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-257959r991589_rule	2025-02-27	2

Description
Source-routed packets allow the source of the packet to suggest 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-routerd traffic, such as when IPv4 forwarding is enabled and the system is functioning as a router. Accepting source-routed packets in the IPv4 protocol has few legitimate uses. It must be disabled unless it is absolutely required.

Description

Source-routed packets allow the source of the packet to suggest 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-routerd traffic, such as when IPv4 forwarding is enabled and the system is functioning as a router. Accepting source-routed packets in the IPv4 protocol has few legitimate uses. It must be disabled unless it is absolutely required.

ℹ️ Check
Verify RHEL 9 will not accept IPv4 source-routed packets. Check the value of the all "accept_source_route" variables with the following command: $ sudo sysctl net.ipv4.conf.all.accept_source_route net.ipv4.conf.all.accept_source_route = 0 If the returned line does not have a value of "0", a line is not returned, or the line is commented out, this is a finding. Check that the configuration files are present to enable this network parameter. $ sudo /usr/lib/systemd/systemd-sysctl --cat-config \| egrep -v '^(#\|;)' \| grep -F net.ipv4.conf.all.accept_source_route \| tail -1 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.

ℹ️ Check

Verify RHEL 9 will not accept IPv4 source-routed packets. Check the value of the all "accept_source_route" variables with the following command: $ sudo sysctl net.ipv4.conf.all.accept_source_route net.ipv4.conf.all.accept_source_route = 0 If the returned line does not have a value of "0", a line is not returned, or the line is commented out, this is a finding. Check that the configuration files are present to enable this network parameter. $ sudo /usr/lib/systemd/systemd-sysctl --cat-config | egrep -v '^(#|;)' | grep -F net.ipv4.conf.all.accept_source_route | tail -1 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. Add or edit the following line in a single system configuration file, in the "/etc/sysctl.d/" directory: net.ipv4.conf.all.accept_source_route = 0 Load settings from all system configuration files with the following command: $ sudo sysctl --system