/// <summary>
/// Resolves a DNS name.
/// </summary>
/// <param name="Name">Domain Name to resolve</param>
/// <param name="TYPE">Resource Record Type of interest.</param>
/// <param name="ExceptionType">If no answer of type <paramref name="TYPE"/> is found, records of this type can also be accepted.</param>
/// <param name="CLASS">Resource Record Class of interest.</param>
/// <returns>Answer to the query</returns>
/// <exception cref="IOException">If the domain name could not be resolved for the TYPE and CLASS provided.</exception>
public static async Task <ResourceRecord[]> Resolve(string Name, QTYPE TYPE,
TYPE?ExceptionType, QCLASS CLASS)
{
LinkedList <KeyValuePair <string, IPEndPoint> > Backup = null;
TYPE? ExpectedType;
IPEndPoint Destination = null;
string LastName = null;
int Timeout = 2000; // Local timeout
if (Enum.TryParse <TYPE>(TYPE.ToString(), out TYPE T))
{
ExpectedType = T;
}
else
{
ExpectedType = null;
}
lock (synchObject)
{
if (nestingDepth == 0 && networkChanged)
{
networkChanged = false;
client?.Dispose();
client = null;
}
if (client is null)
{
client = new DnsUdpClient();
}
nestingDepth++;
}
try
{
while (true)
{
DnsResponse Response = null;
DnsMessage Message;
if (LastName is null || LastName != Name)
{
LastName = Name;
try
{
Response = await Database.FindFirstDeleteRest <DnsResponse>(new FilterAnd(
new FilterFieldEqualTo("Name", Name),
new FilterFieldEqualTo("Type", TYPE),
new FilterFieldEqualTo("Class", CLASS)));
if (!(Response is null) && Response.Expires <= DateTime.Now)
{
await Database.Delete(Response);
Response = null;
}
}
catch (Exception)
{
// Some inconsistency in database. Clear collection to get fresh set of DNS entries.
await Database.Clear("DnsCache");
Response = null;
}
}
if (Response is null)
{
try
{
Message = await client.SendRequestAsync(OpCode.Query, true, new Question[]
{
new Question(Name, TYPE, CLASS)
}, Destination, Timeout);
switch (Message.RCode)
{
case RCode.NXDomain:
throw new ArgumentException("Domain name does not exist.", nameof(Name));
}
}
catch (TimeoutException)
{
Message = null;
}
if (Message is null || Message.RCode != RCode.NoError)
{
Destination = await NextDestination(Backup);
if (Destination is null)
{
throw new IOException("Unable to resolve DNS query.");
}
continue; // Check an alternative
}
uint Ttl = 60 * 60 * 24 * 30; // Maximum TTL = 30 days
Response = new DnsResponse()
{
Name = Name,
Type = TYPE,
Class = CLASS,
Answer = CheckTtl(ref Ttl, Message.Answer),
Authority = CheckTtl(ref Ttl, Message.Authority),
Additional = CheckTtl(ref Ttl, Message.Additional)
};
Response.Expires = DateTime.Now.AddSeconds(Ttl);
await Database.Insert(Response);
}
string CName = null;
if (!(Response.Answer is null))
{
if (!ExpectedType.HasValue)
{
return(Response.Answer);
}
foreach (ResourceRecord RR in Response.Answer)
{
if (RR.Type == ExpectedType.Value)
{
return(Response.Answer);
}
if (CName is null && RR.Type == Enumerations.TYPE.CNAME && RR is CNAME CNAME)
{
CName = CNAME.Name2;
}
}
if (ExceptionType.HasValue)
{
foreach (ResourceRecord RR in Response.Answer)
{
if (RR.Type == ExceptionType.Value)
{
return(Response.Answer);
}
}
}
if (!(CName is null))
{
Name = CName;
Backup = null;
continue;
}
}
if (!(Response.Authority is null))
{
foreach (ResourceRecord RR in Response.Authority)
{
if (RR is NS NS)
{
string Authority = NS.Name2;
IPAddress AuthorityAddress = null;
if (!(Response.Additional is null))
{
foreach (ResourceRecord RR2 in Response.Additional)
{
if (RR2 is A A)
{
AuthorityAddress = A.Address;
break;
}
else if (RR2 is AAAA AAAA)
{
AuthorityAddress = AAAA.Address;
break;
}
}
}
if (Backup is null)
{
Backup = new LinkedList <KeyValuePair <string, IPEndPoint> >();
}
if (AuthorityAddress is null)
{
Backup.AddLast(new KeyValuePair <string, IPEndPoint>(Authority, null));
}
else
{
Backup.AddLast(new KeyValuePair <string, IPEndPoint>(null, new IPEndPoint(AuthorityAddress, DefaultDnsPort)));
}
}
}
}
Destination = await NextDestination(Backup);
if (Destination is null)
{
throw new IOException("Unable to resolve DNS query.");
}
Timeout = 5000;
}
}
finally
{
lock (synchObject)
{
nestingDepth--;
}
}
}
public override string ToString()
{
return(String.Format("{0} {1} {2}", QNAME, QCLASS.ToString(), QTYPE.ToString()));
}