NixOS must terminate all SSH connections after 10 minutes of becoming unresponsive.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-268142	SRG-OS-000163-GPOS-00072	ANIX-00-000990	SV-268142r1131087_rule	2025-08-19	1

Description
Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition, quickly terminating an idle session will also free up resources committed by the managed network element. Terminating network connections associated with communications sessions includes, for example, de-allocating associated TCP/IP address/port pairs at the operating system level, and de-allocating networking assignments at the application level if multiple application sessions are using a single operating system-level network connection. This does not mean that the operating system terminates all sessions or network access; it only ends the inactive session and releases the resources associated with that session. Satisfies: SRG-OS-000163-GPOS-00072, SRG-OS-000279-GPOS-00109, SRG-OS-000395-GPOS-00175

Description

Terminating an idle session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition, quickly terminating an idle session will also free up resources committed by the managed network element. Terminating network connections associated with communications sessions includes, for example, de-allocating associated TCP/IP address/port pairs at the operating system level, and de-allocating networking assignments at the application level if multiple application sessions are using a single operating system-level network connection. This does not mean that the operating system terminates all sessions or network access; it only ends the inactive session and releases the resources associated with that session. Satisfies: SRG-OS-000163-GPOS-00072, SRG-OS-000279-GPOS-00109, SRG-OS-000395-GPOS-00175

ℹ️ Check
Verify that all network connections associated with SSH traffic are automatically terminated after 10 minutes of becoming unresponsive with the following command: $ grep -i clientaliveinterval /etc/ssh/sshd_config ClientAliveInterval 600 If "ClientAliveInterval" does not have a value of "600" or less, is commented out, or is missing, this is a finding.

✔️ Fix
Configure NixOS to automatically terminate all network connections associated with SSH traffic after being unresponsive for 10 minutes. Add the following Nix code to the NixOS Configuration, usually located in /etc/nixos/configuration.nix or /etc/nixos/flake.nix: services.openssh.extraConfig = '' ClientAliveInterval 600 ''; Rebuild and switch to the new NixOS configuration: $ sudo nixos-rebuild switch

✔️ Fix

Configure NixOS to automatically terminate all network connections associated with SSH traffic after being unresponsive for 10 minutes. Add the following Nix code to the NixOS Configuration, usually located in /etc/nixos/configuration.nix or /etc/nixos/flake.nix: services.openssh.extraConfig = '' ClientAliveInterval 600 ''; Rebuild and switch to the new NixOS configuration: $ sudo nixos-rebuild switch