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Re: Retrieving the local address between connect() and HTTP

From: Jean-Francois Dockes via curl-library <curl-library_at_cool.haxx.se>
Date: Sun, 26 Apr 2020 22:44:32 +0200

Ray Satiro via curl-library writes:
> On 4/26/2020 10:40 AM, Jean-Francois Dockes via curl-library wrote:
> > My apologies if this is a frequent question, I could not find an answer.
> >
> > It would be convenient for me to retrieve the local address of the
> > connected socket used for an HTTP GET before the HTTP transaction
> > begins.
> >
> > The pseudo HTTP protocol which my code implements (UPnP) needs to set a
> > HTTP header which contains a callback URL for the remote host to send
> > events to. In case the local machine has multiple interfaces, the easiest
> > way to build an URL which the remote is guaranteed to be able to connect to
> > seems to be just using the connected socket local address.
> >
> > Unfortunately, it seems that the two callbacks in this area,
> > CURLOPT_SOCKOPTFUNCTION and CURLOPT_OPENSOCKETFUNCTION, occur before the
> > socket is connected.
> >
> > I could connect the socket myself (losing the nice CURL code which does it
> > for me), or find another way to derive the right interface from the
> > remote address I am connecting to, but a getsockname() on the connected
> > socket would be much simpler. So I was hoping that there was a way.
> >
> > This is assuming that I can call CURLOPT_HTTPHEADER from this point, which
> > may also not be the case. So, questions...
>
> I don't think that is possible without doing the connect yourself. There
> is CURLINFO_ACTIVESOCKET [1] but it doesn't set the socket until the
> transfer is done. It is really meant to be used with
> CURLOPT_CONNECT_ONLY [2].
>
> Try doing the connection yourself, set CURLOPT_HTTPHEADER and then in
> CURLOPT_SOCKOPTFUNCTION [3] return CURL_SOCKOPT_ALREADY_CONNECTED.

Thanks for the CURLOPT_CONNECT_ONLY pointer.

Is there something to prevent me from using the resulting socket fd with
CURLOPT_OPENSOCKETFUNCTION / CURLOPT_SOCKOPTFUNCTION ? I see that this is a
bit convoluted, but it gains me the use of the libcurl URL parsing and
connect code instead of DIY. Does it make sense ? Performance is no big
issue in my use case.

jf

> [1]: https://curl.haxx.se/libcurl/c/CURLINFO_ACTIVESOCKET.html
> [2]: https://curl.haxx.se/libcurl/c/CURLOPT_CONNECT_ONLY.html
> [3]: https://curl.haxx.se/libcurl/c/CURLOPT_SOCKOPTFUNCTION.html
>
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Received on 2020-04-26