Question

One of our Java program when started, it only listen on IPv6 (8080)

e.g.

# netstat -ntpl

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      -               
tcp6       0      0 :::8080                 :::*                    LISTEN      -               
tcp6       0      0 :::22                   :::*                    LISTEN      -               

The problem is it is not accessible from outside (except localhost), to solve this, I have this manually add

-Djava.net.preferIPv4Stack=true

But this make the program is only for IPv4 network.

Is it possible to do something like the sshd as above, both support IPv4 and IPv6?

Answer 1

I suspect it’s less a Java programming issue than an OS networking stack/OS network configuration issue:

http://coding.derkeiler.com/Archive/Java/comp.lang.java.help/2009-09/msg00087.html

On some OSes, a single native TCP socket can listen to a port on both
IPv4 and IPv6 simultaneously. It is able to accept connections from
remote IPv4 and from remote IPv6 clients. On other OSes (such as WinXP)
an OS native socket CANNOT do that, but can only accept from IPv4 or
IPv6, not both. On those OSes, it is necessary to have two listen
sockets in order to be able to accept connections from both remote IPv4
and IPv6 clients, one socket to listen for IPv4 connections and one for
IPv6.

Windows 7 and Windows Server 2008 handle dual stacks just fine; Windows XP not so much 🙂

You seem to be on Linux – most modern Linux desktops and servers also handle dual ipv4 ipv6 with no problem.

Here’s a good article on interoperability:

• http://ntrg.cs.tcd.ie/undergrad/4ba2.02/ipv6/interop.html

You know how you can “turn off” IPV6 for your Java application: -Djava.net.preferIPv4Stack=true

You can also force your server to use IPV6 like this: echo 0 > /proc/sys/net/ipv6/bindv6only

This is arguably your best source:

• http://docs.oracle.com/javase/6/docs/technotes/guides/net/ipv6_guide/index.html

You should absolutely be able to accomplish what you want (at least at the Java programming level), unless you’re limited by external network issues:

Nodes)      V4 Only  V4/V6  V6 Only
            -------  -----  -------
V4 Only     x        x   
V4/V6       x        x      x
V6 Only              x      x

PS:

Here’s one more good link, which explains what’s happening at the socket level. It’s not Java (it’s C), but exactly the sample principles apply:

• Accept connections from both IPv6 and IPv4 clients