Path MTU daemon
With ECMP enabled the ICMP messages are routed mostly to wrong server. To fix that let's broadcast the ICMP messages that we think are worth it to every machine in colo. Some reading:
$ ./pmtud --help
Usage:
pmtud [options]
Path MTU Daemon is captures and broadcasts ICMP messages related to
MTU detection. It listens on an interface, waiting for ICMP messages
(IPv4 type 3 code 4 or IPv6 type 2 code 0) and it forwards them
verbatim to the broadcast ethernet address.
Options:
--iface Network interface to listen on
--src-rate Pps limit from single source (default=1.0 pss)
--iface-rate Pps limit to send on a single interface (default=10.0 pps)
--verbose Print forwarded packets on screen
--dry-run Don't inject packets, just dry run
--cpu Pin to particular cpu
--ports Forward only ICMP packets with payload
containing L4 source port on this list
(comma separated)
--help Print this message
Example:
pmtud --iface=eth2 --src-rate=1.0 --iface-rate=10.0
Once again, it listens waiting for packets matching:
((icmp and icmp[0] == 3 and icmp[1] == 4) or
(icmp6 and ip6[40+0] == 2 and ip6[40+1] == 0)) and
(ether dst not ff:ff:ff:ff:ff:ff)
And having appropriate length, and forwards them to ethernet broadcast ff:ff:ff:ff:ff:ff.
To debug use tcpdump:
sudo tcpdump -s0 -e -ni eth0 '((icmp and icmp[0] == 3 and icmp[1] == 4) or
(icmp6 and ip6[40+0] == 2 and ip6[40+1] == 0))'
To build type:
git submodule update --init --recursive
make
To test run it in dry-run and verbose mode:
sudo ./pmtud --iface=eth0 --dry-run -v -v -v
If you want to use NFLOG interface:
iptables -I INPUT -i lo -p icmp -m icmp --icmp-type 3/4 --j NFLOG --nflog-group 33
ip6tables -I INPUT -i lo -p icmpv6 -m icmpv6 --icmpv6-type 2/0 -j NFLOG --nflog-group 33
You can add -m pkttype ! --pkt-type broadcast to be even more
specific. Then to use the NFLOG api run:
sudo ./pmtud --iface=eth0 --dry-run -v -v -v --nflog 33
This will cause pmtud to listen to packets from NFLOG and use eth0
to brodcast them if neccesary. Debug by listing this /proc file:
cat /proc/net/netfilter/nfnetlink_log
33 32781 0 2 65535 0 1
Where columns read:
- nflog group number of a given queue (16 bits)
- peer portid: most likely the pid of process
- number of messages buffered on the kernel side
- copy mode: 2 for full packet copy
- copy range: max packet size
- flush timeout in 1/100th of a second
- use count