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

[Sly Gryphon]

I have a branch with my code from ESP-LWIP moved across. Rather than a 
simple compare, I put in the structure of a full sort, and added the RFC 
6724 examples as test cases.

It still takes a few short cuts and only properly handles the LWIP case 
of at most 2 results (one IPv6 and one IPv4), so not all the example 
will work (e.g. can't have 2x IPv6, so comparing precedence only counts 
v4 vs v6).

`getaddrinfo()` now returns both results (linked; which is handled by 
`freeaddrinfo()`), so you may get up to two results (e.g. could be used 
for fallback/happy eyeballs in a dual stack environment).

I still need to run a test app to check it works; I couldn't see 
anything in LWIP (did find some unit test though, which I used).

I will try and getting it running with my ESP-IDF setup.

Branch is here, if anyone is interested:
https://github.com/sgryphon/lwip/tree/sgryphon/dns-dynamic-sort-rfc6724


On 13/3/24 22:18, Sly Gryphon wrote:
> > > 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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