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DNS Extension

akka-docs/src/main/paradox/io-dns.md

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DNS Extension

@@@ warning

async-dns does not support:

Additionally, while search domains are supported through configuration, detection of the system configured Search domains is only supported on systems that provide this configuration through a /etc/resolv.conf file, i.e. it isn't supported on Windows or OSX, and none of the environment variables that are usually supported on most *nix OSes are supported.

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@@@ note

The async-dns API is marked as ApiMayChange as more information is expected to be added to the protocol.

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@@@ warning

The ability to plugin in a custom DNS implementation is expected to be removed in future versions of Akka. Users should pick one of the built in extensions.

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Akka DNS is a pluggable way to interact with DNS. Implementations must implement akka.io.DnsProvider and provide a configuration block that specifies the implementation via provider-object.

@@@ note { title="DNS via Akka Discovery" }

@refAkka Discovery can be backed by the Akka DNS implementation and provides a more general API for service lookups which is not limited to domain name lookup.

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To select which DnsProvider to use set akka.io.dns.resolver to the location of the configuration.

There are currently two implementations:

  • inet-address - Based on the JDK's InetAddress. Using this will be subject to both the JVM's DNS cache and its built in one.
  • async-dns - A native implemention of the DNS protocol that does not use any JDK classes or caches.

inet-address is the default implementation as it pre-dates async-dns, async-dns will likely become the default in the next major release.

DNS lookups can be done via the DNS extension:

Scala : @@snip DnsCompileOnlyDocSpec.scala { #resolve }

Java : @@snip DnsCompileOnlyDocTest.java { #resolve }

Alternatively the IO(Dns) actor can be interacted with directly. However this exposes the different protocols of the DNS provider. inet-adddress uses Dns.Resolve and Dns.Resolved where as the async-dns uses DnsProtocol.Resolve and DnsProtocol.Resolved. The reason for the difference is inet-address predates async-dns and async-dns exposes additional information such as SRV records and it wasn't possible to evolve the original API in a backward compatible way.

Inet-Address API:

Scala : @@snip IODocSpec.scala { #actor-api-inet-address }

Java : @@snip DnsCompileOnlyDocTest.java { #actor-api-inet-address }

Async-DNS API:

Scala : @@snip IODocSpec.scala { #actor-api-async }

Java : @@snip DnsCompileOnlyDocTest.java { #actor-api-async }

The Async DNS provider has the following advantages:

  • No JVM DNS caching. It is expected that future versions will expose more caching related information.
  • No blocking. InetAddress resolving is a blocking operation.
  • Exposes SRV, A and AAAA records.

SRV Records

To get DNS SRV records akka.io.dns.resolver must be set to async-dns and DnsProtocol.Resolve's requestType must be set to DnsProtocol.Srv

Scala : @@snip IODocSpec.scala { #srv }

Java : @@snip DnsCompileOnlyDocTest.java { #srv }

The DnsProtocol.Resolved will contain akka.io.dns.SRVRecords.