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

On Tue, Sep 20, 2011 at 09:29:54AM -0400, Benjamin Poirier wrote:

On 11-09-20 08:14, Michael Kerrisk wrote:

quoted

Hello Benjamin, Neil,

On Mon, Sep 19, 2011 at 3:03 PM, Neil Horman [off-list ref] wrote:

quoted

On Mon, Sep 19, 2011 at 08:19:40AM -0400, Benjamin Poirier wrote:

quoted

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?

How about changing it to:
	IP_MTU_DISCOVER (since Linux 2.2)
	Set or receive the Path MTU Discovery setting for a socket. When
	enabled, the don't-fragment flag is set on all outgoing packets.
	Linux will perform Path MTU Discovery as defined in RFC 1191 on
	SOCK_STREAM sockets. For non-SOCK_STREAM sockets, it is the
	user's responsibility to packetize the data in MTU sized chunks
	and to do the retransmits if necessary. The kernel will reject
	(with EMSGSIZE) datagrams that are bigger than the known path
	MTU. The system-wide default is controlled by the
	/proc/sys/net/ipv4/ip_no_pmtu_disc file. 

	Path MTU discovery flags   Meaning
	[...]

There are some differences between _DO and _WANT that are glossed over
in this description, but I suppose there's only so much detail you can
put in...

Thanks,
-Ben

Yeah, I think thats close, but its only the users responsibility to package
datagrams in mtu sized chunks if they force the dont fragment bit on.  If they
go wtih the default, the stack will fragment a datagram is it sees fit according
to the mtu of the path it traverses
Neil

quoted

Thanks,

Michael

quoted

quoted

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/

--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html