Sunday, January 24, 2010

Generating reverse dns queries

An obvious way of getting a response time out of any dns server is to ask it to perform a reverse dns query on itself. The format of a dns query is documented elsewhere, and a reverse dns query can be generated, captured using tcpdump and analysed in wireshark easily enough. The trick is to generate the dns query programmatically since a fixed string won't work equally well for all dns servers. What we don't want to do is ask a server for information it must get from a remote service. This will vary too much, whereas the objective is to measure the responsiveness of the service itself.

It might be possible to make an irregular query, one that doesn't require it to look up anything. That way the string could be fixed. I'll investigate on Wednesday.

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