Android局域网中如何根据已知主机名获取对应IP地址？

Hey there, let's break down how to fix your issues with getting hostnames for known LAN IPs, and work through each problem you're facing:

Why InetAddress.getHostName() Returns IP Instead of Hostname #

The methods getHostName() and getCanonicalHostName() rely on reverse DNS (PTR) records, which most home LAN devices don't register with a local DNS server. Additionally, Android's network stack might restrict reverse lookups unless the device has been actively communicated with (like a ping to populate the ARP cache).

Fix for This: #

• First, ensure your app has the required permissions in AndroidManifest.xml:

  <uses-permission android:name="android.permission.INTERNET" />
  <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
  

• Try pinging the target IP first to populate the ARP cache, then attempt the hostname lookup. Here's a quick ping implementation snippet:

  private boolean pingHost(String ip) {
      try {
          Process process = Runtime.getRuntime().exec("ping -c 1 " + ip);
          int exitCode = process.waitFor();
          return exitCode == 0;
      } catch (IOException | InterruptedException e) {
          e.printStackTrace();
          return false;
      }
  }
  
  // Usage
  String targetIp = "192.168.1.100";
  if (pingHost(targetIp)) {
      InetAddress address = InetAddress.getByName(targetIp);
      String hostname = address.getHostName();
      // Use the hostname here
  }
  

Why nslookup Doesn't Work in Android #

Android's system environment doesn't include nslookup by default on most devices, unlike desktop Java. Calling system commands like this is also unreliable across different Android versions and OEM skins. Instead, use a dedicated DNS library or Android's built-in APIs for DNS queries.

Fixing NoClassDefFoundError for dnsjava #

Adding the JAR to the libs directory isn't enough in Android Studio—you need to explicitly declare the dependency in your build file, and handle ProGuard if you're using it:

1. Update your app-level build.gradle:

   dependencies {
       // Add this line to include the local JAR
       implementation files('libs/dnsjava-2.1.7.jar')
   }
   

2. Add ProGuard rules (if enabled):
   If you're using code shrinking, add these lines to proguard-rules.pro to prevent dnsjava classes from being obfuscated:

   -keep class org.xbill.** { *; }
   -keep interface org.xbill.** { *; }
   

3. Clean and rebuild your project:
   Go to Build > Clean Project, then Build > Rebuild Project to ensure the dependency is properly linked.

Using dnsjava for Reverse Lookups #

Once the library is set up, you can use it to perform reverse DNS queries directly:

import org.xbill.DNS.*;
import java.net.InetAddress;

public String getHostnameFromIp(String ip) {
    try {
        InetAddress address = InetAddress.getByName(ip);
        Name name = ReverseMap.fromAddress(address);
        Lookup lookup = new Lookup(name, Type.PTR);
        Record[] records = lookup.run();
        if (records != null && records.length > 0) {
            return records[0].rdataToString();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return ip; // Fallback to IP if lookup fails
}

Better Alternative: Android's NsdManager (ZeroConf/mDNS) #

For LAN device discovery, Android's built-in NsdManager is a more reliable option—it uses mDNS (Bonjour) to find devices that advertise their hostnames on the network. Here's a quick implementation outline:

private NsdManager nsdManager;
private NsdManager.DiscoveryListener discoveryListener;

private void startDiscovery() {
    nsdManager = (NsdManager) getSystemService(Context.NSD_SERVICE);
    discoveryListener = new NsdManager.DiscoveryListener() {
        @Override
        public void onDiscoveryStarted(String serviceType) {
            // Discovery started
        }

        @Override
        public void onServiceFound(NsdServiceInfo serviceInfo) {
            // Get hostname and IP
            String hostname = serviceInfo.getServiceName();
            InetAddress ip = serviceInfo.getHost();
            // Process the device info here
        }

        @Override
        public void onServiceLost(NsdServiceInfo serviceInfo) {
            // Device lost
        }

        @Override
        public void onDiscoveryStopped(String serviceType) {
            // Discovery stopped
        }

        @Override
        public void onStartDiscoveryFailed(String serviceType, int errorCode) {
            // Handle error
        }

        @Override
        public void onStopDiscoveryFailed(String serviceType, int errorCode) {
            // Handle error
        }
    };
    nsdManager.discoverServices("_http._tcp.", NsdManager.PROTOCOL_DNS_SD, discoveryListener);
}

// Don't forget to stop discovery when done
private void stopDiscovery() {
    if (nsdManager != null && discoveryListener != null) {
        nsdManager.stopServiceDiscovery(discoveryListener);
    }
}

This method doesn't require third-party libraries and works natively on Android, making it a great choice for LAN device hostname/IP mapping.

内容的提问来源于stack exchange，提问作者Hamza Hajeir