# 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`):

    ```plaintext
    MaxSessions 50  # Increase as necessary
    ```
  * Restart the SSH server:

    ```bash
    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`):

    ```plaintext
    MaxStartups 50:30:200  # Adjust for higher concurrency
    ```
  * Restart the SSH server:

    ```bash
    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`):

    ```bash
    ulimit -n 65535
    ```
  * Edit `/etc/security/limits.conf` to increase the limit for your user:

    ```plaintext
    your-username soft nofile 65535
    your-username hard nofile 65535
    ```
  * Adjust systemd limits if needed (for example, `/etc/systemd/system/ssh.service`):

    ```plaintext
    [Service]
    LimitNOFILE=65535
    LimitNPROC=65535
    ```
  * Reload systemd and restart SSH:

    ```bash
    sudo systemctl daemon-reexec
    sudo systemctl restart sshd
    ```


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