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
75 protocols than IP like e.g.\& AppleTalk.
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.
95 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.*
98 +------ the Internet ------+
101 Assuming router A has the (external) IP address A and the internal address
102 192.168.1.1, while router B has external address B and internal address
103 192.168.2.1, the following commands will configure the tunnel:
106 .Bd -literal -offset indent
108 ifconfig greN inet 192.168.1.1 192.168.2.1
109 ifconfig greN inet tunnel A B
110 route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1
114 .Bd -literal -offset indent
116 ifconfig greN inet 192.168.2.1 192.168.1.1
117 ifconfig greN inet tunnel B A
118 route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1
121 In case when internal and external IP addresses are the same,
122 different routing tables (FIB) should be used.
123 The default FIB will be applied to IP packets before GRE encapsulation.
124 After encapsulation GRE interface should set different FIB number to
126 Then different FIB will be applied to such encapsulated packets.
127 According to this FIB packet should be routed to tunnel endpoint.
129 Host X -- Host A (198.51.100.1) ---tunnel--- Cisco D (203.0.113.1) -- Host E
132 +----- Host B ----- Host C -----+
138 First of multiple FIBs should be configured via loader.conf:
139 .Bd -literal -offset indent
141 net.add_addr_allfibs=0
144 Then routes to the gateway and remote tunnel endpoint via this gateway
145 should be added to the second FIB:
146 .Bd -literal -offset indent
147 route add -net 198.51.100.0 -netmask 255.255.255.0 -fib 1 -iface em0
148 route add -host 203.0.113.1 -fib 1 198.51.100.254
151 And GRE tunnel should be configured to change FIB for encapsulated packets:
152 .Bd -literal -offset indent
154 ifconfig greN inet 198.51.100.1 203.0.113.1
155 ifconfig greN inet tunnel 198.51.100.1 203.0.113.1 tunnelfib 1
161 interfaces is set to 1476 by default, to match the value used by Cisco routers.
162 This may not be an optimal value, depending on the link between the two tunnel
164 It can be adjusted via
167 For correct operation, the
169 device needs a route to the decapsulating host that does not run over the tunnel,
170 as this would be a loop.
172 The kernel must be set to forward datagrams by setting the
173 .Va net.inet.ip.forwarding
175 variable to non-zero.
187 A description of GRE encapsulation can be found in RFC 2784 and RFC 2890.
189 .An Andrey V. Elsukov Aq Mt ae@FreeBSD.org
190 .An Heiko W.Rupp Aq Mt hwr@pilhuhn.de
192 The current implementation uses the key only for outgoing packets.
193 Incoming packets with a different key or without a key will be treated as if they
194 would belong to this interface.
196 The sequence number field also used only for outgoing packets.