MySQL Remote Connections for Replication

Here is another interesting problem:

When trying to set up master to master replication from Server A to Server B, the replication user can login from A to B, but not the other way. The error thrown out by MySQL:

ERROR 1130 (HY000): Host '' is not allowed to connect to this MySQL server

Things to check:
1. Replication user password and host
2. Ability to ping the remote server
3. nslookup the remote server and then nslookup the ip for the reverse lookup

The last one solved the problem.