Re: [Bugme-new] [Bug 15530] New: ipip6 MTU cannot be set lower than 1280

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On Sat, 13 Mar 2010 14:38:54 GMT
bugzilla-daemon@bugzilla.kernel.org wrote:

http://bugzilla.kernel.org/show_bug.cgi?id=15530

           Summary: ipip6 MTU cannot be set lower than 1280
           Product: Networking
           Version: 2.5
    Kernel Version: 2.6.32
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: low
          Priority: P1
         Component: IPV6
        AssignedTo: yoshfuji@linux-ipv6.org
        ReportedBy: berni@birkenwald.de
        Regression: No


The (inner) MTU of a ipip6 (IPv4-in-IPv6) tunnel cannot be set below 1280,
which is the minimum MTU in IPv6. However, there should be no IPv6 on the
tunnel interface at all (ipip6 cannot transport it), so the IPv6 rules should
not apply.

root@pest:~# ip -6 tunnel add test1 mode ipip6 remote 2001:xxxx::26 local
2001:xxx::1:1
root@pest:~# ip link set test1 mtu 1280
root@pest:~# ip link set test1 mtu 1279
RTNETLINK answers: Invalid argument

This prevents the tunnel from work on paths that only support a (outer) IPv6
MTU of 1280, since the tunnel adds an additional overhead of 40 bytes.

Setting net.ipv6.conf.<if>.disable_ipv6=1 and/or removing the IPv6 link-local
address on the tunnel interface does not help.