Enable support for DHCP option 15 (domain name)

[korken89]

This makes smoltcp work in industrial networks with Windows DHCPs as they do not support option 119.

I'm not sure if the result should be directly tied into the dns socket, but the IPs for DNS servers are manually supplied so I follow the same pattern here and assume that users will build the FQDN before doing a DNS request.

Feedback welcome!

[korken89]

I see clippy is a bit angry with the size of the domain name and the impact it has.
Should I move the max size into gen_config.py and set a default that is smaller than the maximum to make clippy happy?
E.g. a domain name size of 64 is probably enough for almost everything in my experience.

[korken89]

I added it in my latest commit. Also realized I was doing wrong in DhcpRepr and moved to use a reference there instead of an owned String.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.29%. Comparing base (6a3f926) to head (9778985).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #972      +/-   ##
==========================================
+ Coverage   79.18%   79.29%   +0.10%     
==========================================
  Files          81       81              
  Lines       26885    26944      +59     
==========================================
+ Hits        21289    21365      +76     
+ Misses       5596     5579      -17     

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[korken89]

This PR builds on top of #974 to make no_std apps build

[korken89]

Also, here is an example of it working from a network I'm testing this PR on

[Dirbaio]

the original design of DHCP was to provide the raw bytes in Config::packet, so the user can parse the options they're interested in from there. The reason for this is I think adding explicit fields in Repr and Config for every single option a user might want is not going to scale, there's a lot of them.

[korken89]

Hi @Dirbaio , I fully agree on the idea of having special options being handled outside as it is designed.
In the PR I was working on for embassy-net I started that path, but changed direction to add it here.

The motivation is that without domain name the DNS implementation is only working on non-FQDN networks or with manual FQDN lookups, and the only discrepancy was networks with domain name and automatically forming non-FQDN -> FQDN.

To me this warranted the direct inclusion in smoltcp instead of a special external handling as it's so fundamental to lookup.
The line-in-the-sand I formed for myself while working on this was that vendor extensions were candidates for direct inclusion in smoltcp and anything else should be handled in the aforementioned API.
If this is not the case I can make a PR to document where the line goes between smoltcp inclusion and not. 🚀

[korken89]

Another idea is to feature gate this so people do not pay for it unless they need it.
That would alleviate the scaling issue. What are your thoughts @Dirbaio ?

[korken89]

I made it optional in my last commit just to show what I mean. I can drop the commit if it's considered as not needed.