1 .\" $NetBSD: gre.4,v 1.28 2002/06/10 02:49:35 itojun Exp $
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6 .\" This code is derived from software contributed to The NetBSD Foundation
7 .\" by Heiko W.Rupp <hwr@pilhuhn.de>
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37 .Nd encapsulating network device
40 driver into the kernel, place the following line in the kernel
42 .Bd -ragged -offset indent
46 Alternatively, to load the
47 driver as a module at boot time, place the following line in
49 .Bd -literal -offset indent
55 network interface pseudo device encapsulates datagrams
57 These encapsulated datagrams are routed to a destination host,
58 where they are decapsulated and further routed to their final destination.
61 appears to the inner datagrams as one hop.
64 interfaces are dynamically created and destroyed with the
71 This driver corresponds to RFC 2784.
72 Encapsulated datagrams are prepended an outer datagram and a GRE header.
73 The GRE header specifies
74 the type of the encapsulated datagram and thus allows for tunneling other
76 GRE mode is also the default tunnel mode on Cisco routers.
78 also supports Cisco WCCP protocol, both version 1 and version 2.
82 interfaces support a number of additional parameters to the
84 .Bl -tag -width "enable_csum"
86 Set the GRE key used for outgoing packets.
87 A value of 0 disables the key option.
89 Enables checksum calculation for outgoing packets.
91 Enables use of sequence number field in the GRE header for outgoing packets.
96 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
99 +------ the Internet ------+
102 Assuming router A has the (external) IP address A and the internal address
103 192.168.1.1, while router B has external address B and internal address
104 192.168.2.1, the following commands will configure the tunnel:
107 .Bd -literal -offset indent
109 ifconfig greN inet 192.168.1.1 192.168.2.1
110 ifconfig greN inet tunnel A B
111 route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
115 .Bd -literal -offset indent
117 ifconfig greN inet 192.168.2.1 192.168.1.1
118 ifconfig greN inet tunnel B A
119 route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
124 interfaces is set to 1476 by default, to match the value used by Cisco routers.
125 This may not be an optimal value, depending on the link between the two tunnel
127 It can be adjusted via
130 For correct operation, the
132 device needs a route to the decapsulating host that does not run over the tunnel,
133 as this would be a loop.
135 The kernel must be set to forward datagrams by setting the
136 .Va net.inet.ip.forwarding
138 variable to non-zero.
149 A description of GRE encapsulation can be found in RFC 2784 and RFC 2890.
151 .An Andrey V. Elsukov Aq Mt ae@FreeBSD.org
152 .An Heiko W.Rupp Aq Mt hwr@pilhuhn.de
155 The current implementation uses the key only for outgoing packets.
156 Incoming packets with a different key or without a key will be treated as if they
157 would belong to this interface.
159 The sequence number field also used only for outgoing packets.