Improve Concurrency on your SSH Bastion

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