gre(4) - NetBSD Manual Pages

Command:
Section:
Arch:
Collection:
>>>
GRE(4)                    NetBSD Programmer's 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 by a 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 (not yet supported). GRE mode is also the default tunnel
          mode on Cisco routers. This is also the default mode of operation of
          the greX interfaces.

     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 network interfaces are named gre0, gre1 and so on, as many as have
     given on the pseudo-device line in the system config file. Each interface
     supports a number of ioctl(2)s, such as:

     GRESADDRS:
          Set the IP address of the local tunnel end.

     GRESADDRD:
          Set the IP address of the remote tunnel end.

     GREGADDRS:
          Query the IP address that is set for the local tunnel end.

     GREGADDRD:
          Query the IP address that is set for the remote tunnel end.

     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
               IPPROTO_GRE

          link2
               IPPROTO_MOBILE
          to ifconfig.  As the linkN flags are not mutually exclusive, modes
          must be set by applying positive and negative flags as e.g.
          ifconfig link0 -link1 -link2

     GREGPROTO:
          Query operation mode.

     Note that the IP addresses of the tunnel endpoints may be the same as the
     ones defined with ifconfig for the interface (as if IP is encapsulated),
     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
        # greconfig -i greN -s A -d 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

     If all goes well, you should see packets flowing ;-)

     If you want to reach Host A over the tunnel (from the Cisco D), 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


NOTES

     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 ..)

     In order to ifconfig 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),
     greconfig(8), ifconfig(8), sysctl(8)

     A description of GRE encapsulation can be found in RFC 1701, RFC 1702.

     A description of MOBILE encapsulation can be found in RFC 2004.


AUTHORS

     Heiko W.Rupp <hwr@pilhuhn.de>


BUGS

     The compute_route() code in if_gre.c toggles the last bit of the IP-ad-
     dress to provoke the search for a less specific route than the one di-
     rectly 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 command line.  This implies that the GRE packet destination
     (set via greconfig -d) and the ifconfig remote host are not the same IP
     addresses, and that the GRE destination does not route over the greX in-
     terface itself.

     GRE RFC not yet fully implemented (no GRE options), no other protocols
     yet than IP over IP.

     BPF does probably not yet work (it might, but last time I looked, it
     bombed, so I #if 0'd it out).

NetBSD 1.6.2                  September 13, 1998                             3