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An async DNS resolver for EventMachine based on udns C library
C Ruby
branch: master

README.markdown

EM-Udns

Overview

EM-Udns is an async DNS resolver for EventMachine based on udns C library. Having most of the code written in C, EM-Udns becomes very fast. It can resolve DNS A, AAAA, PTR, MX, TXT, SRV and NAPTR records, and can handle every kind of errors (domain/record not found, request timeout, malformed response...).

C udns is a stub resolver, so also EM-Udns. This means that it must rely on a recursive name server, usually co-located in local host or local network. A very good choice is Unbound, a validating, recursive and caching DNS resolver.

IMPORTANT: Please read this again: EM-Udns is a stub resolver so you need a recursive nameserver. Probably the DNS nameserver offered via DHCP by your Internet provider is not a recursive nameserver so EM-Udns will NOT work. Please don't attempt to use EM-Udns if you don't properly understand what this note means.

Usage Example

require "em-udns"

EM.run do
  # Set the nameserver rather than using /etc/resolv.conf.
  EM::Udns.nameservers = "127.0.0.1"

  resolver = EM::Udns::Resolver.new
  EM::Udns.run resolver

  query = resolver.submit_A "google.com"

  query.callback do |result|
    puts "result => #{result.inspect}"
  end

  query.errback do |error|
    puts "error => #{error.inspect}"
  end
end

It would produce following output:

result => ["209.85.227.105", "209.85.227.103", "209.85.227.104", "209.85.227.106", "209.85.227.99", "209.85.227.147"]

Setting the Nameservers

EM::Udns.nameservers = nameservers

This class method set the nameservers list to use for all the EM::Udns::Resolver instances. If not used, nameservers are taken from /etc/resolv.conf (default behavior). nameserver parameter can be:

  • String - The IP of a single nameserver.
  • Array of String - IP's of multiple nameservers.

IMPORTANT: This class method must be used before initializing any EM::Udns::Resolver instance.

NOTE: Nameservers must be IPv4 addresses since udns does not listen in IPv6.

Example 1:

EM::Udns.nameservers = "127.0.0.1"

Example 2:

EM::Udns.nameservers = ["192.168.100.1", "192.168.100.2"]

Initializing a Resolver

resolver = EM::Udns::Resolver.new

Returns a EM::Udns::Resolver instance. If there is an error an exception EM::Udns::UdnsError is raised.

Runnig a Resolver

EM::Udns.run resolver

Attaches the UDP socket of the resolver to EventMachine. This method must be called after EventMachine is running.

Async DNS Queries

resolver.submit_XXX(parameters)

DNS queries are performed by invoking EM::Udns::Resolver#submit_XXX(parameters) methods on the resolver. The complete list of submit_XXX methods are shown below. These methods return a EM::Udns::Query instance. Callback and errback can then be assigned to the Query object via the callback and errback methods which accept a code block as single argument.

In case of success, the callback code block is invoked on the EM::Udns::Query object passing the DNS result object as single argument. Definition of those objects are shown below.

In case of error, the errback code block is invoked with the exact error as single argument, which is a Ruby Symbol:

  • :dns_error_nxdomain - The domain name does not exist.
  • :dns_error_nodata - The domain exists, but has no data of requested type.
  • :dns_error_tempfail - Temporary error, the resolver nameserver was not able to process our query or timed out.
  • :dns_error_protocol - Protocol error, a nameserver returned malformed reply.
  • :dns_error_badquery - Bad query, name of dn is invalid.
  • :dns_error_nomem - No memory available to allocate query structure.
  • :dns_error_unknown - An unknown error has occurred.

Type Specific Queries

A Record

resolver.submit_A(domain)

In case of success the callback is invoked passing as argument an array of String objects. Each String represents an IPv4 address.

Example:

resolver.submit_A "google.com"

Callback is called with argument:

["209.85.227.105",
 "209.85.227.103",
 "209.85.227.104",
 "209.85.227.106",
 "209.85.227.99",
 "209.85.227.147"]

AAAA Record

resolver.submit_AAAA(domain)

In case of success the callback is invoked passing as argument an array of String objects. Each String represents an IPv6 address.

Example:

resolver.submit_AAAA "sixxs.net"

Callback is called with argument:

["2001:838:2:1::30:67",
 "2001:838:2:1:2a0:24ff:feab:3b53",
 "2001:960:800::2",
 "2001:1af8:4050::2"]

MX Record

resolver.submit_MX(domain)

In case of success the callback is invoked passing as argument an array of EM::Udns::RR_MX objects. Such object contains the following attribute readers:

  • domain - String representing the domain of the MX record.
  • priority - Fixnum representing the priority of the MX record.

Example:

resolver.submit_MX "gmail.com"

Callback is called with argument:

[#<EventMachine::Udns::RR_MX:0x00000002289090 @domain="alt1.gmail-smtp-in.l.google.com", @priority=10>,
 #<EventMachine::Udns::RR_MX:0x00000002288e60 @domain="alt3.gmail-smtp-in.l.google.com", @priority=30>,
 #<EventMachine::Udns::RR_MX:0x000000022886e0 @domain="gmail-smtp-in.l.google.com", @priority=5>,
 #<EventMachine::Udns::RR_MX:0x00000002288618 @domain="alt2.gmail-smtp-in.l.google.com", @priority=20>,
 #<EventMachine::Udns::RR_MX:0x000000022883c0 @domain="alt4.gmail-smtp-in.l.google.com", @priority=40>]

PTR Record

resolver.submit_PTR(ip)

Argument ip must be a String representing a IPv4 or IPv6. In case of success the callback is invoked passing as argument an array of String objects. Each String represents a domain.

Example 1:

resolver.submit_PTR "8.8.8.8"

Callback is called with argument:

["google-public-dns-a.google.com"]

Example 2:

resolver.submit_PTR "2001:838:2:1:2a0:24ff:feab:3b53"

Callback is called with argument:

["tunnelserver.concepts-ict.net"]

TXT Record

resolver.submit_TXT(domain)

In case of success the callback is invoked passing as argument an array of String objects. Each String represents a text entry in the TXT result.

Example:

resolver.submit_TXT "gmail.com"

Callback is called with argument:

["v=spf1 redirect=_spf.google.com"]

SRV Record

resolver.submit_SRV(domain)
resolver.submit_SRV(domain, service, protocol)

There are two ways to perform a SRV query:

  • By passing as argument a single String (domain) with the format "_service._protocol.domain".
  • By passing three String arguments (domain, service and protocol).

In case of success the callback is invoked passing as argument an array of EM::Udns::RR_SRV objects. Such object contains the following attribute readers:

  • domain - String representing the A domain of the SRV record.
  • port - Fixnum representing the port of the SRV record.
  • priority - Fixnum representing the priority of the SRV record.
  • weight - Fixnum representing the weight of the SRV record.

For more information about these fields check RFC 2782.

Example:

resolver.submit_SRV "_sip._tcp.oversip.net"

or:

resolver.submit_SRV "oversip.net", "sip", "tcp"

Callback is called with argument:

[#<EventMachine::Udns::RR_SRV:0x00000001ea4970 @domain="sip1.oversip.net", @priority=1, @weight=50, @port=5062>,
 #<EventMachine::Udns::RR_SRV:0x00000001ea4b50 @domain="sip2.oversip.net", @priority=2, @weight=50, @port=5060>]

NAPTR Record

resolver.submit_NAPTR(domain)

In case of success the callback is invoked passing as argument an array of EM::Udns::RR_NAPTR objects. Such object contains the following attribute readers:

  • order - Fixnum representing the order of the NAPTR record.
  • preference - Fixnum representing the preference of the NAPTR record.
  • flags - String representing the flags of the NAPTR record.
  • service - String representing the service of the NAPTR record.
  • regexp - String representing the regular expression field of the NAPTR record (nil in case replacement has value).
  • replacement - String representing the replacement string field of the NAPTR record (nil in case regexp has value).

For more information about these fields check RFC 2915.

Example:

resolver.submit_NAPTR "oversip.net"

Callback is called with argument:

[#<EventMachine::Udns::RR_NAPTR:0x00000002472aa0 @order=30, @preference=50, @flags="S", @service="SIPS+D2T", @regexp=nil, @replacement="_sips._tcp.oversip.net">,
 #<EventMachine::Udns::RR_NAPTR:0x00000002472848 @order=40, @preference=50, @flags="S", @service="SIP+D2S", @regexp=nil, @replacement="_sip._sctp.oversip.net">,
 #<EventMachine::Udns::RR_NAPTR:0x000000024723e8 @order=10, @preference=50, @flags="S", @service="SIP+D2T", @regexp=nil, @replacement="_sip._tcp.oversip.net">,
 #<EventMachine::Udns::RR_NAPTR:0x00000002471d80 @order=20, @preference=50, @flags="S", @service="SIP+D2U", @regexp=nil, @replacement="_sip._udp.oversip.net">]

Other Features

Number of Active Queries

resolver.active

EM::Udns::Resolver#active returns the number of pending queries of resolver as a Fixnum.

Cancelling a Pending Query

resolver.cancel query

EM::Udns::Resolver#cancel(query) cancels the EM::Udns::Query given as argument so no callback/errback would be called upon query completion.

Installation

EM-Udns is provided as a Ruby Gem:

~$ gem install em-udns

Supported Platforms

EM-Udns is tested under the following platforms:

  • Linux 32 and 64 bits
  • Mac OSX 32 and 64 bits

See also the extconf.rb file which compiles udns C library according to current platform.

TODO

  • Testing on other platforms.

Acknowledgement

Many thanks to Michael Tokarev (the author of udns C library) for all the help provided in udns mailing list.

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