cyberstrikeus 7.2.7 Enable RFC-recommended Source Route Validation (Scored)
Profile Applicability
- Level 1
Description
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.
Audit Procedure
Using Command Line
Perform the following to determine if RFC-recommended source route validation is enabled.
/sbin/sysctl net.ipv4.conf.all.rp_filter
/sbin/sysctl net.ipv4.conf.default.rp_filter
Expected Result
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
Remediation
Using Command Line
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
Modify 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
Default Value
Reverse path filtering is disabled by default.
References
- CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
Profile
Level 1 - Scored