I have a bind server running and my secondary nameserver provider told me that he "can ask for records using UDP, but not TCP", any idea what might be wrong ?
DNS communication happens over both UDP and TCP, port 53. The UDP version is used for lookups, whereas the your secondary NS provider will need to be able to do zone transfers, which happens over the TCP version.
In the /etc/named.conf, in the options section, there maybe a section of this sort:
allow-transfer { aaa.bbb.ccc.ddd; zzz.yyy.xxx.www; };
You'll need to add the IP of the secondary server there, and depending on your firewall situation add a rule allowing it to contact your machine.
If you're running a firewall (for example, SuSEfirewall2), you might also need to check that TCP is allowed, in addition to UDP, for port 53. Jim