Subject: Which takes precedence - PKTINFO setsockopt or MULTICAST_IF setsockopt?

From: "Lori Napoli" <[email-masked]>
Date: Tue, 31 Jul 2001 16:13:17 -0400

I would like to understand what interface would take precedence if an
application is sending multicast traffic and had done both a PKTINFO
setsockopt() that specified the outgoing interface and a MULTICAST_IF
setsockopt to specify the outgoing interface.  The follow text is from
draft-ietf-ipngwg-rfc2292bis-02.txt which talks about PKTINFO ancillary
data overriding the MULTICAST_IF socket option but does not give direction
on the PKTINFO setsockopt.  Thanks.

    6.1.  Specifying/Receiving the Interface

      Interfaces on an IPv6 node are identified by a small positive
      integer, as described in Section 4 of [RFC-2553].  That document also
      describes a function to map an interface name to its interface index,
      a function to map an interface index to its interface name, and a
      function to return all the interface names and indexes.  Notice from
      this document that no interface is ever assigned an index of 0.

      When specifying the outgoing interface, if the ipi6_ifindex value is
      0, the kernel will choose the outgoing interface.  If the application
      specifies an outgoing interface for a multicast packet, the interface
      specified by the ancillary data overrides any interface specified by
      the IPV6_MULTICAST_IF socket option (described in [RFC-2553]), for
      that call to sendmsg() only.

      When the IPV6_RECVPKTINFO socket option is enabled, the received
      interface index is always returned as the ipi6_ifindex member of the
      in6_pktinfo structure.

   Lori

   z/OS Communications Server Development - TCP/IP Stack

   919-254-6146   T/L 8-444-6146
   E-mail: [email-masked]

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