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
is mainly based on RFC2893
IPv6-over-IPv4 configured tunnel. On NetBSD
can also tunnel ISO traffic over
IPv using EON encapsulation. Note that
does not perform GRE encapsulation; use
for GRE encapsulation.
interface is created at runtime
using interface cloning. This is most easily done with the
” command or using the
, 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 start with
) will be automatically configured whenever
possible. You may need to remove IPv6 link-local addresses manually using
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
device can be configured to be ECN
friendly, as described in
. This is turned
off by default, and can be turned on by the
will show 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
- 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,
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
tunnels may not be nested.
This behavior may be modified at runtime by setting the
desired level of nesting.
R. Gilligan and
E. Nordmark, Transition Mechanisms
for IPv6 Hosts and Routers, RFC2893,
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
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
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
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,
try to perform path MTU discovery for the encapsulated packet (DF bit is set
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,
had a multi-destination
behavior, configurable via
The behavior is obsolete and is no longer supported.