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$ curl -I https://9.185.173.135
curl: (35) Unknown SSL protocol error in connection to 9.185.173.135:443
This is an secured page that I need to access. But I don't know how to obtain its certificate file. I tried to use Firefox, but it says couldn't get any ssl certificate once the url is entered.
$ curl -I http://9.185.173.135
HTTP/1.1 200 OK
Content-Length: 686
Content-Type: text/html
Content-Location: http://9.185.173.135/Default.htm
Last-Modified: Mon, 16 Mar 2009 05:05:38 GMT
Accept-Ranges: bytes
ETag: "a851dbd8f4a5c91:d41"
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 13 Jul 2010 04:09:35 GMT
The server is definitely reachable from my laptop. Once I get the certificate file, I assume I can then import it to Firefox and then use my credentials to pass the authentication (I already got the username/password).
Sorry I am no expert in security at all. Is there anything else I can try?
Many thanks in advance. 
                Can you establish a connection to the server with openssl s_client -host 9.185.173.135 -p 443?
– Rudi
                Jul 13, 2010 at 12:12
                my open_ssl wants me to do -connect <ip>:<port> instead of -host <ip> -p <port> - probably changed in newer version.
– froderik
                Sep 20, 2012 at 11:25
                But this will not work on SPDY. There you have to use spdycat. I am not the author but it's an open-source project.
– Ján Sáreník
                Sep 1, 2015 at 21:28
                FYI: we had this and it didn't work. From another answer we found: curl_setopt($ch, CURLOPT_SSLVERSION, 4); which worked. (Version 4, not 3).
– William Joss Crowcroft
                Sep 27, 2016 at 15:02
                There are security risks with this approach. See the PHP docs. php.net/manual/en/function.curl-setopt.php
– Goose
                Nov 1, 2016 at 18:03
                Please do not use arbitrary numbers for something that is supposed to receive a descriptive enum. @WilliamJossCrowcroft for example, incorrectly refers to 4 as "version 4" (likely this is CURL_SSLVERSION_TLSv1_0). The correct value is CURL_SSLVERSION_SSLv3.
– D.Go
                Nov 19, 2019 at 2:16
To Rudi : Thanks for the hint, that tells me a hell lot of info.
Somehow the admin of the secured page "refreshes" the state of certifications every day. So although I got blocked from accessing it yesterday, it generously lets me to grab another certificate and add it to the exception list of Firefox.
So everything is working, and I really learn something from yesterday's experience.
                Works also on a classic environment (Windows 10 / Apache). You saved me hours of research!
– Pierre Le Bot
                Feb 17, 2021 at 18:00
I got the same error when running curl/httpie against a Tomcat server on my localhost deployed from Eclipse. It turns out that default server.xml deployed by Eclipse disables https. Specifically, the section below is commented out in server.xml.
<Connector port="8443" protocol="org.apache.coyote.http11.Http11NioProtocol"
           maxThreads="150" SSLEnabled="true" scheme="https" secure="true"
           clientAuth="false" sslProtocol="TLS" />
After uncommenting it out and adding the two keystore parameters, the curl command starts working (with --insecure option if the certificate is self-signed).
<Connector port="8443" protocol="org.apache.coyote.http11.Http11NioProtocol"
           maxThreads="150" SSLEnabled="true" scheme="https" secure="true"
           clientAuth="false" sslProtocol="TLS" />
           keystoreFile="/path/to/your/keystore"
           keystorePass="yourpass" />
i have some solutions that fix the issue for me:
1] try update your curl/php/apache [ yum update ]
2] restart apache
Those worked for me!
I had the same error after updating my SSL certificate on the target SSL site.
My source OS was Centos 6 and updating to a new curl version solved it.
*Note I was already using the curl -k (insecure option) but I would still get that error.  Essentially this error is caused by nss or openssl being out of date. 
yum -y install curl nss openssl
Remember if you have a web application like PHP calling curl you will need to restart Apache to make the update take effect.
I've updated based on this guide: http://realtechtalk.com/curl_35_Unknown_SSL_protocol_error_in_connection_Solution_Centos-1988-articles
 curl https://localhost:3000
 curl: (35) Unknown SSL protocol error in connection to localhost:-9847
(not sure where that number -9847came from since I requested port 3000)
fix: turns out my server on port 3000 was running "http" not "https" go figure.
        
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