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From Lucian Adrian Grijincu <lucian.griji...@avira.com>
Subject [Tip of the Day] getaddrinfo is not just for IPv6
Date Wed, 07 Mar 2007 10:22:47 GMT
From http://udrepper.livejournal.com/16116.html :


/*getaddrinfo is not just for IPv6*/

I've heard far too often that getaddrinfo is only interesting for IPv6
and therefore can be ignored since one does not have IPv6.

Aside from the fact that all programs should be protocol independent
this statement is bogus. gethostbyname etc do not perform correctly in
some situations where only ever IPv4 is involved.

Assume you have an internal IPv4 network with, say, 192.168.x.y
addresses. In addition you have a server (web server, for instance)
which is also visible on the Internet. This server has two addresses:
one 192.168.x.y address and one global address. The client is a NATed
machine on the intranet.

Now what happens if the nameserver returns both addresses to a query for
the addresses of said server? With gethostbyname the addresses are
returned to the caller in the order they are received from the DNS
server. Maybe some randomization is applied. In short, it is possible
that the internal machine gets sees the public IPv4 address and then
connects to it. This is not only wasteful (the request has to be routed
through a switch), it might even be dangerous (the traffic might
actually have to go through the Internet).

With getaddrinfo this is not the case. The sorting according to RFC 3484
makes sure that the internal address of the server is returned first.
The sorting function will notice that the source address used on the
client is also an internal address and therefore the internal address of
the server is a better match than the global address.

In summary, gethostbyaddr is not only about IPv6. The old interfaces
were simply completely inadequate and should never be used. If you
/still/ haven't converted your programs to use getaddrinfo instead of
gethostbyname and gethostbyname2 do it now. I have written some time ago
a brief intro <http://people.redhat.com/drepper/userapi-ipv6.html>.

-- 
Best regards,

Lucian Adrian Grijincu
Software Developer
Avira Soft srl

lucian.grijincu@avira.com
http://www.avira.com
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