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Paul
Asked: 2024-11-12 18:37:10 +0800 CST

Automatically kill abandoned SSH sessions in CentOS 7

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From time to time and for various reasons, PyTTY loses connection to SSH sessions on my CentOS 7 server (PuTTY is buggy, SSH server is buggy, router has been rebooted, connection is dropped, cable pulled out etc). These abandoned sessions are a burden on the server and consume resources. Could the CentOS 7 SSH server be configured to automatically terminate any SSH session after 24 hours, regardless of session purpose and state?

Why regardless of session purpose and status? Because in my opinion it can sometimes be difficult to distinguish between a normal session and an abandoned session.

ssh

1 Answers

  1. Best Answer

    First of all, don't run CentOS7. It's EOL

    Second, read man sshd_config:

    ClientAliveCountMax
    Sets the number of client alive messages which may be sent with- out sshd(8) receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from TCPKeepAlive. The client alive messages are sent through the encrypted channel and therefore will not be spoofa- ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa- ble. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become unrespon- sive.

    ClientAliveInterval
    Sets a timeout interval in seconds after which if no data has been received from the client, sshd(8) will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client.

    Setting those to e.g. 3 and 600 respectively will terminate sessions after 3*600 second when the TCP session is dead.

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