Java JNDI DNS錯誤：“接收超時”

Question

import java.net.InetAddress;
import java.net.UnknownHostException;

import javax.naming.NamingEnumeration;
import javax.naming.NamingException;
import javax.naming.directory.Attributes;
import javax.naming.directory.InitialDirContext;

public class DNSLookup
{
    public static void main(String args[])
    {
        String host = "www.google.com";
        try
        {
            InetAddress inetAddress = InetAddress.getByName(host);
            // show the Internet Address as name/address
            System.out.println(inetAddress.getHostName() + " " + inetAddress.getHostAddress());

            // get the default initial Directory Context
            InitialDirContext iDirC = new InitialDirContext();
            // get the DNS records for inetAddress

            Attributes attributes = iDirC.getAttributes("dns://8.8.8.8/www.google.com", new String[] {"A"});
            // get an enumeration of the attributes and print them out
            NamingEnumeration<?> attributeEnumeration = attributes.getAll();
            System.out.println("");
            while (attributeEnumeration.hasMore())
            {
                System.out.println("" + attributeEnumeration.next());
            }
            attributeEnumeration.close();
        }
        catch (UnknownHostException exception)
        {
            System.err.println("ERROR: Cannot access '" + host + "'");
        }
        catch (NamingException exception)
        {
            System.err.println("ERROR: No DNS record for '" + host + "'");
            exception.printStackTrace();
        }
    }
}

如果我運行此代碼，則會收到如下錯誤...

www.google.com 74.125.128.103
ERROR: No DNS record for 'www.google.com'
javax.naming.CommunicationException: DNS error [Root exception is java.net.SocketTimeoutException: Receive timed out]; remaining name 'www.google.com'
    at com.sun.jndi.dns.DnsClient.query(DnsClient.java:300)
    at com.sun.jndi.dns.Resolver.query(Resolver.java:81)
    at com.sun.jndi.dns.DnsContext.c_getAttributes(DnsContext.java:430)
    at com.sun.jndi.toolkit.ctx.ComponentDirContext.p_getAttributes(ComponentDirContext.java:231)
    at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.getAttributes(PartialCompositeDirContext.java:139)
    at com.sun.jndi.toolkit.url.GenericURLDirContext.getAttributes(GenericURLDirContext.java:103)
    at javax.naming.directory.InitialDirContext.getAttributes(InitialDirContext.java:142)
    at DNSLookup.main(DNSLookup.java:24)
Caused by: java.net.SocketTimeoutException: Receive timed out
    at java.net.DualStackPlainDatagramSocketImpl.socketReceiveOrPeekData(Native Method)
    at java.net.DualStackPlainDatagramSocketImpl.receive0(DualStackPlainDatagramSocketImpl.java:121)
    at java.net.AbstractPlainDatagramSocketImpl.receive(AbstractPlainDatagramSocketImpl.java:145)
    at java.net.DatagramSocket.receive(DatagramSocket.java:786)
    at com.sun.jndi.dns.DnsClient.doUdpQuery(DnsClient.java:411)
    at com.sun.jndi.dns.DnsClient.query(DnsClient.java:203)
    ... 7 more

但是，如果我使用“ dns：/www.google.com”作為查詢，而不是“ dns：//8.8.8.8/www.google.com”，它可以完美地工作而不會出現任何錯誤。 僅當我嘗試指定要使用的DNS服務器時，才會發生錯誤。

“ dns：//8.8.8.8/www.google.com”->錯誤

“ dns：/www.google.com”->工作！

http://docs.oracle.com/javase/7/docs/technotes/guides/jndi/jndi-dns.html

在本文檔中，我要使用的功能完全相同。 我不明白為什么會造成問題。

DirContext ictx = new InitialDirContext();
Attributes attrs3 = ictx.getAttributes("dns://server1.example.com/host3.example.com",
                                   new String[] {"MX"});

Answer 1

您的代碼沒有錯，但是我懷疑您是從阻止傳出DNS的代理或防火牆后面運行它的。

當您請求dns:/www.google.com ，實際上是在詢問本地DHCP提供的解析器以獲取www.google.com的IP，就像在瀏覽器地址欄中輸入地址一樣。

但是，在使用dns://8.8.8.8/www.google.com您會要求使用Google DNS來解析google.com ，這要求您的代理/防火牆允許傳出的TCP / UDP端口53流量，但通常不會公司環境中的情況...

干杯，