Question

On Linux using MONO.

How do I use C# to pair a port number to a service ?

example:

port 80 = http
port 443 = https

and output it to the console, I need this for a simple port scanner I built.

I know this function exists in ruby:

Socket.getservbyport(80, "tcp")

Answer 1

Mono Linux version

I had a bit of time on my hands so set up Mono on my Ubuntu Linux box to test with. The Mono PInvoke implementation of getservbyport() and getservbyname() is simpler than on Windows (just load libc which has the networking stuff built in). Here’s the example code for reference in case anyone ever wants it 😉

namespace SocketUtil
{
    using System;
    using System.Net;
    using System.Collections.Generic;
    using System.Runtime.InteropServices;
    using System.Runtime.Serialization;

    [Serializable]
    public class SocketUtilException : Exception
    {

        public SocketUtilException()
        {
        }

        public SocketUtilException(string message)
            : base(message)
        {
        }

        public SocketUtilException(string message, Exception inner)
            : base(message, inner)
        {
        }

        protected SocketUtilException(
            SerializationInfo info, StreamingContext context)
            : base(info, context)
        {
        }
    }

    public static class SocketUtil
    {

        [StructLayoutAttribute(LayoutKind.Sequential)]
        public struct servent
        {
            public string s_name;
            public IntPtr s_aliases;
            public ushort s_port;
            public string s_proto;
        }


        [DllImport("libc", SetLastError = true, CharSet = CharSet.Ansi)]
        private static extern IntPtr getservbyname(string name, string proto);
        [DllImport("libc", SetLastError = true, CharSet = CharSet.Ansi)]
        private static extern IntPtr getservbyport(ushort port, string proto);

        public static string GetServiceByPort(ushort port, string protocol, out List<string> aliases)
        {
            var netport = unchecked((ushort)IPAddress.HostToNetworkOrder(unchecked((short)port)));
            var result = getservbyport(netport, protocol);
            if (IntPtr.Zero == result)
            {
                throw new SocketUtilException(
                    string.Format("Could not resolve service for port {0}", port));
            }
            var srvent = (servent)Marshal.PtrToStructure(result, typeof(servent));
            aliases = GetAliases(srvent);
            return srvent.s_name;

        }

        private static List<string> GetAliases(servent srvent)
        {
            var aliases = new List<string>();
            if (srvent.s_aliases != IntPtr.Zero)
            {
                IntPtr cb;

                for (var i = 0;
                    (cb = Marshal.ReadIntPtr(srvent.s_aliases, i)) != IntPtr.Zero;
                    i += Marshal.SizeOf(cb))
                {
                    aliases.Add(Marshal.PtrToStringAnsi(cb));
                }
            }
            return aliases;
        }

        public static ushort GetServiceByName(string service, string protocol, out List<string> aliases)
        {

            var result = getservbyname(service, protocol);
            if (IntPtr.Zero == result)
            {
                throw new SocketUtilException(
                    string.Format("Could not resolve port for service {0}", service));
            }

            var srvent = (servent)Marshal.PtrToStructure(result, typeof(servent));
            aliases = GetAliases(srvent);
            var hostport = IPAddress.NetworkToHostOrder(unchecked((short)srvent.s_port));
            return unchecked((ushort)hostport);

        }
    }
    class Program
    {

        static void Main(string[] args)
        {

            try
            {
                List<string> aliases;
                var port = SocketUtil.GetServiceByName("https", "tcp", out aliases);

                Console.WriteLine("https runs on port {0}", port);
                foreach (var alias in aliases)
                {
                    Console.WriteLine(alias);
                }

                Console.WriteLine("Reverse call:{0}", SocketUtil.GetServiceByPort(port, "tcp", out aliases));

            }
            catch (SocketUtilException exception)
            {
                Console.WriteLine(exception.Message);
                if (exception.InnerException != null)
                {
                    Console.WriteLine(exception.InnerException.Message);
                }
            }

        }
    }
}