SLEM 5 must not forward Internet Protocol version 4 (IPv4) source-routed packets by default.

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 IPv4 forwarding is enabled and the system is functioning as a router.

Verify SLEM 5 does not accept IPv4 source-routed packets by default with the following command:

     > sudo sysctl net.ipv4.conf.default.accept_source_route
     net.ipv4.conf.default.accept_source_route = 0

If the network parameter "ipv4.conf.default.accept_source_route" is not equal to "0", or nothing is returned, this is a finding.

Configure SLEM 5 to disable IPv4 default source routing by running the following command as an administrator:

     > sudo sysctl -w net.ipv4.conf.default.accept_source_route=0

If "0" is not the system's default value, add or update the following line in "/etc/sysctl.d/99-stig.conf":

     > sudo sh -c 'echo "net.ipv4.conf.default.accept_source_route=0" >> /etc/sysctl.d/99-stig.conf'

     > sudo sysctl --system