Книга: Fedora™ Unleashed, 2008 edition

Reverse Lookup Problems

Reverse Lookup Problems

Reverse lookup problems are often hard to diagnose because they manifest themselves as failures in systems other than DNS. Many security-sensitive services perform reverse lookups on the originating host for all incoming connections and deny the connection if the query fails.

Even if reverse resolution succeeds, many servers might reject connections from your host if your A and PTR records do not match. That is, the PTR record for a particular IP address refers to a name and the A record for that name refers to a different IP address. They perform a double lookup to verify that the PTR and A records match to eliminate spoofing attacks. Carefully maintain your reverse zones at all times.

Delegation problems are a frequent source of woe. Unfortunately, many ISPs appear unable to understand, configure, or delegate reverse zones. In such cases, you often have little choice but to try and tell your ISP what to do to fix the problem. If the ISP staff refuses to listen, find a new ISP (or live with broken DNS).

Another typical symptom of failing reverse lookups is an abnormally long delay on connection attempts. This happens when the server's query for a PTR record is not answered and times out (often because of network problems or the nameserver being down). This can be baffling to diagnose, but you should suspect DNS problems whenever you hear questions such as "Hey! Why is my web browser taking so long to connect?"

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