C＃-IMAP主机名上的TcpClient超时不存在

Question

So I have a very basic script:

#region Get Hostname IP
IPAddress[] dnsRecords = Dns.GetHostAddresses(hostname);
string ipHostname = dnsRecords[0].ToString();
if (ipHostname.Length == 0) {
    responseCode = imapLoginResponse.Failed;
    return false;
}
#endregion
#region Try make a request to the Host, Port
try {
    _Connection = new TcpClient(ipHostname, port);
} catch (SocketException ex) {
    System.Diagnostics.Debug.Write("-" + hostname + "-" + ipHostname + "- " + ex.Message);
    responseCode = ex.Message == "No such host is known" || ex.Message.StartsWith("No connection could be made because the target machine actively refused it") ? imapLoginResponse.BadHostname : imapLoginResponse.Failed;
    return false;
}

The code above simply tries to connect to an IMAP Server.

• Get imap.{domain.ext} DNS Record IP
• Try connect to that IP with port 993
• Voila

What's happening for either dead/bad/abandoned/parked websites is:

• Get imap.{domain.ext} DNS Record IP, somehow will get one even though it doesn't exist
• Try to connect to that IP with port 993
• Server doesn't respond resulting in a timeout

Now what do I do? I cant turn this down to an "BadHostname" as it could just be a connection issue on the user's end or even not connected to internet.

If I were to just retry here, it will end up infinitely looping. While yes, I could do a 5 retries == invalid thing, but that's not accurate and could still be a temporary internet issue.

My question is, why is it getting a DNS Record IP for a record that doesnt exist? And what am I meant to do when something like this occurs?

If you need to test, try it with this: imap.celerityinc.com (no celebrityinc.com) and you can see for yourself.

Answer 1

Name resolution and services provided on an IP address are independent. You get a DNS record because it does exist at some point in the DNS hierarchy with a certain lifetime.

What you can do when something like this occurs? Flush your local DNS cache. Try different DNS servers. Go back to the SOA with your own DNS query solution.