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

(cherry picked from commit 27679a12aa4e03e960112f387640419d29780e5a)

diff --git a/rust/src/dns/dns.rs b/rust/src/dns/dns.rs
index a4a08d8a2fb95d972176747470d6ca3303b1bd64..a678f522064d1fe0e7c5f36caaf6ed9417194976 100644 (file)
--- a/rust/src/dns/dns.rs
+++ b/rust/src/dns/dns.rs
@@ -547,9 +547,23 @@ 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);
         },