From: Sam Roberts <sroberts@...>
Date: 2005-01-20T00:03:35+09:00
Subject: bug -- resolv.rb dies after Resolv::DNS is unsuccessful


resolv.rb:877 does

          raise ResolvError.new("DNS resolv error: #{name}")

This means that when Resolv::DNS fails to find at least one record for
a query, it raises an error, preventing the next resolver in @resolvers
from being checked (see Resolv#each_address()).

Symptoms:

- you can't do a DNS lookup, then fallback to a Hosts lookup:

  resolv = Resolv.new([Resolv::DNS.new, Resolv::Hosts.new])

  resolv.getaddress("localhost")

  # This will fail (unless your DNS server has an entry for "localhost",
  # some do). Doing the opposite (the default) does work, i.e. doing
  # the hosts lookup, then the DNS lookup.


- resolv.rb:228 (and probably 256) are no-ops

  Instead of getting ResolvError.new("no address for #{name}') raised as
  it appears was intended, you will always get the lower level "DNS
  resolv error".

- if you have two DNS resolvers registered the second will never be
  reached

  You might do this if the first uses resolv.conf for its configuration,
  and you add a second with a default config specified with a Hash:

   resolv = Resolv.new([Resolv::DNS.new, Resolv::DNS.new( ... my config ...) ])


- If you write you own resolver module, then the failure of the DNS
  module prevents the next resolver from being used to attempt the
  lookup.


Cheers,
Sam


Example patch:

===================================================================
--- 1.8-resolv.rb       (revision 11)
+++ 1.8-resolv.rb       (working copy)
@@ -226,6 +226,8 @@
   def getaddress(name)
     each_address(name) {|address| return address}
     raise ResolvError.new("no address for #{name}")
+# - this line is a no-op, it isn't reached because an internal DNS error is
+# raised instead.
   end
 
   def getaddresses(name)
@@ -872,7 +874,8 @@
         rescue OtherResolvError
           raise ResolvError.new("DNS error: #{$!.message}")
         end
-        raise ResolvError.new("DNS resolv error: #{name}")
+#       raise ResolvError.new("DNS resolv error: #{name}")
+# - this shouldn't be fatal, the next resolver might be able to resolve this name!
       end
 
       class NXDomain < ResolvError


Test code:

#!/usr/bin/ruby

require 'pp'
require 'socket'
require 'resolv'

puts "(C library resolver) localhost -->"
addr = IPSocket.getaddress("localhost")
pp addr

# This is the default, args = [Resolv::Hosts.new, Resolv::DNS.new]
resolv = Resolv.new

puts "(Hosts, DNS) localhost -->"
addr = resolv.getaddress("localhost")
pp addr
puts "OK - address resolves!"

begin
  puts "(Hosts, DNS) asergh.net -->"
  addr = resolv.getaddress("asergh.net")
  pp addr
rescue
  pp $!
  unless($!.to_s =~ /^no address for/)
    puts "BUG - this should have hit resolv.rb:228, instead it is returning a DNS-specific error!"
  end
end

# This is trying DNS lookup before host lookup
resolv = Resolv.new([Resolv::DNS.new, Resolv::Hosts.new])

begin
  puts "(resolv) localhost -->"
  addr = resolv.getaddress("localhost")
  pp addr
rescue
  pp $!
  puts "BUG - this failed to find localhost when the resolvers were reordered!"
end