4.2.7 Enable RFC-recommended Source Route Validation - net.ipv4.conf.all.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 is 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=1

Run the following commands to set the active kernel parameters:

# /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

Default Value:

OS Default: Yes

See Also

https://workbench.cisecurity.org/files/3096

Item Details

Category: SYSTEM AND INFORMATION INTEGRITY

References: 800-53|SI-4, CSCv6|9.2, CSCv7|9.2

Plugin: Unix

Control ID: 285417d2af5dd1568dcf4083bd484880d2a860f3fa6403939e3c3b8c4292b76c