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Simple, scrappy UDP DNS server in Ruby (with protocol annotations)
# Simple, scrappy UDP DNS server in Ruby (with protocol annotations)
# By Peter Cooper
#
# MIT license
#
# * Not advised to use in your production environment! ;-)
# * Requires Ruby 1.9
# * Supports A and CNAME records
# * See http://www.ietf.org/rfc/rfc1035.txt for protocol guidance
# * All records get the same TTL
require 'socket'
records = {
'example.com.' => '1.2.3.4',
'test.host.' => '127.0.0.2',
'test.cnames.com.' => 'example.com'
}
class DNSRequest
attr_reader :server, :data, :domain
def initialize(server, data)
@server = server
@data = data
extract_domain
end
def extract_domain
@domain = ''
# Check "Opcode" of question header for valid question
if @data[2].ord & 120 == 0
# Read QNAME section of question section
# DNS header section is 12 bytes long, so data starts at offset 12
idx = 12
len = @data[idx].ord
# Strings are rendered as a byte containing length, then text.. repeat until length of 0
until len == 0 do
@domain += @data[idx + 1, len] + '.'
idx += len + 1
len = @data[idx].ord
end
end
end
def response(val)
return empty_response if domain.empty? || !val
cname = val =~ /[a-z]/
# Valid response header
response = "#{data[0,2]}\x81\x00#{data[4,2] * 2}\x00\x00\x00\x00"
# Append original question section
response += data[12..-1]
# Use pointer to refer to domain name in question section
response += "\xc0\x0c"
# Set response type accordingly
response += cname ? "\x00\x05" : "\x00\x01"
# Set response class (IN)
response += "\x00\x01"
# TTL in seconds
response += [server.ttl].pack("N")
# Calculate RDATA - we need its length in advance
if cname
rdata = val.split('.').collect { |a| a.length.chr + a }.join + "\x00"
else
# Append IP address as four 8 bit unsigned bytes
rdata = val.split('.').collect(&:to_i).pack("C*")
end
# RDATA is 4 bytes
response += [rdata.length].pack("n")
response += rdata
end
def empty_response
# Empty response header
# [id * 2, flags, NXDOMAIN, qd count * 2, an count * 2, ns count * 2, ar count * 2]
response = "#{data[0,2]}\x81\x03#{data[4,2]}\x00\x00\x00\x00\x00\x00"
# Append original question section
response += data[12..-1]
end
end
class DNSServer
attr_reader :port, :ttl
attr_accessor :records
def initialize(options = {})
options = {
port: 53,
ttl: 60,
records: {}
}.merge(options)
@port, @records, @ttl = options[:port], options[:records], options[:ttl]
end
def run
Socket.udp_server_loop(@port) do |data, src|
r = DNSRequest.new(self, data)
src.reply r.response(@records[r.domain])
end
end
end
DNSServer.new(records: records, ttl: 120).run
@ghost

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ghost commented Dec 3, 2011

Pretty good!

@squarism

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squarism commented Dec 7, 2011

Amazing. I wonder what awesome things would happen if you kept going with this. DNS views and slave support as modules? The hex binary stuff here is pretty rad, I have never gone that low.

@feyeleanor

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feyeleanor commented Dec 8, 2011

Sweet.

@cayblood

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cayblood commented Dec 8, 2011

Anthony Eden talked about using DNS to do gem versioning. Wonder if this could help with that...

@feyeleanor

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feyeleanor commented Dec 9, 2011

@johnae

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johnae commented Dec 16, 2011

Just wanted to point out that there is a ruby dns library out there already which I've used to build a small dns-server for internal use. It's here: https://github.com/ioquatix/rubydns

@squarism

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squarism commented Dec 18, 2011

@johnae very cool

@djbender

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djbender commented Dec 19, 2011

I like the brevity of this!

@tarcieri

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tarcieri commented Aug 16, 2012

Am I the only one who looks at this and goes hax? I'm looking for DNS server components which map to the primitives in the Resolv module. This is more like barely scraping the packets...

@ioquatix

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ioquatix commented Jan 18, 2013

@tarcieri You might like to try out RubyDNS then.

@peterc Nice work :D

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