MirOS Manual: gre(4)

GRE(4)                     BSD Programmer's Manual                      GRE(4)


     gre - encapsulating network device


     pseudo-device gre [count]


     GRE, WCCPv1, and MobileIP are enabled with the following sysctl(3) vari-
     ables respectively in /etc/sysctl.conf:

     net.inet.gre.allow       Allow GRE packets in and out of the system.

     net.inet.gre.wccp        Allow WCCPv1-style GRE packets into the system
                              (depends on the above).

     net.inet.mobileip.allow  Allow MobileIP packets in and out of the system.


     The gre network interface allows tunnel construction using the Cisco GRE
     or the Mobile-IP (RFC 2004) encapsulation protocols.

     A gre interface can be created at runtime using the ifconfig greN create
     command or by setting up a hostname.if(5) configuration file for

     This driver currently supports the following modes of operation:

     GRE encapsulation (IP protocol number 47).
          Encapsulated datagrams are prepended by an outer datagram and a GRE
          header. The GRE header specifies the type of the encapsulated da-
          tagram and thus allows for tunneling other protocols than IP like
          e.g. AppleTalk. GRE mode is the default tunnel mode on Cisco
          routers. This is also the default mode of operation of the greX in-

     MOBILE encapsulation (IP protocol number 55).
          Datagrams are encapsulated into IP, but with a much smaller encapsu-
          lation header. This protocol only supports IP in IP encapsulation,
          and is intended for use with mobile IP.

     The network interfaces are named gre0, gre1, etc. The number of inter-
     faces is given by the corresponding pseudo-device line in the system con-
     fig file. gre interfaces support the following ioctl(2)s:

          Set the IP address of the local tunnel end.

          Set the IP address of the remote tunnel end.

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

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

          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



          Query operation mode.

     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 encapsulat-
     ed), 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 (OpenBSD):

           # 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


     On Host D (OpenBSD):

           # route add default C
           # ifconfig greN create
           # ifconfig greN D A
           # ifconfig greN tunnel D A

     To reach Host A over the tunnel (from host D), there has to be an alias
     on Host A for the Ethernet interface:
           ifconfig <etherif> alias Y
     and on the Cisco
           ip route Y mask tunnelX


     For correct operation, the gre device needs a route to the destination,
     that is less specific than the one over the tunnel. (There needs to be a
     route to the decapsulating host that does not run over the tunnel, as
     this would create a loop.)

     In order for ifconfig(8) to actually mark the interface as up, the key-
     word ``up'' must be given last on its command line.

     The kernel must be set to forward datagrams by issuing the appropriate
     option to sysctl(8).

     The GRE interface will accept WCCPv1-style GRE encapsulated packets from
     a Cisco router. Some magic with the packet filter configuration and a
     caching proxy like squid are needed to do anything useful with these


     atalk(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, RFC 1702.

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

     A description of WCCPv1 can be found in draft-ietf-wrec-web-pro-00.txt,
     and WCCPv2 in draft-wilson-wrec-wccp-v2-00.txt. Both of these documents
     can be found at http://www.wrec.org/ (at the time of this writing).


     Heiko W.Rupp <hwr@pilhuhn.de>


     The compute_route() code in net/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

     To avoid the address munging described above, turn on the link1 flag on
     the ifconfig command line. This implies that the GRE packet destination
     and the remote host are not the same IP addresses, and that the GRE des-
     tination does not route over the greX interface itself.

     GRE RFC not yet fully implemented (no GRE options).

     For the WCCP GRE encapsulated packets we can only reliably accept WCCPv1
     format; WCCPv2 formatted packets add another header which will skew the
     decode, and results are not defined (i.e. don't do WCCPv2).

MirOS BSD #10-current         September 13, 1998                             2

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