CODEWITHSUNDEEP
c#network-programmingnetwork-protocols
Frecklefoot
Frecklefoot

Reputation: 1692

Get the Default Gateway

I'm writing a program that shows the user their IP address, Subnet mask and Default gateway. I can get the first two, but for the last one, this is what I turned up:

GatewayIPAddressInformationCollection gwc = 
    System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces()[0].GetIPProperties().GatewayAddresses;

That, of course, returns a collection of GatewayIPAddressInformation. So, if a computer has multiple gateways, how can I determine which is the default gateway?

In practice, I've only ever seen this collection contain a single entry, but since it's implemented as a collection, it stands to reason that some computers contain multiple gateways, none of which are marked as "Default". So is there a way to determine the default or is it all just guesswork?

Upvotes: 28

Views: 47883

Answers (9)

caesay
caesay

Reputation: 17213

It will probably be the first valid and enabled gateway address of the first enabled network interface:

public static IPAddress GetDefaultGateway()
{
    return NetworkInterface
        .GetAllNetworkInterfaces()
        .Where(n => n.OperationalStatus == OperationalStatus.Up)
        .Where(n => n.NetworkInterfaceType != NetworkInterfaceType.Loopback)
        .SelectMany(n => n.GetIPProperties()?.GatewayAddresses)
        .Select(g => g?.Address)
        .Where(a => a != null)
         // .Where(a => a.AddressFamily == AddressFamily.InterNetwork)
         // .Where(a => Array.FindIndex(a.GetAddressBytes(), b => b != 0) >= 0)
        .FirstOrDefault();
}

I've also added some further commented checks which have been pointed out as useful by other people here. You can check the AddressFamily one to distinguish between IPv4 and IPv6. The latter one can be used to filter out 0.0.0.0 addresses.

The above solution will give you a valid/connected interface, and is good enough for 99% of situations. That said, if you have multiple valid interfaces that traffic can be routed through, and you need to be 100% accurate, the way to do this uses GetBestInterface to find an interface for routing to a specific IP address. This additionally handles the case where you might have a specific address range routed through a different adapter (e.g. 10.*.*.* going through a VPN, everything else going to your router)

[DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
private static extern int GetBestInterface(UInt32 destAddr, out UInt32 bestIfIndex);

public static IPAddress GetGatewayForDestination(IPAddress destinationAddress)
{
    UInt32 destaddr = BitConverter.ToUInt32(destinationAddress.GetAddressBytes(), 0);

    uint interfaceIndex;
    int result = GetBestInterface(destaddr, out interfaceIndex);
    if (result != 0)
        throw new Win32Exception(result);

    foreach (var ni in NetworkInterface.GetAllNetworkInterfaces())
    {
        var niprops = ni.GetIPProperties();
        if (niprops == null)
            continue;

        var gateway = niprops.GatewayAddresses?.FirstOrDefault()?.Address;
        if (gateway == null)
            continue;

        if (ni.Supports(NetworkInterfaceComponent.IPv4))
        {
            var v4props = niprops.GetIPv4Properties();
            if (v4props == null)
                continue;

            if (v4props.Index == interfaceIndex)
                return gateway;
        }

        if (ni.Supports(NetworkInterfaceComponent.IPv6))
        {
            var v6props = niprops.GetIPv6Properties();
            if (v6props == null)
                continue;

            if (v6props.Index == interfaceIndex)
                return gateway;
        }
    }

    return null;
}

These two examples could be wrapped up into a helper class and used in the appropriate cases: that you do, or do not have a destination address in mind already.

Upvotes: 49

Rich K
Rich K

Reputation: 410

(Just after posting I noticed an earlier answer that mentioned the idea of using traceroute. I probably wouldn't have responded if I'd seen that. But I have included complete/compact code.)

Here's a different approach that finds the default gateway by using a traceroute type method. Ping an address on the internet (or other network you want to get to) with a TTL of 1, and let ping tell you what the first hop IP address was. That ought to be the gateway you're looking for.

I suppose this could go wrong if the gateway was configured not to reply to pings, but I don't think I've ever heard of a network set up that way.

Note: This routine kind-of even works if you pass it an IPV6 address. The potential issue is that it seems to return an IPV6 address with Global scope, rather than Link-Local scope. I don't have enough experience using IPV6 to know if that's good or bad. But I do know that the Windows ipconfig command displays the Link-Local IPV6 addresses of the interface gateway.

using System.Net
using System.Net.NetworkInformation

// ping internet address with ttl=1 and return address of host where ttl expired
public static IPAddress FindDefaultGateway(IPAddress netaddr = null)
{
    // user can provide an ip address that exists on the network they want to connect to, 
    // or this routine will default to 1.1.1.1 (IP of a popular internet dns provider)
    if (netaddr is null)
        netaddr = IPAddress.Parse("1.1.1.1"); 

    PingReply reply = default;  
    using var ping = new Ping();
    var options = new PingOptions(1, true); // ttl=1, dont fragment=true
    try { 
        // I arbitrarily used a 200ms timeout; tune as you see fit.
        reply = ping.Send(netaddr, 200, new byte[0], options); 
    }
    catch (PingException) {
        System.Diagnostics.Debug.WriteLine("Gateway not available");
        return default;
    }
    if (reply.Status != IPStatus.TtlExpired) {
        System.Diagnostics.Debug.WriteLine("Gateway not available");
        return default;
    }
    return reply.Address;
}

Upvotes: 1

harsini
harsini

Reputation: 336

The first IP address returned by traceroute command will be the gateway .You can use this fact for getting gateway.A nice implementation of tracerout can be found here: TraceRoute and Ping in C#

Upvotes: 4

Cameron
Cameron

Reputation: 3063

Been looking for one weird trick to determine the local IP address? Yet do it very robustly vs. the earlier methods?

[See end for response to @caesay]

This may be what you are looking for.

using (var s = new System.Net.Sockets.Socket(AddressFamily.InterNetwork, SocketType.Dgram,ProtocolType.Udp))
{
    s.Bind( new System.Net.IPEndPoint(System.Net.IPAddress.Any,0));
    s.Connect("google.com",0);

    var ipaddr = s.LocalEndPoint as System.Net.IPEndPoint;
    var addr = ipaddr.Address.ToString();
    Console.WriteLine(addr);
}

That will create a UDP socket, but will not send data on it. You can then see what local interface and address the socket was bound to. You must supply an IP address or hostname where I used google.com, that will be used by the OS to determine which interface the socket will be bound to, and what IP address you will find using this method. For 99%+ of IP addresses or hostnames you use, you will get the interface+address to be used for traffic over the default route.

While this is a little obtuse, I suspect it is more reliable than the methods above where people use the PInvoke call and scan through the structures to see if they can find the interface.

I certainly found it more robust on my 3x double interface systems than scanning through the results of NetworkInterface.GetAllNetworkInterfaces() using heuristics to try to figure out the default interface address.

[Update/response to @caesay 2/6/2019] @caesay Has made an excellent point here, so I have switched the sample from using UdpClient to Socket, and explicitly called Socket.Connect() after Bind(). I have also reviewed the Framework source for Connect() and LocalEndPoint, and both call their corresponding Win32 OS system call immediately without deferring or being lazy about calling into the OS. The Win32 bind page says "The application can use getsockname after calling bind to learn the address and the port that has been assigned to the socket. If the Internet address is equal to INADDR_ANY or in6addr_any, getsockname cannot necessarily supply the address until the socket is connected". This condition is now explicitly addressed, and having reviewed the Framework source, the IP address should always be available after the Connect() call.

Upvotes: 3

Code Jockey
Code Jockey

Reputation: 21

I know this is a slightly older question but, I had just come upon the need to retrieve the IPV4 address of the local gateway. The accepted answer doesn't quite fit the bill when it comes to my own system so, I modified it to suite and I'm sure others will be able to use this solution, too.

Since I don't yet have enough rep to comment, I'm forced to add this as a "question":

public static IPAddress GetDefaultGateway()
    {
        IPAddress result = null;
        var cards = NetworkInterface.GetAllNetworkInterfaces().ToList();
        if (cards.Any())
        {
            foreach (var card in cards)
            {
                var props = card.GetIPProperties();
                if (props == null)
                    continue;

                var gateways = props.GatewayAddresses;
                if (!gateways.Any())
                    continue;

                var gateway =
                    gateways.FirstOrDefault(g => g.Address.AddressFamily.ToString() == "InterNetwork");
                if (gateway == null)
                    continue;

                result = gateway.Address;
                break;
            };
        }

        return result;
    }

Upvotes: 2

Nick Randell
Nick Randell

Reputation: 18295

I've just come across this and will be using the following code - basically it looks for interfaces that aren't loopback and are up and have gateways and unicast addresses. From the interface I then get a non transient IPV4 address.

public static class NetworkInterfaces
{
    public static NetworkInterface GetDefaultInterface()
    {
        var interfaces = System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces();
        foreach (var intf in interfaces)
        {
            if (intf.OperationalStatus != OperationalStatus.Up)
            {
                continue;
            }
            if (intf.NetworkInterfaceType == NetworkInterfaceType.Loopback)
            {
                continue;
            }

            var properties = intf.GetIPProperties();
            if (properties == null)
            {
                continue;
            }
            var gateways = properties.GatewayAddresses;
            if ((gateways == null) || (gateways.Count == 0))
            {
                continue;
            }
            var addresses = properties.UnicastAddresses;
            if ((addresses == null) || (addresses.Count == 0))
            {
                continue;
            }
            return intf;
        }
        return null;
    }

    public static IPAddress GetDefaultIPV4Address(NetworkInterface intf)
    {
        if (intf == null)
        {
            return null;
        }
        foreach (var address in intf.GetIPProperties().UnicastAddresses)
        {
            if (address.Address.AddressFamily != AddressFamily.InterNetwork)
            {
                continue;
            }
            if (address.IsTransient)
            {
                continue;
            }
            return address.Address;
        }
        return null;
    }
}

Upvotes: 2

aliep
aliep

Reputation: 1772

according to @midspace's comment on @caesay's answer this is a better answer:

public static IPAddress GetDefaultGateway()
{
    var gateway_address = NetworkInterface.GetAllNetworkInterfaces()
        .Where(e => e.OperationalStatus == OperationalStatus.Up)
        .SelectMany(e => e.GetIPProperties().GatewayAddresses)
        .FirstOrDefault();
    if (gateway_address == null) return null;
    return gateway_address.Address;
}

I'm warning that it's not a complete solution, if your'e looking for the main interface responsible for the internet access, you should combine other approaches like using win32 API GetBestInterface to find the best interface to connect to a destination address.

you can find example usage here: http://www.pinvoke.net/default.aspx/iphlpapi.getbestinterface

Upvotes: 3

Wayne
Wayne

Reputation: 11

NetworkInterface[] allNICs = NetworkInterface.GetAllNetworkInterfaces();
foreach (var nic in allNICs)
{
    var ipProp = nic.GetIPProperties();
    var gwAddresses = ipProp.GatewayAddresses;
    if (gwAddresses.Count > 0 &&
        gwAddresses.FirstOrDefault(g => g.Address.AddressFamily == AddressFamily.InterNetwork) != null)
    {
        localIP = ipProp.UnicastAddresses.First(d => d.Address.AddressFamily == AddressFamily.InterNetwork).Address;
    }
}
Debug.Print(localIP.ToString());

Upvotes: 1

Falcon313
Falcon313

Reputation: 46

I think that you should iterate over this collection and show all the Gateways, since if there are many gateways to an adapter they are all considered Default to that adapter

Upvotes: 0

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