How to enable windows authentication through a reverse proxy?

NTLM won't work if the TCP packets are not forwarded exactly as the reverse proxy received > them. And that's why many reverse proxy doesn't work with NTLM authentication. (like nginx) > They forward HTTP requests correcty but not the TCP packets.

Nginx has the functionality to work with NTLM authentication. Keepalive needs to be enabled which is only available trough the http_upstream_module. Additionally in the location block you need to specify that you will be using HTTP/1.1 and that the "Connection" header field should be cleared for each proxied request. Nginx config should look something like:

upstream http_backend {
    server 1.1.1.1:80;

    keepalive 16;
}

server {
...

location / {
       proxy_pass http://http_backend/;
       proxy_http_version 1.1;
       proxy_set_header Connection "";
    ...
    }
 }

I scratched my head for quite some time with this issue but the above works for me. Note that if you need to proxy HTTPS traffic, a separate upstream block is deemed necessary. To clarify a bit more, "keepalive 16;" specifies the number of simultaneous connections to the upstream your proxy is allowed to keep. Adjust the number as per the expected number of simultaneous visitors on the site.

Although this is an old post, I just want to report that it works for me quite well with an Apache2.2 reverse proxy and the keepalive=on option. Obviously, this keeps the connection between the proxy and the SharePoint host open and "pinned" to the client<>proxy connection. I don't exactly know the mechanisms behind this, but it works fairly well.

But: Sometimes, my users encounter the issue that they're logged in as another user. So there seems to be some mixing-up through sessions. I will have to give this some further testing.

Solution for everything (in case you have a valid, signed SSL certificate): Switch IIS to Basic Auth. This works absolutely fine, and even Windows (i.e. Office with SharePoint connection, all WebClient-based processes etc.) won't complain at all. But they will when you're just using http without SSL/TLS, and also with self-signed certificates.

I found what the problem was (and it is NTLM): in order to have the browser asks the user for its credentials, the response must have a 401 status code. My reverse proxy was forwarding the response to the browser, so IIS was adding a standard HTML code to explain the requested page cannot be accessed thus preventing the browser from asking credentials. The problem was solved by removing the response content when the status code is a 401.

With all due respect I have for the one that answered that some years ago, I must admit this is plainly false. The problem was indeed solved AFTER removing the response content when the status code is a 401, but it had none to do with the initial problem.. The truth is that windows authentication was made to authenticate people over local windows networks, where no proxy server is present or even needed. The main problem with NTLM authentication is that this protocol does not authenticate the HTTP session but the underlying TCP connection, and as far as I know there is no way to access it from asp code. Every proxy server I tried broke NTLM authentication. Windows authentication is comfortable for an user because he won't ever need to enter your password to whatever application may lie in your intranet, frightening for a security guy because there is an auto-login without even a prompt if the site domain is trusted by IE, shocking for a network administrator because it melts the application, transport and network layer into some "windows ball of mug" instead of just plain http traffic.

I confirm that it works with "keep-alive=on" on apache2.2

I examined frames with Wireshark, and I know why it doesn't work. NTLM won't work if the TCP packets are not forwarded exactly as the reverse proxy received them. That's why many reverse proxies, like nginx, don't work with NTLM authentication. Reverse proxies forward HTTP requests correctly but not the TCP packets.

NTLM requires a TCP reverse proxy.