Use Curl with Sni (Server Name Indication)

Use cURL with SNI (Server Name Indication)

To be able to use SNI, three conditions are required:

• Using a version of Curl that supports it, at least 7.18.1, according to the change logs.
• Using a version of Curl compiled against a library that supports SNI, e.g. OpenSSL 0.9.8j (depending on the compilation options some older versions).
• Using TLS 1.0 at least (not SSLv3).

Note that Curl's debug code (-v) only displays the major version number (mainly to distinguish between SSLv2 and SSLv3+ types of messages, see ssl_tls_trace), so it will still display "SSLv3" when you use TLS 1.0 or above (because they're effectively SSL v3.1 or above, 3 is the same major version number).

You could check that your installed version of curl can use SNI using Wireshark. If you make a connection using curl -1 https://something, if you expand the "Client Hello" message, you should be able to see a "server_name" extension.

I'm not sure which SSL/TLS version is used by default (depending on your compilation options) when you use curl without -1 (for TLS 1.0) or -3 (for SSLv3), but you can try to force -1 on your command, since it won't work with SSLv3 anyway.

fake Server Name indication (SNI) in libcurl with OpenSSL backend

If you want to "fake" the SNI then CURLOPT_RESOLVE or CURLOPT_CONNECT_TO are available options to reach the same end goal.

CURLOPT_RESOLVE example

Run a HTTPS server on 127.0.0.1 but make curl think it is example.com when it connects to it (so it sends that as SNI and in the Host: header)

CURL *curl;
struct curl_slist *host = NULL;
host = curl_slist_append(NULL, "example.com:443:127.0.0.1");

curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_RESOLVE, host);
  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

  curl_easy_perform(curl);

  /* always cleanup */
  curl_easy_cleanup(curl);
}

curl_slist_free_all(host);

CURLOPT_CONNECT_TO example

Run a dev HTTPS server on the host name server1.example.com but you want curl to connect to it thinking it is the www.example.org server.

CURL *curl;
struct curl_slist *connect_to = NULL;
connect_to = curl_slist_append(NULL, "www.example.org::server1.example.com:");

curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_CONNECT_TO, connect_to);
  curl_easy_setopt(curl, CURLOPT_URL, "https://www.example.org");

  curl_easy_perform(curl);

  /* always cleanup */
  curl_easy_cleanup(curl);
}

curl_slist_free_all(connect_to);

curl: how to specify target hostname for https request

Indeed SNI in TLS does not work like that. SNI, as everything related to TLS, happens before any kind of HTTP traffic, hence the Host header is not taken into account at that step (but will be useful later on for the webserver to know which host you are connecting too).

So to enable SNI you need a specific switch in your HTTP client to tell it to send the appropriate TLS extension during the handshake with the hostname value you need.

In case of curl, you need at least version 7.18.1 (based on https://curl.haxx.se/changes.html) and then it seems to automatically use the value provided in the Host header. It alo depends on which OpenSSL (or equivalent library on your platform) version it is linked to.

See point 1.10 of https://curl.haxx.se/docs/knownbugs.html that speaks about a bug but explains what happens:

When given a URL with a trailing dot for the host name part: "https://example.com./", libcurl will strip off the dot and use the name without a dot internally and send it dot-less in HTTP Host: headers and in the TLS SNI field.

The --connect-to option could also be useful in your case. Or --resolve as a substitute to /etc/hosts, see https://curl.haxx.se/mail/archive-2015-01/0042.html for am example, or https://makandracards.com/makandra/1613-make-an-http-request-to-a-machine-but-fake-the-hostname
You can add --verbose in all cases to see in more details what is happening. See this example: https://www.claudiokuenzler.com/blog/693/curious-case-of-curl-ssl-tls-sni-http-host-header ; you will also see there how to test directly with openssl.

If you have a.example in your /etc/hosts you should just run curl with https://a.example/ and it should take care of the Host header and hence SNI (or use --resolve instead)

So to answer your question directly, replace

curl --header 'Host: a.example' https://x.example

with

curl --connect-to a.example:443:x.example:443 https://a.example

and it should work perfectly.

RESTful, web-facing service with SNI (Server Name Indication)

If your Spring application is behind an apache web server, acting as a gateway (a.k.a. reverse proxy), you can use the following apache configuration to restrict the access:

<Location "/url/of/the/webservice">
    Require ip 111.222.333.444
</Location>

From the documentation :

The <Location> directive limits the scope of the enclosed directives by URL.

This Require directive tests whether an authenticated user is authorized according to a particular authorization provider and the specified restrictions.

Getting Server/ Server name indication in Python

SNI

You should be able to get there with Python 3.x series, 3.2+ if I understand correctly.

If you're stuck with Python 2, look at what python-requests does with two optional dependencies, ndg-httpsclient and pyasn1.

My understanding is that you can't really do much with plain Python <= 2.7.8 due to devs' perplexing view that SNI is somehow a feature of Python

EDIT

Thanks to PEP466, Python 2.7.9 contains ssl module backported from Python 3.4 and you have all the OpenSSL features.

Other projects

Perhaps sslyze (github) can do what you want already?

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