DNS header length question
Posted on 2006-06-08
My CISCO PIX firewall's default setting includes this:
fixup protocol dns maximum-length 512, which I've increased to 530 to accomodate a URL that I couldn't reach due to this error:
Dropped UDP DNS reply from outside:220.127.116.11/53 to inside:18.104.22.168/79; packet length 520 bytes exceeds configured limit of 512 bytes.
I still get this kind of error, and occasionally I lose DNS resolution altogether, even though comms are still up (I get ICMP replies from external IPs). I could remove the maximum-length entry or increase it until the error goes away, but I'd prefer to know what the standard is. My ISP uses Microsoft, and I'm hosting DNS servers on Linux systems. Are there different standards for DNS header length for MS and Unix/Linux servers, and what is the risk of increasing the DNS maximum length above 512 on the PIX?