man/man4/gre.4

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.\" by Heiko W. Rupp <hwr@pilhuhn.de>
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.Dd $Mdocdate: August 31 2016 $
.Dt GRE 4
.Os
.Sh NAME
.Nm gre ,
.Nm mobileip
.Nd encapsulating network device
.Sh SYNOPSIS
.Cd "pseudo-device gre"
.Sh DESCRIPTION
The
.Nm
driver allows tunnel construction using the Cisco GRE or
the Mobile IP (RFC 2004) encapsulation protocols.
.Pp
.Tn GRE ,
.Tn WCCPv1 ,
and
.Tn Mobile IP
are enabled with the following
.Xr sysctl 3
variables respectively in
.Pa /etc/sysctl.conf :
.Bl -tag -width "net.inet.mobileip.allow"
.It Va net.inet.gre.allow
Allow GRE packets in and out of the system.
.It Va net.inet.gre.wccp
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.
Some magic with the packet filter configuration
and a caching proxy like squid are needed
to do anything useful with these packets.
This sysctl requires
.Va gre.allow
to be set.
.It Va net.inet.mobileip.allow
Allow Mobile IP packets in and out of the system.
.El
.Pp
This driver currently supports the following modes of operation:
.Bl -tag -width mobileipXXX
.It Nm gre
GRE datagrams (IP protocol number 47)
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,
such as AppleTalk.
GRE mode is the default tunnel mode on Cisco routers.
This is also the default mode of operation of the
.Nm
interfaces.
.It Nm mobileip
MOBILE datagrams (IP protocol number 55)
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.
.El
.Pp
A
.Nm gre
or
.Nm mobileip
interface can be created at runtime using the
.Ic ifconfig gre Ns Ar N Ic create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
The MTU 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,
but it can be adjusted via
.Xr ifconfig 8 .
.Pp
For correct operation,
there needs to be 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).
.Pp
Note that the IP addresses of the tunnel endpoints may be the same as the
ones defined with
.Xr ifconfig 8
for the interface (as if IP is encapsulated) but need not be as,
for example, when encapsulating AppleTalk.
.Sh EXAMPLES
Configuration example:
.Bd -literal
Host X ---- Host A ------------ tunnel ------------ Cisco D ---- Host E
               \e                                      /
                \e                                    /
                 +------ Host B ------ Host C ------+
.Ed
.Pp
On Host A
.Pq Ox :
.Bd -literal -offset indent
# route add default B
# ifconfig greN create
# ifconfig greN A D netmask 0xffffffff linkX up
# ifconfig greN tunnel A D
# route add E D
.Ed
.Pp
On Host D (Cisco):
.Bd -literal -offset indent
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
.Ed
.Pp
OR
.Pp
On Host D
.Pq Ox :
.Bd -literal -offset indent
# route add default C
# ifconfig greN create
# ifconfig greN D A
# ifconfig greN tunnel D A
.Ed
.Pp
To reach Host A over the tunnel (from Host D), there has to be an
alias on Host A for the Ethernet interface:
.Pp
.Dl # ifconfig <etherif> alias Y
.Pp
and on the Cisco:
.Pp
.Dl ip route Y mask tunnelX
.Pp
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:
.Bd -literal -offset indent
# ifconfig greN keepalive period count
.Ed
.Pp
This will send a keepalive packet every
.Ar period
seconds.
If no response is received in
.Ar count
*
.Ar period
seconds, the link is considered down.
To return keepalives, the remote host must be configured to forward packets:
.Bd -literal -offset indent
# sysctl net.inet.ip.forwarding=1
.Ed
.Pp
If
.Xr pf 4
is enabled then it is necessary to add a pass rule specific for the keepalive
packets.
The rule must use
.Cm no state
because the keepalive packet is entering the network stack multiple times.
In most cases the following should work:
.Bd -literal -offset indent
pass quick on gre proto gre no state
.Ed
.Sh SEE ALSO
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr options 4 ,
.Xr hostname.if 5 ,
.Xr protocols 5 ,
.Xr ifconfig 8 ,
.Xr netstart 8 ,
.Xr sysctl 8
.Sh STANDARDS
.Rs
.%A S. Hanks
.%A "T. Li"
.%A D. Farinacci
.%A P. Traina
.%D October 1994
.%R RFC 1701
.%T Generic Routing Encapsulation (GRE)
.Re
.Pp
.Rs
.%A S. Hanks
.%A "T. Li"
.%A D. Farinacci
.%A P. Traina
.%D October 1994
.%R RFC 1702
.%T Generic Routing Encapsulation over IPv4 networks
.Re
.Pp
.Rs
.%A C. Perkins
.%D October 1996
.%R RFC 2004
.%T Minimal Encapsulation within IP
.Re
.Pp
.Rs
.%U http://www.wrec.org/Drafts/draft-ietf-wrec-web-pro-00.txt
.%T Web Cache Coordination Protocol V1.0
.Re
.Pp
.Rs
.%U http://www.wrec.org/Drafts/draft-wilson-wrec-wccp-v2-00.txt
.%T Web Cache Coordination Protocol V2.0
.Re
.Sh AUTHORS
.An Heiko W. Rupp Aq Mt hwr@pilhuhn.de
.Sh BUGS
The GRE RFC is not yet fully implemented (no GRE options).
.Pp
The redirect header for WCCPv2 GRE encapsulated packets is skipped.