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Default Value

false

java.net.preferIPv6Addresses Explained

If the operating system that your java application is running on supports IPv6 it will attempt to use IPv6 when connecting to a host that supports IPv6.

Typically this would mean if the host's DNS records include IPv6 addresses, such as in the AAAA records.

Related System Properties

The java.net.preferIPv4Stack system property is related to the java.net.preferIPv6Addresses property, it can be used to prefer IPv4 over IPv6 instead of IPv6 over IPv4.

Here are some other networking Java system properties:

• http.agent
• http.keepAlive
• http.maxConnections
• https.protocols
• java.net.preferIPv4Stack
• javax.net.debug
• javax.net.ssl.trustStore
• jdk.net.hosts.file
• jdk.tls.client.protocols
• jdk.tls.disabledAlgorithms
• networkaddress.cache.ttl
• sun.net.inetaddr.ttl

Supported Since

Java has supported the java.net.preferIPv6Addresses system property since at least version 8, support may go back to even older versions of java.

Setting java.net.preferIPv6Addresses on Startup

You can set the java.net.preferIPv6Addresses java system property during startup of the java runtime using the -D command line argument:

java -Djava.net.preferIPv6Addresses=true MyAppMain

You may also be able to specify java.net.preferIPv6Addresses via the JAVA_TOOL_OPTIONS environment variable:

JAVA_TOOL_OPTIONS=-Djava.net.preferIPv6Addresses=true

Setting / Reading java.net.preferIPv6Addresses at Runtime

You can set java.net.preferIPv6Addresses at runtime with the following Java code:

System.setProperty("java.net.preferIPv6Addresses", "true");

WARNING: Depending on the property and JVM version using setProperty may or may not work if the JDK Java class that uses this variable has already been loaded. The value of the java.net.preferIPv6Addresses system property may be cached within an internal private static variable of the implementing class.

To read the value of java.net.preferIPv6Addresses at runtime, you can use this Java code:

String propertyValue = System.getProperty("java.net.preferIPv6Addresses");
if (propertyValue != null) {
    System.out.println("java.net.preferIPv6Addresses = " + propertyValue);
} else {
    System.out.println("java.net.preferIPv6Addresses was null");
}

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