netdev - Re: IPv6 flow label reflection behave for RST packets

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Date:   Tue, 9 Jul 2019 15:22:39 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marek Majkowski <marek@...udflare.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        Jakub Sitnicki <jakub@...udflare.com>, netdev@...r.kernel.org,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: IPv6 flow label reflection behave for RST packets



On 7/9/19 2:33 PM, Marek Majkowski wrote:
> Ha, thanks. I missed that.
> 
> There is a caveat though. I don't think it's working as intended...


Note that my commit really took a look at a fraction of the cases ;)

commit 323a53c41292a0d7efc8748856c623324c8d7c21

    ipv6: tcp: enable flowlabel reflection in some RST packets
    
    When RST packets are sent because no socket could be found,
    it makes sense to use flowlabel_reflect sysctl to decide
    if a reflection of the flowlabel is requested.
    

In your case, a socket is found, most probably, and np->repflow seems to be ignored.

I'll take a look, thanks.

> Running my script:
> 
> $ sysctl -w net.ipv6.flowlabel_reflect=3
> 
> $ tail reflect.py
> cd2.close()
> cd.send(b"a")
> 
> $ python3 reflect.py
> IP6 (flowlabel 0xf2927, hlim 64) ::1.1235 > ::1.60246: Flags [F.]
> IP6 (flowlabel 0xf2927, hlim 64) ::1.60246 > ::1.1235: Flags [P.]
> IP6 (flowlabel 0x58ecd, hlim 64) ::1.1235 > ::1.60246: Flags [R]
> 
> Note. The RST is opportunistic, depending on timing I sometimes get a
> proper FIN, without RST.
> 
> If I change the script to introduce some delay:
> 
> $ tail reflect.py
> cd2.close()
> time.sleep(0.1)
> cd.send(b"a")
> 
> $ python3 reflect.py
> IP6 (flowlabel 0x2f60c, hlim 64) ::1.60326 > ::1.1235: Flags [.]
> IP6 (flowlabel 0x2f60c, hlim 64) ::1.60326 > ::1.1235: Flags [P.]
> IP6 (flowlabel 0x2f60c, hlim 64) ::1.1235 > ::1.60326: Flags [R]
> 
> Now it seem to work reliably. Tested on net-next under virtme.
> 
> Marek
> 
> On Tue, Jul 9, 2019 at 1:19 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>
>>
>>
>> On 7/9/19 1:10 PM, Marek Majkowski wrote:
>>> Morning,
>>>
>>> I'm experimenting with flow label reflection from a server point of
>>> view. I'm able to get it working in both supported ways:
>>>
>>> (a) per-socket with flow manager IPV6_FL_F_REFLECT and flowlabel_consistency=0
>>>
>>> (b) with global flowlabel_reflect sysctl
>>>
>>> However, I was surprised to see that RST after the connection is torn
>>> down, doesn't have the correct flow label value:
>>>
>>> IP6 (flowlabel 0x3ba3d) ::1.59276 > ::1.1235: Flags [S]
>>> IP6 (flowlabel 0x3ba3d) ::1.1235 > ::1.59276: Flags [S.]
>>> IP6 (flowlabel 0x3ba3d) ::1.59276 > ::1.1235: Flags [.]
>>> IP6 (flowlabel 0x3ba3d) ::1.1235 > ::1.59276: Flags [F.]
>>> IP6 (flowlabel 0x3ba3d) ::1.59276 > ::1.1235: Flags [P.]
>>> IP6 (flowlabel 0xdfc46) ::1.1235 > ::1.59276: Flags [R]
>>>
>>> Notice, the last RST packet has inconsistent flow label. Perhaps we
>>> can argue this behaviour might be acceptable for a per-socket
>>> IPV6_FL_F_REFLECT option, but with global flowlabel_reflect, I would
>>> expect the RST to preserve the reflected flow label value.
>>>
>>> I suspect the same behaviour is true for kernel-generated ICMPv6.
>>>
>>> Prepared test case:
>>> https://gist.github.com/majek/139081b84f9b5b6187c8ccff802e3ab3
>>>
>>> This behaviour is not necessarily a bug, more of a surprise. Flow
>>> label reflection is mostly useful in deployments where Linux servers
>>> stand behind ECMP router, which uses flow-label to compute the hash.
>>> Flow label reflection allows ICMP PTB message to be routed back to
>>> correct server.
>>>
>>> It's hard to imagine a situation where generated RST or ICMP echo
>>> response would trigger a ICMP PTB. Flow label reflection is explained
>>> here:
>>> https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
>>> and:
>>> https://tools.ietf.org/html/rfc7098
>>> https://tools.ietf.org/html/rfc6438
>>>
>>> Cheers,
>>>     Marek
>>>
>>>
>>> (Note: the unrelated "fwmark_reflect" toggle is about something
>>> different - flow marks, but also addresses RST and ICMP generated by
>>> the server)
>>>
>>
>> Please check the recent commits, scheduled for linux-5.3
>>
>> a346abe051bd2bd0d5d0140b2da9ec95639acad7 ipv6: icmp: allow flowlabel reflection in echo replies
>> c67b85558ff20cb1ff20874461d12af456bee5d0 ipv6: tcp: send consistent autoflowlabel in TIME_WAIT state
>> 392096736a06bc9d8f2b42fd4bb1a44b245b9fed ipv6: tcp: fix potential NULL deref in tcp_v6_send_reset()
>> 50a8accf10627b343109a9c9d5c361751bf753b0 ipv6: tcp: send consistent flowlabel in TIME_WAIT state
>> 323a53c41292a0d7efc8748856c623324c8d7c21 ipv6: tcp: enable flowlabel reflection in some RST packets
>>