Posts about ssh

The SSH agent

This is one part in a series on OpenSSH client configuration. Also read Elegant OpenSSH Configuration and Secure OpenSSH Defaults.

As part of another SSH client article we potentially generated a new ssh key for use in ssh public-key authentication.

$ ssh-keygen -t rsa -b 4096 # if you don't already have a key

SSH public-key authentication has intrinsic benefits; but many see it as a mechanism for non-interactive login: you don't have to remember, or type, a password.

This behavior is dependent, however, on having a non-encrypted private key. This is a security risk, because the non-encrypted private key may be compromised, either by accidential mishandling of the file or by unauthorized intrusion into the client system. In almost all cases, ssh private keys should be encrypted with a passphrase.

$ ssh-keygen -t rsa -b 4096 -f test
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again:

If you already have a passphrase that is not encrypted, use the -p argument to ssh-keygen to set one.

$ ssh-keygen -p -f ~/.ssh/id_rsa

Now the private key is protected by a passphrase, which you'll be prompted for each time you use it. This is better than a password, because the passphrase is not transmitted to the server; but we've lost the ability to authenticate without having to type anything.


OpenSSH provides a dedicated agent process for the sole purpose of handling decrypted ssh private keys in-memory. Most Unix and Linux desktop operating systems (including OS X) start and maintain a per-user SSH agent process automatically.

$ pgrep -lfu $USER ssh-agent
815 /usr/bin/ssh-agent -l

Using the ssh-add command, you can decrypt your ssh private key by inputing your passphrase once, adding the decrypted key to the running agent.

$ ssh-add ~/.ssh/id_rsa # the path to the private key may be omitted for default paths
Enter passphrase for /Users/joan5896/.ssh/id_rsa: 
Identity added: /Users/joan5896/.ssh/id_rsa (/Users/joan5896/.ssh/id_rsa)

The decrypted private key remains resident in the ssh-agent process.

$ ssh-add -L
ssh-rsa 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 /Users/joan5896/.ssh/id_rsa

This is better than a non-encrypted on-disk private key for two reasons: first the decrypted private key exists only in memory, not on disk. This makes is more difficult to mishandle, including the fact that it cannot be recovered without re-inputing the passphrase once the workstation is powered off. Second, client applications (like OpenSSH itself) no longer require direct access to the private key, encrypted or otherwise, nor must you provide your (secret) key passphrase to client applications: the agent moderates all use of the key itself.

The default OpenSSH client will use the agent process identified by the SSH_AUTH_SOCK environment variable by default; but you generally don't have to worry about it: your workstation environment should configure it for you.


At this point, there's nothing more to do. With your ssh key added to the agent process, you're back to not needing to type in a password (or passphrase), but without the risk of a non-encrypted private key stored permanently on disk.

Secure OpenSSH defaults

This is one part in a series on OpenSSH client configuration. Also read Elegant OpenSSH configuration and The SSH agent.

It's good practice to harden our ssh client with some secure "defaults". Starting your configuration file with the following directives will apply the directives to all (*) hosts.

(These are listed as multiple Host * stanzas, but they can be combined into a single stanza in your actual configuration file.)

If you prefer, follow along with an example of a complete ~/.ssh/config file.

Require secure algorithms

OpenSSH supports many encryption and authentication algorithms, but some of those algorithms are known to be weak to cryptographic attack. The Mozilla project publishes a list of recommended algorithms that exclude algorithms that are known to be insecure.

Host *

(More information on the the available encryption and authentication algorithms, and how a recommended set is derived, is available in this fantastic blog post, "Secure secure shell.")

Hash your known_hosts file

Every time you connect to an SSH server, your client caches a copy of the remote server's host key in a ~/.ssh/known_hosts file. If your ssh client is ever compromised, this list can expose the remote servers to attack using your compromised credentials. Be a good citizen and hash your known hosts file.

Host *
HashKnownHosts yes

(Hash any existing entries in your ~/.ssh/known_hosts file by running ssh-keygen -H. Don't forget to remove the backup ~/.ssh/known_hosts.old.)

$ ssh-keygen -H
$ rm -i ~/.ssh/known_hosts.old

No roaming

Finally, disable the experimental "roaming" feature to mitigate exposure to a pair of potential vulnerabilities, CVE-2016-0777 and CVE-2016-0778.

Host *
UseRoaming no

Dealing with insecure servers

Some servers are old enough that they may not support the newer, more secure algorithms listed. In the RC environment, for example, the login and other Internet-accessible systems provide relatively modern ssh algorithms; but the host in the domain may not.

To support connection to older hosts while requiring newer algorithms by default, override these settings earlier in the configuration file.

# Internal RC hosts are running an old version of OpenSSH
Match host=*
MACs hmac-sha1,,hmac-ripemd160,,hmac-sha1-96

Elegant OpenSSH configuration

This is one part in a series on OpenSSH client configuration. Also read Secure OpenSSH defaults and The SSH agent.

The OpenSSH client is very robust, verify flexible, and very configurable. Many times I see people struggling to remember server-specific ssh flags or arcane, manual multi-hop procedures. I even see entire scripts written to automate the process.

But the vast majority of what you might want ssh to do can be abstracted away with some configuration in your ~/.ssh/config file.

All (or, at least, most) of these configuration directives are fully documented in the ssh_config manpage.

If you prefer, follow along with an example of a complete ~/.ssh/config file.


One of the first annoyances people have--and one of the first things people try to fix--when using a command-line ssh client is having to type in long hostnames. For example, the Research Computing login service is available at

$ ssh

This particular name isn't too bad; but coupled with usernames and especially when used as part of an scp, these fully-qualified domain names can become cumbersome.

$ scp -r /path/to/src/

OpenSSH supports host aliases through pattern-matching in Host directives.

Host login*.rc

Host *.rc

In this example, %h is substituted with the name specified on the command-line. With a configuration like this in place, connections to login.rc are directed to the full name

$ scp -r /path/to/src/ joan5896@login.rc:dest/

Failing that, other references to hosts with a .rc suffix are directed to the internal Research Computing domain. (We'll use these later.)

(The .rc domain segment could be moved from the Host pattern to the HostName value; but leaving it in the alias helps to distinguish the Research Computing login nodes from other login nodes that you may have access to. You can use arbitrary aliases in the Host directive, too; but then the %h substitution isn't useful: you have to enumerate each targeted host.)


Unless you happen to use the same username on your local workstation as you have on the remove server, you likely specify a username using either the @ syntax or -l argument to the ssh command.

$ ssh joan5896@login.rc

As with specifying a fully-qualified domain name, tracking and specifying a different username for each remote host can become burdensome, especially during an scp operation. Record the correct username in your ~/.ssh/config file in stead.

Match host=*,*
User joan5896

Now all connections to Research Computing hosts use the specified username by default, without it having to be specified on the command-line.

$ scp -r /path/to/src/ login.rc:dest/

Note that we're using a Match directive here, rather than a Host directive. The host= argument to Match matches against the derived hostname, so it reflects the real hostname as determined using the previous Host directives. (Make sure the correct HostName is established earlier in the configuration, though.)


Even if the actual command is simple to type, authenticating to the host may be require manual intervention. The Research Computing login nodes, for example, require two-factor authentication using a password or pin coupled with a one-time VASCO password or Duo credential. If you want to open multiple connections--or, again, copy files using scp--having to authenticate with multiple factors quickly becomes tedious. (Even having to type in a password at all may be unnecessary; but we'll assume, as is the case with the Research Computing login example, that you can't use public-key authentication.)

OpenSSH supports sharing a single network connection for multiple ssh sessions.

ControlMaster auto
ControlPath ~/.ssh/.socket_%h_%p_%r
ControlPersist 4h

With ControlMaster and ControlPath defined, the first ssh connection authenticates and establishes a session normally; but future connections join the active connection, bypassing the need to re-authenticate. The optional ControlPersist option causes this connection to remain active for a period of time even after the last session has been closed.

$ ssh login.rc's password: 
[joan5896@login01 ~]$ logout

$ ssh login.rc
[joan5896@login01 ~]$

(Note that many arguments to the ssh command are effectively ignored after the initial connection is established. Notably, if X11 was not forwarded with -X or -Y during the first session, you cannot use the shared connection to forward X11 in a later session. In this case, use the -S none argument to ssh to ignore the existing connection and explicitly establish a new connection.)


But what if you want to get to a host that isn't directly available from your local workstation? The hosts in the domain referenced above may be accessible from a local network connection; but if you are connecting from elsewhere on the Internet, you won't be able to access them directly.

Except that OpenSSH provides the ProxyCommand option which, when coupled with the OpenSSH client presumed to be available on the intermediate server, supports arbitrary proxy connections through to remotely-accessible servers.

Match host=*
ProxyCommand ssh -W %h:%p

Even though you can't connect directly to Janus compute nodes from the Internet, for example, you can connect to them from a Research Computing login node; so this ProxyCommand configuration allows transparent access to hosts in the internal Research Computing domain.

$ ssh janus-compile1.rc
[joan5896@janus-compile1 ~]$

And it even works with scp.

$ echo 'Hello, world!' >/tmp/hello.txt
$ scp /tmp/hello.txt janus-compile1.rc:/tmp
hello.txt                                     100%   14     0.0KB/s   00:00

$ ssh janus-compile1.rc cat /tmp/hello.txt
Hello, world!

Public-key authentication

If you tried the example above, chances are that you were met with an unexpected password prompt that didn't accept any password that you used. That's because most internal Research Computing hosts don't actually support interactive authentication, two-factor or otherwise. Connections from a CURC login node are authorized by the login node; but a proxied connection must authenticate from your local client.

The best way to authenticate your local workstation to an internal CURC host is using public-key authentication.

If you don't already have an SSH key, generate one now.

$ ssh-keygen -t rsa -b 4096 # if you don't already have a key

Now we have to copy the (new?) public key to the remote CURC ~/.ssh/authorized_keys file. RC provides a global home directory, so copying to any login node will do. Targeting a specific login node is useful, though: the ControlMaster configuration for tends to confuse ssh-copy-id.

$ ssh-copy-id login01.rc

(The ssh-copy-id command doesn't come with OS X, but theres a third-party port available on GitHub. It's usually available on a Linux system, too. Alternatively, you can just edit ~/.ssh/authorized_keys manually.)