Valdis.Kletnieks@vt.edu wrote:
On Thu, 16 Dec 2004 17:18:12 PST, Crist Clark said:
Into a UDP response. A resolver will recieve the first 512 bytes of the truncated response and may then use TCP to get the complete response... unless there is a firewall blocking 53/tcp in the way. But how often does that happpen?
It happens *all* *the* *time* (probably just as often as sites that block all ICMP including 'frag needed' and wonder why PMTU Discovery breaks and connections hang).
The *real* operational problem is that almost 100% of the time that there's a firewall blocking 53/tcp, the person running the firewall is (a) unaware that it's blocking it and (b) doesn't even realize that DNS *can* use TCP....
Quite often, there's even a "(c) they don't even know they have a firewall" just to make things really interesting.
One of the most common misconceptions I've encountered and had heated debates with some would-be admins is the belief that the only "proper" use of 53/tcp for DNS is for zone transfers. For that reason they explicitly block 53/tcp in their firewalls. Same thing with that good old misconception that all forms of ICMP are evil and should be blocked. Doug --*-- Life would be so much easier if we only had the source code... -Anonymous --*--