gre - encapsulating network device
pseudo-device gre [count]
GRE, WCCPv1, and MobileIP are enabled with the following
respectively in /etc/sysctl.conf:
net.inet.gre.allow Allow GRE packets in and out of the
net.inet.gre.wccp Allow WCCPv1-style GRE packets into
(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
command or by setting up a hostname.if(5) configuration file
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 datagram
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 gre
MOBILE encapsulation (IP protocol number 55).
Datagrams are encapsulated into IP, but with a much
header. This protocol only supports IP in IP
and is intended for use with mobile IP.
The network interfaces are named gre0, gre1, etc. The number of interfaces
is given by the corresponding pseudo-device line in
the system configuration
file. gre interfaces support the following
GRESADDRS struct ifreq *
Set the IP address of the local tunnel end.
GRESADDRD struct ifreq *
Set the IP address of the remote tunnel end.
GREGADDRS struct ifreq *
Query the IP address that is set for the local tunnel end.
GREGADDRD struct ifreq *
Query the IP address that is set for the remote tunnel end.
GRESPROTO struct ifreq *
Set the operation mode to the specified IP protocol
protocol is passed to the interface in the ifr_flags
field of the
ifreq structure. The operation mode can also be set
following modifiers to ifconfig(8):
GREGPROTO struct ifreq *
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
but need not be, as e.g. when encapsulating AppleTalk.
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):
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
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
this would create a loop.)
In order for 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 issuing the
option to sysctl(8).
The GRE interface will accept WCCPv1-style GRE encapsulated
a Cisco router. Some magic with the packet filter configuration and a
caching proxy like squid are needed to do anything useful
atalk(4), inet(4), ip(4), netintro(4), options(4), hostname.if(5),
protocols(5), ifconfig(8), netstart(8), sysctl(8)
Generic Routing Encapsulation, RFC 1701.
Generic Routing Encapsulation over IPv4 networks, RFC 1702.
Minimal Encapsulation within IP, RFC 2004.
Web Cache Coordination Protocol V1.0,
Web Cache Coordination Protocol V2.0,
Heiko W.Rupp <email@example.com>
The compute_route() code in sys/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(8) command line. This implies that the GRE
and the remote host are not the same IP addresses, and
that the GRE
destination 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
format; WCCPv2 formatted packets add another header which
will skew the
decode, and results are not defined (i.e. don't do WCCPv2).
OpenBSD 3.6 September 13, 1998
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