AlmaLinux OS 9 must not forward source-routed packets.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-269259	SRG-OS-000480-GPOS-00227	ALMA-09-020260	SV-269259r1050141_rule	2025-02-20	1

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-routed traffic, such as when IP forwarding is enabled and the system is functioning as a router. Accepting source-routed packets 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-routed traffic, such as when IP forwarding is enabled and the system is functioning as a router. Accepting source-routed packets has few legitimate uses. It must be disabled unless it is absolutely required.

ℹ️ Check
Verify AlmaLinux OS 9 will not accept source-routed packets. Check the value of the "accept_source_route" variables with the following command: $ sysctl -a \| grep accept_source_route net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.enp1s0.accept_source_route = 0 net.ipv4.conf.lo.accept_source_route = 0 net.ipv6.conf.all.accept_source_route = 0 net.ipv6.conf.default.accept_source_route = 0 net.ipv6.conf.enp1s0.accept_source_route = 0 net.ipv6.conf.lo.accept_source_route = 0 If the returned lines do not all have a value of "0", this is a finding.

ℹ️ Check

Verify AlmaLinux OS 9 will not accept source-routed packets. Check the value of the "accept_source_route" variables with the following command: $ sysctl -a | grep accept_source_route net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.enp1s0.accept_source_route = 0 net.ipv4.conf.lo.accept_source_route = 0 net.ipv6.conf.all.accept_source_route = 0 net.ipv6.conf.default.accept_source_route = 0 net.ipv6.conf.enp1s0.accept_source_route = 0 net.ipv6.conf.lo.accept_source_route = 0 If the returned lines do not all have a value of "0", this is a finding.

✔️ Fix
Configure AlmaLinux OS 9 to ignore source-routed packets. Create a numbered *.conf file in /etc/sysctl.d/ with the following content: net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.lo.accept_source_route = 0 net.ipv6.conf.all.accept_source_route = 0 net.ipv6.conf.default.accept_source_route = 0 net.ipv6.conf.lo.accept_source_route = 0 The system configuration files must be reloaded for the changes to take effect. To reload the contents of the files, run the following command: $ sysctl –system

✔️ Fix

Configure AlmaLinux OS 9 to ignore source-routed packets. Create a numbered *.conf file in /etc/sysctl.d/ with the following content: net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.lo.accept_source_route = 0 net.ipv6.conf.all.accept_source_route = 0 net.ipv6.conf.default.accept_source_route = 0 net.ipv6.conf.lo.accept_source_route = 0 The system configuration files must be reloaded for the changes to take effect. To reload the contents of the files, run the following command: $ sysctl –system