encapsulating network device
gre network interface allows tunnel
construction using the Cisco GRE or the Mobile IP (RFC 2004) encapsulation
GRE, WCCPv1, and Mobile IP are enabled with the following sysctl(3) variables respectively in /etc/sysctl.conf:
- Allow GRE packets in and out of the system.
- Set to 1 to allow WCCPv1-style GRE packets into the system; set to 2 to handle the packets as WCCPv2-style GRE, truncating the redirect header. This variable depends on gre.allow being set.
- Allow Mobile IP packets in and out of the system.
gre interface can be created at runtime
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 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
- MOBILE encapsulation (IP protocol number 55)
- Datagrams are encapsulated into IP, but with a much smaller encapsulation header. This protocol only supports IP in IP encapsulation, and is intended for use with Mobile IP.
The network interfaces are named
gre1, etc. The number of interfaces is given by the
pseudo-device line in the system
gre interfaces support the
GRESADDRSstruct ifreq *
- Set the IP address of the local tunnel end.
GRESADDRDstruct ifreq *
- Set the IP address of the remote tunnel end.
GREGADDRSstruct ifreq *
- Query the IP address that is set for the local tunnel end.
GREGADDRDstruct ifreq *
- Query the IP address that is set for the remote tunnel end.
GRESPROTOstruct ifreq *
- Set the operation mode to the specified IP protocol value. The protocol is passed to the interface in the ifr_flags field of the ifreq structure. The operation mode can also be set with the following modifiers to ifconfig(8):
GREGPROTOstruct 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 encapsulated), 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):
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
Keepalive packets may optionally be sent to the remote endpoint, which decapsulates and returns them, allowing tunnel failure to be detected. Enable them like this:
# ifconfig greN keepalive period count
This will send a keepalive packet every period seconds. If no response is received in count * period seconds, the link is considered down. To return keepalives, the remote host must be configured to forward packets:
# sysctl net.inet.ip.forwarding=1
If pf(4) is enabled then it is necessary to add a pass rule specific for the keepalive packets. The rule must use no state because the keepalive packet is entering the network stack multiple times. In most cases the following should work:
pass quick on gre proto gre no state
The MTU of
gre 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
For correct operation, the
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 keyword
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 or WWCPv2-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 packets.
inet(4), ip(4), netintro(4), options(4), hostname.if(5), protocols(5), ifconfig(8), netstart(8), sysctl(8)
S. Hanks, T. Li, D. Farinacci, and P. Traina, Generic Routing Encapsulation (GRE), RFC 1701, October 1994.
S. Hanks, T. Li, D. Farinacci, and P. Traina, Generic Routing Encapsulation over IPv4 networks, RFC 1702, October 1994.
C. Perkins, Minimal Encapsulation within IP, RFC 2004, October 1996.
Web Cache Coordination Protocol V1.0, http://www.wrec.org/Drafts/draft-ietf-wrec-web-pro-00.txt.
Web Cache Coordination Protocol V2.0, http://www.wrec.org/Drafts/draft-wilson-wrec-wccp-v2-00.txt.
Heiko W. Rupp <email@example.com>
GRE RFC not yet fully implemented (no GRE options).
The redirect header for WCCPv2 GRE encapsulated packets is skipped.