SSH – SSH Connection Closes Immediately After RemoteCommand

ssh

I have a .ssh config file on my MacBook for connecting to a remote Ubuntu server. The config file looks like this:

Host remote_computer
    HostName 1.2.3.4
    Port 22
    User remote_username
    IdentityFile ~/.ssh/id_rsa
    RemoteCommand echo "Hello, World!"

Here's what happens when I try to connect:

local_computer:/ local_username$ ssh remote_computer
Hello, World!
local_computer:/ local_username$

The RemoteCommand runs successfully, but then the connection closes immediately after that. This happens with any RemoteCommand I try, but if I remove RemoteCommand from my config file, the connection stays open:

local_computer:/ local_username$ ssh remote_computer
remote_username@remote_computer:~$

Why does the connection close after successfully running the RemoteCommand from the config file, and what can I do to prevent that?

Best Answer

force a TTY and enter a shell:

Host remote_computer
    HostName 1.2.3.4
    Port 22
    User remote_username
    IdentityFile ~/.ssh/id_rsa
    RemoteCommand echo "Hello, World!" && bash
    RequestTTY force

Related Solutions

Ssh – Multiple ‘Host *’ in ssh_config

From the ssh_config manual:

Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.

So in your example, all hosts will use User harleypig and IdentityFile ~/.ssh/personal_id_rsa.

Think of Host directives with wildcards as fallbacks: use the following settings only if they haven't been set yet. You need to write something like this:

Host host1
Hostname host1.com
Host host2
Hostname host2.com
Host host*
User harleypig
IdentityFile ~/.ssh/personal_id_rsa

You can put multiple patterns on a Host line if a given set of host aliases can't be matched with wildcards, e.g. Host host* more* outlier.

Ssh – Determine the target of an SSH connection at runtime

Since you don't pass a command to ssh, it runs a shell, which expects commands on its standard input. But you aren't passing it shell commands, you're passing it SSH traffic. Hilarity ensues.

Tunnelling is the whole point of ProxyCommand. This example tunnels an SSH connection inside an SSH connection. If you don't want to tunnel, ProxyCommand is not what you're looking for.

Since OpenSSH 6.4, you can use a Match directive with the exec keyword to include blocks conditionally based on a run-time directive. A typical use case is to have different actual host names or proxy commands depending on where your laptop is. Each Match block replaces a Host block, and you can't use the output of the command to determine the host name, only the fact that it exited with status 0.

# Connection inside ACME network
Match host foo exec on-acme-network
HostName foo.acme.local

# Connection from outside ACME
Host foo
HostName foo.example.com

Proxying is another option. To make a connection to a host name and port that are determined at connection time, you can use netcat to set up the TCP connection.

Host foo
ProxyCommand nc $(determine-target-host-name) 22

If you want a fully dynamic .ssh/config, you can make it a named pipe and ensure that a process is always writing to it. Or put it on scriptfs.

Best Answer

Related Solutions

Ssh – Multiple ‘Host *’ in ssh_config

Ssh – Determine the target of an SSH connection at runtime

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