1 - Use your Original Half-Life 1 on your steam to install the ESF Open Beta (1.3) as a mod.
2 - Seek to create a server without the "LAN GAME" option marked, or if so just open the console and type:
3 - See if your server appears on the list.. if not, type "status" on your console and copy your tcp/ip value to your friend so that he can use "connect <ip>"
4 - Play.
reminder: ESF 1.3 was the so called Open Beta.
The currently in-development ESF version is Earth Special Forces: Final which is NOT released yet.
The currently most played version and still stable is Earth Special Forces 1.2.3
Both Open beta and 1.2.3 versions are avaliable for download on the Website
www.esforces.com and
ONLY WORK WITH ORIGINAL HALF LIFE 1