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1. mdavidsaver commented on Nov 17, 2022

   

   mdavidsaver

   on Nov 17, 2022

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   I've confirmed my suspicion that the java.net.preferIPv4Stack property was involved prior to #93. (I'm assuming that the default SelectorProvider is used, and neglecting to look through the history of the openjdk source) Setting preferIPv4Stack, or if the JVM logic detects no OS support, makes e303750 a no-op.

   There is a bewildering depth of indirection, which I will only summarize here:

   DatagramChannelImpl is reached through the default SelectorProvider.openDatagramChannel()

   https://github.com/openjdk/jdk/blob/134acab5a40b3f927ff6343aa49477a490e410b5/src/java.base/share/classes/sun/nio/ch/DatagramChannelImpl.java#L175-L178

   The Net.isIPv6Available() method ultimately calls through to an OS specific C function IPv6_supported() , which is either.

   https://github.com/openjdk/jdk/blob/134acab5a40b3f927ff6343aa49477a490e410b5/src/java.base/unix/native/libnet/net_util_md.c#L103-L174

   or

   https://github.com/openjdk/jdk/blob/134acab5a40b3f927ff6343aa49477a490e410b5/src/java.base/windows/native/libnet/net_util_md.c#L211-L220

   Looking at the unix/ implementation, I notice that only OS kernel support is being tested, not userspace configuration. With PVXS I have an additional bind() to test if an IPv6 loopback interface is configured. So I think as written, this logic will incorrectly detect IPv6 support in environments like Docker containers, where the Linux kernel supports IPv6, but no interfaces have IPv6 addresses (even the loopback).

   Under unix/ there is also a DONT_ENABLE_IPV6 C macro, which doesn't seem to be set anywhere at present.

   @kasemir @shroffk @juanfem fyi.

   Reacted by Kay Kasemir
2. mdavidsaver commented on Nov 17, 2022

   

   mdavidsaver

   on Nov 17, 2022

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   With pvAccessCPP, exceptions thrown by setMutlicastNIF() are allowed to propagate.

   https://github.com/epics-base/pvAccessCPP/blob/81d79ecc49147452d8d87ec4eceb0975830a93c7/src/remote/blockingUDPTransport.cpp#L791