/// <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="spfStatement">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(SpfStatement spfStatement,
SpfExpression[] spfExpressions)
{
Explanation explanation = null;
string[] spfStatementStrings = null;
string s;
try
{
string[] txt = await DnsResolver.LookupText(spfStatement.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 (!(spfStatementStrings is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError,
"Multiple SPF records found for " + spfStatement.Domain + "."));
}
spfStatementStrings = s.Substring(6).Trim().Split(Space, StringSplitOptions.RemoveEmptyEntries);
}
}
catch (Exception)
{
spfStatementStrings = null;
}
if (spfStatementStrings is null)
{
if (!(spfExpressions is null))
{
foreach (SpfExpression expression in spfExpressions)
{
if (expression.IsApplicable(spfStatement.Domain))
{
if (expression.Spf.StartsWith("v=spf1"))
{
spfStatementStrings = expression.Spf.Substring(6).Trim()
.Split(Space, StringSplitOptions.RemoveEmptyEntries);
break;
}
}
}
}
if (spfStatementStrings is null)
{
return(new KeyValuePair <SpfResult, string>(SpfResult.None,
"No SPF records found " + spfStatement.Domain + "."));
}
}
// Syntax evaluation first, §4.6
int c = spfStatementStrings.Length;
LinkedList <Mechanism> mechanisms = new LinkedList <Mechanism>();
Redirect redirect = null;
int i;
try
{
for (i = 0; i < c; i++)
{
SpfQualifier qualifier;
spfStatement.Reset(spfStatementStrings[i]);
spfStatement.SkipWhitespace();
switch (spfStatement.PeekNextCharacter())
{
case '+':
spfStatement.Position++;
qualifier = SpfQualifier.Pass;
break;
case '-':
spfStatement.Position++;
qualifier = SpfQualifier.Fail;
break;
case '~':
spfStatement.Position++;
qualifier = SpfQualifier.SoftFail;
break;
case '?':
spfStatement.Position++;
qualifier = SpfQualifier.Neutral;
break;
default:
qualifier = SpfQualifier.Pass;
break;
}
switch (spfStatement.NextLabel().ToLower())
{
case "all":
mechanisms.AddLast(new All(spfStatement, qualifier));
break;
case "include":
mechanisms.AddLast(new Include(spfStatement, qualifier, spfExpressions));
break;
case "a":
mechanisms.AddLast(new A(spfStatement, qualifier));
break;
case "mx":
mechanisms.AddLast(new Mx(spfStatement, qualifier));
break;
case "ptr":
mechanisms.AddLast(new Ptr(spfStatement, qualifier));
break;
case "ip4":
mechanisms.AddLast(new Ip4(spfStatement, qualifier));
break;
case "ip6":
mechanisms.AddLast(new Ip6(spfStatement, qualifier));
break;
case "exists":
mechanisms.AddLast(new Exists(spfStatement, qualifier));
break;
case "redirect":
if (!(redirect is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError,
"Multiple redirect modifiers found in SPF record."));
}
redirect = new Redirect(spfStatement, qualifier);
break;
case "exp":
if (!(explanation is null))
{
return(new KeyValuePair <SpfResult, string>(SpfResult.PermanentError,
"Multiple exp modifiers found in SPF record."));
}
explanation = new Explanation(spfStatement, 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 = spfStatement.Domain;
spfStatement.Domain = redirect.Domain;
try
{
KeyValuePair <SpfResult, string> result =
await SpfResolver.CheckHost(spfStatement, 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
{
spfStatement.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));
}