Best way to determine if a domain name would be a valid in a "hosts" file?

Question

The Windows Hosts file allows you to associate an IP to a host name that has far greater freedom than a normal Internet domain name. I'd like to create a function that determines if a given name would be a valid "host" file domain name.

Based on this answer and experimentation of what works and doesn't, I came up with this function:

private static bool IsValidDomainName(string domain)
{
    if (String.IsNullOrEmpty(domain) || domain.Length > 255)
    {
     return false;
    }

    Uri uri;

    if (!Uri.TryCreate("http://" + domain, UriKind.Absolute, out uri))
    {
     return false;
    }

    if (!String.Equals(uri.Host, domain, StringComparison.OrdinalIgnoreCase) || !uri.IsWellFormedOriginalString())
    {
     return false;
    }

    foreach (string part in uri.Host.Split('.'))
    {
     if (part.Length > 63)
     {
      return false;
     }
    }

    return true;
}

It also has the benefit that it should work with Unicode names (where a basic regex would fail).

Is there a better/more elegant way to do this?

UPDATE: As suggested by Bill, the Uri.CheckHostName method almost does what I want, but it doesn't allow for host names like "-test" that Windows allows in a "hosts" file. I would special case the "-" part, but I'm concerned there are more special cases.

Answer 1

How about:

 private static bool IsValidDomainName(string name)
 {
  return Uri.CheckHostName(name) != UriHostNameType.Unknown;
 }

Why do the work yourself?

Answer 2

Some problems I found with this method:

• Allows some punctuation (at least the underscore) that's disallowed in DNS.
• Allows domain names that begin and end with a hyphen.
• Allows invalid TLDs.

There may be more.

Here's one reference - more are easy to find.

Are you adding non-existent names to a host file? If you're adding hostnames that actually exist, a DNS lookup is probably the easiest thing.