net/http: allow processing requests without Host header

[awy]

What version of Go are you using (go version)?

$ go version
go version go1.15.7 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Not relevant

What did you do?

Run up an HTTP server using net/http.Server.ListenAndServe()

Send an illegal HTTP/1.1 request (missing Host header):

POST /endpoint HTTP/1.1
Content-Type: application/octet-stream
Content-Length: 12

Some content

I also looked at #35283 as being a possible solution but clearly not as it has been rejected.

What did you expect to see?

Valid response to request.

I am trying to write a server application that will accept these (invalid) requests from client software over which I have no control. Some of the clients in the field no longer have firmware updates available and so no fix can be expected there. I am looking for a means to modify the behaviour of only a small part of the standard http server.

What did you see instead?

Error response generated by server.go:(c *conn)readRequest()

	hosts, haveHost := req.Header["Host"]
	isH2Upgrade := req.isH2Upgrade()
	if req.ProtoAtLeast(1, 1) && (!haveHost || len(hosts) == 0) && !isH2Upgrade && req.Method != "CONNECT" {
		return nil, badRequestError("missing required Host header")
	}

[networkimprov]

cc @bradfitz @neild @odeke-em