these messages are kernel messages so they should appear in whatever place kernel messages are logged. In my distro (Ubuntu) it’s syslog.
test with
sudo sysctl -a | grep martians
in syslog I can see messages like
…kernel: [391119.150842] IPv4: martian source …
you can try also to use tcpdump on the server interface (not lxdbr0, but the external host interface, eth0 or something else) to check if the third packet (the ACK of the SYN-ACK, in the same direction as the initial SYN) is getting to it.
you can use telnet 80 to simulate a connection, it’s less verbose than Curl, just to make tcpdump trace a bit more clear.