Simple C# wrapper for libresolv to perform DNS SRV lookups.

srv.cs

/*
 * Simple C# wrapper for libresolv to perform DNS SRV lookups.
 * Tested with Mono 2.0 on Linux and OSX.
 *
 * Copyright (c) 2008, Eric Butler <eric@extremeboredom.net>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the name of the <organization> nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 'AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

using System;
using System.Text;
using System.Collections.Generic;
using System.Runtime.InteropServices;

public class SrvTest
{
	public static void Main (string[] args)
	{
		var records = SrvResolver.Resolve(args[0]);
		foreach (SrvRecord record in records) {
			Console.WriteLine(record.ToString());
		}
	}
}

public class SrvResolver
{
	[DllImport("libresolv", EntryPoint="__res_query")]
	private static extern int linux_res_query (string dname, int cls, int type, byte[] header, int headerlen);

	[DllImport("libresolv", EntryPoint="__dn_expand")]
	private unsafe static extern int linux_dn_expand (byte* msg, byte* endorig, byte* comp_dn, byte[] exp_dn, int length);

	[DllImport("libresolv", EntryPoint="res_query")]
	private static extern int bsd_res_query (string dname, int cls, int type, byte[] header, int headerlen);

	[DllImport("libresolv", EntryPoint="dn_expand")]
	private unsafe static extern int bsd_dn_expand (byte* msg, byte* endorig, byte* comp_dn, byte[] exp_dn, int length);

	private unsafe static int res_query (string dname, int cls, int type, byte[] header, int headerlen)
	{
		try {
			return linux_res_query(dname, cls, type, header, headerlen);
		} catch (EntryPointNotFoundException) {
			return bsd_res_query(dname, cls, type, header, headerlen);
		}
	}

	private unsafe static int dn_expand (byte* msg, byte* endorig, byte* comp_dn, byte[] exp_dn, int length)
	{
		try {
			return linux_dn_expand(msg, endorig, comp_dn, exp_dn, length);
		} catch (EntryPointNotFoundException) {
			return bsd_dn_expand(msg, endorig, comp_dn, exp_dn, length);
		}
	}

 	[DllImport("libc")]
	private static extern UInt16 ntohs(UInt16 netshort);

	private static readonly int C_IN  = 1;
	private static readonly int T_SRV = 33;

	[StructLayout(LayoutKind.Explicit)]
	private struct HEADER
	{
		/* The first 4 bytes are a bunch of random crap that
		 * nobody cares about */

		[FieldOffset(4)]
		public UInt16 qdcount; /* number of question entries */

		[FieldOffset(6)]
		public UInt16 ancount; /* number of header entries */

		[FieldOffset(8)]
		public UInt16 nscount; /* number of authority entries */

		[FieldOffset(10)]
		public UInt16 arcount; /* number of resource entries */
	}

	private unsafe static ushort GETSHORT (ref byte* buf)
	{
		byte *t_cp = (byte*)(buf);
		ushort s = (ushort) (((ushort)t_cp[0] << 8) | ((ushort)t_cp[1]));
		buf += sizeof(ushort);
		return s;
	}

	public static SrvRecord[] Resolve (string query)
	{
		byte[] buffer = new byte[1024];
		byte[] name = new byte[256];
		ushort type, dlen, priority, weight, port;
		int size;
		GCHandle handle;
		List<SrvRecord> results = new List<SrvRecord>();

		size = res_query(query, C_IN, T_SRV, buffer, buffer.Length);

		handle = GCHandle.Alloc(buffer, GCHandleType.Pinned);

		try {
			HEADER header = (HEADER)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(HEADER));

			int qdcount = ntohs(header.qdcount);
			int ancount = ntohs(header.ancount);

			int headerSize = Marshal.SizeOf(header);

			unsafe {
				fixed (byte* pBuffer = buffer) {

					byte *pos = pBuffer + headerSize;
					byte *end = pBuffer + size;

					// We don't care about the question section.
					while (qdcount-- > 0 && pos < end) {
						size = dn_expand(pBuffer, end, pos, name, 256);
						if (size < 0) return null;
						pos += size + 4;
					}

					// The answers, however, we do care about!
					while (ancount-- > 0 && pos < end) {
						size = dn_expand(pBuffer, end, pos, name, 256);
						if (size < 0) return null;

						pos += size;

						type = GETSHORT(ref pos);

						// Skip TTL
						pos += 6;

						dlen = GETSHORT(ref pos);

						if (type == T_SRV) {
							priority = GETSHORT(ref pos);
							weight = GETSHORT(ref pos);
							port = GETSHORT(ref pos);

							size = dn_expand(pBuffer, end, pos, name, 256);
							if (size < 0) return null;

							string nameStr = null;
							fixed (byte* pName = name) {
								nameStr = new String((sbyte*)pName);
							}

							results.Add(new SrvRecord(nameStr, port, priority, weight));

							pos += size;
						} else {
							pos += dlen;
						}
					}
				}
			}

		} finally {
			handle.Free();
		}

		return results.ToArray();
	}
}

public class SrvRecord
{
	string m_Name;
	int    m_Priority;
	int    m_Port;
	int    m_Weight;

	public SrvRecord (string name, int port, int priority, int weight)
	{
		m_Name   = name;
		m_Port   = port;
		m_Priority   = priority;
		m_Weight = weight;
	}

	public string Name {
		get {
			return m_Name;
		}
	}

	public int Port {
		get {
			return m_Port;
		}
	}

	public int Priority {
		get {
			return m_Priority;
		}
	}
	
	public int Weight {
		get {
			return m_Weight;
		}
	}

	public override string ToString ()
	{
		return String.Format("{0} {1} {2} {3}", m_Priority, m_Weight, m_Port, m_Name);
	}
}