— generic tunnel interface
interface is a generic tunnelling device for IPv4 and
IPv6. It can tunnel IPv traffic over IPv. Therefore, there can be four
possible configurations. The behavior of gif
is mainly based
on RFC2893 IPv6-over-IPv4 configured tunnel. On
can also tunnel ISO
traffic over IPv using EON encapsulation. Note that gif
does not perform GRE encapsulation; use gre(4)
interface is created at runtime using interface
cloning. This is most easily done with the “ifconfig
” command or using the
variable in rc.conf(5)
To use gif
, the administrator needs to configure the protocol
and addresses used for the outer header. This can be done by using
ioctl. The administrator also needs to
configure the protocol and addresses for the inner header, with
. Note that IPv6 link-local addresses (those that
) will be automatically configured
whenever possible. You may need to remove IPv6 link-local addresses manually
, if you want to disable the use of IPv6 as
the inner header (for example, if you need a pure IPv4-over-IPv6 tunnel).
Finally, you must modify the routing table to route the packets through the
device can be configured to be ECN friendly. This can
be configured by
ECN friendly behavior¶
device can be configured to be ECN friendly, as
. This is
turned off by default, and can be turned on by the
normal behavior, as described in RFC2893. This can be summarized as follows:
- Set outer TOS bit to
- Drop outer TOS bit.
will copy ECN bits
on IPv4 TOS
byte or IPv6 traffic class byte) on egress and ingress, as follows:
- Copy TOS bits except for ECN CE (masked with
0xfe) from inner to outer. Set ECN CE bit to
- Use inner TOS bits with some change. If outer ECN CE bit is
1, enable ECN CE bit on the inner.
Note that the ECN friendly behavior violates RFC2893. This should be used in
mutual agreement with the peer.
A malicious party may try to circumvent security filters by using tunnelled
packets. For better protection, gif
performs both martian
and ingress filtering against the outer source address on egress. Note that
martian/ingress filters are in no way complete. You may want to secure your
node by using packet filters. Ingress filtering can break tunnel operation in
an asymmetrically routed network. It can be turned off by
Processing each packet requires two route lookups: first on the packet itself,
and second on the tunnel destination. This second route can be cached,
increasing tunnel performance. However, in a dynamically routed network, the
tunnel will stick to the cached route, ignoring routing table updates. Route
caching can be enabled with the
By default, gif
tunnels may not be nested. This behavior may
be modified at runtime by setting the sysctl(8)
to the desired level of
nesting. Additionally, gif
tunnels are restricted to one per
pair of end points. Parallel tunnels may be enabled by setting the
R. Gilligan and E.
Nordmark, Transition Mechanisms for IPv6 Hosts and
Routers, RFC2893, August
Sally Floyd, David
L. Black, and K. K. Ramakrishnan,
IPsec Interactions with ECN,
device first appeared in the WIDE hydrangea IPv6 kit.
There are many tunnelling protocol specifications, all defined differently from
each other. The gif
device may not interoperate with peers
which are based on different specifications, and are picky about outer header
fields. For example, you cannot usually use gif
to talk with
IPsec devices that use IPsec tunnel mode.
The current code does not check if the ingress address (outer source address)
configured in the gif
interface makes sense. Make sure to
specify an address which belongs to your node. Otherwise, your node will not
be able to receive packets from the peer, and it will generate packets with a
spoofed source address.
If the outer protocol is IPv4, gif
does not try to perform
path MTU discovery for the encapsulated packet (DF bit is set to 0).
If the outer protocol is IPv6, path MTU discovery for encapsulated packets may
affect communication over the interface. The first bigger-than-pmtu packet may
be lost. To avoid the problem, you may want to set the interface MTU for
to 1240 or smaller, when the outer header is IPv6 and
the inner header is IPv4.
device does not translate ICMP messages for the outer
header into the inner header.
In the past, gif
had a multi-destination behavior,
flag. The behavior is
obsolete and is no longer supported.
6.1, 6.2, 6.3, 7.0, 7.1, and 7.2 the
sends and receives incorrect EtherIP packets with
reversed version field when if_bridge(4)
is used together.
As a workaround on this interoperability issue, the following two
flags can be used:
- accepts both correct EtherIP packets and ones with reversed
version field, if enabled. If disabled, the gif accepts
the correct packets only. This flag is enabled by default.
- sends EtherIP packets with reversed version field
intentionally, if enabled. If disabled, the gif sends
the correct packets only. This flag is disabled by default.
If interoperability with the older FreeBSD
needed, both of these two flags must be enabled.