我有一段用来切换设备网络IP模式（静态/DHCP）的Java代码，在独立类中运行完全正常，但部署到Tomcat的Web应用后，每次执行都会打印一堆JmDNS相关的警告和错误日志，想请大家帮忙分析原因和解决办法。

执行的代码 #

MulticastSocket multicastSocket = new MulticastSocket(5353);
InetAddress address = InetAddress.getByName("224.0.0.251");
multicastSocket.joinGroup(address);
multicastSocket.setLoopbackMode(true);
StringBuilder command = new StringBuilder();
command.append("SET_");
command.append(macaddress);
command.append("_<IPNetworkSettings><IPAddress>");
command.append("0.0.0.0");
command.append("</IPAddress><SubnetMask>");
command.append("x.x.x.x");
command.append("</SubnetMask><Gateway>");
command.append("x.x.x.x");
command.append("</Gateway></IPNetworkSettings>");
byte[] message = command.toString().getBytes();
DatagramPacket packet = new DatagramPacket(message, message.length, address, MULTICAST_PORT);
multicastSocket.send(packet);
byte[] buf = new byte[1000];
multicastSocket.setSoTimeout(30000);
DatagramPacket recv = new DatagramPacket(buf, buf.length);
multicastSocket.receive(recv);
String recmessage = new String(recv.getData(), US_ASCII).trim();
multicastSocket.leaveGroup(address);

出现的异常日志 #

Dec 19, 2017 7:46:50 PM javax.jmdns.impl.constants.DNSRecordClass classForIndex WARNING: Could not find record class for index: -1
Dec 19, 2017 7:46:50 PM javax.jmdns.impl.DNSIncoming$MessageInputStream readName SEVERE: bad domain name: possible circular name detected. Bad offset: 0xffffffff at 0x91
Dec 19, 2017 7:46:50 PM javax.jmdns.impl.constants.DNSRecordType typeForIndex SEVERE: Could not find record type for index: -1
Dec 19, 2017 7:46:50 PM javax.jmdns.impl.DNSIncoming readQuestion SEVERE: Could not find record type: dns[query,199.xxx.xxx.xxx:5353, length=147, id=0x0, flags=0x545f:aa, questions=935 questions: [DNSQuestion@19280069 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: 199.xxx.xxx.xxx255.255.255.0199.xxx.xxx.xxx??????.] [DNSQuestion@5804116 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@15634375 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@17093854 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@23358883 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@30187115 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@25734882 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@75113 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ] [DNSQuestion@15094207 type: TYPE_IGNORE index 0, class: CLASS_UNKNOWN index 0, name: ]

原因分析 #

这堆日志其实是JmDNS（Java Multicast DNS）组件抛出的解析错误，问题根源在于：

• 你的代码使用了5353端口，这是mDNS（多播DNS）的默认端口，而Tomcat环境中很可能已经有其他应用或组件在使用JmDNS监听这个端口做服务发现。
• 当你的代码发送自定义格式的多播数据包到5353端口时，JmDNS会尝试把这些数据包当作标准的mDNS协议包来解析，但你的数据包是自定义的XML命令格式，完全不符合mDNS规范，所以JmDNS解析失败，就打印了这些错误日志。

解决方案 #

最直接有效的办法是更换多播端口，避开mDNS的默认5353端口，具体步骤如下：

1. 修改代码中的端口配置：

   • 将new MulticastSocket(5353)替换为其他未被占用的端口，比如5354
   • 同时确保MULTICAST_PORT常量的值也改成对应的新端口（比如5354）
   • 还要确认你的目标设备也配置为监听这个新的多播端口，能接收并解析你的命令

2. 修改后的代码片段示例：

// 更换为非5353的端口
int CUSTOM_MULTICAST_PORT = 5354;
MulticastSocket multicastSocket = new MulticastSocket(CUSTOM_MULTICAST_PORT);
InetAddress address = InetAddress.getByName("224.0.0.251");
multicastSocket.joinGroup(address);
multicastSocket.setLoopbackMode(true);
// ... 其他代码不变 ...
DatagramPacket packet = new DatagramPacket(message, message.length, address, CUSTOM_MULTICAST_PORT);
// ... 其他代码不变 ...

如果因为设备限制必须使用5353端口，那可以尝试在代码中明确指定绑定的网卡，避免和JmDNS的监听冲突，但这种方式兼容性较差，不如换端口来得稳妥。

内容的提问来源于stack exchange，提问作者Srikanth