IPv6 fragmented packets are being reassembled as they pass through the linux qbr bridge in Red Hat OpenStack Platform

Solution In Progress - Updated -


IPv6 fragmented packets are being reassembled as they pass through the linux qbr bridge in Red Hat OpenStack Platform. Note that this behavior creates problems in OpenStack, where we have:

Host A on external network with MTU 1500
<blackbox network with MTU 1500>
Hypervisor with all external interfaces set to MTU 9000 and all bridges and tap interfaces to MTU 9000
Instance on this hypervisor with MTU 1500

As a detailed view:

qvb (veth MTU 9000) <-> qbr (Linux bridge MTU 9000) <-> tap ( tap interface MTU 9000) <-> instance (MTU 1500)

The issue seems to be related to the backport of this upstream commit https://github.com/torvalds/linux/commit/efb6de9b4ba0092b2c55f6a52d16294a8a698edd
Referred by OpenStack upstream bug: https://bugs.launchpad.net/neutron/+bug/1463911

The upstream bugfix was backported into the RHEL 7.3 kernel

The following was fixed and already backported into the kernel in question:

These were fixed here:

[root@rhospbl-1 ~]# rpm -q kernel --changelog | grep 1265259 | grep -i v6
- [net] netfilter: bridge: Use __in6_dev_get rather than in6_dev_get in br_validate_ipv6 (Paolo Abeni) [1265259]
- [net] netfilter: bridge: split ipv6 code into separated file (Paolo Abeni) [1265259]
- [net] netfilter: bridge: forward IPv6 fragmented packets (Paolo Abeni) [1265259]
- [net] netfilter: bridge: re-order br_nf_pre_routing_finish_ipv6() (Paolo Abeni) [1265259]
- [net] netfilter: nf_tables_bridge: set the pktinfo for IPv4/IPv6 traffic (Paolo Abeni) [1265259]


It seems that https://github.com/torvalds/linux/commit/efb6de9b4ba0092b2c55f6a52d16294a8a698edd resolved an issue with packet drops, but introduced a problem with unwanted packet reassembly on the network. So, now, instead of dropping fragments, these are reassembled, although this behavior may be undesired,

At host A with an MTU of 1500 one is fragmenting 3.5K Bytes in 2x1.5K + 0.5K
At point these packets are recombined to 3.5K Bytes, because the next leg is having 9K Bytes MTU.
At host B with an MTU of 1500 the 3.5K packets arrive, but are refused due to the 1500 Byte MTU.

The instance is configured with MTU 1500, and therefore refuses the packets.

[user@wks-user 01805825]$ tshark -nn -r tap4130202d-c5.pcap sip | head -n2
198  15.026115 0.000000 e8:f7:24:9c:e7:26 → fa:16:3e:19:bf:3a 2607:f160:10:230d:ce:104:0:5 → 2607:f160:10:24ff::8 5090 5060 SIP/SDP 3234  Request: INVITE sip:16692481053@vzimstest.com | 
199  15.026337 0.000222 fa:16:3e:19:bf:3a → e8:f7:24:9c:e7:26 2607:f160:10:24fd::47 → 2607:f160:10:230d:ce:104:0:5 5090 5060 ICMPv6 1294  Packet Too Big
[user@wks-user 01805825]$ tshark -nn -r tap4130202d-c5.pcap -V frame.number == 199  | grep -i MTU -C5
    [Destination GeoIP: Unknown]
Internet Control Message Protocol v6
    Type: Packet Too Big (2)
    Code: 0
    Checksum: 0x50bf [correct]
    MTU: 1500
    Internet Protocol Version 6, Src: 2607:f160:10:230d:ce:104:0:5, Dst: 2607:f160:10:24ff::8
        0110 .... = Version: 6
        .... 1000 0000 .... .... .... .... .... = Traffic class: 0x80 (DSCP: CS4, ECN: Not-ECT)
            .... 1000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Class Selector 4 (32)
            .... .... ..00 .... .... .... .... .... = Explicit Congestion Notification: Not ECN-Capable Transport (0)


Red Hat Enterprise Linux 7.3
Red Hat OpenStack Platform 7.0
Red Hat OpenStack Platform 8.0
Red Hat OpenStack Platform 9.0
Red Hat OpenStack Platform 10.0

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