[bug #59319] ICMPv6 neighbor solicitation fails if LWIP_IPV6_MLD is set

[yuanjianmin]

URL:
  <https://savannah.nongnu.org/bugs/?59319>

                 Summary: ICMPv6 neighbor solicitation fails if LWIP_IPV6_MLD
is set
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: cyberdeck
            Submitted on: Thu 22 Oct 2020 02:29:34 PM UTC
                Category: IPv6
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

    _______________________________________________________

Details:

In the IPv6 stack neighbor solicitation packages are dropped if LWIP_IPV6_MLD
is set. For me it seems that this is a logical bug included in the ip6.c
source. Please see the included patch.

Description of the bug:
If lwIP is compiled with LWIP_IPV6_MLD set, ICMPv6 neighbor solicitation
messages are dropped if they are targeted for the solicited-node multicast
address (e.g. "ff02::1:ff28:9c5a", cf. [1]).

That drop is by purpose because the logic for checking the received packets
against the solicited-node multicast address is excluded. In my opinion that
behavior is wrong. The attached patch will fix this behavior.

Effect of the bug:
Bring up an interface and try to ping it's global IPv6 address. As the
interface is not replying to neighbor solicitation it cannot be reached.

Effect after the patch:
Bring up an interface and try to ping it's global IPv6 address. Now the
interface is replying to neighbor solicitation and this, it can be reached
afterwards.

[1]: https://en.wikipedia.org/wiki/Solicited-node_multicast_address



    _______________________________________________________

File Attachments:


-------------------------------------------------------
Date: Thu 22 Oct 2020 02:29:34 PM UTC  Name: ipv6.patch  Size: 658B   By:
cyberdeck

<http://savannah.nongnu.org/bugs/download.php?file_id=50049>

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[yuanjianmin]

Follow-up Comment #1, bug #59319 (project lwip):

I agree that this seems strange.

Originally the code was like this (commit 4bfbe7ebe):


  /* match packet against an interface, i.e. is this packet for us? */
  if (ip6_addr_ismulticast(ip6_current_dest_addr())) {
    /* Always joined to multicast if-local and link-local all-nodes group. */
    if (ip6_addr_isallnodes_iflocal(ip6_current_dest_addr()) ||
        ip6_addr_isallnodes_linklocal(ip6_current_dest_addr())) {
      netif = inp;
    }
#if LWIP_IPV6_MLD
    else if (mld6_lookfor_group(inp, ip6_current_dest_addr())) {
      netif = inp;
    }
#else /* LWIP_IPV6_MLD */
    else if (ip6_addr_issolicitednode(ip6_current_dest_addr())) {
      /* Accept all solicited node packets when MLD is not enabled
       * (for Neighbor discovery). */
      netif = inp;
    }
#endif /* LWIP_IPV6_MLD */
    else {
      netif = NULL;



Then in commit 6e81f722a the solicited node handling was added:


#else /* LWIP_IPV6_MLD */
    else if (ip6_addr_issolicitednode(ip6_current_dest_addr())) {
      /* Filter solicited node packets when MLD is not enabled
       * (for Neighbor discovery). */
      netif = NULL;
      for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
        if (ip6_addr_isvalid(netif_ip6_addr_state(inp, i)) &&
            ip6_addr_cmp_solicitednode(ip6_current_dest_addr(),
netif_ip6_addr(inp, i))) {
          netif = inp;
          LWIP_DEBUGF(IP6_DEBUG, ("ip6_input: solicited node packet accepted
on interface %c%c\n",
              netif->name[0], netif->name[1]));
          break;
        }
      }
    }
#endif /* LWIP_IPV6_MLD */


Since solicited nodes are not checked inside mld6 code, it seems this code
should always be active.

Simon, do you have an opinion here?

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