net: add note about disabling loopback in ListenMulticastUDP()

Fixes #41752

Change-Id: I83520d2303e5fd2e5f6329f092b40e73c13771a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/271908
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

diff --git a/src/net/udpsock.go b/src/net/udpsock.go
index ec2bcfa607bea18d142d3d52abf52665857a9fc0..571e099abd81c0e60fa5e51a1f79afe90a085b86 100644 (file)
--- a/src/net/udpsock.go
+++ b/src/net/udpsock.go
@@ -259,6 +259,9 @@ func ListenUDP(network string, laddr *UDPAddr) (*UDPConn, error) {
 // ListenMulticastUDP is just for convenience of simple, small
 // applications. There are golang.org/x/net/ipv4 and
 // golang.org/x/net/ipv6 packages for general purpose uses.
+//
+// Note that ListenMulticastUDP will set the IP_MULTICAST_LOOP socket option
+// to 0 under IPPROTO_IP, to disable loopback of multicast packets.
 func ListenMulticastUDP(network string, ifi *Interface, gaddr *UDPAddr) (*UDPConn, error) {
        switch network {
        case "udp", "udp4", "udp6":