GIF(7P)GIF(7P)NAMEgif - Generic tunnel interface
SYNOPSIS
pseudo-device gif [count]
DESCRIPTION
The gif interface is a generic tunnelling pseudo device for IPv4 and
IPv6. It can tunnel IPv[46] traffic over IPv[46]. Therefore, there can
be four possible configurations. The behavior of gif is mainly based on
RFC1933 IPv6-over-IPv4 configured tunnel.
To use gif, administrator needs to configure protocol and addresses used
for the outer header. This can be done by using gifconfig(1), or
SIOCSIFPHYADDR ioctl. Also, the administrator needs to configure
protocol and addresses used for the inner header by using ifconfig(1).
Note that IPv6 link-local address (those start with fe80::) will be
automatically configured whenever possible. You may need to remove IPv6
link-local address manually using ifconfig(1), when you would like to
disable the use of IPv6 as inner header (like when you need pure IPv4-
over-IPv6 tunnel). Finally, use routing table to route the packets
toward gif interface.
gif interface can be configued to perform bidirectional tunnel, or
multi-destination tunnel. This is controlled by IFF_LINK0 interface
flag.
Bidirectional and multi-destination mode
Usually, gif implements bidirectional tunnel. gifconfig(1) should
configure a tunnel ingress point (this node) and an egress point (tunnel
endpoint), and one gif interface will tunnel to only a single tunnel
endpoint, and accept from only a single tunnel endpoint. Source and
destination address for outer IP header is always the ingress and the
egress point configued by gifconfig(1).
With IFF_LINK0 interface flag, gif can be configured to implement multi-
destination tunnel. With IFF_LINK0, it is able to configure egress point
to IPv4 wildcard address (0.0.0.0) or IPv6 unspecified address (0::0). In
this case, destination address for the outer IP header is determined
based on the routing table setup. Therefore, one gif interface can
tunnel to multiple destinations. Also, gif will accept tunneled traffic
from any outer source address.
When finding a gif interface from the inbound tunneled traffic,
bidirectional mode interface is preferred than multi-destination mode
interface. For example, if you have the following three gif interfaces
on node A, tunneled traffic from C to A will match the second gif
interface, not the third one.
Page 1
GIF(7P)GIF(7P)
o bidirectional, A to B
o bidirectional, A to C
o multi-destination, A to any
Please note that multi-destination mode is far less secure than
bidirectional mode. Multi-destination mode gif can accept tunneled
packet from anybody, and can be attacked from a malicious node.
Security
Malicious party may try to circumvent security filters by using tunnelled
packets. For better protection, gif performs martian filter and ingress
filter against outer source address, on egress. Note that
martian/ingress filters are no way complete. You may want to secure your
node by using packet filters.
As mentioned above, multi-destination mode (IFF_LINK0) is far less secure
than bidirectional mode.
SEE ALSOinet(7), inet6(7), gifconfig(1)
R. Gilligan, and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
Routers", RFC1933, April 1996, ftp://ftp.isi.edu/in-notes/rfc1933.txt.
Sally Floyd, David L. Black, and K. K. Ramakrishnan, IPsec Interactions
with ECN, December 1999, draft-ietf-ipsec-ecn-02.txt.
HISTORY
The gif device first appeared in WIDE hydrangea IPv6 kit.
BUGS
There are many tunnelling protocol specifications, defined differently
from each other. gif 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 to gif makes sense. Make sure to configure an
address which belongs to your node. Otherwise, your node will not be
able to receive packets from the peer, and your node will generate
packets with a spoofed source address.
gif(7) is an IFF_POINTOPOINT device, however, it supports NBMA behavior
in multi-destination mode.
Page 2
