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.\"     $OpenBSD: divert.4,v 1.20 2022/09/10 10:22:46 jsg Exp $
.\"
.\" Copyright (c) 2009 Michele Marchetto <michele@openbsd.org>
.\" Copyright (c) 2012-2014 Lawrence Teo <lteo@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: September 10 2022 $
.Dt DIVERT 4
.Os
.Sh NAME
.Nm divert
.Nd kernel packet diversion mechanism
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.Ft int
.Fn socket AF_INET SOCK_RAW IPPROTO_DIVERT
.Ft int
.Fn socket AF_INET6 SOCK_RAW IPPROTO_DIVERT
.Sh DESCRIPTION
Divert sockets are part of a mechanism completely integrated with
.Xr pf 4
that queues raw packets from the kernel stack to userspace applications,
and vice versa.
.Pp
A divert socket must be bound to a divert port through
.Xr bind 2 ,
which only the superuser can do.
Divert ports have their own number space, completely separated from
.Xr tcp 4
and
.Xr udp 4 .
When
.Xr pf 4
processes a packet that matches a rule with the
.Ar divert-packet
parameter
(see
.Xr pf.conf 5
for details), it is sent to the divert socket listening on the
divert port specified in the rule.
Note that
.Ar divert-packet
should not be confused with
.Ar divert-to
or
.Ar divert-reply ,
which do not use divert sockets.
If there are no divert sockets listening, the packets are dropped.
.Pp
Packets can be read via
.Xr read 2 ,
.Xr recv 2 ,
or
.Xr recvfrom 2
from the divert socket.
The application that is processing the packets can then reinject them into the
kernel.
With
.Xr recvfrom 2 ,
an interface IP address is passed if it is an inbound packet.
Outbound packets provide the unspecified address.
When reinjecting, use this address as argument to
.Xr sendto 2 .
This allows the kernel to guess the original incoming interface and
process it as an incoming packet.
If no interface IP address is given, the reinjected packet is treated
as an outgoing packet.
Since the userspace application could have modified the packets, upon
reinjection basic sanity checks are done to ensure that the packets are still
valid.
The packets' IPv4 and protocol checksums (TCP, UDP, ICMP, and ICMPv6) are also
recalculated.
.Pp
Writing to a divert socket can be achieved using
.Xr sendto 2
and it will skip
.Xr pf 4
filters to avoid loops.
Note that this means that a reinjected inbound packet will also not
run through the pf out rules after being forwarded.
A diverted packet that is not reinjected into the kernel stack is lost.
.Pp
Receive and send divert socket buffer space can be tuned through
.Xr sysctl 8 .
.Xr netstat 1
shows information relevant to divert sockets.
Note that the default is 64k and too short to handle full sized UDP
packets.
.Sh EXAMPLES
The following PF rule queues outbound IPv4 packets to TCP port 80,
as well as the return traffic, on the em0 interface to divert port 700:
.Bd -literal -offset indent
pass out on em0 inet proto tcp to port 80 divert-packet port 700
.Ed
.Pp
The following program reads packets on divert port 700 and reinjects them
back into the kernel.
This program does not perform any processing of the packets,
apart from discarding invalid IP packets.
.Bd -literal
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>
#include <err.h>

#define DIVERT_PORT 700

int
main(int argc, char *argv[])
{
	int fd, s;
	struct sockaddr_in sin;
	socklen_t sin_len;

	fd = socket(AF_INET, SOCK_RAW, IPPROTO_DIVERT);
	if (fd == -1)
		err(1, "socket");

	memset(&sin, 0, sizeof(sin));
	sin.sin_family = AF_INET;
	sin.sin_port = htons(DIVERT_PORT);
	sin.sin_addr.s_addr = 0;

	sin_len = sizeof(struct sockaddr_in);

	s = bind(fd, (struct sockaddr *) &sin, sin_len);
	if (s == -1)
		err(1, "bind");

	for (;;) {
		ssize_t n;
		char packet[IP_MAXPACKET];
		struct ip *ip;
		struct tcphdr *th;
		int hlen;
		char src[48], dst[48];

		memset(packet, 0, sizeof(packet));
		n = recvfrom(fd, packet, sizeof(packet), 0,
		    (struct sockaddr *) &sin, &sin_len);
		if (n == -1) {
			warn("recvfrom");
			continue;
		}
		if (n < sizeof(struct ip)) {
			warnx("packet is too short");
			continue;
		}

		ip = (struct ip *) packet;
		hlen = ip->ip_hl << 2;
		if (hlen < sizeof(struct ip) || ntohs(ip->ip_len) < hlen ||
		    n < ntohs(ip->ip_len)) {
			warnx("invalid IPv4 packet");
			continue;
		}

		th = (struct tcphdr *) (packet + hlen);

		if (inet_ntop(AF_INET, &ip->ip_src, src,
		    sizeof(src)) == NULL)
			(void)strlcpy(src, "?", sizeof(src));

		if (inet_ntop(AF_INET, &ip->ip_dst, dst,
		    sizeof(dst)) == NULL)
			(void)strlcpy(dst, "?", sizeof(dst));

		printf("%s:%u -> %s:%u\en",
		    src,
		    ntohs(th->th_sport),
		    dst,
		    ntohs(th->th_dport)
		);

		n = sendto(fd, packet, n, 0, (struct sockaddr *) &sin,
		    sin_len);
		if (n == -1)
			warn("sendto");
	}

	return 0;
}
.Ed
.Sh SEE ALSO
.Xr socket 2 ,
.Xr ip 4 ,
.Xr pf.conf 5
.Sh HISTORY
The
.Nm
protocol first appeared in
.Ox 4.7 .