/// <summary>
/// Attempts a lookup from DNS.
/// </summary>
/// <param name="uri"></param>
/// <param name="startQuery"></param>
/// <param name="preferIPv6"></param>
/// <param name="ct"></param>
/// <returns></returns>
private static async Task <SIPEndPoint> SIPLookupAsync(
SIPURI uri,
QueryType startQuery,
bool preferIPv6,
CancellationToken ct)
{
SIPEndPoint result = null;
QueryType queryType = startQuery;
string host = uri.MAddrOrHostAddress;
int port = SIPConstants.GetDefaultPort(uri.Protocol);
if (ushort.TryParse(uri.HostPort, out var uriPort))
{
port = uriPort;
}
bool isDone = false;
while (!isDone && !ct.IsCancellationRequested)
{
switch (queryType)
{
case QueryType.SRV:
try
{
var srvProtocol = SIPServices.GetSRVProtocolForSIPURI(uri);
var srvResult = await _lookupClient.ResolveServiceAsync(uri.MAddrOrHostAddress, uri.Scheme.ToString(), srvProtocol.ToString()).ConfigureAwait(false);
(var srvHost, var srvPort) = GetHostAndPortFromSrvResult(srvResult);
if (srvHost != null)
{
host = srvHost;
port = srvPort != 0 ? srvPort : port;
logger.LogDebug($"SIP DNS SRV for {uri} resolved to {host} and port {port}.");
}
}
catch (Exception srvExcp)
{
logger.LogWarning(srvExcp, $"SIPDNS exception on SRV lookup. {srvExcp.Message}.");
}
queryType = preferIPv6 ? QueryType.AAAA : QueryType.A;
break;
case QueryType.AAAA:
try
{
var aaaaResult = await _lookupClient.QueryAsync(host, UseANYLookups?QueryType.ANY : QueryType.AAAA, QueryClass.IN, ct).ConfigureAwait(false);
if (aaaaResult?.Answers?.Count > 0)
{
result = GetFromLookupResult(uri.Protocol, aaaaResult.Answers.AddressRecords().OrderByDescending(x => x.RecordType).First(), port);
isDone = true;
}
else
{
queryType = QueryType.A;
}
}
catch (Exception srvExcp)
{
logger.LogWarning(srvExcp, $"SIPDNS exception on AAAA lookup. {srvExcp.Message}.");
queryType = QueryType.A;
}
break;
default:
// A record lookup.
try
{
var aResult = await _lookupClient.QueryAsync(host, QueryType.A, QueryClass.IN, ct).ConfigureAwait(false);
if (aResult != null)
{
if (aResult.Answers?.Count > 0)
{
result = GetFromLookupResult(uri.Protocol, aResult.Answers.AddressRecords().First(), port);
}
else
{
// We got a result back but it was empty indicating an unresolvable host or
// some other DNS error condition.
result = SIPEndPoint.Empty;
}
}
}
catch (Exception srvExcp)
{
logger.LogWarning(srvExcp, $"SIPDNS exception on A lookup. {srvExcp.Message}.");
result = SIPEndPoint.Empty;
}
isDone = true;
break;
}
}
return(result);
}