To add another thing thats broken: if this plugin is enabled it pings a little bit too much, i count 10 pings/s and more and it doesn't stop. Tested with olsr 0.5.6 on Debian (x86, 2.6) and OpenWrt on x86-2.6 and brcm47xx-2.6. I used the following config:

LoadPlugin "olsrd_dyn_gw.so.0.4"
{
PlParam "Interval" "40"
PlParam "Ping" "151.1.1.1"
}

Started with -d1 olsr tells me:

Do ping on 151.1.1.1 ...
PING 151.1.1.1 (151.1.1.1) 56(84) bytes of data.

--- 151.1.1.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 69.734/69.734/69.734/0.000 ms
...OK

HNA[00000000/00000000](ping is possible) VIA eth0 detected in routing table.

Do ping on 151.1.1.1 ...
PING 151.1.1.1 (151.1.1.1) 56(84) bytes of data.

--- 151.1.1.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 70.378/70.378/70.378/0.000 ms
...OK

.... and so on. tcpdump tells me:

23:11:47.422588 IP 192.168.0.123 > dns.it.net: ICMP echo request, id 51725, seq 1, length 64
23:11:47.490778 IP dns.it.net > 192.168.0.123: ICMP echo reply, id 51725, seq 1, length 64
23:11:47.499851 IP 192.168.0.123 > dns.it.net: ICMP echo request, id 52493, seq 1, length 64
23:11:47.569899 IP dns.it.net > 192.168.0.123: ICMP echo reply, id 52493, seq 1, length 64
23:11:47.578821 IP 192.168.0.123 > dns.it.net: ICMP echo request, id 52749, seq 1, length 64
23:11:47.647743 IP dns.it.net > 192.168.0.123: ICMP echo reply, id 52749, seq 1, length 64
23:11:47.656549 IP 192.168.0.123 > dns.it.net: ICMP echo request, id 53005, seq 1, length 64
23:11:47.729543 IP dns.it.net > 192.168.0.123: ICMP echo reply, id 53005, seq 1, length 64
23:11:47.739088 IP 192.168.0.123 > dns.it.net: ICMP echo request, id 53773, seq 1, length 64