A tool to check if a host takes care of UDP checksum

Hello,

I've written a small tool to see if a host takes care of UDP
checksums. This may be important for DNS operation, as corrupted
packets without UDP checksums can break some DNS servers.

Thanks to an idea of Francis Dupont, the motus operandi is to send a
packet with a BAD udp checksum and to see if the host processes it,
usually responding with an ICMP PORT UNREACHABLE. If yes, the host is
broken. If no ICMP message is received, either the network is broken
or the host is fine.

This work because most of the host that do not generate UDP checksums
will not check UDP checksums of incoming packets (this clearly
violates the RFCs...).

It is a lot easier to do things this way by sending bad packets and
waiting for a reply than trying to look into received packets to see
if the checksum is corret. The latter solution requires some network
tapping, and may not be easily portable.

ckudpcksum is available at:

ftp://ftp.nic.fr/pub/autres/dns-wg/ckudpcksum.tar.gz

Read the manual, and please DO NOT overuse the program as remote hosts
will increment MIB counters each time a wrong packet is received.

-- 
Benoit Grange

NIC France E-Mail: nic at nic.fr
Personnal E-Mail : Benoit.Grange at inria.fr