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ip.7: Improve description of IP_MTU_DISCOVER 

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).

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.

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.

Reviewed-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Benjamin Poirier 2011-09-22 06:08:30 +02:00 committed by Michael Kerrisk
parent 4023450d6c
commit 8341e3a766
1 changed files with 24 additions and 14 deletions
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38
man7/ip.7
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@ -9,7 +9,7 @@
.\"
.\" FIXME: Document IP_MINTTL, added in Linux 2.6.34
.\"
.TH IP 7 2010-10-24 "Linux" "Linux Programmer's Manual"
.TH IP 7 2011-09-22 "Linux" "Linux Programmer's Manual"
.SH NAME
ip \- Linux IPv4 protocol implementation
.SH SYNOPSIS
@ -310,22 +310,32 @@ Only valid as a
.\" Precisely: 2.1.124
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
.I /proc/sys/net/ipv4/ip_no_pmtu_disc
file for
as defined in RFC\ 1191 on
.B SOCK_STREAM
sockets, and disabled on all others.
sockets.
For
.RB 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 packets that are bigger than the known
path MTU if this flag is set (with
.B EMSGSIZE
).
sockets,
.B IP_PMTUDISC_DO
forces the don't-fragment flag to be set on all outgoing packets.
The don't-fragment flag is set on all outgoing datagrams.
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
.BR EMSGSIZE )
datagrams that are bigger than the known path MTU.
.B IP_PMTUDISC_WANT
will fragment a datagram if needed according to the path MTU,
or will set the don't-fragment flag otherwise.
The system-wide default can be toggled between
.B IP_PMTUDISC_WANT
and
.B IP_PMTUDISC_DONT
by writing (respectively, zero and nonzero values) to the
.I /proc/sys/net/ipv4/ip_no_pmtu_disc
file.
.TS
tab(:);
c l