source: binary-improvements2/MapRendering/Web/Handlers/SessionHandler.cs@ 384

using System;
using System.IO;
using System.Net;
using System.Text;
using HttpListenerRequest = SpaceWizards.HttpListener.HttpListenerRequest;
using HttpListenerResponse = SpaceWizards.HttpListener.HttpListenerResponse;

namespace AllocsFixes.NetConnections.Servers.Web.Handlers {
        public class SessionHandler : AbsHandler {
                private const string pageBasePath = "/";
                private const string steamOpenIdVerifyUrl = "verifysteamopenid";
                private const string steamLoginUrl = "loginsteam";
                
                private readonly string footer = "";
                private readonly string header = "";

                private readonly ConnectionHandler connectionHandler;

                public SessionHandler (string _dataFolder, ConnectionHandler _connectionHandler) : base (null) {
                        connectionHandler = _connectionHandler;
                        
                        if (File.Exists (_dataFolder + "/sessionheader.tmpl")) {
                                header = File.ReadAllText (_dataFolder + "/sessionheader.tmpl");
                        }

                        if (File.Exists (_dataFolder + "/sessionfooter.tmpl")) {
                                footer = File.ReadAllText (_dataFolder + "/sessionfooter.tmpl");
                        }
                }

                public override void HandleRequest (string _requestPath, HttpListenerRequest _req, HttpListenerResponse _resp, WebConnection _con,
                        int _permissionLevel) {
                        
                        IPEndPoint reqRemoteEndPoint = _req.RemoteEndPoint;
                        if (reqRemoteEndPoint == null) {
                                _resp.Redirect (pageBasePath);
                                return;
                        }

                        string subpath = _requestPath.Remove (0, urlBasePath.Length);

                        StringBuilder result = new StringBuilder ();
                        result.Append (header);

                        if (subpath.StartsWith (steamOpenIdVerifyUrl)) {
                                string remoteEndpointString = reqRemoteEndPoint.ToString ();

                                try {
                                        ulong id = OpenID.Validate (_req);
                                        if (id > 0) {
                                                WebConnection con = connectionHandler.LogIn (id, reqRemoteEndPoint.Address);
                                                int level = GameManager.Instance.adminTools.GetUserPermissionLevel (con.UserId);
                                                Log.Out ("Steam OpenID login from {0} with ID {1}, permission level {2}",
                                                        remoteEndpointString, con.UserId, level);
                                                
                                                Cookie cookie = new Cookie ("sid", con.SessionID, "/") {
                                                        Expired = false,
                                                        Expires = DateTime.MinValue,
                                                        HttpOnly = true,
                                                        Secure = false
                                                };
                                                _resp.AppendCookie (cookie);
                                                _resp.Redirect (pageBasePath);

                                                return;
                                        }
                                } catch (Exception e) {
                                        Log.Error ("Error validating login:");
                                        Log.Exception (e);
                                }

                                Log.Out ($"Steam OpenID login failed from {remoteEndpointString}");
                                result.Append ($"<h1>Login failed, <a href=\"{pageBasePath}\">click to return to main page</a>.</h1>");
                        } else if (subpath.StartsWith ("logout")) {
                                if (_con != null) {
                                        connectionHandler.LogOut (_con.SessionID);
                                        Cookie cookie = new Cookie ("sid", "", "/") {
                                                Expired = true
                                        };
                                        _resp.AppendCookie (cookie);
                                        _resp.Redirect (pageBasePath);
                                        return;
                                }

                                result.Append ($"<h1>Not logged in, <a href=\"{pageBasePath}\">click to return to main page</a>.</h1>");
                        } else if (subpath.StartsWith (steamLoginUrl)) {
                                string host = (Web.IsSslRedirected (_req) ? "https://" : "http://") + _req.UserHostName;
                                string url = OpenID.GetOpenIdLoginUrl (host, host + urlBasePath + steamOpenIdVerifyUrl);
                                _resp.Redirect (url);
                                return;
                        } else {
                                result.Append ($"<h1>Unknown command, <a href=\"{pageBasePath}\">click to return to main page</a>.</h1>");
                        }

                        result.Append (footer);

                        _resp.ContentType = MimeType.GetMimeType (".html");
                        _resp.ContentEncoding = Encoding.UTF8;
                        byte[] buf = Encoding.UTF8.GetBytes (result.ToString ());
                        _resp.ContentLength64 = buf.Length;
                        _resp.OutputStream.Write (buf, 0, buf.Length);
                }
        }
}