Okay, so I have two computers with Minecraft and Forge 1.7.2 and related profiles set up in the launcher. I have my mod which I built and it works great in single player on both computers.  I am logged into each with a different valid account (one is my daughter's).

 

However, if I open to LAN in either direction the other computer can see the game but when it attempts to join it gets a "Mod Rejection" error.  If I look at the console of the computer that had opened the game to LAN there is following error messages:

 

20:00:39 INFO]: Client> [20:00:39] [Client thread/INFO]: Started on 58404

[20:00:39 INFO]: Client> [20:00:39] [Client thread/INFO]: [CHAT] Local game hosted on port 58404

[20:00:42 INFO]: Client> [20:00:42] [Netty IO #5/INFO] [FML]: Client protocol version 1

[20:00:42 INFO]: Client> [20:00:42] [Netty IO #5/INFO] [FML]: Client attempting to join with 4 mods : [email protected],[email protected],[email protected],[email protected]

[20:00:42 INFO]: Client> [20:00:42] [Netty IO #5/INFO] [FML]: Attempting connection with missing mods [] at CLIENT

[20:00:42 INFO]: Client> [20:00:42] [Netty IO #5/INFO] [FML]: Rejecting connection CLIENT: [Forge]

[20:00:42 INFO]: Client> [20:00:42] [server thread/INFO]: Taliaailat lost connection: TextComponent{text='Mod rejections [Forge]', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}}

[20:00:42 INFO]: Client> [20:00:42] [server thread/INFO]: Taliaailat left the game

 

I get same problem if I run server on one of the computers and also run client on same computer and connect to localhost.

 

What am I doing wrong?

A related question: What is the actual process where server and client agree that the mods match to allow connection?   What is it comparing?

Okay, I solved this.  It wasn't my mod that was mis-matched, it was Forge itself.  And I missed it because it was really close -- my server was 1073 and client was 1075.  Getting them both to 1075 fixed the issue.

 

Still makes me wonder what is sufficient for client and server to consider a mod matched though...if anyone knows I'd still be interested.