A Mutable Log

Enable IP multicast routing in Linux kernel

In this post I discuss how to enable multicast routing in a Linux system. It is a continuation to the post Wireless Router with Buildroot and Raspberry Pi, where I discussed how to build a basic Wi-Fi router with a Raspberry Pi. You’ll want to read that first.

Linux kernel configuration

Besides the Kernel modules mentioned in the post(s) linked above, you’ll need a few additional modules.

Under Networking support, Networking options, enable

IP multicast routing and tunneling

IPv6 protocol

In the absence of IPv6 smcroute fails with an error such as

Starting static multicast router daemon: INIT: ICMPv6 socket open; Errno(97): Address family not supported by protocol
INIT: MRT6_INIT failed; Errno(97): Address family not supported by protocol

Under Networking support, Networking options, The IPv6 protocol, enable IPv6: multicast routing and its sub-options.

IPv6 Multicast Routing

Enable packet mangling with TTL target support if you require support for changing TTL values with iptables.

Packet Mangling

Buildroot package configuration

The following Buildroot packages provide daemons for performing multicast routing. Enable mrouted and smcroute under Target packages, Networking applications. mrouted requires a glibc based toolchain, you will have to enable it instead of uClibc if you want to use mrouted.



Perform build and prepare the SD card.

Setup multicast routing

The following procedure is performed from a root console. I usually use the serial console through the expansion header.

Use mrouted when proper IGMP signaling exists


The default configuration file /etc/mrouted.conf should be enough, unless you want to perform tunneling.

If you don’t have proper IGMP signaling happening, you can still perform static multicast routing using

smcroute -d

smcroute requires a configuration file, which in my case is /etc/smcroute.conf and looks something like

mgroup from wlan0 group
mroute from wlan0 group to usb0

If you don’t have an application and want to use ping to test mutlicast, you can enable ICMP echo responses thus

echo "0" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

You can use ping requests and receive responses from destination hosts

ping -t 10

Note the use of the time to live (TTL) parameter -t. Linux and Mac OS X will set TTL to 1 before forwarding message to the default gateway. You can dump ping messages with TTL parameter using

tcpdump -v host or

Note change in TTL from 10 to 1 in a packet routed through Mac OS X in the following dump

14:49:19.642140 IP (tos 0x0, ttl 10, id 0, offset 0, flags [DF], proto ICMP (1), length 84) > ICMP echo request, id 4113, seq 92, length 64
14:49:19.642190 IP (tos 0x0, ttl 1, id 32573, offset 0, flags [none], proto ICMP (1), length 84) > ICMP echo request, id 4113, seq 92, length 64

If all is well with the routing daemon, IP variable /proc/sys/net/ipv4/conf/{all,interface}/mc_forwarding will be set to 1.

Some other files offer useful hints related to multicast routing. The following lists interfaces where multicast routing is active

cat /proc/net/ip_mr_vif

This lists multicast routing cache entries

cat /proc/net/ip_mr_cache

When using static multicast routing with smcroute, routing will work only when TTL is greater than 1. If the downstream hosts are transmitting packets with TTL at 1, you can use iptables to set TTL thus

iptables -t mangle -A PREROUTING -i wlan0 -j TTL --ttl-set 64

I’ve also had to wait a while after executing smcroute for NAT to kick in, so that source IP address is translated to address of interface on the destination network. Note the change in source IP address in a message sequence captured using

tcpdump -v -i usb0 host or
03:03:25.393056 IP (tos 0x0, ttl 63, id 16103, offset 0, flags [none], proto UDP (17), length 41) > UDP, length 13
03:04:22.277348 IP (tos 0x0, ttl 63, id 8095, offset 0, flags [none], proto UDP (17), length 41) > UDP, length 13