RHEL 9 must not forward IPv6 source-routed packets by default.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-257977	SRG-OS-000480-GPOS-00227	RHEL-09-254040	SV-257977r1155772_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. Accepting source-routed packets in the IPv6 protocol has few legitimate uses. It must be disabled unless it is absolutely required. 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 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. Accepting source-routed packets in the IPv6 protocol has few legitimate uses. It must be disabled unless it is absolutely required. 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 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 does not accept IPv6 source-routed packets by default. Note: If IPv6 is disabled on the system, this requirement is Not Applicable. Check the value of the "net.ipv6.conf.default.accept_source_route" variables with the following command: $ sudo sysctl net.ipv6.conf.default.accept_source_route net.ipv6.conf.default.accept_source_route = 0 If "net.ipv6.conf.default.accept_source_route" is not set to "0" or is missing, this is a finding.

ℹ️ Check

Verify RHEL 9 does not accept IPv6 source-routed packets by default. Note: If IPv6 is disabled on the system, this requirement is Not Applicable. Check the value of the "net.ipv6.conf.default.accept_source_route" variables with the following command: $ sudo sysctl net.ipv6.conf.default.accept_source_route net.ipv6.conf.default.accept_source_route = 0 If "net.ipv6.conf.default.accept_source_route" is not set to "0" or is missing, this is a finding.

✔️ Fix
Configure RHEL 9 to not accept IPv6 source-routed packets by default. Create a configuration file if it does not already exist: $ sudo vi /etc/sysctl.d/ipv6_accept_source_route.conf Add the following line to the file: net.ipv6.conf.default.accept_source_route = 0 Reload settings from all system configuration files with the following command: $ sudo sysctl --system