.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH in_pktinfo 2type 2025-11-26 "Linux man-pages 6.17"
.SH NAME
in_pktinfo
\-
information about the incoming packet
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <netinet/in.h>
.fi
.P
.EX
struct in_pktinfo {
    unsigned int    ipi_ifindex;   /* Interface index */
    struct in_addr  ipi_spec_dst;  /* Local address */
    struct in_addr  ipi_addr;      /* Header Destination address */
};
.EE
.SH DESCRIPTION
.SS recvmsg(2)
When returned by
.BR recvmsg (2) ,
.I .ipi_ifindex
is the unique index of the interface the packet was received on.
.P
.I .ipi_spec_dst
is the preferred source address for replies to the given packet.
.P
.I .ipi_addr
is the destination address in the packet header.
.P
These addresses are usually the same,
but can differ for broadcast or multicast packets.
Depending on the configured routes,
.I .ipi_spec_dst
might belong to a different interface from the one that received the packet.
.SS sendmsg(2)
If
.B IP_PKTINFO
is passed to
.BR sendmsg (2)
and
.\" This field is grossly misnamed
.I .ipi_spec_dst
is not zero,
then it is used as the local source address,
for the routing table lookup,
and for setting up IP source route options.
.P
When
.I .ipi_ifindex
is not zero,
the primary local address of the interface specified by the index
overwrites
.I .ipi_spec_dst
for the routing table lookup.
.P
.I .ipi_addr
is ignored.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.2.
.\" Precisely: since Linux 2.1.68
.SH SEE ALSO
.BR IP_PKTINFO (2const),
.BR setsockopt (2),
.BR ip (7)