Tracking IPv6 link local support in browsers
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Nico Schottelius
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content/u/blog/ipv6-link-local-support-in-browsers/contents.lr
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title: Support for IPv6 link local addresses in browsers
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---
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pub_date: 2021-06-14
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author: ungleich
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twitter_handle: ungleich
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_hidden: no
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---
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_discoverable: yes
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---
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abstract:
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Tracking the progress of browser support for link local addresses
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---
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body:
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## Introduction
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Link Local addresses
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([fe80::/10](https://en.wikipedia.org/wiki/Link-local_address)) are
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used for addressing devices in your local subnet. They can be
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automatically generated and using the IPv6 multicast address
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**ff02::1**, all hosts on the local subnet can easily be located.
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However browsers like Chrome or Firefox do not support **entering link
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local addresses inside a URL**, which prevents accessing devices
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locally with a browser, for instance for configuring them.
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Link local addresses need **zone identifiers** to specify which
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network device to use as an outgoing interface. This is because
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**you have link local addresses on every interface** and your network
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stack does not know on its own, which interface to use. So typically a
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link local address is something on the line of
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**fe80::fae4:e3ff:fee2:37a4%eth0**, where **eth0** is the zone
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identifier.
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Them problem is becoming more emphasised, as the world is moving more
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and more towards **IPv6 only networks**.
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You might not even know the address of your network equipment anymore,
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but you can easily locate iit using the **ff02::1 multicast
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address**. So we need support in browsers, to allow network
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configurations.
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## Status of implementation
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The main purpose of this document is to track the status of the
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link-local address support in the different browsers and related
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standards. The current status is:
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* Firefox says whatwg did not define it
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* Whatwg says zone id is intentionally omitted and and reference w3.org
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* w3.org has a longer reasoning, but it basically boils down to
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"Chrome does not do it and it's complicated and nobody needs it"
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* Chromium says it seems not to be worth the effort
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## IPv6 link local address support in Firefox
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The progress of IPv6 link local addresses for Firefox is tracked
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on [the mozilla
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bugzilla](https://bugzilla.mozilla.org/show_bug.cgi?id=700999). The
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current situation is that Firefox references to the lack of
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standardisation by whatwg as a reason for not implementing it. Quoting
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Valentin Gosu from the Mozilla team:
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```
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The main reason the zone identifier is not supported in Firefox is
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that parsing URLs is hard. You'd think we can just pass whatever
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string to the system API and it will work or fail depending on whether
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it's valid or not, but that's not the case. In bug 1199430 for example
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it was apparent that we need to make sure that the hostname string is
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really valid before passing it to the OS.
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I have no reason to oppose zone identifiers in URLs as long as the URL
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spec defines how to parse them. As such, I encourage you to engage
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with the standard at https://github.com/whatwg/url/issues/392 instead
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of here.
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Thank you!
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```
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## IPv6 link local address support in whatwg
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The situation at [whatwg](https://whatwg.org/) is that there is a
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[closed bugreport on github](https://github.com/whatwg/url/issues/392)
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and [in the spec it says](https://url.spec.whatwg.org/#concept-ipv6)
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that
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Support for <zone_id> is intentionally omitted.
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That paragraph links to a bug registered at w3.org (see next chapter).
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## IPv6 link local address support at w3.org
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At [w3.org](https://www.w3.org/) there is a
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bug titled
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[Support IPv6 link-local
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addresses?](https://www.w3.org/Bugs/Public/show_bug.cgi?id=27234#c2)
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that is set to status **RESOLVED WONTFIX**. It is closed basically
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based on the following statement from Ryan Sleevi:
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```
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Yes, we're especially not keen to support these in Chrome and have
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repeatedly decided not to. The platform-specific nature of <zone_id>
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makes it difficult to impossible to validate the well-formedness of
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the URL (see https://tools.ietf.org/html/rfc4007#section-11.2 , as
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referenced in 6874, to fully appreciate this special hell). Even if we
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could reliably parse these (from a URL spec standpoint), it then has
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to be handed 'somewhere', and that opens a new can of worms.
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Even 6874 notes how unlikely it is to encounter these in practice -
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"Thus, URIs including a
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ZoneID are unlikely to be encountered in HTML documents. However, if
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they do (for example, in a diagnostic script coded in HTML), it would
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be appropriate to treat them exactly as above."
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Note that a 'dumb' parser may not be sufficient, as the Security Considerations of 6874 note:
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"To limit this risk, implementations MUST NOT allow use of this format
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except for well-defined usages, such as sending to link-local
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addresses under prefix fe80::/10. At the time of writing, this is
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the only well-defined usage known."
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And also
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"An HTTP client, proxy, or other intermediary MUST remove any ZoneID
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attached to an outgoing URI, as it has only local significance at the
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sending host."
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This requires a transformative rewrite of any URLs going out the
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wire. That's pretty substantial. Anne, do you recall the bug talking
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about IP canonicalization (e.g. http://127.0.0.1 vs
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http://[::127.0.0.1] vs http://012345 and friends?) This is
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conceptually a similar issue - except it's explicitly required in the
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context of <zone_id> that the <zone_id> not be emitted.
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There's also the issue that zone_id precludes/requires the use of APIs
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that user agents would otherwise prefer to avoid, in order to
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'properly' handle the zone_id interpretation. For example, Chromium on
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some platforms uses a built in DNS resolver, and so our address lookup
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functions would need to define and support <zone_id>'s and map them to
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system concepts. In doing so, you could end up with weird situations
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where a URL works in Firefox but not Chrome, even though both
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'hypothetically' supported <zone_id>'s, because FF may use an OS
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routine and Chrome may use a built-in routine and they diverge.
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Overall, our internal consensus is that <zone_id>'s are bonkers on
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many grounds - the technical ambiguity (and RFC 6874 doesn't really
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resolve the ambiguity as much as it fully owns it and just says
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#YOLOSWAG) - and supporting them would add a lot of complexity for
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what is explicitly and admittedly a limited value use case.
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```
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This bug references the Mozilla Firefox bug above and
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[RFC3986 (replaced by RFC
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6874)](https://datatracker.ietf.org/doc/html/rfc6874#section-2).
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## IPv6 link local address support in Chrome / Chromium
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On the chrome side there is a
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[huge bug
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report](https://bugs.chromium.org/p/chromium/issues/detail?id=70762)
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which again references a huge number of other bugs that try to request
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IPv6 link local support, too.
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The bug was closed by cbentzel@chromium.org stating:
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```
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There are a large number of special cases which are required on core
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networking/navigation/etc. and it does not seem like it is worth the
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up-front and ongoing maintenance costs given that this is a very
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niche - albeit legitimate - need.
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```
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