GRE(4)                  NetBSD Kernel Interfaces Manual                 GRE(4)

NAME
     gre -- encapsulating network device

SYNOPSIS
     pseudo-device gre

DESCRIPTION
     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.

EXAMPLES
     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.

NOTES
     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).

SEE ALSO
     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.

AUTHORS
     Heiko W.Rupp <hwr@pilhuhn.de>
     David Young <dyoung@NetBSD.org> (GRE in UDP encapsulation, bug fixes)

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.

     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

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