Bug 247718 - FreeBSD incorrectly drops IPv6 packets looping back to the same p2p interface
Summary: FreeBSD incorrectly drops IPv6 packets looping back to the same p2p interface
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: Unspecified
Hardware: Any Any
: --- Affects Only Me
Assignee: Alexander V. Chernikov
Keywords: patch
Depends on:
Reported: 2020-07-02 15:06 UTC by Mira Ressel
Modified: 2020-07-05 12:44 UTC (History)
3 users (show)

See Also:

suggested patch for sys/netinet6/ip6_forward.c (3.17 KB, patch)
2020-07-02 15:15 UTC, Mira Ressel
no flags Details | Diff

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Description Mira Ressel 2020-07-02 15:06:00 UTC
FreeBSD's IPv6 stack explicitly drops packets which were received on an
IFF_POINTOPOINT interface and are then looped back to the same interface [1].

This piece of code originated in the KAME project, and was controversial even
when it was enabled there by default back in 2001 [2]. Notably, there is no
equivalent logic in FreeBSD's IPv4 stack -- it also generates ICMP Redirect
messages under these circumstances, but doesn't drop any packets.

While this logic may arguably make sense for true point-to-point links, not all
interfaces with the IFF_POINTOPOINT flag are such links. In particular, tun
interfaces are always marked with this flag, even though the userland
application behind a tun interface might well be forwarding packets to multiple

One example for an application that uses tun interfaces to present a
point-to-multipoint/NBMA link rather than a point-to-point link is wireguard-go,
the userland implementation of the wireguard protocol [3]. In this case, the
aforementioned logic causes packet drops in a perfectly valid network topology,
as reported in [4].

Therefore, I would suggest removing the entire "if ((nh->nh_ifp->if_flags &
IFF_POINTOPOINT) != 0) { ... }" block that drops these packets. Even in the case
of true point-to-point links, its only function is to avoid transient looping of
packets sent my misconfigured applications or attackers. This is a well-known
problem [5] that can be easily avoided by proper route configuration, rather
than by hardcoded kernel logic to drop packets.

[1] https://svnweb.freebsd.org/base/head/sys/netinet6/ip6_forward.c?view=markup&pathrev=362338#l265
[2] lengthy ML thread archived at
    https://www.ietf.org/mail-archive/text/ipngwg/2001-04.mail and -05,
    subject "nonexisting destination on p2p link"
[3] https://www.wireguard.com/
[4] https://forums.freebsd.org/threads/wireguard-peers-cant-reach-each-other-on-ipv6.75991/
[5] https://tools.ietf.org/html/rfc6164#section-5.1
Comment 1 Mira Ressel 2020-07-02 15:15:40 UTC
Created attachment 216144 [details]
suggested patch for sys/netinet6/ip6_forward.c
Comment 2 lutz 2020-07-05 12:44:16 UTC
Open an internal Code review: https://reviews.freebsd.org/D25567