Java DNS查找SRV记录

问题描述

在下面的java代码中，我正在进行DNS SRV记录查找，以解决给定域名的目标域名和关联端口，如[email protected]。用/ HERE / below表示的查找功能以某种方式返回null，我无法获取查询结果（即，记录数组为空）。因此，在for循环中访问记录数组时抛出空指针异常。

In the below java code, I am making a DNS SRV record lookup to resolve the target domain name and associated port for a given domain name such as [email protected]. The lookup function indicated with /HERE/ below returns null somehow and I cannot get a a query result (i.e. record array is null). As a consequence, a null pointer exception is thrown when records array is accessed in the for loop.

您认为我错过了以下代码如何工作？我正在使用dnsjava，相关的API jar文件可从（在页面的底部）。感谢您的建议。

What do you think I am missing to make the following code work. I am using using dnsjava and the related API jar file is available at http://www.dnsjava.org/download/ (at the bottom of the page). Thanks in advance for your suggestion.

     import org.xbill.DNS.Lookup;
     import org.xbill.DNS.Record;
     import org.xbill.DNS.SRVRecord;
     import org.xbill.DNS.TextParseException;
     import org.xbill.DNS.Type;

     public class DNSLookup {

        public static void main(String[] args) {

          if (args.length < 1) {
             System.err.println("Usage: java -jar DNSLookup <hostname>");
             System.exit(1);
          }

          String query = "_ssh._tcp." + args[0];

          try {
            /*****HERE*********/
            Record[] records = new Lookup(query, Type.SRV).run();

            for (Record record : records) {
                 SRVRecord srv = (SRVRecord) record;

            String hostname = srv.getTarget().toString().replaceFirst("\\.$", "");
            int port = srv.getPort();

            System.out.println(hostname + ":" + port);
          }
          } catch (TextParseException e) {
             e.printStackTrace();
          }
       }
      }

推荐答案

你的代码很好，除了 Lookup.run（）返回 null （ not 一个空数组）如果没有找到记录。

Your code is fine, except that Lookup.run() returns null (not an empty array) if no records are found.

例如，如果替换 _ssh._tcp 使用 _nicname._tcp ，然后查找 uk ，您将会找到.UK的SRV记录 whois 服务器。

For example, if you replace _ssh._tcp with _nicname._tcp and then look up uk you'll find an SRV record for the .UK whois server.

如果有一个问题，你的输入参数是。

If there's a problem it's with your input parameters.

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