Some additional data:

I don't see this functionality documented in POSIX for getaddrinfo.

As outlined in #716, the HTTP/1.1 spec section 5.4 clearly says "If the target URI includes an authority component, then a client MUST send a field-value for Host that is identical to that authority component". (emphasis added by me). Sending a Host: header with such a trailing dot causes breakage with numerous http servers out there and we should not do that.

I am completely fine with stripping the dot of the Host: header.
The problem is in resolving the name for connecting to the server.

getadddrinfo specification is on a higher level (above nsswitch) and only referencing rfc1034 when it comes to name-resolving. This makes sense as there could as well be a non dns backends for resolving hosts.

getadddrinfo specification is on a higher level

getadddrinfo is exactly the API we work with (mostly) so that's the specification and docs we have to work with. It not being specified in POSIX makes it unreliable to depend on, even if there are (several?) implementations that work like this.

I am completely fine with stripping the dot of the Host: header.

It's just a violation of the HTTP/1.1 spec...

(if someone wants to work on a PR for this, I'll just warn that #3017 might land soon and will change hostname handling a little bit)

getadddrinfo is exactly the API we work with (mostly) so that's the specification and docs we have to work with. It not being specified in POSIX makes it unreliable to depend on, even if there are (several?) implementations that work like this.

POSIX is the interface to the operating system. But beyond that there are numerous standards (be it HTTP, IP, DNS, ...). The thing about trailing dots in domain names is covered in RFC1034 3.1.

The POSIX interface of getaddrinfo takes care of translating names to addresses. These are not necessarily names from DNS but could also be from NIS/yp, ldap and others. Still, DNS is quite prominent here and the documentation refers to it ("In many cases it is implemented by the Domain Name System, as documented in RFC 1034, RFC 1035, and RFC 1886.").
So POSIX does not specify the trailing-dot-topic, because it is not the right scope. Following the references will lead to the respective standard, though.

I am completely fine with stripping the dot of the Host: header.

It's just a violation of the HTTP/1.1 spec...

"I am completely fine" was highly exaggerated. What I meant is, that I do not care in the scope of this issue, because this issue is not about the Host-Header at all.
This issue is only about the name used as input for dns-resolving (or getaddrinfo) to establish the tcp connection. Currently curl removes the trailing dot before name resolution and that causes issues.

In combination with --resolve this feels even worse:

% ltrace -e 'getaddrinfo' -f curl --resolve example.com.:1234:192.0.2.1 http://example.com.:1234/foo
[pid 28812] libcurl.so.4->getaddrinfo("example.com", "1234", 0x555ddbb0d810, 0x7f8ee0356e50)        = 0
[pid 28812] +++ exited (status 0) +++

The stripping happens in the URI but not in the cache-entry from --resolve. So I use --resolve with the exact same host as found in the URI, but it will not apply.

So the question about dot-stripping or not is to be considered in at least 3 layers:

• SNI (nothing to do with HTTP, see https://lists.w3.org/Archives/Public/ietf-http-wg/2016JanMar/0444.html)
• TLS and Host: header
• name resolving for establishing a TCP/L4 connection

Are you interested in working on a pull-request for this?

Which kind of approach would be acceptable?
Changing the dot-behavior for the resolving part only, but "fixing" it by not stripping the dot?

Having the dot present for name resolving, but stripping it for SNI and Host: header I'd say.

Then, yes.
I would like to look into this, but cannot promise much capacity.