NetworkInterface.getNetworkInterfaces() not listing all interfaces

Question

I have three interfaces `(eth0,Loopback,wlan0)` on my machine and i want to get use Java-API to get the mac address. I use this code. ``` Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces(); for (NetworkInterface netint : Collections.list(nets)) displayInterfaceInformation(netint); } static void displayInterfaceInformation(NetworkInterface netint) throws SocketException { System.out.println("Display name: " + netint.getDisplayName()); System.out.println("Hardware address: " + Arrays.toString(netint.getHardwareAddress())); } ``` But that code print `wlan0,loopback`, but missed eth0 . - My OS Ubuntu, any help. Update The o/p (`strace -f java Networks 2>&1| grep ioctl`).. blank(empty). `java -version` java version "1.7.0_21" Java(TM) SE Runtime Environment (build 1.7.0_21-b11) Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode) - strace ifconfig 2>&1 | grep ioctl ` ``` ioctl(4, SIOCGIFCONF, {80, {{"lo", {AF_INET, inet_addr("127.0.0.1")}}, {"wlan0", {A…

Accepted Answer

Obviously, I was wrong in the first place: even though both `ifconfig` and the Java API are using the same `ioctl()` syscalls, they behave differently. First of all, the `SIOCGIFCONF ioctl()` is documented as follows (see http://linux.die.net/man/7/netdevice): ``` SIOCGIFCONF Return a list of interface (transport layer) addresses. ... The kernel fills the ifreqs with all current L3 interface addresses that are running. ``` So, the `SIOCGIFCONF ioctl()` which is used by both `ifconfig` and the JAVA API only returns the running interfaces. This can also be seen in the `strace ifconfig ...` output from the question - the very first `ioctl` only returns `lo` and `wlan0`, but not eth0. Then, where does `ifconfig` get the `eth0` from at all? Checking the `ifconfig` source code (from the `net-tools` package on Debian/Ubuntu), we see that `ifconfig` is not using the result from the `ioctl()` as the basis for the network device enumeration, but first of all reads the `/proc` filesystem to determine all network interfaces. Then, it uses the `ioctl()` syscalls to determine further information about each interface. Unfortunately, the `java.net.NetworkInterface.getByName()` method does not even return a network interface object for an unconfigured interface if we explicitly pass the name, like `eth0`. Essentially, there remain three different approaches to get the hardware addresses of all devices on Linux: - Call `ifconfig` and parse the output (should be last resort) - Implement a JNI library to do the same what `ifconfig` does (requires an architecture dependent shared library) - Read the data directly from the `/proc` and the `/sys` filesystems. All of these approaches are system dependant and not portable. The benefit of the third approach is that it can be implemented in pure Java. The following is a sample implementation of the third approach which worked well in my environment: ``` static void printHardwareAddresses() throws SocketException { if (System.getProperty("os.name").equals("Linux")) { // Read all available device names List devices = new ArrayList<>(); Pattern pattern = Pattern.compile("^ *(.*):"); try (FileReader reader = new FileReader("/proc/net/dev")) { BufferedReader in = new BufferedReader(reader); String line = null; while( (line = in.readLine()) != null) { Matcher m = pattern.matcher(line); if (m.find()) { devices.add(m.group(1)); } } } catch (IOException e) { e.printStackTrace(); } // read the hardware address for each device for (String device : devices) { try (FileReader reader = new FileReader("/sys/class/net/" + device + "/address")) { BufferedReader in = new BufferedReader(reader); String addr = in.readLine(); System.out.println(String.format("%5s: %s", device, addr)); } catch (IOException e) { e.printStackTrace(); } } } else { // use standard API for Windows & Others (need to test on each platform, though!!) ... } } ```

NetworkInterface.getNetworkInterfaces() not listing all interfaces

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