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
