'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH IP_MTU_DISCOVER 2const 2025-11-25 "Linux man-pages 6.17"
.SH NAME
IP_MTU_DISCOVER
\-
path maximum transfer unit size discovery
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <netinet/in.h>" "  /* Definition of " IP* " constants */"
.B #include <sys/socket.h>
.P
.BI int\~setsockopt(int\~ sockfd ,\~IPPROTO_IP,\~IP_MTU_DISCOVER,
.BI "               const\~int\~*" val ,\~sizeof(int));
.BI int\~getsockopt(int\~ sockfd ,\~IPPROTO_IP,\~IP_MTU_DISCOVER,
.BI "               int\~*" val ,\~sizeof(int));
.fi
.SH DESCRIPTION
Set or receive the Path MTU Discovery setting for a socket.
.P
When enabled,
Linux will perform Path MTU Discovery
as defined in RFC\ 1191 on
.B SOCK_STREAM
sockets.
.P
For
.RB non- SOCK_STREAM
sockets,
.B IP_PMTUDISC_DO
forces the don't-fragment flag to be set on all outgoing packets.
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.
.P
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
l l.
Path MTU discovery value:Meaning
IP_PMTUDISC_WANT:Use per-route settings.
IP_PMTUDISC_DONT:Never do Path MTU Discovery.
IP_PMTUDISC_DO:Always do Path MTU Discovery.
IP_PMTUDISC_PROBE:Set DF but ignore Path MTU.
.TE
.P
When PMTU discovery is enabled, the kernel automatically keeps track of
the path MTU per destination host.
When it is connected to a specific peer with
.BR connect (2),
the currently known path MTU can be retrieved conveniently using the
.BR IP_MTU (2const)
socket option (e.g., after an
.B EMSGSIZE
error occurred).
The path MTU may change over time.
For connectionless sockets with many destinations,
the new MTU for a given destination can also be accessed using the
error queue (see
.BR IP_RECVERR (2const)).
A new error will be queued for every incoming MTU update.
.P
While MTU discovery is in progress, initial packets from datagram sockets
may be dropped.
Applications using UDP should be aware of this and not
take it into account for their packet retransmit strategy.
.P
To bootstrap the path MTU discovery process on unconnected sockets, it
is possible to start with a big datagram size
(headers up to 64 kilobytes long) and let it shrink by updates of the path MTU.
.P
To get an initial estimate of the
path MTU, connect a datagram socket to the destination address using
.BR connect (2)
and retrieve the MTU by calling
.BR getsockopt (2)
with the
.BR IP_MTU (2const)
option.
.P
It is possible to implement RFC 4821 MTU probing with
.B SOCK_DGRAM
or
.B SOCK_RAW
sockets by setting a value of
.BR IP_PMTUDISC_PROBE .
This is also particularly useful for diagnostic tools such as
.BR tracepath (8)
that wish to deliberately send probe packets larger than
the observed Path MTU.
.SH ERRORS
See
.BR IPPROTO_IP (2const).
See
.BR setsockopt (2).
See
.BR ip (7).
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.2.
.\" Precisely: since Linux 2.1.124
.TP
.B IP_PMTUDISC_PROBE
Linux 2.6.22.
.SH SEE ALSO
.BR IPPROTO_IP (2const),
.BR setsockopt (2),
.BR ip (7)