Recommendations to Improve SSH Tunnel Concurrency on SSH Bastion

Recommendations to Improve SSH Tunnel Concurrency on SSH Bastion

To improve concurrency and manage multiple SSH tunnels more efficiently, consider the following recommendations:

1. Increase MaxSessions on SSH Bastion

• Purpose: The MaxSessions parameter limits the number of concurrent sessions (logical channels) allowed over a single SSH connection. Increasing this value allows more port forwarding sessions to run simultaneously without opening multiple SSH connections.

• Action:

  • Edit the SSH server configuration on the bastion host (/etc/ssh/sshd_config):

    MaxSessions 50  # Increase as necessary

  • Restart the SSH server:

    sudo systemctl restart sshd

2. Increase MaxStartups on SSH Bastion

• Purpose: The MaxStartups parameter controls the number of simultaneous unauthenticated SSH connections allowed. Increasing this value prevents new connections from being rejected when a large number of SSH sessions are established.

• Action:

  • Edit the SSH server configuration on the bastion host (/etc/ssh/sshd_config):

    MaxStartups 50:30:200  # Adjust for higher concurrency

  • Restart the SSH server:

    sudo systemctl restart sshd

3. Increase System Resource Limits

• Purpose: System limits on file descriptors and processes can impact the number of concurrent SSH connections or tunnels. Increasing these limits can help support more concurrent sessions.

• Action:

  • Check and increase the file descriptor limit (ulimit -n):

    ulimit -n 65535

  • Edit /etc/security/limits.conf to increase the limit for your user:

    your-username soft nofile 65535
    your-username hard nofile 65535

  • Adjust systemd limits if needed (for example, /etc/systemd/system/ssh.service):

    [Service]
    LimitNOFILE=65535
    LimitNPROC=65535

  • Reload systemd and restart SSH:

    sudo systemctl daemon-reexec
    sudo systemctl restart sshd