Okay, so I have one machine with this config:
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.default.log_martians = 0
And, after a sysctl -p, it stopped flooding the /var/log/messages (as it should).
But I have this other machine, with an sdiff sysctl, identical to the first machine, that keeps flooding the /var/log/messages with the martian source warnings.
I had to do this
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.default.log_martians = 0
net.ipv4.conf.eth0.log_martians = 0
net.ipv4.conf.eth1.log_martians = 0
in order to stop the messages from appearing.
And I can't really understand why it needed the additional specific .ethx lines.
Any help would be most welcome.