How many A records can fit in a single DNS response?

Question

What are the size limits on DNS responses? For instance how many \'A\' resource records can be present in a single DNS response? The DNS response should still be cache-able.

Answer 1

According to this RFC, the limit is based on the UDP message size limit, which is 512 octets. The EDNS standard supports a negotiated response with a virtually unlimited response size, but at the time of that writing (March 2011), only 65% of clients supported it (which means you can't really rely on it)