GRE(4) NetBSD Kernel Interfaces Manual GRE(4)
gre -- encapsulating network device
The gre network interface pseudo device encapsulates datagrams into IP. These encapsulated datagrams are routed to a destination host, where they are decapsulated and further routed to their final destination. The ``tunnel'' appears to the inner datagrams as one hop. gre interfaces are dynamically created and destroyed with the ifconfig(8) create and destroy subcommands. This driver currently supports the following modes of operation: GRE encapsulation (IP protocol number 47) Encapsulated datagrams are prepended an outer datagram and a GRE header. The GRE header specifies the type of the encapsulated data- gram and thus allows for tunneling other protocols than IP like e.g. AppleTalk. GRE mode is also the default tunnel mode on Cisco routers. This is also the default mode of operation of the greX interfaces. GRE in UDP encapsulation Encapsulated datagrams are prepended a GRE header, and then they are sent over a UDP socket. Userland may create the socket and ``delegate'' it to the kernel using the GRESSOCK ioctl(2). If user- land does not supply a socket, then the kernel will create one using the addresses and ports supplied by ioctl(2)s SIOCSLIFPHYADDR, GRESADDRD, and/or GRESADDRS. MOBILE encapsulation (IP protocol number 55) Datagrams are encapsulated into IP, but with a shorter encapsula- tion. The original IP header is modified and the modifications are inserted between the so modified header and the original payload. Like gif(4), only for IP in IP encapsulation. The greX interfaces support a number of ioctl(2)s, such as: GRESADDRS: Set the IP address of the local tunnel end. This is the source address set by or displayed by ifconfig for the greX interface. GRESADDRD: Set the IP address of the remote tunnel end. This is the destina- tion address set by or displayed by ifconfig for the greX interface. GREGADDRS: Query the IP address that is set for the local tunnel end. This is the address the encapsulation header carries as local address (i.e. the real address of the tunnel start point.) GREGADDRD: Query the IP address that is set for the remote tunnel end. This is the address the encapsulated packets are sent to (i.e. the real address of the remote tunnel endpoint.) GRESPROTO: Set the operation mode to the specified IP protocol value. The pro- tocol is passed to the interface in (struct ifreq)->ifr_flags. The operation mode can also be given as link0 link2 IPPROTO_UDP link0 -link2 IPPROTO_GRE -link0 -link2 IPPROTO_MOBILE to ifconfig(8). The link1 flag is not used to choose encapsulation, but to modify the internal route search for the remote tunnel endpoint, see the BUGS section below. GREGPROTO: Query operation mode. GRESSOCK: Delegate a socket from userland to a tunnel interface in UDP encap- sulation mode. The file descriptor for the socket is passed in (struct ifreq)->ifr_value. Note that the IP addresses of the tunnel endpoints may be the same as the ones defined with ifconfig(8) for the interface (as if IP is encapsu- lated), but need not be, as e.g. when encapsulating AppleTalk.
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 create # ifconfig greN A D netmask 0xffffffff linkX up # ifconfig greN tunnel A 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 create # ifconfig greN D A # ifconfig tunnel greN D A If all goes well, you should see packets flowing ;-) If you want to reach Host A over the tunnel (from Host D (Cisco)), 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 A similar setup can be used to create a link between two private networks (for example in the 192.168 subnet) over the Internet: 192.168.1.* --- Router A -------tunnel-------- Router B --- 192.168.2.* \ / \ / +----- the Internet ------+ Assuming router A has the (external) IP address A and the internal address 192.168.1.1, while router B has external address B and internal address 192.168.2.1, the following commands will configure the tunnel: On router A: # ifconfig greN create # ifconfig greN 192.168.1.1 192.168.2.1 link1 # ifconfig greN tunnel A B # route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 On router B: # ifconfig greN create # ifconfig greN 192.168.2.1 192.168.1.1 link1 # ifconfig greN tunnel B A # route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 Note that this is a safe situation where the link1 flag (as discussed in the BUGS section below) may (and probably should) be set. To setup the same tunnel as above, but using GRE in UDP encapsulation instead of GRE encapsulation, set flags link0 and link2, and specify source and destination UDP ports. On router A: # ifconfig greN create # ifconfig greN link0 link2 # ifconfig greN 192.168.1.1 192.168.2.1 # ifconfig greN tunnel A,port-A B,port-B # route add -net 192.168.2 -netmask 255.255.255.0 192.168.2.1 On router B: # ifconfig greN create # ifconfig greN link0 link2 # ifconfig greN 192.168.2.1 192.168.1.1 # ifconfig greN tunnel B,port-B A,port-A # route add -net 192.168.1 -netmask 255.255.255.0 192.168.1.1 Along these lines, you can use GRE tunnels to interconnect two IPv6 networks over an IPv4 infrastructure, or to hook up to the IPv6 internet via an IPv4 tunnel to a Cisco router. 2001:db8:1::/64 -- NetBSD A -----tunnel----- Cisco B --- IPv6 Internet \ / \ / +----- the Internet ------+ The example will use the following addressing: NetBSD A has the IPv4 address A and the IPv6 address 2001:db8:1::1 (connects to internal network 2001:db8:1::/64). Cisco B has external IPv4 address B. All the IPv6 internet world is behind B, so A wants to route 0::0/0 (the IPv6 default route) into the tunnel. The GRE tunnel will use a transit network: 2001:db8:ffff::1/64 on the ,Nx side, and ::2/64 on the Cisco side. Then the following commands will configure the tunnel: On router A (NetBSD): # ifconfig greN create # ifconfig greN inet6 2001:db8:ffff::1/64 # ifconfig greN tunnel A B # route add -inet6 2001:db8:ffff::/64 2001:db8:ffff::2 -ifp greN # route add -inet6 0::0/0 2001:db8:ffff::2 -ifp greN On router B (Cisco): Interface TunnelX tunnel mode gre ip ipv6 address 2001:db8:ffff::2/64 ! transfer network tunnel source B ! e.g. address from LAN interface tunnel destination A ! where the tunnel is connected to ipv6 route 2001:db8::/64 TunnelX ! route this network through tunnel Note that this is a safe situation where the link1 flag (as discussed in the BUGS section below) may be set.
The MTU of greX 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 ifconfig(8). For correct operation, the gre 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. This is not relevant for IPv6-over-IPv4 tunnels, of course.) If the addresses are ambiguous, doing the ifconfig(8) tunnel step before the ifconfig(8) call to set the greX IP addresses will help to find a route outside the tunnel. In order to tell ifconfig(8) to actually mark the interface as up, the keyword ``up'' must be given last on its command line. The kernel must be set to forward datagrams by either option GATEWAY in the kernel config file or by issuing the appropriate option to sysctl(8).
atalk(4), gif(4), inet(4), ip(4), netintro(4), options(4), protocols(5), ifconfig(8), sysctl(8) A description of GRE encapsulation can be found in RFC 1701 and RFC 1702. A description of MOBILE encapsulation can be found in RFC 2004.
Heiko W.Rupp <firstname.lastname@example.org> David Young <dyoung@NetBSD.org> (GRE in UDP encapsulation, bug fixes)
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. To avoid the address munging described above, turn on the link1 flag on the ifconfig(8) command line. This implies that the GRE packet destina- tion and the ifconfig remote host are not the same IP addresses, and that the GRE destination does not route over the greX interface itself. The GRE RFCs are not yet fully implemented (no GRE options). The MOBILE encapsulation appears to have been broken since it was first added to NetBSD, until August 2006. It is known to interoperate with another gre(4) in MOBILE mode, however, it has not been tested for inter- operability with any other implementation of RFC 2004. The NetBSD base system does not (yet) contain a daemon for automatically establishing a UDP tunnel between a host behind a NAT router and a host on the Internet. NetBSD 4.0 August 30, 2006 NetBSD 4.0
You can also request any man page by name and (optionally) by section: