RHEL 9 must not forward Internet Protocol version 4 (IPv4) source-routed packets.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-257959	SRG-OS-000480-GPOS-00227	RHEL-09-253020	SV-257959r1155724_rule	2025-11-24	2

Description
Source-routed packets allow the source of the packet to suggest that routers forward the packet along a different path than configured on the router, which can be used to bypass network security measures. This requirement applies only to the forwarding of source-routed traffic, such as when forwarding is enabled and the system is functioning as a router. The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographical order, regardless of which of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored. /etc/sysctl.d/.conf /run/sysctl.d/.conf /usr/local/lib/sysctl.d/.conf /usr/lib/sysctl.d/.conf /lib/sysctl.d/*.conf /etc/sysctl.conf

Description

Source-routed packets allow the source of the packet to suggest that routers forward the packet along a different path than configured on the router, which can be used to bypass network security measures. This requirement applies only to the forwarding of source-routed traffic, such as when forwarding is enabled and the system is functioning as a router. The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographical order, regardless of which of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored. /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf /etc/sysctl.conf

ℹ️ Check
Verify RHEL 9 will not accept IPv4 source-routed packets. Check the value of the "accept_source_route" variable with the following command: $ sudo sysctl net.ipv4.conf.all.accept_source_route net.ipv4.conf.all.accept_source_route = 0 If "net.ipv4.conf.all.accept_source_route" is not set to "0" or is missing, this is a finding.

✔️ Fix
Configure RHEL 9 to ignore IPv4 source-routed packets. Create a configuration file if it does not already exist: $ sudo vi /etc/sysctl.d/99-ipv4_accept_source.conf Add the following line to the file: net.ipv4.conf.all.accept_source_route = 0 Reload settings from all system configuration files with the following command: $ sudo sysctl --system