Initial commit

.gitignore

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+*.swp
+*.*~
+project.lock.json
+.DS_Store
+*.pyc
+
+# Visual Studio Code
+.vscode
+
+# User-specific files
+*.suo
+*.user
+*.userosscache
+*.sln.docstates
+
+# Build results
+[Dd]ebug/
+[Dd]ebugPublic/
+[Rr]elease/
+[Rr]eleases/
+x64/
+x86/
+build/
+bld/
+[Bb]in/
+[Oo]bj/
+msbuild.log
+msbuild.err
+msbuild.wrn
+
+# Visual Studio 2015
+.vs/

Program.cs

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+using System;
+using System.CommandLine;
+using System.CommandLine.Invocation;
+using System.Linq;
+using System.Net;
+using System.Net.NetworkInformation;
+using DotNetProjects.DhcpServer;
+
+namespace dhcpshot
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            var rootCommand = new RootCommand
+            {
+                new Option(
+                    new string[] { "--interface-ip", "-i" },
+                    "IPv4 for the interface we are going to listen to")
+                {
+                    Argument = new Argument<string>{ Arity = ArgumentArity.ExactlyOne },
+                    Required = true
+                },
+                new Option(
+                    new string[] { "--target-ip", "-t" },
+                    "IPv4 that will be given to the DHCP client, if none given it will be generated from interface IP")
+                {
+                    Argument = new Argument<string>()
+                }
+            };
+            rootCommand.Description = "DHCPshot - Assign IP address to first client asking on the specified interface";
+            rootCommand.Handler = CommandHandler.Create<string, string>(StartServer);
+            rootCommand.InvokeAsync(args).Wait();
+        }
+
+        public static void StartServer(string interfaceIp, string targetIp)
+        {
+            Console.WriteLine($"Looking for interface for IP {interfaceIp}");
+            var intf = NetworkInterface.GetAllNetworkInterfaces()
+                .Where(
+                    x => x.GetIPProperties().UnicastAddresses
+                        .Any(y => y.Address.Equals(IPAddress.Parse(interfaceIp)))
+                    ).FirstOrDefault();
+            if (intf == null)
+            {
+                Console.WriteLine($"Interface for IP {interfaceIp} not found!");
+                return;
+            }
+            Console.WriteLine($"Starting DHCP server on interface {intf.Name}");
+            var server = new DHCPServer();
+
+            if (targetIp == null)
+            {
+                var rnd = new Random();
+                var ip_bytes = interfaceIp.Split(".");
+                var lastByte = int.Parse(ip_bytes[3]);
+                int newByte = rnd.Next(2, 254);
+                while (newByte == lastByte)
+                {
+                    newByte = rnd.Next(2, 254);
+                }
+                ip_bytes[3] = newByte.ToString();
+                targetIp = string.Join(".", ip_bytes);
+            }
+            Console.WriteLine($"Client will receive IP {targetIp}");
+            server.OnDataReceived += delegate (DHCPRequest dhcpRequest)
+            {
+                Request(dhcpRequest, IPAddress.Parse(targetIp));
+            };
+            server.BroadcastAddress = IPAddress.Broadcast;
+            server.SendDhcpAnswerNetworkInterface = intf;
+            server.Start();
+            Console.WriteLine("Running DHCP server... Press CTRL-C to exit");
+        }
+
+        static void Request(DHCPRequest dhcpRequest, IPAddress targetIp)
+        {
+            try
+            {
+                var type = dhcpRequest.GetMsgType();
+                var replyOptions = new DHCPReplyOptions();
+                replyOptions.SubnetMask = IPAddress.Parse("255.255.255.0");
+
+                if (type == DHCPMsgType.DHCPDISCOVER)
+                    dhcpRequest.SendDHCPReply(DHCPMsgType.DHCPOFFER, targetIp, replyOptions);
+                if (type == DHCPMsgType.DHCPREQUEST)
+                    dhcpRequest.SendDHCPReply(DHCPMsgType.DHCPACK, targetIp, replyOptions);
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine(ex);
+            }
+        }
+    }
+}

dhcpshot.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="DotNetProjects.DhcpServer" Version="1.0.15" />
+    <PackageReference Include="System.CommandLine.Experimental" Version="0.3.0-alpha.19573.2" />
+  </ItemGroup>
+
+</Project>