$NetBSD: gre.4,v 1.10 1999/12/22 14:55:49 kleink Exp $
Copyright 1998 (c) The NetBSD Foundation, Inc.
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This code is derived from software contributed to The NetBSD Foundation
by Heiko W. Rupp <hwr@pilhuhn.de>
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.Dd $Mdocdate: February 22 2018 $ .Dt GRE 4 .Os .Sh NAME .Nm gre , .Nm mgre ,
.Nm egre , .Nm nvgre .Nd Generic Routing Encapsulation network device .Sh SYNOPSIS .Cd "pseudo-device gre" .Sh DESCRIPTION The .Nm gre pseudo-device provides interfaces for tunnelling protocols across IPv4 and IPv6 networks using the Generic Routing Encapsulation (GRE) encapsulation protocol.
p GRE datagrams (IP protocol number 47) consist of a GRE header and an outer IP header for encapsulating another protocol's datagram. The GRE header specifies the type of the encapsulated datagram, allowing for the tunnelling of multiple protocols.
p Different tunnels between the same endpoints may be distinguished by an optional Key field in the GRE header. The Key field may be partitioned to carry flow information about the encapsulated traffic to allow better use of multipath links.
p
This pseudo driver provides the clonable network interfaces:
l -tag -width nvgreX t Nm gre Point-to-point Layer 3 tunnel interfaces.
.It Nm mgre
Point-to-multipoint Layer 3 tunnel interfaces.
t Nm egre Point-to-point Ethernet tunnel interfaces.
t Nm nvgre Network Virtualization Using Generic Routing Encapsulation
(NVGRE) overlay Ethernet network interfaces.
.El
p All GRE packet processing in the system is allowed or denied by setting the .Va net.inet.gre.allow .Xr sysctl 8 variable. To allow GRE packet processing, set .Va net.inet.gre.allow to 1.
p
.Nm gre ,
.Nm mgre ,
.Nm egre ,
and
.Nm nvgre
interfaces can be created at runtime using the
c ifconfig iface Ns Ar N Ic create command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
p For correct operation, encapsulated traffic must not be routed over the interface itself. This can be implemented by adding a distinct or a more specific route to the tunnel destination than the hosts or networks routed via the tunnel interface. Alternatively, the tunnel traffic may be configured in a separate routing table to the encapsulated traffic. .Ss Point-to-Point Layer 3 GRE tunnel interfaces (gre) A .Nm gre tunnel can encapsulate IPv4, IPv6, and MPLS packets. The MTU is set to 1476 by default to match the value used by Cisco routers.
p
.Nm gre
supports sending keepalive packets to the remote endpoint, which
allows tunnel failure to be detected.
talk about keepalive packet construction?
To return keepalives, the remote host must be configured to forward
IP packets received from inside the tunnel back to the address of
the local tunnel endpoint.
For example, an
.Ox
remote host with a GRE tunnel over IPv4 would need the following
to ensure packets received inside the tunnel can be forwarded back
to the local tunnel.
d -literal -offset indent # sysctl net.inet.ip.forwarding=1
# ifconfig greN rdomain X
# ifconfig greN tunneldomain X
.Ed
talk about caveats with rdomains
p
.Nm gre
interfaces may be configured to receive IPv4 packets in
Web Cache Communication Protocol (WCCP)
encapsulation by setting the
.Cm link0
flag on the interface.
WCCP reception may be enabled globally by setting the
.Va net.inet.gre.wccp
sysctl value to 1.
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 net.inet.gre.allow
to also be set.
.Ss Point-to-Multipoint Layer 3 GRE tunnel interfaces (mgre)
.Ss Point-to-Point Ethernet over GRE tunnel interfaces (egre)
A
.Nm egre
tunnel interface carries Ethernet over GRE (EoGRE).
Ethernet traffic is encapsulated using Transparent Ethernet (0x6558)
as the protocol identifier in the GRE header, as per RFC 1701.
The MTU is set to 1500 by default.
.Ss Network Virtualization Using GRE interfaces (nvgre)
.Nm nvgre
interfaces allow construction of virtual overlay Ethernet networks
on top of an IPv4 or IPv6 underlay network as per RFC 7367.
Ethernet traffic is encapsulated using Transparent Ethernet (0x6558)
as the protocol identifier in the GRE header, a 24-bit Virtual
Subnet ID (VSID), and an 8-bit FlowID.
A
.Nm nvgre
interface represents a Network Virtualization Edge (NVE), and
transports traffic in the overlay network by encapsulating it
in the underlay.
p .Nm nvgre interfaces are configured with a unicast tunnel source address, a multicast tunnel destination address, and a parent interface to use for sending and receiving multicast traffic on the underlay network. The unicast source address is used as the NVE Provider Address (PA) on the underlay network. The multicast group address in the underlay is used to transport broadcast and multicast traffic from the overlay to other participating NVGRE endpoints. It is also used to flood unicast traffic to Ethernet addresses in the overlay with an unknown association to a NVGRE endpoint. Traffic received from other NVGRE endpoints, either to the unicast PA address or multicast group address, is used to learn associations between Ethernet addresses in the overlay network and the Provider Addresses of NVGRE endpoints.
p
The MTU is set to 1500 by default.
talk about DF and MTU of underlay network.
.Ss Programming Interface
.Nm gre ,
.Nm egre ,
and
.Nm nvgre
interfaces support the following
.Xr ioctl 2
calls for configuring tunnel options:
l -tag -width indent -offset 3n t Dv SIOCSLIFPHYADDR Fa "struct if_laddrreq *" Set the IPv4 or IPv6 addresses for the encapsulating IP packets.
The addresses may only be configured while the interface is down.
p .Nm gre and .Nm egre interfaces support configuration of unicast IP addresses as the tunnel endpoints.
p .Nm nvgre interfaces support configuration of a unicast IP address as the local endpoint and a multicast group address as the destination address. t Dv SIOCGLIFPHYADDR Fa "struct if_laddrreq *" Get the addresses used for the encapsulating IP packets. t Dv SIOCDIFPHYADDR Fa "struct ifreq *" Clear the addresses used for the encapsulating IP packets. The addresses may only be cleared while the interface is down. t Dv SIOCSVNETID Fa "struct ifreq *" Configure a virtual network identifier for use in the GRE Key header. The virtual network identifier may only be configured while the interface is down.
p .Nm gre and .Nm egre interfaces configured with a virtual network identifier will enable the use of the GRE Key header. The Key is a 32-bit value by default, or a 24-bit value when the virtual network flow identifier is enabled.
p .Nm nvgre interfaces use the virtual network identifier in the 24-bit Virtual Subnet Identifer (VSID) aka Tenant Network Identifier (TNI) field in of the GRE Key header. t Dv SIOCGVNETID Fa "struct ifreq *" Get the virtual network identifer used in the GRE Key header. t Dv SIOCDVNETID Fa "struct ifreq *" Disable the use of the virtual network identifier. The virtual network identifer may only be disabled while the interface is down.
p When the virtual network identifier is disabled on .Nm gre and .Nm egre interfaces, it disables the use of the GRE Key header.
p .Nm nvgre interfaces do not support this ioctl as a Virtual Subnet Identifier is required by the protocol. t Dv SIOCSLIFPHYRTABLE Fa "struct ifreq *" Set the routing table the tunnel traffic operates in. The routing table may only be configured while the interface is down. t Dv SIOCGLIFPHYRTABLE Fa "struct ifreq *" Get the routing table the tunnel traffic operates in. t Dv SIOCSLIFPHYTTL Fa "struct ifreq *" Set the Time-To-Live field in IPv4 encapsulation headers, or the Hop Limit field in IPv6 encapsulation headers.
p .Nm gre interfaces configured with a TTL of -1 will copy the TTL in and out of the encapsulated protocol headers. t Dv SIOCGLIFPHYTTL Fa "struct ifreq *" Get the value used in the Time-To-Live field in a IPv4 encapsulation header or the Hop Limit field in a IPv6 encapsulation header. t Dv SIOCSLIFPHYDF Fa "struct ifreq *" Configure whether the tunnel traffic sent by the interface can be fragmented or not. This sets the Don't Fragment (DF) bit on IPv4 packets, and disables fragmentation of IPv6 packets. t Dv SIOCGLIFPHYDF Fa "struct ifreq *" Get whether the tunnel traffic sent by the interface can be fragmented or not. .El
p .Nm gre and .Nm egre interfaces support the following .Xr ioctl 2 calls: l -tag -width indent -offset 3n t Dv SIOCSVNEFLOWID Fa "struct ifreq *" Enable or disable the partitioning of the optional GRE Key header into a 24-bit virtual network identifier and an 8-bit flow identifier.
p .Nm gre and .Nm egre must already be configured with a virtual network identifer before enabling flow identifiers in the GRE Key header. The configured virtual network identify must also fit into 24 bits. t Dv SIOCDVNETFLOWID Fa "struct ifreq *" Get the status of the partitioning of the GRE key. .El
p
.Nm gre
interfaces support the following
.Xr ioctl 2
calls:
l -tag -width indent -offset 3n t Dv SIOCSETKALIVE Fa "struct ifkalivereq *" Enable the transmission of keepalive packets to detect tunnel failure.
Keepalives may only be configured while the interace is down.
p Setting the keepalive period or count to 0 disables keepalives on the tunnel. t Dv SIOCGETKALIVE Fa "struct ifkalivereq *" Get the configuration of keepalive packets. .El
p
.Nm nvgre
interfaces support the following
.Xr ioctl 2
calls:
l -tag -width indent -offset 3n t Dv SIOCSIFPARENT Fa "struct if_parent *" Configure which interface will be joined to the multicast group
specified by the tunnel destination address.
The parent interface may only be configured while the interface is
down.
t Dv SIOCGIFPARENT Fa "struct if_parent *" Get the name of the interface used for multicast communication.
t Dv SIOCGIFPARENT Fa "struct ireq *" Remove the configuration of the interface used for multicast
communication.
bridge(4) ioctls should go here too.
.El
.Sh EXAMPLES
.Nm gre
Configuration example:
d -literal Host X ---- Host A ------------ tunnel ------------ Cisco D ---- Host E
\e /
\e /
+------ Host B ------ Host C ------+
.Ed
p On Host A
q Ox : d -literal -offset indent # route add default B # ifconfig greN create # ifconfig greN A D netmask 0xffffffff up # ifconfig greN tunnel A D # route add E D .Ed
p On Host D (Cisco): d -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
p OR
p On Host D
q Ox : d -literal -offset indent # route add default C # ifconfig greN create # ifconfig greN D A # ifconfig greN tunnel D A .Ed
p To reach Host A over the tunnel (from Host D), there has to be an alias on Host A for the Ethernet interface:
p .Dl # ifconfig <etherif> alias Y
p and on the Cisco:
p .Dl ip route Y mask tunnelX
p Keepalive packets may be enabled like this: d -literal -offset indent # ifconfig greN keepalive period count .Ed
p 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: d -literal -offset indent # sysctl net.inet.ip.forwarding=1 .Ed
p 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: d -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
p .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
p .Rs .%A D. Farinacci .%A "T. Li" .%A S. Hanks .%A D. Meyer .%A P. Traina .%D March 2000 .%R RFC 2784 .%T Generic Routing Encapsulation (GRE) .Re
p .Rs .%A G. Dommety .%D September 2000 .%R RFC 2890 .%T Key and Sequence Number Extensions to GRE .Re
p .Rs .%A P. Garg .%A Y. Wang .%D September 2015 .%R RFC 7647 .%T NVGRE: Network Virtualization Using Generic Routing Encapsulation .Re
p .Rs .%U https://tools.ietf.org/html/draft-ietf-wrec-web-pro-00.txt .%T Web Cache Coordination Protocol V1.0 .Re
p .Rs .%U https://tools.ietf.org/html/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 CAVEATS RFC 1701 and RFC 2890 describe a variety of optional GRE header fields in the protocol that are not implemented in the .Nm gre and .Nm egre interface drivers. The only optional field the drivers implement support for is the Key header.
p .Nm gre interfaces skip the redirect header in WCCPv2 GRE encapsulated packets.
p The NVGRE RFC specifies VSIDs 0 (0x0) to 4095 (0xfff) as reserved for future use, and VSID 16777215 (0xffffff) for use for vendor-specific endpoint communication. The NVGRE RFC also explicitly states encapsulated Ethernet packets must not contain IEEE 802.1Q (VLAN) tags. The .Nm nvgre driver does not restrict the use of these VSIDs, and does not prevent the configuration of child .Xr vlan 4 interfaces or the bridging of VLAN tagged traffic across the tunnel. These non-restrictions allow non-compliant tunnels to be configured which may not interoperate with other vendors.