Linux Kernel Option: rp

Accidentally find a weird problem in RHEL: when there are more than 1 active network devices, the system will only response to ping from the one through which DHCP DISCOVER is last sent. However, packets sent from the IP address of both devices could be received with no problem.

At first, I thought they are blocked by firewall. It was not resolved though after disabling system firewall. After discussing with a Kernel QE, I was advised to run

echo "0" > /proc/sys/net/ipv4/conf/all/rp_filter

to set rp_filter to 0 and it was resolved.

There are explanation about rp_filter in Kernel documentation and Linux Advanced Routing & Traffic Control HOWTO. It is used to verify the routes of reverse traffic packets and expected to prevent IP spoofing from DDoS attacks.

The default value according to Kernel documentation would be 0, which denotes no validation. For confirmation, the default value in Debian is indeed 0 (no validation). Yet the default value in Fedora Server in 1 (strict validation), leading to discarding all packets that fail the tests against the reverse path.

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It is indeed very safe to do so by default in a server-oriented distribution. It will, however, become a rabbit hole for those who are not very familiar with this feature.