cb9cbce235
PR: 228465 Submitted by: Sergey Akhmatov <sergey at akhmatov dot ru> Reviewed by: bcr Approved by: bcr MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D26097
278 lines
8.4 KiB
Groff
278 lines
8.4 KiB
Groff
.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
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.\"
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.\" Copyright 1998 (c) The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Heiko W.Rupp <hwr@pilhuhn.de>
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd August 21, 2020
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.Dt GRE 4
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.Os
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.Sh NAME
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.Nm gre
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.Nd encapsulating network device
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.Sh SYNOPSIS
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To compile the
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driver into the kernel, place the following line in the kernel
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configuration file:
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.Bd -ragged -offset indent
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.Cd "device gre"
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.Ed
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.Pp
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Alternatively, to load the
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driver as a module at boot time, place the following line in
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.Xr loader.conf 5 :
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.Bd -literal -offset indent
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if_gre_load="YES"
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.Ed
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.Sh DESCRIPTION
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The
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.Nm
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network interface pseudo device encapsulates datagrams
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into IP.
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These encapsulated datagrams are routed to a destination host,
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where they are decapsulated and further routed to their final destination.
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The
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.Dq tunnel
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appears to the inner datagrams as one hop.
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.Pp
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.Nm
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interfaces are dynamically created and destroyed with the
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.Xr ifconfig 8
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.Cm create
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and
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.Cm destroy
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subcommands.
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.Pp
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This driver corresponds to RFC 2784.
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Encapsulated datagrams are prepended an outer datagram and a GRE header.
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The GRE header specifies
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the type of the encapsulated datagram and thus allows for tunneling other
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protocols than IP.
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GRE mode is also the default tunnel mode on Cisco routers.
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.Nm
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also supports Cisco WCCP protocol, both version 1 and version 2.
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.Pp
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The
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.Nm
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interfaces support a number of additional parameters to the
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.Xr ifconfig 8 :
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.Bl -tag -width "enable_csum"
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.It Ar grekey
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Set the GRE key used for outgoing packets.
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A value of 0 disables the key option.
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.It Ar enable_csum
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Enables checksum calculation for outgoing packets.
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.It Ar enable_seq
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Enables use of sequence number field in the GRE header for outgoing packets.
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.It Ar udpencap
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Enables UDP-in-GRE encapsulation (see the
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.Sx GRE-IN-UDP ENCAPSULATION
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Section below for details).
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.It Ar udpport
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Set the source UDP port for outgoing packets.
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A value of 0 disables the persistence of source UDP port for outgoing packets.
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See the
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.Sx GRE-IN-UDP ENCAPSULATION
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Section below for details.
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.El
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.Sh GRE-IN-UDP ENCAPSULATION
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The
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.Nm
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supports GRE in UDP encapsulation as defined in RFC 8086.
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A GRE in UDP tunnel offers the possibility of better performance for
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load-balancing GRE traffic in transit networks.
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Encapsulating GRE in UDP enables use of the UDP source port to provide
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entropy to ECMP hashing.
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.Pp
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The GRE in UDP tunnel uses single value 4754 as UDP destination port.
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The UDP source port contains a 14-bit entropy value that is generated
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by the encapsulator to identify a flow for the encapsulated packet.
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The
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.Ar udpport
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option can be used to disable this behaviour and use single source UDP
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port value.
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The value of
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.Ar udpport
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should be within the ephemeral port range, i.e., 49152 to 65535 by default.
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.Pp
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Note that a GRE in UDP tunnel is unidirectional; the tunnel traffic is not
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expected to be returned back to the UDP source port values used to generate
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entropy.
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This may impact NAPT (Network Address Port Translator) middleboxes.
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If such tunnels are expected to be used on a path with a middlebox,
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the tunnel can be configured either to disable use of the UDP source port
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for entropy or to enable middleboxes to pass packets with UDP source port
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entropy.
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.Sh EXAMPLES
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.Bd -literal
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192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
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\\ /
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\\ /
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+------ the Internet ------+
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.Ed
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.Pp
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Assuming router A has the (external) IP address A and the internal address
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192.168.1.1, while router B has external address B and internal address
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192.168.2.1, the following commands will configure the tunnel:
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.Pp
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On router A:
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.Bd -literal -offset indent
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ifconfig greN create
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ifconfig greN inet 192.168.1.1 192.168.2.1
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ifconfig greN inet tunnel A B
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route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
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.Ed
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.Pp
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On router B:
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.Bd -literal -offset indent
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ifconfig greN create
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ifconfig greN inet 192.168.2.1 192.168.1.1
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ifconfig greN inet tunnel B A
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route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
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.Ed
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.Pp
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In case when internal and external IP addresses are the same,
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different routing tables (FIB) should be used.
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The default FIB will be applied to IP packets before GRE encapsulation.
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After encapsulation GRE interface should set different FIB number to
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outgoing packet.
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Then different FIB will be applied to such encapsulated packets.
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According to this FIB packet should be routed to tunnel endpoint.
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.Bd -literal
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Host X -- Host A (198.51.100.1) ---tunnel--- Cisco D (203.0.113.1) -- Host E
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\\ /
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\\ /
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+----- Host B ----- Host C -----+
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(198.51.100.254)
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.Ed
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.Pp
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On Host A (FreeBSD):
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.Pp
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First of multiple FIBs should be configured via loader.conf:
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.Bd -literal -offset indent
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net.fibs=2
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net.add_addr_allfibs=0
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.Ed
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.Pp
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Then routes to the gateway and remote tunnel endpoint via this gateway
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should be added to the second FIB:
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.Bd -literal -offset indent
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route add -net 198.51.100.0 -netmask 255.255.255.0 -fib 1 -iface em0
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route add -host 203.0.113.1 -fib 1 198.51.100.254
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.Ed
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.Pp
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And GRE tunnel should be configured to change FIB for encapsulated packets:
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.Bd -literal -offset indent
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ifconfig greN create
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ifconfig greN inet 198.51.100.1 203.0.113.1
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ifconfig greN inet tunnel 198.51.100.1 203.0.113.1 tunnelfib 1
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.Ed
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.Sh NOTES
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The MTU of
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.Nm
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interfaces is set to 1476 by default, to match the value used by Cisco routers.
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This may not be an optimal value, depending on the link between the two tunnel
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endpoints.
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It can be adjusted via
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.Xr ifconfig 8 .
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.Pp
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For correct operation, the
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.Nm
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device needs a route to the decapsulating host that does not run over the tunnel,
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as this would be a loop.
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.Pp
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The kernel must be set to forward datagrams by setting the
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.Va net.inet.ip.forwarding
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.Xr sysctl 8
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variable to non-zero.
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.Pp
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By default,
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.Nm
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tunnels may not be nested.
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This behavior may be modified at runtime by setting the
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.Xr sysctl 8
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variable
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.Va net.link.gre.max_nesting
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to the desired level of nesting.
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.Sh SEE ALSO
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.Xr gif 4 ,
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.Xr inet 4 ,
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.Xr ip 4 ,
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.Xr me 4 ,
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.Xr netintro 4 ,
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.Xr protocols 5 ,
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.Xr ifconfig 8 ,
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.Xr sysctl 8
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.Sh STANDARDS
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.Rs
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.%A S. Hanks
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.%A "T. Li"
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.%A D. Farinacci
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.%A P. Traina
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.%D October 1994
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.%R RFC 1701
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.%T Generic Routing Encapsulation (GRE)
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.Re
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.Pp
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.Rs
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.%A S. Hanks
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.%A "T. Li"
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.%A D. Farinacci
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.%A P. Traina
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.%D October 1994
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.%R RFC 1702
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.%T Generic Routing Encapsulation over IPv4 networks
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.Re
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.Pp
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.Rs
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.%A D. Farinacci
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.%A "T. Li"
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.%A S. Hanks
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.%A D. Meyer
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.%A P. Traina
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.%D March 2000
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.%R RFC 2784
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.%T Generic Routing Encapsulation (GRE)
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.Re
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.Pp
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.Rs
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.%A G. Dommety
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.%D September 2000
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.%R RFC 2890
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.%T Key and Sequence Number Extensions to GRE
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.Re
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.Sh AUTHORS
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.An Andrey V. Elsukov Aq Mt ae@FreeBSD.org
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.An Heiko W.Rupp Aq Mt hwr@pilhuhn.de
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.Sh BUGS
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The current implementation uses the key only for outgoing packets.
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Incoming packets with a different key or without a key will be treated as if they
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would belong to this interface.
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.Pp
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The sequence number field also used only for outgoing packets.
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