RFC compliant address selection vs. home made getaddrinfo

[Michael McMahon]

Hi Bernd,

On 12/06/13 22:05, Bernd Eckenfels wrote:
> Hello,
>
> I have given a (german) talk about Java and IPv6 at the largest 
> european IPv6 Congress last week in Frankfurt. As part of my 
> preparation for that talk was looking at some of the details of Java 
> IPv6 networking. I have some points to discuss because of that:
>
> RFC 3484 defines the default address selection for IPv6. It is 
> basically defining a number of rules for ordering the result from 
> getaddrinfo() to observe standard behaviour (prefer ipv6 if supported, 
> allow administrative redefinition). Linux and Windows have elaborate 
> mechanisms to influence getaddrinfo (net sh int ipv6 prefixpolicy on 
> Windows and /etc/gai.conf on Linux).
>
> Unfortunatelly Java is re-ordering that answer (in 
> Java_java_net_Inet6AddressImpl_lookupAllHostAddr based on preverIPv6). 
> It would be good if instead of preferIPV6=true/false there would be a 
> preferIPV6=true/false/donottouch, with donottouch beening the RFC 3484 
> compliant default.
>

This seems like a good idea. I will create a bug report for it. Adding a 
third option
to the preferIPv6 property could be a good way to support it. Though, 
I'm less
certain that we could make this the default behavior, due to 
compatibility requirements.

> I am talking about
>
> http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/272483f6650b/src/solaris/native/java/net/Inet6AddressImpl.c 
> Line 140ff
>
> According to RFC 3493 you should instead use the AI_ADDRCONFIG to 
> automatically be sensitive to hosts stack configuration.
>
> There is also another problem, the code is using the AI_CANONNAME 
> hint. This hint is by definition returning the canonical name in the 
> FIRST result. However the res[0].ai_canonname is not queried in that 
> code. So you can skip it. Or even better use the result, because the 
> InetAddress#getCanonicalHostName() is not very helpful. (For example 
> using getaddrinfo on "mail.google.com" will return 
> "googlemail.l.google.com" because it is a CNAME but Java returns 
> lga15s34-in-f21.1e100.net as it is the PTR for 173.194.43.21.
>

This does cause some confusion. The method getCanonicalHostName() was 
never intended
to return the CNAME from DNS. Rather it was intended to return the name 
that results
from a reverse lookup of the IP address (ie the PTR).

If we were to add support for the CNAME, then that would have to be 
through a new method
on InetAddress I think.

Michael