Generic Routing Encapsulation network
gre pseudo-device provides interfaces
for tunnelling protocols across IPv4 and IPv6 networks using the Generic
Routing Encapsulation (GRE) encapsulation protocol.
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.
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.
This pseudo driver provides the clonable network interfaces:
- Point-to-point Layer 3 tunnel interfaces.
- Point-to-multipoint Layer 3 tunnel interfaces.
- Point-to-point Ethernet tunnel interfaces.
- Network Virtualization using Generic Routing Encapsulation (NVGRE) overlay Ethernet network interfaces.
- MikroTik Ethernet over IP tunnel interfaces.
See eoip(4) for information regarding MikroTik Ethernet over IP interfaces.
All GRE packet processing in the system is allowed or denied by setting the net.inet.gre.allow sysctl(8) variable. To allow GRE packet processing, set net.inet.gre.allow to 1.
can be created at runtime using the
or by setting up a
hostname.if(5) configuration file for
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.
Point-to-Point Layer 3 GRE tunnel interfaces (gre)
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.
gre supports sending keepalive packets to
the remote endpoint which allows tunnel failure to be detected. 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
gre interfaces may be configured to
receive IPv4 packets in Web Cache Communication Protocol (WCCP)
encapsulation by setting the
link0 flag on the
interface. WCCP reception may be enabled globally by setting the
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
net.inet.gre.allow to also be set.
Point-to-Multipoint Layer 3 GRE tunnel interfaces (mgre)
mgre interfaces can encapsulate IPv4,
IPv6, and MPLS packets. Unlike a point-to-point interface,
mgre interfaces are configured with an address on an
IP subnet. Peers on that subnet are mapped to the addresses of multiple
The MTU is set to 1476 by default to match the value used by Cisco routers.
Point-to-Point Ethernet over GRE tunnel interfaces (egre)
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.
Network Virtualization using GRE interfaces (nvgre)
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.
By default the MTU of an
is set to 1500, and the Don't Fragment flag is set. The MTU on the network
interfaces carrying underlay network traffic must be raised to accommodate
this and the overhead of the NVGRE encapsulation, or the
nvgre interface must be reconfigured for less
The underlay network parameters on a
interface are a unicast tunnel source address, a multicast tunnel
destination address, and a parent network interface. The unicast source
address is used as the NVE Provider Address (PA) on the underlay network.
The parent interface is used to identify which interface the multicast group
should be joined to.
The multicast group 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 Provider Address or via the multicast group, is used to learn associations between Ethernet addresses in the overlay network and the Provider Addresses of NVGRE endpoints in the underlay.
support the following ioctl(2) calls for configuring tunnel options:
SIOCSLIFPHYADDRstruct if_laddrreq *
- Set the IPv4 or IPv6 addresses for the encapsulating IP packets. The
addresses may only be configured while the interface is down.
egreinterfaces support configuration of unicast IP addresses as the tunnel endpoints.
mgreinterfaces support configuration of a unicast local IP address, and require an
nvgreinterfaces support configuration of a unicast IP address as the local endpoint and a multicast group address as the destination address.
SIOCGLIFPHYADDRstruct if_laddrreq *
- Get the addresses used for the encapsulating IP packets.
SIOCDIFPHYADDRstruct ifreq *
- Clear the addresses used for the encapsulating IP packets. The addresses may only be cleared while the interface is down.
SIOCSVNETIDstruct 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
egreinterfaces 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.
nvgreinterfaces use the virtual network identifier in the 24-bit Virtual Subnet Identifier (VSID) aka Tenant Network Identifier (TNI) field in of the GRE Key header.
SIOCGVNETIDstruct ifreq *
- Get the virtual network identifier used in the GRE Key header.
SIOCDVNETIDstruct ifreq *
- Disable the use of the virtual network identifier. The virtual network
identifier may only be disabled while the interface is down.
When the virtual network identifier is disabled on
egreinterfaces, it disables the use of the GRE Key header.
nvgreinterfaces do not support this ioctl as a Virtual Subnet Identifier is required by the protocol.
SIOCSLIFPHYRTABLEstruct ifreq *
- Set the routing table the tunnel traffic operates in. The routing table may only be configured while the interface is down.
SIOCGLIFPHYRTABLEstruct ifreq *
- Get the routing table the tunnel traffic operates in.
SIOCSLIFPHYTTLstruct ifreq *
- Set the Time-To-Live field in IPv4 encapsulation headers, or the Hop Limit
field in IPv6 encapsulation headers.
mgreinterfaces configured with a TTL of -1 will copy the TTL in and out of the encapsulated protocol headers.
SIOCGLIFPHYTTLstruct ifreq *
- Get the value used in the Time-To-Live field in an IPv4 encapsulation header or the Hop Limit field in an IPv6 encapsulation header.
SIOCSLIFPHYDFstruct 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.
SIOCGLIFPHYDFstruct ifreq *
- Get whether the tunnel traffic sent by the interface can be fragmented or not.
SIOCSTXHPRIOstruct ifreq *
- Set the priority value used in the Type of Service field in IPv4 headers,
or the Traffic Class field in IPv6 headers. Values may be from 0 to 7, or
IF_HDRPRIO_PACKETto specify that the current priority of a packet should be used.
mgreinterfaces configured with a value of
IF_HDRPRIO_PAYLOADwill copy the priority from encapsulated protocol headers.
SIOCGTXHPRIOstruct ifreq *
- Get the priority value used in the Type of Service field in IPv4 headers, or the Traffic Class field in IPv6 headers.
egre interfaces support the following
SIOCSVNETFLOWIDstruct 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.
The interface must already be configured with a virtual network identifier before enabling flow identifiers in the GRE Key header. The configured virtual network identify must also fit into 24 bits.
SIOCDVNETFLOWIDstruct ifreq *
- Get the status of the partitioning of the GRE key.
gre interfaces support the following
nvgre interfaces support the following
SIOCSIFPARENTstruct 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.
SIOCGIFPARENTstruct if_parent *
- Get the name of the interface used for multicast communication.
SIOCDIFPARENTstruct ifreq *
- Remove the configuration of the interface used for multicast communication.
The GRE protocol in all its flavours does not provide any integrated security features. GRE should only be deployed on trusted private networks, or protected with IPsec to add authentication and encryption for confidentiality. IPsec is especially recommended when transporting GRE over the public internet.
The Packet Filter pf(4) can be used to filter tunnel traffic with endpoint policies pf.conf(5).
The Time-to-Live (TTL) value of a tunnel can be set to 1 or a low value to restrict the traffic to the local network:
# ifconfig gre0 tunnelttl 1
Point-to-Point Layer 3 GRE tunnel interfaces (gre) 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 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
gre keepalive packets may be enabled with
ifconfig(8) 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
is enabled then it is necessary to add a pass rule specific for the
keepalive packets. The rule must use
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
Point-to-Multipoint Layer 3 GRE tunnel interfaces (mgre) example
mgre can be used to build a
point-to-multipoint tunnel network to several hosts using a single
In this example the host A has an outer IP of 198.51.100.12, host B has 203.0.113.27, and host C has 203.0.113.254.
Addressing within the tunnel is done using 192.0.2.0/24:
+--- Host B / / Host A --- tunnel ---+ \ \ +--- Host C
On Host A:
# ifconfig mgreN create # ifconfig mgreN tunneladdr 198.51.100.12 # ifconfig mgreN inet 192.0.2.1 netmask 0xffffff00 up
On Host B:
# ifconfig mgreN create # ifconfig mgreN tunneladdr 203.0.113.27 # ifconfig mgreN inet 192.0.2.2 netmask 0xffffff00 up
On Host C:
# ifconfig mgreN create # ifconfig mgreN tunneladdr 203.0.113.254 # ifconfig mgreN inet 192.0.2.3 netmask 0xffffff00 up
To reach Host B over the tunnel (from Host A), there has to be a route on Host A specifying the next-hop:
# route add -host 192.0.2.2 203.0.113.27 -iface -ifp mgreN
Similarly, to reach Host A over the tunnel from Host B, a route must be present on B with A's outer IP as next-hop:
# route add -host 192.0.2.1 198.51.100.12 -iface -ifp mgreN
The same tunnel interface can then be used between host B and C by adding the appropriate routes, making the network any-to-any instead of hub-and-spoke:
On Host B:
# route add -host 192.0.2.3 203.0.113.254 -iface -ifp mgreN
On Host C:
# route add -host 192.0.2.2 203.0.113.27 -iface -ifp mgreN
Point-to-Point Ethernet over GRE tunnel interfaces (egre) example
egre can be used to carry Ethernet traffic
between two endpoints over an IP network, including the public internet.
This can also be achieved using
egre offers the ability to
carry different Ethernet networks between the same endpoints by using
virtual network identifiers to distinguish between them.
For example, a pair of routers separated by the internet could
bridge several Ethernet networks using
In this example the first router has a public IP of 192.0.2.1, and
the second router has 203.0.113.2. They are connecting the Ethernet networks
on two vlan(4)
interfaces over the internet. A separate
is created for each VLAN and given different virtual network identifiers so
the routers can tell which network the encapsulated traffic is for. The
egre interfaces are explicitly configured to provide
the same MTU as the vlan(4) interfaces (1500 bytes) with fragmentation enabled so they
can be carried over the internet, which has the same or lower MTU.
At the first site:
# ifconfig vlan0 vnetid 100 # ifconfig egre0 create # ifconfig egre0 tunnel 192.0.2.1 203.0.113.2 # ifconfig egre0 vnetid 100 # ifconfig egre0 mtu 1500 -tunneldf # ifconfig egre0 up # ifconfig bridge0 add vlan0 add egre0 up # ifconfig vlan1 vnetid 200 # ifconfig egre1 create # ifconfig egre1 tunnel 192.0.2.1 203.0.113.2 # ifconfig egre1 vnetid 200 # ifconfig egre1 mtu 1500 -tunneldf # ifconfig egre1 up # ifconfig bridge1 add vlan1 add egre1 up
At the second site:
# ifconfig vlan0 vnetid 100 # ifconfig egre0 create # ifconfig egre0 tunnel 203.0.113.2 192.0.2.1 # ifconfig egre0 vnetid 100 # ifconfig egre0 mtu 1500 -tunneldf # ifconfig egre0 up # ifconfig bridge0 add vlan0 add egre0 up # ifconfig vlan1 vnetid 200 # ifconfig egre1 create # ifconfig egre1 tunnel 203.0.113.2 192.0.2.1 # ifconfig egre1 vnetid 200 # ifconfig egre1 mtu 1500 -tunneldf # ifconfig egre1 up # ifconfig bridge1 add vlan1 add egre1 up
Network Virtualization using GRE interfaces (nvgre) example
NVGRE can be used to build a distinct logical Ethernet network on
top of another network.
nvgre is therefore like a
interface configured on top of a physical Ethernet interface, except it can
sit on any IP network capable of multicast.
The following shows a basic
configuration and an equivalent
configuration. In the examples, 192.168.0.1/24 will be the network
configured on the relevant virtual interfaces. The NVGRE underlay network
will be configured on 100.64.10.0/24, and will use 184.108.40.206 as the
multicast group address.
The vlan(4) interface only relies on Ethernet, it does not rely on IP configuration on the parent interface:
# ifconfig em0 up # ifconfig vlan0 create # ifconfig vlan0 parent em0 # ifconfig vlan0 vnetid 10 # ifconfig vlan0 inet 192.168.0.1/24 # ifconfig vlan0 up
nvgre relies on IP configuration on the
parent interface, and an MTU large enough to carry the encapsulated
# ifconfig em0 mtu 1600 # ifconfig em0 inet 100.64.10.1/24 # ifconfig em0 up # ifconfig nvgre0 create # ifconfig nvgre0 parent em0 tunnel 100.64.10.1 220.127.116.11 # ifconfig nvgre0 vnetid 10010 # ifconfig nvgre0 inet 192.168.0.1/24 # ifconfig nvgre0 up
NVGRE is intended for use in a multitenant datacentre environment to provide each customer with distinct Ethernet networks as needed, but without running into the limit on the number of VLAN tags, and without requiring reconfiguration of the underlying Ethernet infrastructure. Another way to look at it is NVGRE can be used to construct multipoint Ethernet VPNs across an IP core.
For example, if a customer has multiple virtual machines running
in vmm(4) on
distinct physical hosts,
can be used to provide network connectivity between the
interfaces connected to the virtual machines. If there are 3 virtual
machines, all using tap0 on each hosts, and those hosts are connected to the
same network described above,
nvgre with a distinct
virtual network identifier and multicast group can be created for them. The
following assumes nvgre1 and bridge1 have already been created on each host,
and em0 has had the MTU raised:
On physical host 1:
# ifconfig em0 inet 100.64.10.10/24 # ifconfig nvgre1 parent em0 tunnel 100.64.10.10 18.104.22.168 # ifconfig nvgre1 vnetid 10011 # ifconfig bridge1 add nvgre1 add tap0 up
On physical host 2:
# ifconfig em0 inet 100.64.10.11/24 # ifconfig nvgre1 parent em0 tunnel 100.64.10.11 22.214.171.124 # ifconfig nvgre1 vnetid 10011 # ifconfig bridge1 add nvgre1 add tap0 up
On physical host 3:
# ifconfig em0 inet 100.64.10.12/24 # ifconfig nvgre1 parent em0 tunnel 100.64.10.12 126.96.36.199 # ifconfig nvgre1 vnetid 10011 # ifconfig bridge1 add nvgre1 add tap0 up
Being able to carry working multicast and jumbo frames over the
public internet is unlikely, which makes it difficult to use NVGRE to
extended Ethernet VPNs between different sites.
egre can be
bridged together to provide such connectivity. See the
egre section for an example.
eoip(4), 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.
D. Farinacci, T. Li, S. Hanks, D. Meyer, and P. Traina, Generic Routing Encapsulation (GRE), RFC 2784, March 2000.
G. Dommety, Key and Sequence Number Extensions to GRE, RFC 2890, September 2000.
T. Worster, Y. Rekhter, and E. Rosen, Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE), RFC 4023, March 2005.
P. Garg and Y. Wang, NVGRE: Network Virtualization Using Generic Routing Encapsulation, RFC 7637, September 2015.
Web Cache Coordination Protocol V1.0, https://tools.ietf.org/html/draft-ietf-wrec-web-pro-00.txt.
Web Cache Coordination Protocol V2.0, https://tools.ietf.org/html/draft-wilson-wrec-wccp-v2-00.txt.
Heiko W. Rupp <firstname.lastname@example.org>
RFC 1701 and RFC 2890 describe a variety of optional GRE header
fields in the protocol that are not implemented in the
drivers. The only optional field the drivers implement support for is the
gre interfaces skip the redirect header in
WCCPv2 GRE encapsulated packets.
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
nvgre driver does not restrict the use of these
VSIDs, and does not prevent the configuration of child
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.