GIF(4)

📦 version 12.2, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 GIF(4) — the generic tunnel interface page, bestie edition 💖

okay so gather round, we're talking about gif — the generic tunnel interface. No, not the file format, bestie, put the reaction image down. 🫶

✨ SYNOPSIS ✨

device gif

That's it. That's the whole vibe. 👑

🌈 DESCRIPTION 🌈

The gif interface is a generic tunnelling device for IPv4 and IPv6. It can tunnel IPv[46] traffic over IPv[46]. Do the math, that's four possible configurations, and honestly? Versatile queen behavior. 💅

The behavior of gif is mainly based on RFC2893, the IPv6-over-IPv4 configured tunnel spec. And on NetBSD, gif can also tunnel ISO traffic over IPv[46] using EON encapsulation. Iconic.

⚠️ Not me letting you get this wrong: gif does not perform GRE encapsulation. For that you want gre(4). Different device, different slay.

Each gif interface is created at runtime using interface cloning. Easiest way to do it is the “ifconfig create” command, or the ifconfig_⟨interface⟩ variable in rc.conf(5). No cap, that's the move. ✨

To actually use gif, the administrator needs to configure the protocol and addresses used for the outer header. You do that with ifconfig(8) tunnel, or the SIOCSIFPHYADDR ioctl. Then you also need to configure the protocol and addresses for the inner header, with ifconfig(8).

Little heads up though: IPv6 link-local addresses (the ones that start with fe80::) will be automatically configured whenever possible. So if you want to disable IPv6 as the inner header — say, you need a pure IPv4-over-IPv6 tunnel — you may need to remove those IPv6 link-local addresses manually using ifconfig(8). 🙈

And finally? You must modify the routing table to route the packets through the gif interface. It's giving "do the homework or the tunnel doesn't work." 😭

The gif device can be configured to be ECN friendly. This can be configured by IFF_LINK1. More on that below. 💖

🌸 ECN friendly behavior 🌸

The gif device 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 the IFF_LINK1 interface flag. 🎀

Without IFF_LINK1, gif will show normal behavior, as described in RFC2893. Summarized, it's:

  • 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, like this:

  • 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.
🚨 Okay important: the ECN friendly behavior violates RFC2893. This should be used in mutual agreement with the peer. Do not just turn this on and ghost them, bestie.

🛡️ Security 🛡️

A malicious party may try to circumvent security filters by using tunnelled packets. Rude. 😤

For better protection, gif 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. Also — ingress filtering can break tunnel operation in an asymmetrically routed network. It can be turned off by the IFF_LINK2 bit.

🦋 Miscellaneous 🦋

By default, gif tunnels may not be nested. This behavior may be modified at runtime by setting the sysctl(8) variable net.link.gif.max_nesting to the desired level of nesting. Nest away, bestie. 🪺

👀 SEE ALSO 👀

R. Gilligan and E. Nordmark, “Transition Mechanisms for IPv6 Hosts and Routers”, RFC2893, http://tools.ietf.org/html/rfc2893, August 2000.

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 the WIDE hydrangea IPv6 kit. A true elder icon. 🌸

💀 BUGS 💀

Okay so here's the messy part, and we're being honest about it:

  • There are many tunnelling protocol specifications, all defined differently from each other. The gif 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 gif to talk with IPsec devices that use IPsec tunnel mode. It's giving "we're not compatible, sorry babe." 😭
  • 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 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 gif to 1240 or smaller, when the outer header is IPv6 and the inner header is IPv4. Write that down. 📝
  • The gif device does not translate ICMP messages for the outer header into the inner header.
  • In the past, gif had a multi-destination behavior, configurable via the IFF_LINK0 flag. That behavior is obsolete and is no longer supported. Rip to a real one. 🪦

And that's the whole page, bestie! This one came from the Debian folks, dated October 21, 2018, and it's the GIF(4) page living in the Kernel Interfaces Manual. Go configure your tunnel, drink some water, and remember: ECN friendly mode is a conversation, not a monologue. 💖✨🎀🦋

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