1.\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $ 2.\" 3.\" Copyright 1998 (c) The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Heiko W.Rupp <hwr@pilhuhn.de> 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.\" $FreeBSD$ 31.\" 32.Dd June 2, 2015 33.Dt GRE 4 34.Os 35.Sh NAME 36.Nm gre 37.Nd encapsulating network device 38.Sh SYNOPSIS 39To compile the 40driver into the kernel, place the following line in the kernel 41configuration file: 42.Bd -ragged -offset indent 43.Cd "device gre" 44.Ed 45.Pp 46Alternatively, to load the 47driver as a module at boot time, place the following line in 48.Xr loader.conf 5 : 49.Bd -literal -offset indent 50if_gre_load="YES" 51.Ed 52.Sh DESCRIPTION 53The 54.Nm 55network interface pseudo device encapsulates datagrams 56into IP. 57These encapsulated datagrams are routed to a destination host, 58where they are decapsulated and further routed to their final destination. 59The 60.Dq tunnel 61appears to the inner datagrams as one hop. 62.Pp 63.Nm 64interfaces are dynamically created and destroyed with the 65.Xr ifconfig 8 66.Cm create 67and 68.Cm destroy 69subcommands. 70.Pp 71This driver corresponds to RFC 2784. 72Encapsulated datagrams are prepended an outer datagram and a GRE header. 73The GRE header specifies 74the type of the encapsulated datagram and thus allows for tunneling other 75protocols than IP. 76GRE mode is also the default tunnel mode on Cisco routers. 77.Nm 78also supports Cisco WCCP protocol, both version 1 and version 2. 79.Pp 80The 81.Nm 82interfaces support a number of additional parameters to the 83.Xr ifconfig 8 : 84.Bl -tag -width "enable_csum" 85.It Ar grekey 86Set the GRE key used for outgoing packets. 87A value of 0 disables the key option. 88.It Ar enable_csum 89Enables checksum calculation for outgoing packets. 90.It Ar enable_seq 91Enables use of sequence number field in the GRE header for outgoing packets. 92.El 93.Sh EXAMPLES 94.Bd -literal 95192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* 96 \\ / 97 \\ / 98 +------ the Internet ------+ 99.Ed 100.Pp 101Assuming router A has the (external) IP address A and the internal address 102192.168.1.1, while router B has external address B and internal address 103192.168.2.1, the following commands will configure the tunnel: 104.Pp 105On router A: 106.Bd -literal -offset indent 107ifconfig greN create 108ifconfig greN inet 192.168.1.1 192.168.2.1 109ifconfig greN inet tunnel A B 110route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 111.Ed 112.Pp 113On router B: 114.Bd -literal -offset indent 115ifconfig greN create 116ifconfig greN inet 192.168.2.1 192.168.1.1 117ifconfig greN inet tunnel B A 118route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 119.Ed 120.Pp 121In case when internal and external IP addresses are the same, 122different routing tables (FIB) should be used. 123The default FIB will be applied to IP packets before GRE encapsulation. 124After encapsulation GRE interface should set different FIB number to 125outgoing packet. 126Then different FIB will be applied to such encapsulated packets. 127According to this FIB packet should be routed to tunnel endpoint. 128.Bd -literal 129Host X -- Host A (198.51.100.1) ---tunnel--- Cisco D (203.0.113.1) -- Host E 130 \\ / 131 \\ / 132 +----- Host B ----- Host C -----+ 133 (198.51.100.254) 134.Ed 135.Pp 136On Host A (FreeBSD): 137.Pp 138First of multiple FIBs should be configured via loader.conf: 139.Bd -literal -offset indent 140net.fibs=2 141net.add_addr_allfibs=0 142.Ed 143.Pp 144Then routes to the gateway and remote tunnel endpoint via this gateway 145should be added to the second FIB: 146.Bd -literal -offset indent 147route add -net 198.51.100.0 -netmask 255.255.255.0 -fib 1 -iface em0 148route add -host 203.0.113.1 -fib 1 198.51.100.254 149.Ed 150.Pp 151And GRE tunnel should be configured to change FIB for encapsulated packets: 152.Bd -literal -offset indent 153ifconfig greN create 154ifconfig greN inet 198.51.100.1 203.0.113.1 155ifconfig greN inet tunnel 198.51.100.1 203.0.113.1 tunnelfib 1 156.Ed 157.Sh NOTES 158The MTU of 159.Nm 160interfaces is set to 1476 by default, to match the value used by Cisco routers. 161This may not be an optimal value, depending on the link between the two tunnel 162endpoints. 163It can be adjusted via 164.Xr ifconfig 8 . 165.Pp 166For correct operation, the 167.Nm 168device needs a route to the decapsulating host that does not run over the tunnel, 169as this would be a loop. 170.Pp 171The kernel must be set to forward datagrams by setting the 172.Va net.inet.ip.forwarding 173.Xr sysctl 8 174variable to non-zero. 175.Sh SEE ALSO 176.Xr gif 4 , 177.Xr inet 4 , 178.Xr ip 4 , 179.Xr me 4 , 180.Xr netintro 4 , 181.Xr protocols 5 , 182.Xr ifconfig 8 , 183.Xr sysctl 8 184.Pp 185A description of GRE encapsulation can be found in RFC 2784 and RFC 2890. 186.Sh AUTHORS 187.An Andrey V. Elsukov Aq Mt ae@FreeBSD.org 188.An Heiko W.Rupp Aq Mt hwr@pilhuhn.de 189.Sh BUGS 190The current implementation uses the key only for outgoing packets. 191Incoming packets with a different key or without a key will be treated as if they 192would belong to this interface. 193.Pp 194The sequence number field also used only for outgoing packets. 195