Using the proxy_command option with ssh

This page explains how to use the proxy_command feature of ssh. This feature is needed when you want to connect to a computer B, but you are not allowed to connect directly to it; instead, you have to connect to computer A first, and then perform a further connection from A to B.


The idea is that you ask ssh to connect to computer B by using a proxy to create a sort of tunnel. One way to perform such an operation is to use netcat, a tool that simply takes the standard input and redirects it to a given TCP port.

Therefore, a requirement is to install netcat on computer A. You can already check if the netcat or nc command is available on you computer, since some distributions include it (if it is already installed, the output of the command:

which netcat


which nc

will return the absolute path to the executable).

If this is not the case, you will need to install it on your own. Typically, it will be sufficient to look for a netcat distribution on the web, unzip the downloaded package, cd into the folder and execute something like:

./configure --prefix=.
make install

This usually creates a subfolder bin, containing the netcat and nc executables. Write down the full path to nc that we will need later.


You can now test the proxy command with ssh. Edit the ~/.ssh/config file on the computer on which you installed AiiDA (or create it if missing) and add the following lines:


where you have to replace:

  • FULLHOSTNAMEA and FULLHOSTNAMEB with the fully-qualified hostnames of computer A and B (remembering that B is the computer you want to actually connect to, and A is the intermediate computer to which you have direct access)
  • USER_A and USER_B are the usernames on the two machines (that can possibly be the same).
  • ABSPATH_NETCAT is the absolute path to the nc executable that you obtained in the previous step.

Remember also to configure passwordless ssh connections using ssh keys both from your computer to A, and from A to B.

Once you add this lines and save the file, try to execute:


which should allow you to directly connect to B.


There are several versions of netcat available on the web. We found at least one case in which the executable wasn’t working properly. At the end of the connection, the netcat executable might still be running: as a result, you may rapidly leave the cluster with hundreds of opened ssh connections, one for every time you connect to the cluster B. Therefore, check on both computers A and B that the number of processes netcat and ssh are disappearing if you close the connection. To check if such processes are running, you can execute:

ps -aux | grep <username>

Remember that a cluster might have more than one login node, and the ssh connection will randomly connect to any of them.

AiiDA config

If the above steps work, setup and configure now the computer as explained here.

If you properly set up the ~/.ssh/config file in the previous step, AiiDA should properly parse the information in the file and provide the correct default value for the proxy_command during the verdi computer configure step.

Some notes on the proxy_command option

  • In the ~/.ssh/config file, you can leave the %h and %p placeholders, that are then automatically replaced by ssh with the hostname and the port of the machine B when creating the proxy. However, in the AiiDA proxy_command option, you need to put the actual hostname and port. If you start from a properly configured ~/.ssh/config file, AiiDA will already replace these placeholders with the correct values. However, if you input the proxy_command value manually, remember to write the hostname and the port and not %h and %p.
  • In the ~/.ssh/config file, you can also insert stdout and stderr redirection, e.g. 2> /dev/null to hide any error that may occur during the proxying/tunneling. However, you should only give AiiDA the actual command to be executed, without any redirection. Again, AiiDA will remove the redirection when it automatically reads the ~/.ssh/config file, but be careful if entering manually the content in this field.