问题
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?
回答1:
It should 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 if you're having issues with a 0.0.0.0 address being returned.
That said, the recommended way to do this uses GetBestInterface to find an interface for routing to a specific IP address. If you have a destination IP address in mind already, then it's better to use this - and so I've also included an example of that below:
[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;
}
回答2:
The first IP address returned by traceroute command will be the gateway .You can also using this fact for getting gateway.You can find a nice implementation of tracerout here:
TraceRoute and Ping in C#
回答3:
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;
}
回答4:
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.
回答5:
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());
回答6:
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
回答7:
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;
}
}
回答8:
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
来源：https://stackoverflow.com/questions/13634868/get-the-default-gateway