[PATCH] IGMP: Inhibit reports for local multicast groups

IGMP reports are generated for link local multicast groups (224.0.0.1
- 224.0.0.255) used by the routing protocols such as RIP, OSPF etc.
In general routers do not generate reports for local multicast groups.

IGMP reports for local multicast groups can now be inhibited by means
of a system control variable (setting the value to zero).

To retain backwards compatibility the previous behaviour is retained by
default on system boot.

Signed-off-by: Philip Downey <redacted>
---
 include/linux/igmp.h       |    1 +
 net/ipv4/igmp.c            |   29 ++++++++++++++++++++++++++++-
 net/ipv4/sysctl_net_ipv4.c |    7 +++++++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 193ad48..e3e0dae 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h

@@ -37,6 +37,7 @@ static inline struct igmpv3_query *
 	return (struct igmpv3_query *)skb_transport_header(skb);
 }
 
+extern int sysctl_igmp_link_local_reports;
 extern int sysctl_igmp_max_memberships;
 extern int sysctl_igmp_max_msf;
 extern int sysctl_igmp_qrv;

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 651cdf6..2e4d5b8 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c

@@ -110,6 +110,15 @@
 #define IP_MAX_MEMBERSHIPS	20
 #define IP_MAX_MSF		10
 
+/* IGMP reports for link-local multicast groups are enabled by default */
+#define IGMP_ENABLE_LLM         1
+
+int sysctl_igmp_link_local_reports __read_mostly = IGMP_ENABLE_LLM;
+
+#define IGMP_INHIBIT_LINK_LOCAL_REPORTS(_ipaddr) \
+	(ipv4_is_local_multicast(_ipaddr) && \
+	 (sysctl_igmp_link_local_reports == 0))
+
 #ifdef CONFIG_IP_MULTICAST
 /* Parameter names and values are taken from igmp-v2-06 draft */
 

@@ -437,6 +446,8 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 
 	if (pmc->multiaddr == IGMP_ALL_HOSTS)
 		return skb;
+	if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(pmc->multiaddr))
+		return skb;
 
 	isquery = type == IGMPV3_MODE_IS_INCLUDE ||
 		  type == IGMPV3_MODE_IS_EXCLUDE;

@@ -545,6 +556,8 @@ static int igmpv3_send_report(struct in_device *in_dev, struct ip_mc_list *pmc)
 		for_each_pmc_rcu(in_dev, pmc) {
 			if (pmc->multiaddr == IGMP_ALL_HOSTS)
 				continue;
+			if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(pmc->multiaddr))
+				continue;
 			spin_lock_bh(&pmc->lock);
 			if (pmc->sfcount[MCAST_EXCLUDE])
 				type = IGMPV3_MODE_IS_EXCLUDE;

@@ -678,7 +691,11 @@ static int igmp_send_report(struct in_device *in_dev, struct ip_mc_list *pmc,
 
 	if (type == IGMPV3_HOST_MEMBERSHIP_REPORT)
 		return igmpv3_send_report(in_dev, pmc);
-	else if (type == IGMP_HOST_LEAVE_MESSAGE)
+
+	if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(group))
+		return 0;
+
+	if (type == IGMP_HOST_LEAVE_MESSAGE)
 		dst = IGMP_ALL_ROUTER;
 	else
 		dst = group;

@@ -851,6 +868,8 @@ static bool igmp_heard_report(struct in_device *in_dev, __be32 group)
 
 	if (group == IGMP_ALL_HOSTS)
 		return false;
+	if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(group))
+		return false;
 
 	rcu_read_lock();
 	for_each_pmc_rcu(in_dev, im) {

@@ -957,6 +976,8 @@ static bool igmp_heard_query(struct in_device *in_dev, struct sk_buff *skb,
 			continue;
 		if (im->multiaddr == IGMP_ALL_HOSTS)
 			continue;
+		if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(im->multiaddr))
+			continue;
 		spin_lock_bh(&im->lock);
 		if (im->tm_running)
 			im->gsquery = im->gsquery && mark;

@@ -1181,6 +1202,8 @@ static void igmp_group_dropped(struct ip_mc_list *im)
 #ifdef CONFIG_IP_MULTICAST
 	if (im->multiaddr == IGMP_ALL_HOSTS)
 		return;
+	if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(im->multiaddr))
+		return;
 
 	reporter = im->reporter;
 	igmp_stop_timer(im);

@@ -1213,6 +1236,8 @@ static void igmp_group_added(struct ip_mc_list *im)
 #ifdef CONFIG_IP_MULTICAST
 	if (im->multiaddr == IGMP_ALL_HOSTS)
 		return;
+	if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(im->multiaddr))
+		return;
 
 	if (in_dev->dead)
 		return;

@@ -1515,6 +1540,8 @@ static void ip_mc_rejoin_groups(struct in_device *in_dev)
 	for_each_pmc_rtnl(in_dev, im) {
 		if (im->multiaddr == IGMP_ALL_HOSTS)
 			continue;
+		if (IGMP_INHIBIT_LINK_LOCAL_REPORTS(im->multiaddr))
+			continue;
 
 		/* a failover is happening and switches
 		 * must be notified immediately

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 433231c..e7fd509 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c

@@ -912,6 +912,13 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "igmp_link_local_reports",
+		.data		= &sysctl_igmp_link_local_reports,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
 	{ }
 };
 

-- 
1.7.10.4

From: Philip Downey <redacted>
Date: Wed, 12 Aug 2015 17:13:53 +0100

IGMP reports are generated for link local multicast groups (224.0.0.1
- 224.0.0.255) used by the routing protocols such as RIP, OSPF etc.
In general routers do not generate reports for local multicast groups.

IGMP reports for local multicast groups can now be inhibited by means
of a system control variable (setting the value to zero).

To retain backwards compatibility the previous behaviour is retained by
default on system boot.

Signed-off-by: Philip Downey <redacted>

I'm always hesitent to apply patches like this.

I can't even understand from your explanation:

1) what about local reporting behavior is so bad

2) why you want to inhibit them at all

For example, this:

In general routers do not generate reports for local multicast groups.

Doesn't tell me anything.  You need to go into more detail about
this, and explain the situation sufficiently.

Hi David
Thanks for taking the time to review and comment.
This is my first upstream request so please forgive any ignorance on my part.   I have added a new proposed commit wording below with a view to agreeing the content before resubmitting the patch.
I hope it is sufficient to address your concerns.

   IGMP: Inhibit reports for local multicast groups
    
    The range of addresses between 224.0.0.0 and 224.0.0.255
    inclusive, is reserved for the use of routing protocols and other
    low-level topology discovery or maintenance protocols, such as
    gateway discovery and group membership reporting.  Multicast
    routers should not forward any multicast datagram with   destination
    addresses in this range, regardless of its TTL.

    Currently, IGMP reports are generated for this reserved range of
    addresses even though a router will ignore this information since
    it has no purpose.  However, the presence of reserved group
    addresses in an IGMP membership report uses up network bandwidth
    and can also obscure addresses of interest when inspecting
    membership reports using packet inspection or debug messages.

    IGMP reports for local multicast groups can now be inhibited by means
    of a system control variable (setting the value to zero).
    
    To retain backwards compatibility the previous behaviour is retained by
    default on system boot.
    
    Signed-off-by: Philip Downey [off-list ref]


Regards

Philip

-----Original Message-----
From: David Miller [mailto:davem@davemloft.net]
Sent: Thursday, August 13, 2015 12:45 AM
To: Philip Downey
Cc: kuznet@ms2.inr.ac.ru; jmorris@namei.org; yoshfuji@linux-ipv6.org;
kaber@trash.net; linux-kernel@vger.kernel.org; netdev@vger.kernel.org
Subject: Re: [PATCH] IGMP: Inhibit reports for local multicast groups

From: Philip Downey <redacted>
Date: Wed, 12 Aug 2015 17:13:53 +0100

quoted

IGMP reports are generated for link local multicast groups (224.0.0.1
- 224.0.0.255) used by the routing protocols such as RIP, OSPF etc.
In general routers do not generate reports for local multicast groups.

IGMP reports for local multicast groups can now be inhibited by means
of a system control variable (setting the value to zero).

To retain backwards compatibility the previous behaviour is retained
by default on system boot.

Signed-off-by: Philip Downey <redacted>

I'm always hesitent to apply patches like this.

I can't even understand from your explanation:

1) what about local reporting behavior is so bad

2) why you want to inhibit them at all

For example, this:

quoted

In general routers do not generate reports for local multicast groups.

Doesn't tell me anything.  You need to go into more detail about this, and
explain the situation sufficiently.

On Thu, Aug 13, 2015 at 02:48:23PM +0000, Philip Downey wrote:

Hi David
Thanks for taking the time to review and comment.
This is my first upstream request so please forgive any ignorance on my part.   I have added a new proposed commit wording below with a view to agreeing the content before resubmitting the patch.
I hope it is sufficient to address your concerns.

   IGMP: Inhibit reports for local multicast groups
    
    The range of addresses between 224.0.0.0 and 224.0.0.255
    inclusive, is reserved for the use of routing protocols and other
    low-level topology discovery or maintenance protocols, such as
    gateway discovery and group membership reporting.  Multicast
    routers should not forward any multicast datagram with   destination
    addresses in this range, regardless of its TTL.

    Currently, IGMP reports are generated for this reserved range of
    addresses even though a router will ignore this information since
    it has no purpose.

Hi Philip

What about switches which are doing IGMP snooping?

     Andrew

Hi Andrew
IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined.   Link-Local multicast traffic should not have an IGMP client since it is reserved for routing protocols.  One would expect that IGMP snooping needs to ignore local multicast traffic in the reserved range intended for routers since there should be no IGMP client to make "join" requests.

Regards

Philip

-----Original Message-----
From: Andrew Lunn [mailto:andrew@lunn.ch]
Sent: Thursday, August 13, 2015 5:06 PM
To: Philip Downey
Cc: David Miller; kuznet@ms2.inr.ac.ru; jmorris@namei.org; yoshfuji@linux-
ipv6.org; kaber@trash.net; linux-kernel@vger.kernel.org;
netdev@vger.kernel.org
Subject: Re: [PATCH] IGMP: Inhibit reports for local multicast groups

On Thu, Aug 13, 2015 at 02:48:23PM +0000, Philip Downey wrote:

quoted

Hi David
Thanks for taking the time to review and comment.
This is my first upstream request so please forgive any ignorance on my

part.   I have added a new proposed commit wording below with a view to
agreeing the content before resubmitting the patch.

quoted

I hope it is sufficient to address your concerns.

   IGMP: Inhibit reports for local multicast groups

    The range of addresses between 224.0.0.0 and 224.0.0.255
    inclusive, is reserved for the use of routing protocols and other
    low-level topology discovery or maintenance protocols, such as
    gateway discovery and group membership reporting.  Multicast
    routers should not forward any multicast datagram with   destination
    addresses in this range, regardless of its TTL.

    Currently, IGMP reports are generated for this reserved range of
    addresses even though a router will ignore this information since
    it has no purpose.

Hi Philip

What about switches which are doing IGMP snooping?

     Andrew

On Thu, Aug 13, 2015 at 04:52:32PM +0000, Philip Downey wrote:

Hi Andrew
IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined.   Link-Local multicast traffic should not have an IGMP client since it is reserved for routing protocols.  One would expect that IGMP snooping needs to ignore local multicast traffic in the reserved range intended for routers since there should be no IGMP client to make "join" requests.

The point of this patch is that Linux is sending out group membership
for these addresses, it is acting as a client. What happens with a
switch which is applying IGMP snooping to link-local multicast groups?
You turn on this feature, and you no longer get your routing protocol
messages.

I had a quick look at RFC 3376. The only mention i spotted for not
sending IGMP messages is:

   The all-systems multicast address, 224.0.0.1, is handled as a special
   case.  On all systems -- that is all hosts and routers, including
   multicast routers -- reception of packets destined to the all-systems
   multicast address, from all sources, is permanently enabled on all
   interfaces on which multicast reception is supported.  No IGMP
   messages are ever sent regarding the all-systems multicast address.

IGMP v2 has something similar:

   The all-systems group (address 224.0.0.1) is handled as a special
   case.  The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

But i did not find anything which says all other link-local addresses
don't need member reports. Did i miss something?

      Andrew

On Thu, Aug 13, 2015 at 07:01:37PM +0200, Andrew Lunn wrote:

On Thu, Aug 13, 2015 at 04:52:32PM +0000, Philip Downey wrote:

quoted

Hi Andrew
IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined.   Link-Local multicast traffic should not have an IGMP client since it is reserved for routing protocols.  One would expect that IGMP snooping needs to ignore local multicast traffic in the reserved range intended for routers since there should be no IGMP client to make "join" requests.

The point of this patch is that Linux is sending out group membership
for these addresses, it is acting as a client. What happens with a
switch which is applying IGMP snooping to link-local multicast groups?
You turn on this feature, and you no longer get your routing protocol
messages.

I had a quick look at RFC 3376. The only mention i spotted for not
sending IGMP messages is:

   The all-systems multicast address, 224.0.0.1, is handled as a special
   case.  On all systems -- that is all hosts and routers, including
   multicast routers -- reception of packets destined to the all-systems
   multicast address, from all sources, is permanently enabled on all
   interfaces on which multicast reception is supported.  No IGMP
   messages are ever sent regarding the all-systems multicast address.

IGMP v2 has something similar:

   The all-systems group (address 224.0.0.1) is handled as a special
   case.  The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

But i did not find anything which says all other link-local addresses
don't need member reports. Did i miss something?

      Andrew

From RFC 4541 (Considerations for Internet Group Management Protocol (IGMP) and

Multicast Listener Discovery (MLD) Snooping Switches):

 2) Packets with a destination IP (DIP) address in the 224.0.0.X range
      which are not IGMP must be forwarded on all ports.

      This recommendation is based on the fact that many host systems do
      not send Join IP multicast addresses in this range before sending
      or listening to IP multicast packets.  Furthermore, since the
      224.0.0.X address range is defined as link-local (not to be
      routed), it seems unnecessary to keep the state for each address
      in this range.  Additionally, some routers operate in the
      224.0.0.X address range without issuing IGMP Joins, and these
      applications would break if the switch were to prune them due to
      not having seen a Join Group message from the router.

So, it looks like some hosts and routers out there in the field do not send
joins for those local addresses. In fact, IPv4 local multicast addresses are
ignored when Linux bridge multicast snooping adds a new group.

static int br_ip4_multicast_add_group(struct net_bridge *br,
...
	if (ipv4_is_local_multicast(group))
		return 0;

Cascardo.

Sorry for the duplication - I responded in a similar manner before seeing this.

Thanks

Philip

-----Original Message-----
From: Thadeu Lima de Souza Cascardo [mailto:cascardo@redhat.com]
Sent: Thursday, August 13, 2015 7:08 PM
To: Andrew Lunn
Cc: Philip Downey; David Miller; kuznet@ms2.inr.ac.ru; jmorris@namei.org;
yoshfuji@linux-ipv6.org; kaber@trash.net; linux-kernel@vger.kernel.org;
netdev@vger.kernel.org
Subject: Re: [PATCH] IGMP: Inhibit reports for local multicast groups

On Thu, Aug 13, 2015 at 07:01:37PM +0200, Andrew Lunn wrote:

quoted

On Thu, Aug 13, 2015 at 04:52:32PM +0000, Philip Downey wrote:

quoted

Hi Andrew
IGMP snooping is designed to prevent hosts on a local network from

receiving traffic for a multicast group they have not explicitly joined.   Link-
Local multicast traffic should not have an IGMP client since it is reserved for
routing protocols.  One would expect that IGMP snooping needs to ignore
local multicast traffic in the reserved range intended for routers since there
should be no IGMP client to make "join" requests.

quoted

The point of this patch is that Linux is sending out group membership
for these addresses, it is acting as a client. What happens with a
switch which is applying IGMP snooping to link-local multicast groups?
You turn on this feature, and you no longer get your routing protocol
messages.

I had a quick look at RFC 3376. The only mention i spotted for not
sending IGMP messages is:

   The all-systems multicast address, 224.0.0.1, is handled as a special
   case.  On all systems -- that is all hosts and routers, including
   multicast routers -- reception of packets destined to the all-systems
   multicast address, from all sources, is permanently enabled on all
   interfaces on which multicast reception is supported.  No IGMP
   messages are ever sent regarding the all-systems multicast address.

IGMP v2 has something similar:

   The all-systems group (address 224.0.0.1) is handled as a special
   case.  The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

But i did not find anything which says all other link-local addresses
don't need member reports. Did i miss something?

      Andrew

From RFC 4541 (Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping Switches):

 2) Packets with a destination IP (DIP) address in the 224.0.0.X range
      which are not IGMP must be forwarded on all ports.

      This recommendation is based on the fact that many host systems do
      not send Join IP multicast addresses in this range before sending
      or listening to IP multicast packets.  Furthermore, since the
      224.0.0.X address range is defined as link-local (not to be
      routed), it seems unnecessary to keep the state for each address
      in this range.  Additionally, some routers operate in the
      224.0.0.X address range without issuing IGMP Joins, and these
      applications would break if the switch were to prune them due to
      not having seen a Join Group message from the router.

So, it looks like some hosts and routers out there in the field do not send
joins for those local addresses. In fact, IPv4 local multicast addresses are
ignored when Linux bridge multicast snooping adds a new group.

static int br_ip4_multicast_add_group(struct net_bridge *br, ...
	if (ipv4_is_local_multicast(group))
		return 0;

Cascardo.

Hi Andrew
Answers inline...

-----Original Message-----
From: Andrew Lunn [mailto:andrew@lunn.ch]
Sent: Thursday, August 13, 2015 6:02 PM
To: Philip Downey
Cc: David Miller; kuznet@ms2.inr.ac.ru; jmorris@namei.org; yoshfuji@linux-
ipv6.org; kaber@trash.net; linux-kernel@vger.kernel.org;
netdev@vger.kernel.org
Subject: Re: [PATCH] IGMP: Inhibit reports for local multicast groups

On Thu, Aug 13, 2015 at 04:52:32PM +0000, Philip Downey wrote:

quoted

Hi Andrew
IGMP snooping is designed to prevent hosts on a local network from

receiving traffic for a multicast group they have not explicitly joined.   Link-
Local multicast traffic should not have an IGMP client since it is reserved for
routing protocols.  One would expect that IGMP snooping needs to ignore
local multicast traffic in the reserved range intended for routers since there
should be no IGMP client to make "join" requests.

The point of this patch is that Linux is sending out group membership for
these addresses, it is acting as a client. What happens with a switch which is
applying IGMP snooping to link-local multicast groups?
You turn on this feature, and you no longer get your routing protocol
messages.

It is expected that link-local multicast is always forwarded by switches otherwise routers may not function correctly.

From the relevant RFC:

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006


2.1.2. Data Forwarding Rules


   1) Packets with a destination IP address outside 224.0.0.X which are
      not IGMP should be forwarded according to group-based port
      membership tables and must also be forwarded on router ports.

      This is the main IGMP snooping functionality for the data path.
      One approach that an implementation could take would be to
      maintain separate membership and multicast router tables in
      software and then "merge" these tables into a forwarding cache.

   2) Packets with a destination IP (DIP) address in the 224.0.0.X range
      which are not IGMP must be forwarded on all ports.

      This recommendation is based on the fact that many host systems do
      not send Join IP multicast addresses in this range before sending
      or listening to IP multicast packets.  Furthermore, since the
      224.0.0.X address range is defined as link-local (not to be
      routed), it seems unnecessary to keep the state for each address
      in this range.  Additionally, some routers operate in the
      224.0.0.X address range without issuing IGMP Joins, and these
      applications would break if the switch were to prune them due to
      not having seen a Join Group message from the router.

I had a quick look at RFC 3376. The only mention i spotted for not sending
IGMP messages is:

   The all-systems multicast address, 224.0.0.1, is handled as a special
   case.  On all systems -- that is all hosts and routers, including
   multicast routers -- reception of packets destined to the all-systems
   multicast address, from all sources, is permanently enabled on all
   interfaces on which multicast reception is supported.  No IGMP
   messages are ever sent regarding the all-systems multicast address.

IGMP v2 has something similar:

   The all-systems group (address 224.0.0.1) is handled as a special
   case.  The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

But i did not find anything which says all other link-local addresses don't
need member reports. Did i miss something?

No you did not miss anything - that is correct.
However, the RFCs don't really cover the behavior of routers well in some areas.   Routing protocols which use the 224.0.0.x address space do not need IGMP therefore it makes no sense to distribute membership reports for these groups.  A router which receives an IGMP membership report which includes groups from this reserved address range will ignore it -and probably generate debug messages highlighting an invalid address.

Regards

Philip

      Andrew

Hi Philip

So with a bit of poking and prodding, we have a much better
understanding as to why this is O.K. Maybe your next patch can quote
the relevant RFCs and have a much fuller commit message?

Thanks
	Andrew

Hi Andrew
Will resubmit with the information you suggest.
There might be a slight delay for this as I am on holiday now for a week.

Rest regards

Philip

-----Original Message-----
From: Andrew Lunn [mailto:andrew@lunn.ch]
Sent: Friday, August 14, 2015 2:35 PM
To: Philip Downey
Cc: David Miller; kuznet@ms2.inr.ac.ru; jmorris@namei.org; yoshfuji@linux-
ipv6.org; kaber@trash.net; linux-kernel@vger.kernel.org;
netdev@vger.kernel.org
Subject: Re: [PATCH] IGMP: Inhibit reports for local multicast groups

Hi Philip

So with a bit of poking and prodding, we have a much better understanding
as to why this is O.K. Maybe your next patch can quote the relevant RFCs
and have a much fuller commit message?

Thanks
	Andrew