$NetBSD: gre.4,v 1.10 1999/12/22 14:55:49 kleink Exp $
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 .Sh NAME .Nm gre .Nd encapsulating network device .Sh SYNOPSIS .Cd pseudo-device gre Op Ar count .Sh NOTE .Tn GRE and .Tn MobileIP are enabled with the following .Xr sysctl 3 variables respectively in
a /etc/sysctl.conf : l -tag -width xxxxxxxxxxxxxxxxxxxxx t net.inet.gre.allow Allow GRE packets in and out of the system. t net.inet.mobileip.allow Allow MobileIP packets in and out of the system. .El
p .Sh DESCRIPTION The .Nm gre network interface allows tunnel construction using the Cisco GRE or the Mobile-IP (RFC 2004) encapsulation protocols.
p This driver currently supports the following modes of operation: l -tag -width abc 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 the default tunnel mode on Cisco routers. This is also the default mode of operation of the .Sy gre Ns Ar X interfaces. t MOBILE encapsulation (IP protocol number 55). Datagrams 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 The network interfaces are named .Sy gre Ns Ar 0 , .Sy gre Ns Ar 1 , etc. The number of interfaces is given by the corresponding .Sy pseudo-device line in the system config file. .Xr gre 4 interfaces support the following .Xr ioctl 2 Ns s : 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 IP address that is set for the local tunnel end. t GREGADDRD: Query the IP address that is set for the remote tunnel end. t GRESPROTO: Set the operation mode to the specified IP protocol value. The protocol is passed to the interface in (struct ifreq)->ifr_flags. The operation mode can also be given as l -tag -width bbb t link0 IPPROTO_GRE t link2 IPPROTO_MOBILE .El
p to .Xr ifconfig 8 . As the linkN flags are not mutually exclusive, modes must be set by applying positive and negative flags, e.g., .Xr ifconfig 8 link0 -link1 -link2 t GREGPROTO: Query operation mode. .El
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.
p .Sh EXAMPLE Configuration example: d -literal Host X-- Host A ----------------tunnel---------- cisco D------Host E \ | \ / +------Host B----------Host C----------+ .Ed On host A (OpenBSD): # route add default B # ifconfig greN A D netmask 0xffffffff linkX up # 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 (OpenBSD): # route add default C # ifconfig greN D A
p To reach Host A over the tunnel (from host D), there has to be an alias on Host A for the ethernet interface: 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. (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 ``up'' must be given last on its command line.
p The kernel must be set to forward datagrams by including option ``GATEWAY'' in the kernel config file and issuing the appropriate option to .Xr sysctl 8 . .Sh SEE ALSO .Xr netintro 4 , .Xr ip 4 , .Xr atalk 4 , .Xr inet 4 , .Xr ifconfig 8 , .Xr options 4 , .Xr protocols 5 , .Xr sysctl 8
p A description of GRE encapsulation can be found in RFC 1701, RFC 1702.
p A description of MOBILE encapsulation can be found in RFC 2004. .Sh BUGS The compute_route() code in net/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 To avoid the address munging described above, turn on the link1 flag on the ifconfig command line. This implies that the GRE packet destination and the remote host are not the same IP addresses, and that the GRE destination does not route over the greX interface itself.
p GRE RFC not yet fully implemented (no GRE options), no other protocols yet than IP over IP.
p .Sh AUTHOR Heiko W.Rupp <hwr@pilhuhn.de>