I was running into a problem, when trying to join multiple multicast groups
on a single socket and thus binding to the any-address on said socket. I
received traffic from multicast groups, I did not join on that socket and
was at first surprised by that. After reading some old e-mails/threads,
which came to the conclusion "It is, as it is."
(e.g https://marc.info/?l=linux-kernel&m=115815686626791&w=2), I discovered
the IPv4 socketoption IP_MULTICAST_ALL, which, when disabled, does exactly
what I would expect from a socket by default.

I propose a socket option for IPv6, which does the same and has the same
default as the IPv4 version. My first thought was, to just apply
IP_MULTICAST_ALL to a ipv6 socket, but that would change the behavior of
current applications and would probably be a big no-no.


>From 473653086c05a3de839c3504885053f6254c7bc5 Mon Sep 17 00:00:00 2001
From: Andre Naujoks <nauts...@gmail.com>
Date: Wed, 11 Apr 2018 12:38:28 +0200
Subject: [PATCH] Add a socketoption IPV6_MULTICAST_ALL analogue to the IPV4

The socket option will be enabled by default to ensure current behaviour
is not changed. This is the same for the IPv4 version.

A socket bound to in6addr_any and a specific port will receive all traffic
on that port. Analogue to IP_MULTICAST_ALL, disable this behaviour, if
one or more multicast groups were joined (using said socket) and only
pass on multicast traffic from groups, which were explicitly joined via
this socket.

Without this option disabled a socket (system even) joined to multiple
multicast groups is very hard to get right. Filtering by destination
address has to take place in user space to avoid receiving multicast
traffic from other multicast groups, which might have traffic on the same

The extension of the IP_MULTICAST_ALL socketoption to just apply to ipv6,
too, is not done to avoid changing the behaviour of current applications.

Signed-off-by: Andre Naujoks <nauts...@gmail.com>
 include/linux/ipv6.h     |  3 ++-
 include/uapi/linux/in6.h |  1 +
 net/ipv6/af_inet6.c      |  1 +
 net/ipv6/ipv6_sockglue.c | 11 +++++++++++
 net/ipv6/mcast.c         |  2 +-
 5 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 8415bf1a9776..495e834c1367 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -274,7 +274,8 @@ struct ipv6_pinfo {
-                               autoflowlabel_set:1;
+                               autoflowlabel_set:1,
+                               mc_all:1;
        __u8                    min_hopcount;
        __u8                    tclass;
        __be32                  rcv_flowinfo;
diff --git a/include/uapi/linux/in6.h b/include/uapi/linux/in6.h
index ed291e55f024..71d82fe15b03 100644
--- a/include/uapi/linux/in6.h
+++ b/include/uapi/linux/in6.h
@@ -177,6 +177,7 @@ struct in6_flowlabel_req {
 #define IPV6_V6ONLY            26
 #define IPV6_JOIN_ANYCAST      27
 #define IPV6_LEAVE_ANYCAST     28
+#define IPV6_MULTICAST_ALL     29
 /* IPV6_MTU_DISCOVER values */
 #define IPV6_PMTUDISC_DONT             0
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 8da0b513f188..7844cd9d2f10 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -209,6 +209,7 @@ static int inet6_create(struct net *net, struct socket 
*sock, int protocol,
        np->hop_limit   = -1;
        np->mcast_hops  = IPV6_DEFAULT_MCASTHOPS;
        np->mc_loop     = 1;
+       np->mc_all      = 1;
        np->pmtudisc    = IPV6_PMTUDISC_WANT;
        np->repflow     = net->ipv6.sysctl.flowlabel_reflect;
        sk->sk_ipv6only = net->ipv6.sysctl.bindv6only;
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 4d780c7f0130..b2bc1942a2ee 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -664,6 +664,13 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, 
int optname,
                        retv = ipv6_sock_ac_drop(sk, mreq.ipv6mr_ifindex, 
+       case IPV6_MULTICAST_ALL:
+               if (optlen < sizeof(int))
+                       goto e_inval;
+               np->mc_all = valbool;
+               retv = 0;
+               break;
        case MCAST_JOIN_GROUP:
        case MCAST_LEAVE_GROUP:
@@ -1255,6 +1262,10 @@ static int do_ipv6_getsockopt(struct sock *sk, int 
level, int optname,
                val = np->mcast_oif;
+       case IPV6_MULTICAST_ALL:
+               val = np->mc_all;
+               break;
        case IPV6_UNICAST_IF:
                val = (__force int)htonl((__u32) np->ucast_oif);
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 793159d77d8a..623ad00eb3c2 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -622,7 +622,7 @@ bool inet6_mc_check(struct sock *sk, const struct in6_addr 
        if (!mc) {
-               return true;
+               return np->mc_all;
        psl = mc->sflist;

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