ProxyJump and ProxyCommand
When connecting to servers that are behind firewalls or only accessible through an intermediate server (jump host), ProxyJump and ProxyCommand provide efficient methods for managing these multi-hop SSH connections. These options simplify the process of accessing internal systems through a bastion or jump server without needing to manually tunnel connections.
Accessing Hosts Behind Firewalls
In environments where servers are located behind firewalls or restricted networks, a jump host (also known as a bastion host) serves as an intermediary that allows access to internal systems. Instead of opening additional ports or using VPNs, SSH’s ProxyJump and ProxyCommand features allow secure access to the internal hosts by routing traffic through the jump host.
Traditionally, connecting through a jump host required manually tunneling the connection using commands like this:
ssh -t user@jump-host ssh user@internal-server
However, this method can be cumbersome to manage. With ProxyJump and ProxyCommand, the process becomes much simpler.
Using ProxyJump
ProxyJump (-J option) provides a streamlined way to route an SSH connection through one or more jump hosts. Instead of manually chaining SSH connections, ProxyJump automates this process.
Example:
ssh -J user@jump-host user@internal-server
In this example, the SSH connection first routes through the jump host (jump-host), then continues to the internal server (internal-server). ProxyJump manages the connection through the jump host, eliminating the need for multiple manual SSH commands.
Configuring ProxyJump in the SSH Configuration File
To simplify the process even further, ProxyJump can be configured in the SSH configuration file (~/.ssh/config):
Host internal-server
ProxyJump user@jump-host
With this configuration, connecting to internal-server will automatically route through jump-host, making it unnecessary to specify the -J option each time.
Using ProxyCommand
Before ProxyJump was introduced, ProxyCommand was used to achieve similar functionality. ProxyCommand allows users to define a custom command that handles the SSH connection, which can be used to route through a jump host or even use custom protocols for connecting to remote servers.
Example with ProxyCommand:
ssh -o ProxyCommand="ssh -W %h:%p user@jump-host" user@internal-server
This command tells SSH to first connect to jump-host using SSH and then forward the connection to internal-server. The -W option in the nested SSH command forwards the connection between the two hosts.
Configuring ProxyCommand in the SSH Configuration File
Like ProxyJump, ProxyCommand can also be set in the SSH configuration file for ease of use:
Host internal-server
ProxyCommand ssh -W %h:%p user@jump-host
With this configuration, connecting to internal-server will automatically forward traffic through the jump host without needing to specify additional options on the command line.
Benefits of ProxyJump and ProxyCommand
Both ProxyJump and ProxyCommand offer secure, efficient ways to access internal servers without exposing them to the internet or requiring complex networking configurations. They provide flexible solutions for managing SSH connections in environments where direct access to internal systems is restricted, such as enterprise networks or cloud environments with strict firewall rules.
When to Use ProxyJump vs ProxyCommand
- ProxyJump is easier to use and requires less configuration for common cases where traffic is forwarded through one or more jump hosts. It is the preferred method for most users.
- ProxyCommand offers more flexibility and customization, making it suitable for more complex setups, such as when integrating SSH with non-standard protocols or using multiple steps for the connection.
Security Considerations
When using ProxyJump or ProxyCommand, it’s important to ensure that the jump host is trusted and properly secured. Since all traffic is routed through the jump host, compromising the jump host could expose internal systems. Therefore, ensuring that the jump host is properly configured, monitored, and secured is critical for maintaining the integrity of the network.
Additionally, it is advisable to use key-based authentication when working with jump hosts to minimize the risk of password-based attacks. Regularly reviewing security logs on both the jump host and internal servers is essential to detect any unauthorized access attempts.