34:48 user@examplehost ## This is the fingerprint of your new key
Note
Please be sure to set a strong passphrase on your private key. Ideally, this passphrase should be at least eight characters and contain a mixture of letters, numbers and symbols.
Warning
Do not set an empty passphrase on your ssh key. If infra finds out this is the case; your account will be suspended.

Created files:

user $ ls ~/.ssh
id_rsa id_rsa.pub

You may have more files than this, but the two files listed above are the ones that are really important.

The first file, id_rsa , is your private key. Don't give this to anyone; never decrypt it on an untrusted machine. Gentoo Staff will never ask you for a copy of your private key.

Warning
Be very careful which machines you put your private key on. If you have several ( trusted! ) hosts from which you want to connect to cvs.gentoo.org, you should copy id_rsa to the ~/.ssh directories on those hosts. Trusted machines are machines that only you have root on; these machines are not shared with other users.

The second file, id_rsa.pub , is your public key. Distribute this file amongst all hosts that you want to be able to access through SSH pubkey authentification. This file should be appended to ~/.ssh/authorized_keys on those remote hosts. Also add it to your local host so you can connect to that one too if you have several boxes.

CodeAdding the SSH key to the box

$ cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

Installing your public key on a machine using LDAP authentication for SSH

Note
If you are a new developer, your recruiter will put your first SSH key into LDAP, so that you can login. You can then add any additional SSH keys yourself using the following procedure.
Note
For most of the Gentoo infrastructure, we use LDAP to distribute user information including SSH public keys. On these machines, ~/.ssh/authorized_keys should generally not contain your key.

You should place your public key into LDAP, using perl_ldap , or ldapmodify directly. The Infrastructure LDAP guide describes this in more detail.

CodeAdding the SSH key with perl_ldap on dev.gentoo.org

$ perl_ldap -b user -C sshPublicKey "$(cat ~/.ssh/id_rsa.pub)" <username>
Warning
Each sshPublicKey attribute must contain exactly one public key. If you have multiple public keys, you must have multiple attributes!

Using keychain

Every time you want to log on to a remote host using SSH public key authentification, you will be asked to enter your passphrase. As much as everybody likes typing, too much is sometimes too much. Luckily, there is keychain to the rescue. There is an document on this one here , but I'll give you a quick introduction.

First, install keychain :

root # emerge --ask keychain

Now have keychain load up your private ssh key when you log on to your local box. To do so, add the following to ~/.bash_profile . Again, this should be done on your local machine where you work at the Gentoo CVS.

Warning
NEVER run keychain or decrypt your private key on an untrusted host.
CodeAdd this to .bash_profile

keychain ~/.ssh/id_rsa
. .keychain/hostname-sh

Be sure to substitute hostname with your hostname.

Acknowledgements

We would like to thank the following authors and editors for their contributions to this guide:

  • swift
  • robbat2
  • antarus
  • nightmorph