mjhjmtu

I have mrd6 installed on my raspberry pi. It registers with a local interface (tun0) and periodically transmits MLDv2 queries over it.

According to [RFC3810], MLDv2 message types are a subset of ICMPv6 messages, and are identified in IPv6 packets by a preceding Next Header value of 58 (0x3a). They are sent with a link-local IPv6 Source Address, an IPv6 Hop Limit of 1, and an IPv6 Router Alert option [RFC2711] in a Hop-by-Hop Options header.

I can confirm that I'm seeing these packets periodically over tun0:

pi@machine:~ $ sudo tcpdump -i tun0 ip6 -vv -XX

01:22:52.125915 IP6 (flowlabel 0x71df6, hlim 1, next-header Options (0)
payload length: 36) 
fe80::69bf:be2d:e087:9921 > ip6-allnodes: HBH (rtalert: 0x0000) (padn)
[icmp6 sum ok] ICMP6, multicast listener query v2 [max resp delay=10000]
[gaddr :: robustness=2 qqi=125]
 0x0000: 6007 1df6 0024 0001 fe80 0000 0000 0000 `....$..........
 0x0010: 69bf be2d e087 9921 ff02 0000 0000 0000 i..-...!........
 0x0020: 0000 0000 0000 0001 3a00 0502 0000 0100 ........:.......
 0x0030: 8200 b500 2710 0000 0000 0000 0000 0000 ....'...........
 0x0040: 0000 0000 0000 0000 027d 0000 .........}..

I have a socket set up in my application on tun0 as follows, since I expect these to be ICMP packets:

int fd = socket(AF_INET6, SOCK_RAW, IPPROTO_ICMPV6); // ICMP

// ... bind this socket to tun0

 int interfaceIndex = // tun0 interface Index
 int mcastTTL = 10;
 int loopBack = 1;

 if (setsockopt(listener->socket,
 IPPROTO_IPV6,
 IPV6_MULTICAST_IF,
 &interfaceIndex,
 sizeof(interfaceIndex))
 < 0) 
 perror("setsockopt:: IPV6_MULTICAST_IF:: ");
 

 if (setsockopt(listener->socket,
 IPPROTO_IPV6,
 IPV6_MULTICAST_LOOP,
 &loopBack,
 sizeof(loopBack))
 < 0) 
 perror("setsockopt:: IPV6_MULTICAST_LOOP:: ");
 

 if (setsockopt(listener->socket,
 IPPROTO_IPV6,
 IPV6_MULTICAST_HOPS,
 &mcastTTL,
 sizeof(mcastTTL))
 < 0) 
 perror("setsockopt:: IPV6_MULTICAST_HOPS:: ");
 

 struct ipv6_mreq mreq6 = 0;
 MEMCOPY(&mreq6.ipv6mr_multiaddr.s6_addr, sourceAddress, 16);
 mreq6.ipv6mr_interface = interfaceIndex;

 if (setsockopt(listener->socket,
 IPPROTO_IPV6,
 IPV6_JOIN_GROUP,
 &mreq6,
 sizeof(mreq6))
 < 0) 
 perror("setsockopt:: IPV6_JOIN_GROUP:: ");
 

Setting up the socket this way, I can receive ICMP echo requests, replies to my own address, and multicasts sent using the link-local multicast address. However, I don't see any MLDv2 queries.

Here's my receive loop:

 uint8_t received[1000] = 0 ;
 struct sockaddr_storage peerAddress = 0 ;
 socklen_t addressLength = sizeof(peerAddress);
 socklen_t addressLength = sizeof(peerAddress);

 int receivedLength = recvfrom(sockfd,
 received,
 sizeof(received),
 0,
 (struct sockaddr *)&peerAddress,
 &addressLength);

 if (receivedLength > 0) 
 // Never get here for MLDv2 queries.
 

Researching this a bit further, I discovered the IPV6_ROUTER_ALERT socket option, which the man page describes as follows:

IPV6_ROUTER_ALERT
Pass forwarded packets containing a router alert hop-by-hop option to this socket.
Only allowed for SOCK_RAW sockets. The tapped packets are not forwarded by the
kernel, it is the user's responsibility to send them out again. Argument is a
pointer to an integer. A positive integer indicates a router alert option value
to intercept. Packets carrying a router alert option with a value field
containing this integer will be delivered to the socket. A negative integer
disables delivery of packets with router alert options to this socket.

So I figured I was missing this option, and tried setting it as follows. [RFC2710] 0 means Multicast Listener Discovery message.

 int routerAlertOption = 0;

 if (setsockopt(listener->socket,
 IPPROTO_IPV6,
 IPV6_ROUTER_ALERT,
 &routerAlertOption,
 sizeof(routerAlertOption))
 < 0) 
 perror("setsockopt:: IPV6_ROUTER_ALERT:: ");
 

However, this gives me the ENOPROTOOPT error (errno 92). Some more Googling (http://www.atm.tut.fi/list-archive/usagi-users-2005/msg00317.html) led me to the fact that you can't set the IPV6_ROUTER_ALERT option with the IPPROTO_ICMPV6 protocol. It needs a socket defined using the IPPROTO_RAW protocol.

However, defining my socket as:

int fd = socket(AF_INET6, SOCK_RAW, IPPROTO_RAW);

means I'm not able to receive any ICMP packets in my recvfrom anymore.

TL;DR: How do I read MLDv2 queries using an IPv6 socket?

edit (answer):
It appears conventional implementations of Linux will drop MLDv2 packets when passing them to an ICMPV6 socket. Why this is, I'm not sure. (Could be because of the next-header option.)

I went with an approach of reading raw packets on the tun0 interface. I followed the ping6_ll.c example here: http://www.pdbuchan.com/rawsock/rawsock.html.

It uses a socket with (SOCK_RAW, ETH_P_ALL). You can also set some SOL_PACKET options to filter on specific multicast rules on your interface.