dns: don't parse a full request during probe if not enough data

If there is more data than a header, but not enough for a complete DNS
message, the hostname parser could return an error causing the probe to
fail on valid DNS messages.

So only parse the complete message if we have enough input data. This is
reliable for TCP as DNS messages are prefixed, but for UDP its just
going to be the size of the input buffer presented to the parser, so
incomplete could still happen.

Ticket #5034

rust/src/dns/dns.rs

@@ -598,9 +598,23 @@ fn probe_header_validity(header: DNSHeader, rlen: usize) -> (bool, bool, bool) {
 }
 
 /// Probe input to see if it looks like DNS.
+///
+/// Returns a tuple of booleans: (is_dns, is_request, incomplete)
 fn probe(input: &[u8], dlen: usize) -> (bool, bool, bool) {
-    let i2 = if input.len() <= dlen { input } else { &input[..dlen] };
-    match parser::dns_parse_request(i2) {
+    // Trim input to dlen if larger.
+    let input = if input.len() <= dlen { input } else { &input[..dlen] };
+
+    // If input is less than dlen then we know we don't have enough data to
+    // parse a complete message, so perform header validation only.
+    if input.len() < dlen {
+        if let Ok((_, header)) = parser::dns_parse_header(input) {
+            return probe_header_validity(header, dlen);
+        } else {
+            return (false, false, false);
+        }
+    }
+
+    match parser::dns_parse_request(input) {
         Ok((_, request)) => {
             return probe_header_validity(request.header, dlen);
         },