libcurl acting differently to curl.exe

[Josh via Digitalmars-d-learn]

I'm trying to use libcurl in D to download a page that requires 
logging in first. At the moment though, I can't even get the 
logging in working. I tried with curl.exe, got it working, and 
used the --libcurl command to export C code that I then turned 
into (I think) equivalent D code.

The problem is that when I POST the login form, curl.exe gets a 
HTTP 200 to the correct page, whereas libcurl gets a HTTP 302 
back to the login page, which is the same behaviour I noticed 
when cookies weren't being saved/reused.

I've tried with the HTTP struct functions, and also setting curl 
options explicitly like the C code does. Is there something I've 
missed?

import std.file;
import std.net.curl;
import std.stdio;

void main() {
        getLogon();
        postLogon();
        getIndex();
}

void getLogon()
{
        auto http = HTTP();
	http.onReceive = (ubyte[] data) { /+ drop +/ 
std.file.write("logon1.html", data); 
/+writeln(cast(char[])(data)); stdout.flush;+/ return 
data.length; };
        
        http.handle.set(CurlOption.tcp_nodelay, 1);
        http.handle.set(CurlOption.buffersize, 102400);
        http.handle.set(CurlOption.noprogress, 1);
        //http.handle.set(CurlOption.useragent, "curl/7.57.0");
        http.handle.set(CurlOption.maxredirs, 50);
	//http.handle.set(CurlOption.cainfo, 
"C:\\Users\\Josh\\Downloads\\curl-7.57.0-win64-mingw\\bin\\curl-ca-bundle.crt");
        http.handle.set(CurlOption.cookiejar, "cookie.dat");
        http.handle.set(CurlOption.cookiefile, "cookie.dat");
        http.handle.set(CurlOption.verbose, 1);
        
        http.handle.set(CurlOption.url, "https://foo.com/logon.php";);
        http.method(HTTP.Method.get);
        
        http.perform();
}

void doLogon()
{
        auto http = HTTP();
	http.onReceive = (ubyte[] data) { /+ drop +/ 
std.file.write("logon2.html", data); 
/+writeln(cast(char[])(data)); stdout.flush;+/ return 
data.length; };
        
        http.handle.set(CurlOption.tcp_nodelay, 1);
        http.handle.set(CurlOption.buffersize, 102400);
        http.handle.set(CurlOption.noprogress, 1);
        //http.handle.set(CurlOption.useragent, "curl/7.57.0");
        http.handle.set(CurlOption.maxredirs, 50);
	//http.handle.set(CurlOption.cainfo, 
"C:\\Users\\Josh\\Downloads\\curl-7.57.0-win64-mingw\\bin\\curl-ca-bundle.crt");
        http.handle.set(CurlOption.cookiejar, "cookie.dat");
        http.handle.set(CurlOption.cookiefile, "cookie.dat");
        http.handle.set(CurlOption.verbose, 1);
        
        http.handle.set(CurlOption.url, "https://foo.com/logon.php";);
	http.handle.set(CurlOption.postfields, 
"username=user&password=pass&Logon=submit");
        http.handle.set(CurlOption.postfieldsize_large, 52);
        http.method(HTTP.Method.post);
        
        http.perform();
}

void getIndex()
{
        auto http = HTTP();
	http.onReceive = (ubyte[] data) { /+ drop +/ 
std.file.write("index.html", data); 
/+writeln(cast(char[])(data)); stdout.flush;+/ return 
data.length; };
        
        http.handle.set(CurlOption.tcp_nodelay, 1);
        http.handle.set(CurlOption.buffersize, 102400);
        http.handle.set(CurlOption.noprogress, 1);
        //http.handle.set(CurlOption.useragent, "curl/7.57.0");
        http.handle.set(CurlOption.maxredirs, 50);
	//http.handle.set(CurlOption.cainfo, 
"C:\\Users\\Josh\\Downloads\\curl-7.57.0-win64-mingw\\bin\\curl-ca-bundle.crt");
        http.handle.set(CurlOption.cookiejar, "cookie.dat");
        http.handle.set(CurlOption.cookiefile, "cookie.dat");
        http.handle.set(CurlOption.verbose, 1);
        
        http.handle.set(CurlOption.url, "https://foo.com/index.php";);
        http.method(HTTP.Method.get);
        
        http.perform();
}

libcurl verbose dump: https://pastebin.com/Sq60CLHV
curl.exe verbose dump: https://pastebin.com/KBDDNq9k

Thanks,

Josh