I've recently been introduced to the caching behaviour of InetAddress,
and I think it may be improved.
The javadoc reads:
The InetAddress class has a cache to store successful as well as
unsuccessful host name resolutions. The positive caching is there to
guard against DNS spoofing attacks; while the negative caching is used
to improve performance.
And that is all fine and well, but for multihomed hosts, I believe the
current behaviour is
1) Not documented properly
2) Not correct
Coming from a unix-world, I'm used to the resolver handing out
RR-replies in random order, and thus I would expect InetAddress to do
the same, but a short test (courtesy of Arne Vajhøj) shows that
InetAddress.getByName() will return the same IP for a lookup when the
lookup is performed within the 10-second range of the cache:
public class DNSLookup {
private static final String MS = "www.microsoft.com";
public static void main(String[] args) throws Exception {
Map<String, Integer> m = new HashMap<String, Integer>();
InetAddress[] all = InetAddress.getAllByName(MS);
for(InetAddress one : all) {
m.put(one.getHostAddress(), 0);
}
for(int i = 0; i < 10000; i++) {
String ms = InetAddress.getByName(MS).getHostAddress();
m.put(ms, m.get(ms) + 1);
}
for(Entry<String, Integer> e : m.entrySet()) {
System.out.println(e.getKey() + ": " + e.getValue());
}
}
}
Outputs:
207.46.19.190: 0
207.46.193.254: 10000
207.46.192.254: 0
207.46.19.254: 0
While I would expect the replies to distribute to about 2500 on each
(which it will if the cache isn't used).
--
Andreas
