Added config comments and variable expansion

If we want Kerberos.NET to be completely compliant with MIT Kerberos
defaults, then we can change some of the paths. The location of
krb5.conf is usually system-wide (/etc/krb5.conf) and the kdc.conf is in
the $LOCALSTATEDIR/krb5kdc. /etc isn't a user configuration location so
I changed it to $XDG_CONFIG_HOME, and I added the ability to expand
variables set in the krb5.conf. That can allow putting the krb5ccache in
the home directory with %HOME%. We should consider honoring the
%KRB5CCNAME% environment variable.

Author: petrsnd

.gitignore

@@ -26,6 +26,8 @@ bld/
 
 # Visual Studio 2015 cache/options directory
 .vs/
+# Visual Studio Code directory
+.vscode/
 # Uncomment if you have tasks that create the project's static files in wwwroot
 #wwwroot/
 
@@ -285,4 +287,4 @@ __pycache__/
 *.btp.cs
 *.btm.cs
 *.odx.cs
-*.xsd.cs
+*.xsd.cs

Kerberos.NET/Configuration/Krb5Config.cs

@@ -74,14 +74,18 @@ public Krb5Config()
         public Krb5Logging Logging { get; private set; }
 
         public static string UserConfigurationPath => GetFilePath(
-            envVar: "%KRB5_CONFIG%",
+            envVar: "%KRB5_CONFIG%", // %KRB_CONFIG% is being used to specify the user
+                                     // configuration path, but it should be the path to krb5.conf itself
+                                     // (see https://web.mit.edu/kerberos/krb5-1.12/doc/admin/env_variables.html)
             winPath: "%APPDATA%\\Kerberos.NET\\",
             osxPath: "Library/Preferences/Kerberos.NET/",
             linuxPath: "%HOME%/.config/Kerberos.NET/" // use XDG_CONFIG_HOME default
         );
 
         public static string ServiceConfigurationPath => GetFilePath(
-            envVar: "%KRB5_KDC_PROFILE%",
+            envVar: "%KRB5_KDC_PROFILE%", // %KRB5_KDC_PROFILE% is being used to specify the service
+                                          // configuration path, but it should be the path to kdc.conf itself
+                                          // (see https://web.mit.edu/kerberos/krb5-1.12/doc/admin/env_variables.html)
             winPath: "%APPDATA%\\Kerberos.NET\\",
             osxPath: "Library/Preferences/Kerberos.NET/",
             linuxPath: "/var/krb5kdc"
@@ -117,6 +121,7 @@ public static Krb5Config CurrentUser(string path = null)
                 path = DefaultUserConfigurationPath;
             }
 
+            // Expansion allows the use of environment variables in krb5.conf, but they must be in %VAR% format
             var expandedPath = Environment.ExpandEnvironmentVariables(path);
             if (File.Exists(expandedPath))
             {
@@ -181,7 +186,9 @@ private static string GetFilePath(string envVar, string winPath, string osxPath,
             }
             else if (OSPlatform.IsLinux)
             {
-                return linuxPath;
+                // Environment variables use %VAR% format
+                // (see https://github.com/dotnet/runtime/issues/25792)
+                return Environment.ExpandEnvironmentVariables(linuxPath);
             }
 
             return string.Empty;