/// <summary>
/// Fetches SPF records, parses them, and
/// evaluates them to determine whether a particular host is or is not
/// permitted to send mail with a given identity.
/// </summary>
/// <param name="Term">Information about current query.</param>
/// <param name="SpfExpressions">SPF Expressions that can be used, in case a domain lacks SPF records in the DNS.</param>
/// <returns>Result of SPF evaluation, together with an optional explanation string,
/// if one exists, and if the result indicates a failure.</returns>
internal static async Task <KeyValuePair <SpfResult, string> > CheckHost(Term Term, SpfExpression[] SpfExpressions)
{
Exp Explanation = null;
string[] TermStrings = null;
string s;
try
{
string[] TXT = await DnsResolver.LookupText(Term.domain);
foreach (string Row in TXT)
{
s = Row.Trim();
if (s.Length > 1 && s[0] == '"' && s[s.Length - 1] == '"')
{
s = s.Substring(1, s.Length - 2);
}
if (!s.StartsWith("v=spf1"))
{
continue;
}
if (!(TermStrings is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, "Multiple SPF records found for " + Term.domain + "."));
}
TermStrings = s.Substring(6).Trim().Split(space, StringSplitOptions.RemoveEmptyEntries);
}
}
catch (Exception)
{
TermStrings = null;
}
if (TermStrings is null)
{
if (!(SpfExpressions is null))
{
foreach (SpfExpression Expression in SpfExpressions)
{
if (Expression.IsApplicable(Term.domain))
{
if (Expression.Spf.StartsWith("v=spf1"))
{
TermStrings = Expression.Spf.Substring(6).Trim().Split(space, StringSplitOptions.RemoveEmptyEntries);
break;
}
}
}
}
if (TermStrings is null)
{
return(new KeyValuePair <SpfResult, string>(SpfResult.None, "No SPF records found " + Term.domain + "."));
}
}
// Syntax evaluation first, §4.6
int c = TermStrings.Length;
LinkedList <Mechanism> Mechanisms = new LinkedList <Mechanism>();
Redirect Redirect = null;
int i;
try
{
for (i = 0; i < c; i++)
{
SpfQualifier Qualifier;
Term.Reset(TermStrings[i]);
Term.SkipWhiteSpace();
switch (Term.PeekNextChar())
{
case '+':
Term.pos++;
Qualifier = SpfQualifier.Pass;
break;
case '-':
Term.pos++;
Qualifier = SpfQualifier.Fail;
break;
case '~':
Term.pos++;
Qualifier = SpfQualifier.SoftFail;
break;
case '?':
Term.pos++;
Qualifier = SpfQualifier.Neutral;
break;
default:
Qualifier = SpfQualifier.Pass;
break;
}
switch (Term.NextLabel().ToLower())
{
case "all":
Mechanisms.AddLast(new All(Term, Qualifier));
break;
case "include":
Mechanisms.AddLast(new Include(Term, Qualifier, SpfExpressions));
break;
case "a":
Mechanisms.AddLast(new A(Term, Qualifier));
break;
case "mx":
Mechanisms.AddLast(new Mx(Term, Qualifier));
break;
case "ptr":
Mechanisms.AddLast(new Ptr(Term, Qualifier));
break;
case "ip4":
Mechanisms.AddLast(new Ip4(Term, Qualifier));
break;
case "ip6":
Mechanisms.AddLast(new Ip6(Term, Qualifier));
break;
case "exists":
Mechanisms.AddLast(new Exists(Term, Qualifier));
break;
case "redirect":
if (!(Redirect is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, "Multiple redirect modifiers foundin SPF record."));
}
Redirect = new Redirect(Term, Qualifier);
break;
case "exp":
if (!(Explanation is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, "Multiple exp modifiers foundin SPF record."));
}
Explanation = new Exp(Term, Qualifier);
break;
default:
throw new Exception("Syntax error.");
}
}
foreach (Mechanism Mechanism in Mechanisms)
{
await Mechanism.Expand();
SpfResult Result = await Mechanism.Matches();
switch (Result)
{
case SpfResult.Pass:
switch (Mechanism.Qualifier)
{
case SpfQualifier.Pass: return(new KeyValuePair <SpfResult, string>(SpfResult.Pass, null));
case SpfQualifier.Fail: return(new KeyValuePair <SpfResult, string>(SpfResult.Fail, Explanation == null ? null : await Explanation.Evaluate()));
case SpfQualifier.Neutral: return(new KeyValuePair <SpfResult, string>(SpfResult.Neutral, null));
case SpfQualifier.SoftFail: return(new KeyValuePair <SpfResult, string>(SpfResult.SoftFail, Explanation == null ? null : await Explanation.Evaluate()));
}
break;
case SpfResult.TemporaryError:
return(new KeyValuePair <SpfResult, string>(SpfResult.TemporaryError, Explanation == null ? null : await Explanation.Evaluate()));
case SpfResult.None:
case SpfResult.PermanentError:
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, Explanation == null ? null : await Explanation.Evaluate()));
}
}
if (!(Redirect is null))
{
await Redirect.Expand();
string Bak = Term.domain;
Term.domain = Redirect.Domain;
try
{
KeyValuePair <SpfResult, string> Result = await SpfResolver.CheckHost(Term, SpfExpressions);
if (Result.Key == SpfResult.None)
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, Explanation == null ? null : await Explanation.Evaluate()));
}
else if (Result.Key != SpfResult.Pass && Result.Key != SpfResult.Neutral &&
string.IsNullOrEmpty(Result.Value))
{
return(new KeyValuePair <SpfResult, string>(Result.Key, Explanation == null ? null : await Explanation.Evaluate()));
}
else
{
return(Result);
}
}
finally
{
Term.domain = Bak;
}
}
}
catch (Exception ex)
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError, "Unable to evaluate SPF record: " + FirstRow(ex.Message)));
}
return(new KeyValuePair <SpfResult, string>(SpfResult.Neutral, null));
}