dnsjava

Overview

dnsjava is an implementation of DNS in Java. It

• supports almost all defined record types (including the DNSSEC types), and unknown types.

• can be used for queries, zone transfers, and dynamic updates.

• includes a cache which can be used by clients, and an authoritative only server.

• supports TSIG authenticated messages, DNSSEC verification, and EDNS0.

• is fully thread safe.

Getting started

Have a look at the basic examples.

Config options

Some settings of dnsjava can be configured via Java system properties:

Property	Explanation
	Type	Default	Example
dns[.fallback].server	DNS server(s) to use for resolving. Comma separated list. Can be IPv4/IPv6 addresses or hostnames (which are resolved using Java’s built in DNS support).
	String	-	8.8.8.8,[2001:4860:4860::8888]:853,dns.google
dns[.fallback].search	Comma separated list of DNS search paths.
	String	-	ds.example.com,example.com
dns[.fallback].ndots	Sets a threshold for the number of dots which must appear in a name given to resolve before an initial absolute query will be made.
	Integer	1	2
dnsjava.options	Comma separated key-value pairs, see dnsjava.options pairs.
	option list	-	BINDTTL,tsigfudge=1
dnsjava.configprovider.skipinit	Set to true to disable static ResolverConfig initialization.
	Boolean	false	true
dnsjava.configprovider.sunjvm.enabled	Set to true to enable the reflection based DNS server lookup, see Limitations.
	Boolean	false	true
dnsjava.udp.ephemeral.start	First ephemeral port for UDP-based DNS queries.
	Integer	49152 (Linux: 32768)	50000
dnsjava.udp.ephemeral.end	Last ephemeral port for UDP-based DNS queries.
	Integer	65535 (Linux: 60999)	60000
dnsjava.udp.ephemeral.use_ephemeral_port	Use an OS-assigned ephemeral port for UDP queries. Enabling this option is insecure! Do NOT use it.
	Boolean	false	true
dnsjava.lookup.max_iterations	Maximum number of CNAMEs to follow in a chain.
	Integer	16	20
dnsjava.lookup.use_hosts_file	Use the system’s hosts file for lookups before resorting to a resolver.
	Boolean	true	false
dnsjava.nio.selector_timeout	Set selector timeout in milliseconds. Default/Max 1000, Min 1.
	Integer	1000	700
dnsjava.harden_unknown_additional	Harden against unknown records in the authority section and additional section. If disabled, such records are copied from the upstream and presented to the client together with the answer.
	Boolean	True	False
DNSSEC Options
dnsjava.dnssec.keycache.max_ttl	Maximum time-to-live (TTL) of entries in the key cache in seconds.
	Integer	900	1800
dnsjava.dnssec.keycache.max_size	Maximum number of entries in the key cache.
	Integer	1000	5000
org.jitsi.dnssec.nsec3.iterations.N	Maximum iteration count for the NSEC3 hashing function depending on the key size N. The defaults are from RFC5155.
	Integer	1024 bit keys: 150 iterations 2048 bit keys: 500 iterations 4096 bit keys: 2500 iterations e.g. dnsjava.dnssec.nsec3.iterations.1024=200
dnsjava.dnssec.trust_anchor_file	The file from which the trust anchor should be loaded. The file must be formatted like a DNS zone master file. It can only contain DS or DNSKEY records.
	String	-	/etc/dnssec-root-anchors
dnsjava.dnssec.digest_preference	Defines the preferred DS record digest algorithm if a zone has registered multiple DS records. The list is comma-separated, the highest preference first. If this property is not specified, the DS record with the highest digest ID is chosen. To stay compliant with the RFCs, the mandatory digest IDs must be listed in this property. The GOST digest requires BouncyCastle on the classpath.
	String	-	2,1,4
dnsjava.dnssec.harden_algo_downgrade	Prevent algorithm downgrade when multiple algorithms are advertised in a zone’s DS records. If false, allows any algorithm to validate the zone.
	Boolean	true	false
dnsjava.dnssec.max_validate_rrsigs	Maximum number of RRSig records to validate until the response is considered bogus. This is limited to avoid the 'KeyTrap' vulnerability (CVE-2023-50387).
	Integer	8	4
dnsjava.dnssec.max_ds_match_failures	Maximum number of DS records to validate until the response is considered bogus. This is limited to avoid the 'KeyTrap' vulnerability (CVE-2023-50387).
	Integer	4	2
dnsjava.dnssec.algorithm_enabled.ID	Enable or disable a DS/DNSKEY algorithm. See RFC8624 for recommended values. Note that algorithm number 1, RSAMD5, is disabled and cannot be enabled with this property.
	Boolean	Disable ED448: dnsjava.dnssec.algorithm_enabled.16=false
dnsjava.dnssec.algorithm_rsa_min_key_size	Set the minimum size, in bits, for RSA keys.
	Integer	1024	512
dnsjava.dnssec.digest_enabled.ID	Enable or disable a DS record digest algorithm. See RFC8624 for recommended values.
	Boolean	Disable SHA.1: dnsjava.dnssec.digest_enabled.1=false

dnsjava.options pairs

The dnsjava.options configuration options can also be set programmatically through the Options class. Please refer to the Javadoc for details.

Key	Type	Default	Explanation
BINDTTL	Boolean	false	Print TTLs in BIND format
multiline	Boolean	false	Print records in multiline format
noPrintIN	Boolean	false	Do not print the class of a record if it is IN
tsigfudge	Integer	300	Sets the default TSIG fudge value (in seconds)
sig0validity	Integer	300	Sets the default SIG(0) validity period (in seconds)

Resolvers

SimpleResolver

Basic resolver that uses UDP by default and falls back to TCP if required.

ExtendedResolver

A Resolver that uses multiple Resolvers to send the queries, defaulting to SimpleResolvers. Can be configured to query the servers in a round-robin order. Blacklists a server if it times out.

DohResolver

Proof-of-concept DNS over HTTP resolver, e.g. to use https://dns.google/query.

ValidatingResolver

DNSSEC validating stub resolver. Originally based on the work of the Unbound Java prototype from 2005/2006. The Unbound prototype was stripped from all unnecessary parts, heavily modified, complemented with more than 300 unit test and found bugs were fixed. Before the import into dnsjava, the resolver was developed as an independent library at https://github.com/ibauersachs/dnssecjava. To migrate from dnssecjava, replace org.jitsi with org.xbill.DNS in Java packages and org.jitsi with dnsjava in property prefixes.

Validated, secure responses contain the DNS AD-flag, while responses that failed validation return the SERVFAIL-RCode. Insecure responses return the actual return code without the AD-flag set. The reason why the validation failed or is insecure is provided as a localized string in the additional section under the record ./65280/TXT (a TXT record for the owner name of the root zone in the private query class ValidatingResolver.VALIDATION_REASON_QCLASS). The Extended DNS Errors (EDE, RFC8914) also provides the failure reason, although in less detail.

The examples contain a small demo.

Important

Do not use the ValidatingResolver standalone. A response will need CNAME/DNAME post-processing, and DNS messages can still be manipulated with DNSSEC alone. Subsequent processing and validation of messages is intricate and best done using the built-in LookupSession (or the legacy Lookup) class.

Migrating from version 2.1.x to v3

dnsjava v3 has significant API changes compared to version 2.1.x and is neither source nor binary compatible. The most important changes are:

• Requires at least Java 8

• Uses slf4j for logging and thus needs slf4j-api on the classpath

• The command line tools were moved to the org.xbill.DNS.tools package

• On Windows, JNA should be on the classpath for the search path and proper DNS server finding

• The Resolver API for custom resolvers has changed to use CompletionStage<Message> for asynchronous resolving. The built-in resolvers are now fully non-blocking and do not start a thread per query anymore.

• Many methods return a List<T> instead of an array. Ideally, use a for-each loop. If this is not possible, call size() instead of using length:

  • Cache#findAnyRecords

  • Cache#findRecords

  • Lookup#getDefaultSearchPath

  • Message#getSectionRRsets

  • SetResponse#answers

  • ResolverConfig

• RRset returns a List<T> instead of an Iterator. Ideally, modify your code to use a for-each loop. If this is not possible, create an iterator on the returned list:

  • RRset#rrs

  • RRset#sigs

• Methods using java.util.Date are deprecated. Use the new versions with java.time.Instant or java.time.Duration instead

• The type hierarchy of SMIMEARecord changed, it now inherits from TLSARecord and constants are shared

• Records are no longer marked as Serializable after 3.0. While 3.5 reintroduced Serializable, it is preferred to use the RFC defined serialization formats directly:

  • toString(), rrToString() ↔ fromString()

  • toWire() ↔ fromWire(), newRecord()

• Message and Header properly support clone()

Replacing the standard Java DNS functionality

Java 1.4 to 8

Java versions from 1.4 to 8 can load DNS service providers at runtime. To load the dnsjava service provider, build dnsjava on JDK 8 and set the system property:

sun.net.spi.nameservice.provider.1=dns,dnsjava

This instructs the JVM to use the dnsjava service provide for DNS at the highest priority.

Java 9 to 17

The functionality to load a DNS SPI was removed in JDK 9 and a replacement API was requested.

Java 18+

JEP 418: Internet-Address Resolution SPI reintroduces a DNS SPI. See #245 for the support status in dnsjava.

Build

dnsjava uses Maven as the build system. Run mvn package from the toplevel directory to build dnsjava. JDK 8 or higher is required.

Testing dnsjava

Matt Rutherford contributed a number of unit tests, which are in the tests subdirectory.

The hierarchy under tests mirrors the org.xbill.DNS classes. To run the unit tests, execute mvn test.

Limitations

There is no standard way to determine what the local nameserver or DNS search path is at runtime from within the JVM. dnsjava attempts several methods until one succeeds.

• The properties dns.server and dns.search (comma delimited lists) are checked. The servers can either be IP addresses or hostnames (which are resolved using Java’s built in DNS support).

• On Unix/Solaris, /etc/resolv.conf is parsed.

• On Windows, if JNA is available on the classpath, the GetAdaptersAddresses API is used.

• On Android the ConnectivityManager is used (requires initialization using org.xbill.DNS.config.AndroidResolverConfigProvider.setContext).

• The sun.net.dns.ResolverConfiguration class is queried if enabled. As of Java 16 the JVM flag --add-opens java.base/sun.net.dns=ALL-UNNAMED (classpath) or --add-opens java.base/sun.net.dns=org.dnsjava (modules) is also required.

• If available and no servers have been found yet, JNDI-DNS is used.

• If still no servers have been found yet, use the fallback properties. This can be used to query e.g. a well-known public DNS server instead of localhost.

• As a last resort, localhost is used as the nameserver, and the search path is empty.

Additional documentation

Javadoc documentation can be built with mvn javadoc:javadoc or viewed online at javadoc.io. See the examples for some basic usage information.

License

dnsjava is placed under the BSD-3-Clause license.

History

dnsjava was started as an excuse to learn Java. It was useful for testing new features in BIND without rewriting the C resolver. It was then cleaned up and extended in order to be used as a testing framework for DNS interoperability testing. The high level API and caching resolver were added to make it useful to a wider audience. The authoritative only server was added as proof of concept.

dnsjava on GitHub

This repository has been a mirror of the dnsjava project at Sourceforge since 2014 to maintain the Maven build for publishing to Maven Central. As of 2019-05-15, GitHub is officially the new home of dnsjava. The dnsjava-users mailing list (archive) still exists but is mostly inactive.

Please use the GitHub issue tracker and send - well tested - pull requests.

Authors

• Brian Wellington (@bwelling), March 12, 2004

• Various contributors, see the Changelog

• Ingo Bauersachs (@ibauersachs), current maintainer

Final notes

• Thanks to Network Associates, Inc. for sponsoring some of the original dnsjava work in 1999-2000.

• Thanks to Nominum, Inc. for sponsoring some work on dnsjava from 2000 through 2017.