private static SRVRecord PickSRV(SRVRecord[] srv)
{
// TODO: keep track of connection failures, and try the next priority down.
if ((srv == null) || (srv.Length == 0))
throw new ArgumentException();
if (srv.Length == 1)
return srv[0];
// randomize order. One might wish that the OS would have done this for us.
// cf. Bob Schriter's Grandfather.
Random rnd = new Random();
byte[] keys = new byte[srv.Length];
rnd.NextBytes(keys);
Array.Sort(keys, srv); // Permute me, Knuth! (I wish I had a good anagram for that)
int minpri = int.MaxValue;
foreach (SRVRecord rec in srv)
{
if (rec.Priority < minpri)
minpri = rec.Priority;
}
int weight = 0;
foreach (SRVRecord rec in srv)
{
if (rec.Priority == minpri)
weight += rec.Weight;
}
int pos = rnd.Next(weight);
weight = 0;
foreach (SRVRecord rec in srv)
{
if (rec.Priority == minpri)
{
weight += rec.Weight;
if ((pos < weight) || (weight == 0))
return rec;
}
}
throw new DnsException("No matching SRV");
}
public static SRVRecord[] ResolveSRV(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);
}
var record = new SRVRecord();
record.NameNext = nameStr;
record.Priority = priority;
record.Weight = weight;
record.Port = port;
results.Add(record);
pos += size;
}
else
{
pos += dlen;
}
}
}
}
}
finally
{
handle.Free();
}
return results.ToArray();
}
