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

[Sly Gryphon]

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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