/// <summary>
/// Finds the Autodiscover host from DNS SRV records.
/// </summary>
/// <remarks>
/// If the domain to lookup is "contoso.com", Autodiscover will use DnsQuery on SRV records
/// for "_autodiscover._tcp.contoso.com". If the query is successful it will return a target
/// domain (e.g. "mail.contoso.com") which will be tried as an Autodiscover endpoint.
/// </remarks>
/// <param name="domain">The domain.</param>
/// <returns>Autodiscover hostname (will be null if lookup failed).</returns>
internal string FindAutodiscoverHostFromSrv(string domain)
{
string domainToMatch = AutoDiscoverSrvPrefix + domain;
DnsSrvRecord dnsSrvRecord = this.FindBestMatchingSrvRecord(domainToMatch);
if ((dnsSrvRecord == null) || string.IsNullOrEmpty(dnsSrvRecord.NameTarget))
{
this.service.TraceMessage(
TraceFlags.AutodiscoverConfiguration,
"No appropriate SRV record was found.");
return(null);
}
else
{
this.service.TraceMessage(
TraceFlags.AutodiscoverConfiguration,
string.Format("DNS query for SRV record for domain {0} found {1}", domain, dnsSrvRecord.NameTarget));
return(dnsSrvRecord.NameTarget);
}
}
/// <summary>
/// Finds the best matching SRV record.
/// </summary>
/// <param name="domain">The domain.</param>
/// <returns>DnsSrvRecord(will be null if lookup failed).</returns>
private DnsSrvRecord FindBestMatchingSrvRecord(string domain)
{
List <DnsSrvRecord> dnsSrvRecordList;
try
{
// Make DnsQuery call to get collection of SRV records.
dnsSrvRecordList = DnsClient.DnsQuery <DnsSrvRecord>(domain, this.service.DnsServerAddress);
}
catch (DnsException ex)
{
string dnsExcMessage = string.Format(
"DnsQuery returned error error '{0}' error code 0x{1:X8}.",
ex.Message,
ex.NativeErrorCode);
this.service.TraceMessage(TraceFlags.AutodiscoverConfiguration, dnsExcMessage);
return(null);
}
catch (SecurityException ex)
{
// In restricted environments, we may not be allowed to call unmanaged code.
this.service.TraceMessage(
TraceFlags.AutodiscoverConfiguration,
string.Format("DnsQuery cannot be called. Security error: {0}.", ex.Message));
return(null);
}
this.service.TraceMessage(
TraceFlags.AutodiscoverConfiguration,
string.Format("{0} SRV records were returned.", dnsSrvRecordList.Count));
// If multiple records were returned, they will be returned sorted by priority
// (and weight) order. Need to find the index of the first record that supports SSL.
int priority = int.MinValue;
int weight = int.MinValue;
bool recordFound = false;
foreach (DnsSrvRecord dnsSrvRecord in dnsSrvRecordList)
{
if (dnsSrvRecord.Port == SslPort)
{
priority = dnsSrvRecord.Priority;
weight = dnsSrvRecord.Weight;
recordFound = true;
break;
}
}
// Records were returned but nothing matched our criteria.
if (!recordFound)
{
this.service.TraceMessage(
TraceFlags.AutodiscoverConfiguration,
"No appropriate SRV records were found.");
return(null);
}
// Collect all records with the same (highest) priority.
// (Aren't lambda expressions cool? ;-)
List <DnsSrvRecord> bestDnsSrvRecordList = dnsSrvRecordList.FindAll(
record => (record.Port == SslPort) && (record.Priority == priority) && (record.Weight == weight));
// The list must contain at least one matching record since we found one earlier.
EwsUtilities.Assert(
dnsSrvRecordList.Count > 0,
"AutodiscoverDnsClient.FindBestMatchingSrvRecord",
"At least one DNS SRV record must match the criteria.");
// If we have multiple records with the same priority and weight, randomly pick one.
int recordIndex = (bestDnsSrvRecordList.Count > 1)
? randomTieBreakerSelector.Next(bestDnsSrvRecordList.Count)
: 0;
DnsSrvRecord bestDnsSrvRecord = bestDnsSrvRecordList[recordIndex];
string traceMessage = string.Format(
"Returning SRV record {0} of {1} records. Target: {2}, Priority: {3}, Weight: {4}",
recordIndex,
dnsSrvRecordList.Count,
bestDnsSrvRecord.NameTarget,
bestDnsSrvRecord.Priority,
bestDnsSrvRecord.Weight);
this.service.TraceMessage(TraceFlags.AutodiscoverConfiguration, traceMessage);
return(bestDnsSrvRecord);
}
