igmp: Document sysctl force_igmp_version

There is some difference between force_igmp_version and force_mld_version.
Add document to make users aware of this.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 3db8c67d2c8db9bd952c551010805b1f92d8f777..5af48dd7c5fc106705d4ef0034a4f3663c17294e 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -967,6 +967,21 @@ igmp_qrv - INTEGER
        Default: 2 (as specified by RFC2236 8.1)
        Minimum: 1 (as specified by RFC6636 4.5)
 
+force_igmp_version - INTEGER
+       0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
+           allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
+           Present timer expires.
+       1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
+           receive IGMPv2/v3 query.
+       2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
+           IGMPv1 query message. Will reply report if receive IGMPv3 query.
+       3 - Enforce to use IGMP version 3. The same react with default 0.
+
+       Note: this is not the same with force_mld_version because IGMPv3 RFC3376
+       Security Considerations does not have clear description that we could
+       ignore other version messages completely as MLDv2 RFC3810. So make
+       this value as default 0 is recommended.
+
 conf/interface/*  changes special settings per interface (where
 "interface" is the name of your network interface)