Understanding SSH Behavior with ControlPersist in Cygwin and Git Bash

Understanding SSH Behavior Control in Cygwin Git Bash

Secure Shell (SSH) is a cryptographic network protocol used for secure data communication between a client and a server. It is widely used in various applications such as remote login, remote command execution, and data transfer. When it comes to using SSH in Cygwin Git Bash on Windows, there are some unique behaviors and configurations that users should be aware of.

SSH Control Persist and Forking Behavior

One of the common issues that users encounter is the behavior of the SSH control master (-M) and the forking behavior (-f) options in Git Bash. When running the following command:

it seems to behave as if it was running with the following command:

The reason for this behavior is that Git Bash implicitly forks the SSH process to the background when running in a non-interactive mode. This behavior is different from the standard SSH client, which does not fork the process when the -M option is used.

Understanding ControlPersist Option

The ControlPersist option in SSH is used to keep the master connection alive for a specified amount of time, even if there is no activity. This is useful in scenarios where multiple SSH connections are made to the same server, as it reduces the overhead of establishing a new connection for each command. The default value for ControlPersist is 0, which means that the master connection is closed as soon as there is no activity.

In Git Bash, the ControlPersist option is used in conjunction with the -M option to create a master connection that is kept alive for a specified amount of time. However, due to the forking behavior of Git Bash, the master connection is not kept alive as expected. This can be resolved by adding the -N option, which tells SSH to not execute a remote command.

Creating a Persistent SSH Connection in Git Bash

To create a persistent SSH connection in Git Bash, follow these steps:

1. Open Git Bash.
2. Run the following command to create a master connection:

This command will create a master connection that is kept alive for 60 seconds. You can adjust the time as needed.

2. In a new Git Bash window, run your SSH commands as needed. The master connection will be reused, reducing the overhead of establishing a new connection.

• SSH behavior in Git Bash is different from the standard SSH client due to the forking behavior of Git Bash.
• The ControlPersist option in SSH is used to keep the master connection alive for a specified amount of time.
• To create a persistent SSH connection in Git Bash, use the -M, -S, -N, and ControlPersist options as shown in the example.

References

• SSH Manual Page
• Git for Windows
• Cygwin ssh fails with ControlPersist when run from a batch file