With earlier Half-Life versions authentication was made by WON. This mechanism doesn't exist anymore and has been replaced by Steam. When launching older executables the WON authentication is not possible anymore. The server is not authentified and players cannot connect.

The trick is to launch server with +sv_lan 1 / -nomaster options and specify an Internet address via the command line. But it leads to LAN servers are restricted to local clients (class C ) message … which is normal because the server is supposed to run on a LAN and clients use Internet address.

2 solutions :

1. use the binary¹⁾ , it allows to bypass WON by faking an authentication. However these binary stands on master.ini which references down servers … So you get a timeout which causes lag. You could modify master.ini file with listening port and address whatever the service is. It's just to avoid timeout even if you get message like “the server is not public”. Don't worry.
2. in some cases lan_fw could be hard to use. We can simply use this script which bypasses WON authentication and suppresses network checks. This is simple ans this solution must be used instead.