$NetBSD: gre.4,v 1.10 1999/12/22 14:55:49 kleink Exp $
Copyright 1998 (c) The NetBSD Foundation, Inc.
All rights reserved.
This code is derived from software contributed to The NetBSD Foundation
by Heiko W. Rupp <hwr@pilhuhn.de>
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
.Dd $Mdocdate: August 28 2016 $ .Dt GRE 4 .Os .Sh NAME .Nm gre , .Nm mobileip .Nd encapsulating network device .Sh SYNOPSIS .Cd "pseudo-device gre" .Sh DESCRIPTION The .Nm driver allows tunnel construction using the Cisco GRE or the Mobile IP (RFC 2004) encapsulation protocols.
p .Tn GRE , .Tn WCCPv1 , and .Tn Mobile IP are enabled with the following .Xr sysctl 3 variables respectively in
a /etc/sysctl.conf : l -tag -width "net.inet.mobileip.allow" t Va net.inet.gre.allow Allow GRE packets in and out of the system. t Va net.inet.gre.wccp Set to 1 to allow WCCPv1-style GRE packets into the system; set to 2 to handle the packets as WCCPv2-style GRE, truncating the redirect header. This variable depends on .Va gre.allow being set. t Va net.inet.mobileip.allow Allow Mobile IP packets in and out of the system. .El
p This driver currently supports the following modes of operation: l -tag -width mobileipXXX t Nm gre GRE datagrams (IP protocol number 47) are prepended by an outer datagram and a GRE header. The GRE header specifies the type of the encapsulated datagram and thus allows for tunneling other protocols than IP like e.g. AppleTalk. GRE mode is the default tunnel mode on Cisco routers. This is also the default mode of operation of the .Nm interfaces. t Nm mobileip MOBILE datagrams (IP protocol number 55) are encapsulated into IP, but with a much smaller encapsulation header. This protocol only supports IP in IP encapsulation, and is intended for use with Mobile IP. .El
p A .Nm gre or .Nm mobileip interface can be created at runtime using the c ifconfig Ar gre Ns Ar N Ic create command or by setting up a .Xr hostname.if 5 configuration file for .Xr netstart 8 .
p Note that the IP addresses of the tunnel endpoints may be the same as the ones defined with .Xr ifconfig 8 for the interface (as if IP is encapsulated), but need not be, as e.g. when encapsulating AppleTalk. .Sh EXAMPLES Configuration example: d -literal Host X ---- Host A ------------ tunnel ------------ Cisco D ---- Host E \e / \e / +------ Host B ------ Host C ------+ .Ed
p On Host A (OpenBSD): d -literal -offset indent # route add default B # ifconfig greN create # ifconfig greN A D netmask 0xffffffff linkX up # ifconfig greN tunnel A D # route add E D .Ed
p On Host D (Cisco): d -literal -offset indent Interface TunnelX ip unnumbered D ! e.g. address from Ethernet interface tunnel source D ! e.g. address from Ethernet interface tunnel destination A ip route C <some interface and mask> ip route A mask C ip route X mask tunnelX .Ed
p OR
p On Host D (OpenBSD): d -literal -offset indent # route add default C # ifconfig greN create # ifconfig greN D A # ifconfig greN tunnel D A .Ed
p To reach Host A over the tunnel (from Host D), there has to be an alias on Host A for the Ethernet interface:
p .Dl # ifconfig <etherif> alias Y
p and on the Cisco:
p .Dl ip route Y mask tunnelX
p Keepalive packets may optionally be sent to the remote endpoint, which decapsulates and returns them, allowing tunnel failure to be detected. Enable them like this: d -literal -offset indent # ifconfig greN keepalive period count .Ed
p This will send a keepalive packet every .Ar period seconds. If no response is received in .Ar count * .Ar period seconds, the link is considered down. To return keepalives, the remote host must be configured to forward packets: d -literal -offset indent # sysctl net.inet.ip.forwarding=1 .Ed
p If .Xr pf 4 is enabled then it is necessary to add a pass rule specific for the keepalive packets. The rule must use .Em no state because the keepalive packet is entering the network stack multiple times. In most cases the following should work: d -literal -offset indent pass quick on gre proto gre no state .Ed .Sh NOTES The MTU of .Nm interfaces is set to 1476 by default to match the value used by Cisco routers. This may not be an optimal value, depending on the link between the two tunnel endpoints. It can be adjusted via .Xr ifconfig 8 .
p For correct operation, the .Nm device needs a route to the destination, that is less specific than the one over the tunnel. (There needs to be a route to the decapsulating host that does not run over the tunnel, as this would create a loop.)
p In order for .Xr ifconfig 8 to actually mark the interface as up, the keyword .Cm up must be given last on its command line.
p The kernel must be set to forward datagrams by issuing the appropriate option to .Xr sysctl 8 .
p The GRE interface will accept WCCPv1-style or WWCPv2-style GRE encapsulated packets from a Cisco router. Some magic with the packet filter configuration and a caching proxy like squid are needed to do anything useful with these packets. .Sh SEE ALSO .Xr inet 4 , .Xr ip 4 , .Xr netintro 4 , .Xr options 4 , .Xr hostname.if 5 , .Xr protocols 5 , .Xr ifconfig 8 , .Xr netstart 8 , .Xr sysctl 8 .Sh STANDARDS .Rs .%A S. Hanks .%A "T. Li" .%A D. Farinacci .%A P. Traina .%D October 1994 .%R RFC 1701 .%T Generic Routing Encapsulation (GRE) .Re
p .Rs .%A S. Hanks .%A "T. Li" .%A D. Farinacci .%A P. Traina .%D October 1994 .%R RFC 1702 .%T Generic Routing Encapsulation over IPv4 networks .Re
p .Rs .%A C. Perkins .%D October 1996 .%R RFC 2004 .%T Minimal Encapsulation within IP .Re
p .Rs .%U http://www.wrec.org/Drafts/draft-ietf-wrec-web-pro-00.txt .%T Web Cache Coordination Protocol V1.0 .Re
p .Rs .%U http://www.wrec.org/Drafts/draft-wilson-wrec-wccp-v2-00.txt .%T Web Cache Coordination Protocol V2.0 .Re .Sh AUTHORS .An Heiko W. Rupp Aq Mt hwr@pilhuhn.de .Sh BUGS GRE RFC not yet fully implemented (no GRE options).
p The redirect header for WCCPv2 GRE encapsulated packets is skipped.