7.2.7 Enable RFC-recommended Source Route Validation - 'net.ipv4.conf.default.rp_filter = 1'

Information

Setting net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter to 1 forces the
Linux kernel to utilize reverse path filtering on a received packet to determine if the packet
was valid. Essentially, with reverse path filtering, if the return packet does not go out the
same interface that the corresponding source packet came from, the packet is dropped
(and logged if log_martians is set).

*Rationale*

Setting these flags is a good way to deter attackers from sending your server bogus packets
that cannot be responded to. One instance where this feature breaks down is if
asymmetrical routing is employed. This would occur when using dynamic routing protocols
(bgp, ospf, etc) on your system. If you are using asymmetrical routing on your server, you
will not be able to enable this feature without breaking the routing.

Solution

Set the net.ipv4.conf.all.rp_filter and net.ipv4.conf.default.rp_filter parameters to
1 in /etc/sysctl.conf-net.ipv4.conf.all.rp_filter=1
net.ipv4.conf.default.rp_filter=1Modify active kernel parameters to match-# /sbin/sysctl -w net.ipv4.conf.all.rp_filter=1
# /sbin/sysctl -w net.ipv4.conf.default.rp_filter=1
# /sbin/sysctl -w net.ipv4.route.flush=1

See Also

https://workbench.cisecurity.org/files/91

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-7(12), CSCv6|9.2

Plugin: Unix

Control ID: b4d14f1593c7c9b122c6aca0f09ed86ca0ecda73c9d1ca250251733def37d8e1