Resolv::DNS -如何强制TCP？

Question

有人知道如何在使用Resolv::DNS时强制TCP吗？

似乎当我请求ANY记录时，输出被截断了，我得到了部分结果。当我执行多个查询(每个记录类型一个)时，我会得到更多的结果。我也得到了不一致的结果(不同的机器，两个连续的查询返回不同的结果，...)

我认为这可能与UDP受数据包大小限制有关。

你知道我该怎么强制它使用TCP吗？还有没有别的DNS包我可以用？

Answer 1

我也遇到了同样的问题，我希望使用Resolv进行仅限TCP的查询，因为我期望得到的结果集非常大。我最终深入研究了Resolv的源代码，了解到在默认情况下，只有在UDP查询失败的情况下才会执行TCP查询。我发现我可以子类化Resolv::DNS并覆盖each_resource方法。这是我的资料来源：

require 'resolv'

# A TCP-only resolver built from `Resolv::DNS`. See the docs for what it's about.
# http://ruby-doc.org/stdlib-1.9.3/libdoc/resolv/rdoc/Resolv/DNS.html
class TcpDNS < Resolv::DNS
  # Override fetch_resource to use a TCP requester instead of a UDP requester. This
  # is mostly borrowed from `lib/resolv.rb` with the UDP->TCP fallback logic removed.
  def each_resource(name, typeclass, &proc)
    lazy_initialize
    senders = {}
    requester = nil
    begin
      @config.resolv(name) { |candidate, tout, nameserver, port|
        requester = make_tcp_requester(nameserver, port)
        msg = Message.new
        msg.rd = 1
        msg.add_question(candidate, typeclass)
        unless sender = senders[[candidate, nameserver, port]]
          sender = senders[[candidate, nameserver, port]] =
            requester.sender(msg, candidate, nameserver, port)
        end

        begin # HACK
          reply, reply_name = requester.request(sender, tout)
        rescue
          return
        end

        case reply.rcode
        when RCode::NoError
          extract_resources(reply, reply_name, typeclass, &proc)
          return
        when RCode::NXDomain
          raise Config::NXDomain.new(reply_name.to_s)
        else
          raise Config::OtherResolvError.new(reply_name.to_s)
        end
      }
    ensure
      requester.close
    end
  end
end

然后，使用它就像下面这样简单：

TcpDNS.open :nameserver => ns_addrs, :search => '', :ndots => 1 do |dns|
  resp = dns.getresources target, Resolv::DNS::Resource::IN::ANY
end