GIF(4) BSD Kernel Interfaces Manual GIF(4)NAMEgif — generic tunnel interface
SYNOPSIS
pseudo-device gifDESCRIPTION
The gif interface is a generic tunneling 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 RFC 2893
IPv6-over-IPv4 configured tunnel. gif can also tunnel ISO traffic over
IPv[46] using EON encapsulation.
To use gif, the administrator must first create the interface and then
configure protocol and addresses used for the outer header. This can be
done by using ifconfig(8) create and tunnel subcommands, or SIOCIFCREATE
and SIOCSIFPHYADDR ioctls. Also, administrator needs to configure proto‐
col and addresses used for the inner header, by using ifconfig(8). Note
that IPv6 link-local address (those start with fe80::) will be automati‐
cally configured whenever possible. You may need to remove IPv6 link-
local address manually using ifconfig(8), 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 can be configured to be ECN friendly. This can be configured by
IFF_LINK1.
ECN friendly behavior
gif can be configured to be ECN friendly, as described in
draft-ietf-ipsec-ecn-02.txt. This is turned off by default, and can be
turned on by IFF_LINK1 interface flag.
Without IFF_LINK1, gif will show a normal behavior, like described in RFC
2893. This can be summarized as follows:
Ingress Set outer TOS bit to 0.
Egress Drop outer TOS bit.
With IFF_LINK1, gif will copy ECN bits (0x02 and 0x01 on IPv4 TOS byte or
IPv6 traffic class byte) on egress and ingress, as follows:
Ingress Copy TOS bits except for ECN CE (masked with 0xfe) from
inner to outer. set ECN CE bit to 0.
Egress 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 RFC 2893. This should be
used in mutual agreement with the peer.
Packet format
Every inner packet is encapsulated in an outer packet. The inner packet
may be IPv4, IPv6, or ISO CLNP. The outer packet may be IPv4 or IPv6,
and has all the usual IP headers, including a protocol field that identi‐
fies the type of inner packet.
When the inner packet is IPv4, the protocol field of the outer packet is
4 (IPPROTO_IPV4). When the inner packet is IPv6, the protocol field of
the outer packet is 41 (IPPROTO_IPV6). When the inner packet is ISO
CNLP, the protocol field of the outer packet is 80 (IPPROTO_EON).
Security
Malicious party may try to circumvent security filters by using tunneled
packets. For better protection, gif performs martian filter and ingress
filter against outer source address, on egress. Note that mar‐
tian/ingress filters are no way complete. You may want to secure your
node by using packet filters. Ingress filter can be turned off by
IFF_LINK2 bit.
EXAMPLES
Configuration example:
Host X--NetBSD A ----------------tunnel---------- cisco D------Host E
\ |
\ /
+-----Router B--------Router C---------+
On NetBSD system A (NetBSD):
# route add default B
# ifconfig gifN create
# ifconfig gifN A netmask 0xffffffff tunnel A D up
# route add E 0
# route change E -ifp gif0
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
or on Host D (NetBSD):
# route add default C
# ifconfig gifN D A
If all goes well, you should see packets flowing.
If you want to reach Host A over the tunnel (from the Cisco D), then you
have to have an alias on Host A for e.g. the Ethernet interface like:
ifconfig <etherif> alias Y and on the cisco ip route Y mask tunnelX.
SEE ALSOetherip(4), inet(4), inet6(4), ifconfig(8)
C. Perkins, "IP Encapsulation within IP", RFC 2003, ftp://ftp.isi.edu/in-
notes/rfc2003.txt, October 1996.
R. Gilligan and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
Routers", RFC 2893, ftp://ftp.isi.edu/in-notes/rfc2893.txt, August 2000.
Sally Floyd, David L. Black, and K. K. Ramakrishnan, IPsec Interactions
with ECN, http://datatracker.ietf.org/internet-drafts/draft-ietf-ipsec-
ecn/, December 1999.
F. Baker and P. Savola, "Ingress Filtering for Multihomed Networks", RFC
3704, ftp://ftp.isi.edu/in-notes/rfc3704.txt, March 2004.
STANDARDS
IPv4 over IPv4 encapsulation is compatible with RFC 2003. IPv6 over IPv4
encapsulation is compatible with RFC 2893.
HISTORY
The gif device first appeared in WIDE hydrangea IPv6 kit.
BUGS
There are many tunneling 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 pack‐
ets with a spoofed source address.
If the outer protocol is IPv6, path MTU discovery for encapsulated packet
may affect communication over the interface.
In the past, gif had a multi-destination behavior, configurable via
IFF_LINK0 flag. The behavior was obsoleted and is no longer supported.
BSD January 15, 2009 BSD
