在 Windows 平台上使用 Java 查找特定路由的传出 IP 地址

Question

Although there is a lot of threads for finding machine IP addresses using combinations of NetworkInterface , InterfaceAddress , InetAddress , yet my case goes a bit beyond that.

I need to determine outgoing IP address which will be used during communication with particular (given) target. I have to assume that several interfaces with many ip addresses may be available at the host. This is the best approximation I can imagine for real life scenario where I have to supply to the remote machine which is known only by IP address an own IP that it may use to reach my host.

So far I have found no pure Java solution for this task. With my very limited Java knowledge I see no easy way to analyse possible routing options. That's why on Linux I will just execute externally and parse something like that:

ip -o route get 10.10.xx.xx

Which in turn will give me desired value ("src xx.xx.xx.xx"):

10.10.xx.xx dev eth1  src 10.20.xx.xx \    cache  mtu 1500 advmss 1460 hoplimit 64

My question is if there is a native way to achieve the same result without calling external commands like above. If not then what do you suggest for Windows equivalent?

---

I have also tried using InetAddress.isReachable(...) and external ping with given source address but former fails on firewall in my environment when falling back to TCP echo, while the other provides false positives. (On my test VM I have one interface bound directly to the host network and second one as a bridge (NAT) via VM host. So for ping both routes do the trick by route via host is not supported in opposite direction as required.)

I would appreciate any suggestions how to proceed with that...

Answer 1

Finally I ended up with parsing routing table (IP4 only, no static routes). Here is a code snippet, maybe it can be useful to somebody. More specific routes are preferred over more generic ones. For the same network part lengths the best metric wins.

private static String findOutgoingIpForGivenAdress(String remoteIP) {
    final String COMMAND = "route print -4";
    List<RouteInfo> routes = new ArrayList<>();
    try {
        Process exec = Runtime.getRuntime().exec(COMMAND);
        BufferedReader reader = new BufferedReader(new InputStreamReader(exec.getInputStream()));

        String localIP = null;
        String line;
        /* examples:
                0.0.0.0          0.0.0.0     10.172.180.1    10.172.180.36     20
                0.0.0.0          0.0.0.0      10.187.20.1    10.187.20.225     25
           10.172.180.0    255.255.255.0         On-link     10.172.180.36    276
          10.172.180.36  255.255.255.255         On-link     10.172.180.36    276
        */
        Pattern p = Pattern.compile("^\\s*(\\d+\\.\\d+\\.\\d+\\.\\d+)\\s+(\\d+\\.\\d+\\.\\d+\\.\\d+)\\s+\\S+?\\s+(\\d+\\.\\d+\\.\\d+\\.\\d+)\\s+(\\d+)\\s*$");
        while ((line = reader.readLine()) != null) {
            Matcher match = p.matcher(line);
            if (match.matches()) {
                String network = match.group(1);
                String mask = match.group(2);
                String address = match.group(3);
                short maskLength = 0;
                boolean networkMatch = network.contentEquals("0.0.0.0");
                if (!networkMatch) {
                    SubnetUtils subnet = new SubnetUtils(network, mask);
                    SubnetUtils.SubnetInfo info = subnet.getInfo();
                    networkMatch = info.isInRange(remoteIP);
                    maskLength = Short.valueOf(info.getCidrSignature().split("/")[1]);
                }
                if (networkMatch) {
                    short metric = Short.valueOf(match.group(4));
                    routes.add(new RouteInfo(address, maskLength, metric));
                }
            }
        }
        Collections.sort(routes);
        for (RouteInfo route : routes) {
        }
        if (!routes.isEmpty())
            return routes.get(0).source;
        if (localIP != null)
            return localIP;
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return null;
}

And there is RouteInfo helper class:

public class RouteInfo implements Comparable<RouteInfo> {
    public final String source;
    public final short maskLength;
    public final short metric;

    public RouteInfo(String src, short mask, short mtr) {
        source = src;
        maskLength = mask;
        metric = mtr;
    }

    @Override
    public int compareTo(RouteInfo ri) {
        if (ri.maskLength != maskLength)
            return ri.maskLength - maskLength;
        return metric - ri.metric;
    }
}