Usually, the route taken from Host A to Host B is the same as the route used to get back from Host B to Host A; the route is then called symmetric . In complex setups, the route back may be different; in this case, it is asymmetric.
关于反向路径过滤，参考《Understanding Linux Network Internals》三十一章，
31.7. Reverse Path Filtering
We saw what an asymmetric route is in the section “Essential Elements of Routing in Chapter 30. Asymmetric routes are not common, but may be necessary in certain cases. The default behavior of Linux is to consider asymmetric routing suspicious and therefore to drop any packet whose source IP address is not reachable through the device the packet was received from, according to the routing table.
However, this behavior can be tuned via /proc on a per-device basis, as we will see in Chapter 36. See also the section “Input Routing” in Chapter 35.
rp_filter - INTEGER
0 - No source validation.
1 - Strict mode as defined in RFC3704 Strict Reverse Path
Each incoming packet is tested against the FIB and if the interface
is not the best reverse path the packet check will fail.
By default failed packets are discarded.
2 - Loose mode as defined in RFC3704 Loose Reverse Path
Each incoming packet's source address is also tested against the FIB
and if the source address is not reachable via any interface
the packet check will fail.
Current recommended practice in RFC3704 is to enable strict mode
to prevent IP spoofing from DDos attacks. If using asymmetric routing
or other complicated routing, then loose mode is recommended.
The max value from conf/{all,interface}/rp_filter is used
when doing source validation on the {interface}.
Default value is 0. Note that some distributions enable it
in startup scripts.
