GIF(4)

GIF(4) | Device Drivers Manual | GIF(4)

# NAME

`gif` — generic tunnel interface

# SYNOPSIS

`pseudo-device gif`

# DESCRIPTION

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.

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 protocol and addresses used for the inner header, by using ifconfig(8). 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(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 or IPv6. The outer packet may be IPv4 or IPv6, and has all the usual IP headers, including a protocol field that identifies 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`).

## 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 martian/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
        tunnel mode ipip
       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 ALSO

inet(4), inet6(4), l2tp(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 packets 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.

August 14, 2018 | NetBSD 11.0