---
name: "gif"
section: "4"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/gif"
---

# gif(4)

generic tunnel interface

## 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 RFC2893 IPv6-over-IPv4 configured tunnel. On NetBSD, **gif** can also tunnel ISO traffic over IPv\[46\] using EON encapsulation.

Each **gif** interface is created at runtime using interface cloning. This is done with the [ifconfig(8)](/ifconfig/8.md) **create** command.

To use **gif**, administrator needs to configure protocol and addresses used for the outer header. This can be done by using the SIOCSIFPHYADDR ioctl. Also, administrator needs to configure protocol and addresses used for the inner header, by using [ifconfig(8)](/ifconfig/8.md). 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)](/ifconfig/8.md), 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 RFC2893. 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 RFC2893. This should be used in mutual agreement with the peer.

### 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. Ingress filter can be turned off by IFF\_LINK2 bit.

### Miscellaneous

By default, **gif** tunnels may not be nested. This behavior may be modified at runtime by setting the [sysctl(8)](/sysctl/8.md) variable *net.link.gif.max\_nesting* 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 [sysctl(8)](/sysctl/8.md) variable *net.link.gif.parallel\_tunnels* to 1.

## SEE ALSO

[inet(4)](/inet/4.md), [inet6(4)](/inet6/4.md), R. Gilligan and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers”, *RFC2893*, August 2000, ftp://ftp.isi.edu/in-notes/rfc2893.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 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 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 packet 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 **gif** to 1240 or smaller, when outer header is IPv6 and inner header is IPv4.

**gif** does not translate ICMP messages for outer header into inner header.

In the past, **gif** had a multi-destination behavior, configurable via IFF\_LINK0 flag. The behavior was obsoleted and is no longer supported.

It is thought that this is not actually a bug in gif, but rather lies somewhere around a manipulation of an IPv6 routing table.