.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH IP_ROUTER_ALERT 2const 2025-11-25 "Linux man-pages 6.17"
.SH NAME
IP_ROUTER_ALERT
\-
pass all to-be forwarded packets
.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_ROUTER_ALERT,
.BI "               const\~int\~*" enable ,\~sizeof(int));
.fi
.SH DESCRIPTION
Pass all to-be forwarded packets with the
IP Router Alert option set to this socket.
.P
Valid only for raw sockets.
.P
This is useful, for instance, for user-space RSVP daemons.
.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 2.1.68
.SH CAVEATS
The tapped packets are not forwarded by the kernel;
it is the user's responsibility to send them out again.
.P
Socket binding is ignored,
such packets are filtered only by protocol.
.SH SEE ALSO
.BR IPPROTO_IP (2const),
.BR setsockopt (2),
.BR ip (7)