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Re: gethostby* users



Ulrich Drepper wrote:

Simo Sorce wrote:
Ulrich, some people claim that glibc's getaddrinfo breaks IPv4 DNS round
robin as it reorders also IPv4 address, can you comment on this? In some
cases it is important.

It is not necessary (and quite wrong) to implement RR in the server.
It's an approximation at best anyway.  In no way better than
randomization.  And nothing stops programs from randomizing the results
returned by getaddrinfo before using them.  Normal programs won't do
that but there is nothing which stops programs like ntpd from doing this.

A client has no possible way to know what the "best" choice from a list of RR DNS IP's might be, whereas server products like F5's 3DNS/global load balancer monitor server load/availability and and an assortment of other things to offer the best choice. I don't know if any of these products send an ordered list in the preferred choice, but things would be more robust if they did and the client walked the list in the presented order. If the server must send only one IP to control the client's choice, it causes a problem when circumstances change during the client application's run time. Many clients (particularly IE - the most popular one..) will cache the list given in the initial lookup and on subsequent failure will retry others from that list but will not do another DNS lookup as part of the error recovery. Thus if they have been given a preferred and backup IP in the first lookup, they'll keep working even if the preferred server becomes unreachable. But, if they resort the list, half of them will pick the wrong first choice even when it is reachable.

--
  Les Mikesell
   lesmikesell gmail com


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