I run a server which amongst other things uses tinydns for DNS and axfrdns for handling transfer requests from our secondary DNS (another system). I understand that tinydns uses port 53 on UDP and axfrdns uses port 53 on TCP.
I've configured axfrdns to only allow connections from my agreed secondary host. I run logcheck to monitor my logs and every day I see spurious connections on port 53 (TCP) from seemingly random hosts. They usually turn out to be from ADSL connections.
My question is; are these innocent requests or a security risk? I am happy to block repeat offenders using iptables but don't want to block innocent users of one of the websites I host.