kernel: nd6_setmtu0: new link MTU on ng29 (1218) is too small for IPv6

[Hiroki Sato]

Sami Halabi <sodynet1 at gmail.com> wrote
  in <CAEW+ogbY_ARtc4mVDQHpYPKgaLWHj64EhmFrQDNXuMcbn5Kz8Q at mail.gmail.com>:

so> Hi,
so> I'm using a FreeBSD8.2-R-p5 in conjunction with MPD5.5 port for creating
so> pptp/l2tp tunnels.
so>
so> I'm using MPPC (Compression & Encryption), my current onfiguration i use
so> only IPv4.
so>
so> I keep getting in the logs the following:
so> Jan  3 19:15:21 mpd2 kernel: nd6_setmtu0: new link MTU on ng120 (1218) is
so> too small for IPv6
so> Jan  3 20:00:40 mpd2 kernel: nd6_setmtu0: new link MTU on ng151 (1218) is
so> too small for IPv6
so> Jan  3 20:09:27 mpd2 kernel: nd6_setmtu0: new link MTU on ng128 (1218) is
so> too small for IPv6
so> Jan  3 20:30:13 mpd2 kernel: nd6_setmtu0: new link MTU on ng128 (1218) is
so> too small for IPv6
so> Jan  3 20:34:33 mpd2 kernel: nd6_setmtu0: new link MTU on ng137 (1218) is
so> too small for IPv6
so> Jan  3 21:06:46 mpd2 kernel: nd6_setmtu0: new link MTU on ng105 (1218) is
so> too small for IPv6
so> Jan  3 21:42:48 mpd2 kernel: nd6_setmtu0: new link MTU on ng92 (1218) is
so> too small for IPv6
so> Jan  3 22:12:49 mpd2 kernel: nd6_setmtu0: new link MTU on ng137 (1218) is
so> too small for IPv6
so> Jan  3 23:21:50 mpd2 kernel: nd6_setmtu0: new link MTU on ng92 (1218) is
so> too small for IPv6
so> Jan  4 00:00:36 mpd2 kernel: nd6_setmtu0: new link MTU on ng105 (1218) is
so> too small for IPv6
so> Jan  4 00:34:48 mpd2 kernel: nd6_setmtu0: new link MTU on ng105 (1218) is
so> too small for IPv6
so> Jan  4 07:47:37 mpd2 kernel: nd6_setmtu0: new link MTU on ng100 (1218) is
so> too small for IPv6
so> Jan  4 08:31:55 mpd2 kernel: nd6_setmtu0: new link MTU on ng116 (1218) is
so> too small for IPv6
so> Jan  4 09:16:21 mpd2 kernel: nd6_setmtu0: new link MTU on ng123 (1218) is
so> too small for IPv6
so> Jan  4 12:55:32 mpd2 kernel: nd6_setmtu0: new link MTU on ng53 (1218) is
so> too small for IPv6
so>
so> although the NG tunnels don't negotiate IPv6.
so>
so> a close look to the MPD log i see that this happens for connections that
so> set MRU/MTU 1400.
so>
so> I talked to MPD developer (Alexander Motin) and this isn't a MPD problem,
so> rather than a kernel issue as the logs say.
so>
so> why this problem happens when no IPv6 is in work?
so>
so> I don't want to disable ipv6 completely since i have plans in using it in
so> the near future.

 It is because the end point of an L2TPv2 tunnel is implemented in MPD
 by using ng_iface(4), and it always has an inet6 hook when the kernel
 has IPv6 support (note that GENERIC kernel supports IPv6).  This
 means an ngNNN interface is always IPv6-capable regardless of whether
 the tunnel supports IPv6 as the inner protocol (typically via IPV6CP
 in the case of L2TPv2) or not.  IPv6 communication is denied when the
 tunnel does not support IPv6, but the interface ngNNN itself always
 supports IPv6.

 Thus, the warning you got is caused by a side-effect of IPv6
 initialization of ngNNN.  Since IPv6 specification requires the
 minimum MTU as 1280, an warning message will be displayed if you try
 to use a smaller MTU as the link MTU than that.  You can ignore the
 messages safely, or increasing the MTU will suppress them.

 I am still wondering why the message appears when MTU is set to 1400,
 however.  In the case of L2TPv2 over UDPv4, MTU reduction should be
 46 because of encapsulation by IPv4, UDP, L2TP, and PPP.

-- Hiroki
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