netdev - Re: discrepancy in ip(7) wrt. IP DF flag for UDP sockets

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Message-ID: <CAKgNAkgWRdkxgTXNnMi4PVGGsOd-8EHF1siBrk-Bj=rnYenVmQ@mail.gmail.com>
Date:	Tue, 20 Sep 2011 08:14:24 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Benjamin Poirier <benjamin.poirier@...il.com>,
	linux-man@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: discrepancy in ip(7) wrt. IP DF flag for UDP sockets

Hello Benjamin, Neil,

On Mon, Sep 19, 2011 at 3:03 PM, Neil Horman <nhorman@...driver.com> wrote:
> On Mon, Sep 19, 2011 at 08:19:40AM -0400, Benjamin Poirier wrote:
>> Hi,
>>
>> I noticed what appears to be a discrepancy between the ip(7) man page
>> and the kernel code with regards to the IP DF flag for UDP sockets.
>>
>> The man page says that "The don't-fragment flag is set on all outgoing
>> datagrams" and that the ip_no_pmtu_disc sysctl affects only SOCK_STREAM
>> sockets. This is quickly disproved by doing:
>> echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc
>> firing up netcat and looking at a few outgoing udp packets in wireshark
>> (they don't have the DF flag set).

Could you describe the required change in terms of how the man page
text should look--i.e., rewrite the passage as you think it should
look?

Thanks,

Michael


>> 1) in the words of `man 7 ip`:
>> IP_MTU_DISCOVER (since Linux 2.2)
>>       Set or receive the Path MTU Discovery  setting  for  a  socket.
>>       When  enabled, Linux will perform Path MTU Discovery as defined
>>       in RFC 1191 on this socket.  The don't-fragment flag is set  on
>>       all  outgoing datagrams.  The system-wide default is controlled
>>       by the /proc/sys/net/ipv4/ip_no_pmtu_disc file for  SOCK_STREAM
>>       sockets, and disabled on all others.
>>
>> This is the text present in the latest version of the online manpages,
>> http://webcache.googleusercontent.com/search?q=cache:http://www.kernel.org/doc/man-pages/reporting_bugs.html&ie=UTF-8
>>
>> 2) in net/ipv4/af_inet.c:inet_create():
>>       if (ipv4_config.no_pmtu_disc)
>>               inet->pmtudisc = IP_PMTUDISC_DONT;
>>       else
>>               inet->pmtudisc = IP_PMTUDISC_WANT;
>>
>> and pmtudisc is left alone from there on for UDP sockets.
>>
>> What should be adjusted, the man page or the code?
>>
> The man page is wrong I think
>
> By my read, the code:
> 1) Affects UDP and TCP the same way (which makes sense to me)
>
> 2) Is doing exactly what you asked it to, since you set no_pmtu_disc, which
> means the stack should be free to fragment a frame as it sees fit according to
> the MTU metric of the route its traversing, hence the cleared DF bit in the
> fraem.
>
> RFC 1191 can apply equally well to udp, as tcp, and is evident in that you can
> set the per-socket option IP_MTU_DISCOVER to any of the 4 acceptible values
> offered (DONT/WANT/DO/PROBE), so theres no reason the sysctl governing the
> default value at creation shouldn't apply as well.
> Neil
>
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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