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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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IP_MTU_DISCOVER(2const) IP_MTU_DISCOVER(2const)
IP_MTU_DISCOVER - path maximum transfer unit size discovery
Standard C library (libc, -lc)
#include <netinet/in.h> /* Definition of IP* constants */
#include <sys/socket.h>
int setsockopt(int sockfd, IPPROTO_IP, IP_MTU_DISCOVER,
const int *val, sizeof(int));
int getsockopt(int sockfd, IPPROTO_IP, IP_MTU_DISCOVER,
int *val, sizeof(int));
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 SOCK_STREAM sockets.
For non-SOCK_STREAM sockets, 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
EMSGSIZE) datagrams that are bigger than the known path MTU.
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 IP_PMTUDISC_WANT
and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero
values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
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.
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 connect(2), the currently known path MTU
can be retrieved conveniently using the IP_MTU(2const) socket
option (e.g., after an 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 IP_RECVERR(2const)). A new
error will be queued for every incoming MTU update.
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.
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.
To get an initial estimate of the path MTU, connect a datagram
socket to the destination address using connect(2) and retrieve
the MTU by calling getsockopt(2) with the IP_MTU(2const) option.
It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM
or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE. This
is also particularly useful for diagnostic tools such as
tracepath(8) that wish to deliberately send probe packets larger
than the observed Path MTU.
See IPPROTO_IP(2const). See setsockopt(2). See ip(7).
Linux.
Linux 2.2.
IP_PMTUDISC_PROBE
Linux 2.6.22.
IPPROTO_IP(2const), setsockopt(2), ip(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2026-05-24. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.18 2025-11-25 IP_MTU_DISCOVER(2const)
Pages that refer to this page: IPPROTO_IP(2const), IPV6_MTU_DISCOVER(2const)
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HTML rendering created 2026-05-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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