The presence of "martian" packets (which have impossible addresses) as well as spoofed packets, source-routed packets, and redirects could be a sign of nefarious network activity. Logging these packets enables this activity to be detected.
The status of the "net.ipv4.conf.all.log_martians" kernel parameter can be queried by running the following command:
$ sysctl net.ipv4.conf.all.log_martians
The output of the command should indicate a value of "1". If this value is not the default value, investigate how it could have been adjusted at runtime, and verify it is not set improperly in "/etc/sysctl.conf".