Copyright 1998 (c) The NetBSD Foundation, Inc.
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This code is derived from software contributed to The NetBSD Foundation
by Heiko W.Rupp <hwr@pilhuhn.de>
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.Dd 13 September 1998 .Dt GRE 4 .Os NetBSD .Sh NAME .Nm gre .Nd encapsulating network device .Sh SYNOPSIS .Cd pseudo-device gre Op Ar count .Sh DESCRIPTION The .Nm gre network interface is a pseudo device that allows to encapsulate datagrams into IP. These encapsulated datagrams are routed to a destination host, where they are decapsulated and further routed to their final destination. The so called ``tunnel'' appears to the inner datagrams like one hop.
p This driver currently supports two modes of operation: l -tag -width abc t IPIP encapsulation (IP protocol number 4). Here packets are directly encapsulated into IP packets and sent to the tunnel destionation. This mode allows only for IP in IP tunneling. t GRE encapsulation (IP protocol number 47). Encapsulated datagrams are prepended by a 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 (not yet supported). GRE mode is also the default tunnel mode on Cisco routers. .El
p GRE mode is the default mode of operation.
p The network interfaces are named .Sy gre Ns Ar 0 , .Sy gre Ns Ar 1 and so on, as many as have given on the .Sy pseudo-device line in the system config file. Each interface supports a number of .Xr ioctl 2 Ns s, such as: l -tag -width aaa t GRESADDRS: Set the IP address of the local tunnel end. t GRESADDRD: Set the IP address of the remote tunnel end. t GREGADDRS: Query the the IP address that is set for the local tunnel end. t GREGADDRD: Query the the IP address that is set for the remote tunnel end. t GRESPROTO: Set the operation mode. The operation mode can (in a limited manner) also be given as ``-link0'' (gre) or ``link0'' (ipip) flag on the command line of .Xr ifconfig . t GREGPROTO: Query operation mode. .El
p Note that the IP adresses of the tunnel endpoints may be the same as the ones defined with .Xr ifconfig for the interface (as if IP is encapsulated), but need not be, as e.g. when encapsulating AppleTalk.
p .Sh EXAMPLE Configuration example: Host X-- Host A ----------------tunnel---------- cisco D------Host E \ | \ / +------Host B----------Host C----------+ On host A (NetBSD): # route add default B # ifconfig greN A D # greconfig -i greN -s A -d D # route add E D On Host D (Cisco): 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 OR On Host D (NetBSD): # route add default C # ifconfig greN D A
p If all goes well, you should see packets flowing ;-)
p If you want to reach Host A over the tunnel (from the Cisco D), then you have to have an alias on Host A for e.g. the ethernet interface like: ifconfig <etherif> alias Y and on the cisco ip route Y mask tunnelX .Sh NOTE For correct operation, the .Nm device needs a route to the destination, that is less specific than the one over the tunnel. (Basically, there needs to be a route to the decapsulating host that does not run over the tunnel, as this would be a loop ..) .Sh SEE ALSO .Xr netintro 4 , .Xr ip 4 , .Xr atalk 4 , .Xr inet 4 , .Xr ifconfig 8 , .Xr greconfig 8 , .Xr protocols 5
p A description of GRE encapsulation can be found in RFC 1701, RFC 1702.
p A description of IPIP encapsulation can be found in RFC 2003. .Sh BUGS The compute_route() code in if_gre.c toggles the last bit of the IP-address to provoke the search for a less specific route than the one directly over the tunnel to prevent loops. This is possibly not the best solution.
p GRE RFC not yet fully implemented (no GRE options), no other protocols yet than IP over IP. RFC 2003 also not yet fully supportet wrt. options etc.
p Code quality is surely not best and possibly not near anything like KNF
p Traceroute does not work yet over the tunnel :(
p BPF does probably not yet work (it might, but last time I looked, it bombed, so I #if 0'd it out).
p .Sh AUTHOR Heiko W.Rupp <hwr@pilhuhn.de>