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[lwip-devel] DNS destination address selection improvements for IPv6


From: Sly Gryphon
Subject: [lwip-devel] DNS destination address selection improvements for IPv6
Date: Wed, 13 Mar 2024 22:18:48 +1000

Hi,

I have been working on IPv6 in ESP32 Arduino, which led back to ESP-IDF, and then some changes in ESP-LWIP.

As ESP-LWIP is based on LWIP, I thought it would be best to contribute those changes upstream, as they will benefit/apply to all users of LWIP.

The problem was certain specific scenarios don't work, e.g. with both protocols enabled, looking up a dual-stack server from an IPv6 only network, doesn't work.

The client has IPv6 enabled, the network supports IPv6, and the destination supports IPv6, but DNS lookup gives the wrong result.

If you don't have IPv4 enabled, then it works fine; i.e. enabling IPv4 breaks the IPv6 scenario.

An example of the problem (ESP-IDF example) is detailed here: https://github.com/espressif/esp-idf/issues/13255

Ultimately it came down to the current static ordering of IPv4 vs IPv6 in LWIP DNS. (Changing the static ordering would have the same problem in reverse.)

So, I have a working solution, using RFC 6724 destination address selection, which dynamically selects which destination address to use based on available source addresses. i.e. if an IPv6 global scope address (and not IPv4) is available it will return IPv6 and vice-versa. Posix/linux getaddrinfo() has a similar sorting documented (RFC 6724 replaces RFC 3484): https://man7.org/linux/man-pages/man3/getaddrinfo.3.html

So rather than a static order it has dynamic ordering. I put the changes behind an option flag so that the old behaviour can be preserved. The functions are also written in such a way that we could easily add a hook to replace the default ordering if we want to add that. The current code knows you currently only get one result from DNS lookup, so maximum of 2 (one IPv6, one IPv4), so is simplified for that case, but could be extended in the future to compare multiple and keep the best (or even sort results).

Note that RFC 6724 _source_ address selection is already implemented in `ip6_select_source_address()` in `ip6.c`; this code implementing destination address selection.

Proposed implementation of RFC 6724 destination address selection (this is what I would like to contribute): https://github.com/espressif/esp-lwip/pull/66

I am not sure how to raise a pull request against upstream LWIP.

Sly Gryphon

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