Streamlining two-factor authentication with ssh¶

Do you tire of responding to Duo pushes every single time you log in to NCAR's HPC resources? While two-factor authentication (2FA) is a hard requirement for user access, there are techniques for "sharing" authentication, allowing multiple ssh or scp processes to reuse a pre-established connection. This technique uses the ControlPath and ControlMaster functionality built in to ssh and requires some straightforward local configuration.

ControlPath and ControlMaster are ssh client options that work together along with an automatically generated socket file, allowing an initial ssh connection to be reused and optionally persist after the initial session has disconnected. In practice this means the first connection to a host will be authenticated normally, but subsequent connections will simply reuse the initial connection without requiring additional authentication.

Local configuration¶

Nothing special is required on the HPC systems to enable this functionality, however you will need to perform some local configuration of your ssh client. In the example below I'm using the default OpenSSH on my Macbook laptop, and all that is required is editing the config file in my ~/.ssh directory:

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host casper
     HostName casper.hpc.ucar.edu

host derecho
     HostName derecho.hpc.ucar.edu

host *
    ControlMaster auto
    ControlPath ~/.ssh/controlpath-%r@%h:%p
    ControlPersist 12h
    ServerAliveInterval 120s
    ServerAliveCountMax 5

The first two sections are simply convenience: they define aliases for the host names casper and derecho to their fully-qualified domain names. The idea then is I can ssh casper without needing to use the full host and domain name.

The final section (host *) contains the specific configuration of interest:

• ControlPath ~/.ssh/controlpath-%r@%h:%p specifies the path to a controling file "socket" used for connection sharing. This makes use of some special tokens:

  • %r : username on the remote system,
  • %h : the remote system host name, and
  • %p : the remote port.

  This allows for a unique ControlPath file to be placed in our ~/.ssh/ directory for each unique user/host/port combination.

• ControlMaster auto enables "opportunistic ssh multiplexing," meaning ssh will attempt to use an existing established connection if possible, and establish a new one if required.

• ControlPersist 12h specifies that the controlling connection should remain open in the background (waiting for future client connections) after the initial client connection has been closed.

  Without this option, closing a controlling ssh session will abruptly terminate any other active, shared authentication connections.

• The final two lines, ServerAliveInterval 120s and ServerAliveCountMax 5, are useful for maintaining ssh connections.

  When our client is idle, eventually we will be disconnected from the server. These settings send very minimal traffic at intervals even when we are not actively using ssh, triggering the server to respond. If the server does not respond after ServerAliveCountMax steps, our client will finally give up and disconnect.

  See here for additional discussion.

Your ~/.ssh/config file supports many, many more options. See here for additional details.

Demonstration¶

To see how these pieces work together, consider the following examples:

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ssh-client(1)$ ls ~/.ssh/control*
ls: /Users/benkirk/.ssh/control*: No such file or directory

ssh-client(2)$ ssh casper uptime
(benkirk@casper.hpc.ucar.edu) ncar-two-factor:
 07:53:13  up 22 days 12:30,  89 users,  load average: 7.74, 7.87, 8.09

ssh-client(3)$ file ~/.ssh/control*
/Users/benkirk/.ssh/controlpath-benkirk@casper.hpc.ucar.edu:22: socket

ssh-client(4)$ ps aux | grep "ssh: " | grep -v grep | awk '{print $11 " " $12 " " $13}'
ssh: /Users/benkirk/.ssh/controlpath-benkirk@casper.hpc.ucar.edu:22 [mux]

ssh-client(5)$ ssh casper uptime
 07:53:34  up 22 days 12:31,  87 users,  load average: 7.66, 7.85, 8.08

The same process applies when adding a new connection to Derecho as seen in the following example.

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ssh-client(6)$ ssh derecho "uname -a"
Access to and use of this UCAR computer system is limited to authorized use by
UCAR Policies 1-7 and 3-6 and all applicable federal laws, executive orders,
policies and directives. UCAR computer systems are subject to monitoring at all
times to ensure proper functioning of equipment and systems including security
devices, to prevent unauthorized use and violations of statutes and security
regulations, to deter criminal activity, and for other similar purposes.

Users should be aware that information placed in the system is subject to
monitoring and is not subject to any expectation of privacy. Unauthorized use
or abuse will be dealt with according to UCAR Policy, up to and including
criminal or civil penalties as warranted.

By logging in, you are agreeing to these terms.

(benkirk@derecho.hpc.ucar.edu) ncar-two-factor:
Linux derecho4 5.14.21-150400.24.18-default #1 SMP PREEMPT_DYNAMIC Thu Aug 4 14:17:48 UTC 2022 (e9f7bfc) x86_64 x86_64 x86_64 GNU/Linux

ssh-client(7)$ file ~/.ssh/control*
/Users/benkirk/.ssh/controlpath-benkirk@casper.hpc.ucar.edu:22:  socket
/Users/benkirk/.ssh/controlpath-benkirk@derecho.hpc.ucar.edu:22: socket

ssh-client(8)$ ps aux | grep "ssh: " | grep -v grep | awk '{print $11 " " $12 " " $13}'
ssh: /Users/benkirk/.ssh/controlpath-benkirk@derecho.hpc.ucar.edu:22 [mux]
ssh: /Users/benkirk/.ssh/controlpath-benkirk@casper.hpc.ucar.edu:22 [mux]

ssh-client(9)$ ssh derecho "uname -a"
Linux derecho4 5.14.21-150400.24.18-default #1 SMP PREEMPT_DYNAMIC Thu Aug 4 14:17:48 UTC 2022 (e9f7bfc) x86_64 x86_64 x86_64 GNU/Linux

In the examples above we have used ssh to run a remote command, but the same functionality applies with terminal sessions, and extends to scp and sftp as well.

Happy ssh'ing!!