milestone 16: behavioral fixes for silent failures, locks, counters and the query log

src/cache/dns_cache.zig

@@ -103,8 +103,15 @@ pub const Class = struct {
 ///
 /// `negative_ttl_max == 0` disables negative caching outright: the response is
 /// not stored at all (milestone-6 ruling 6).
+///
+/// A TC=1 response is never cached, whatever its rcode. It is a partial message
+/// whose tail the sender dropped, and RFC 2181 §9 forbids keeping one: served
+/// from the cache the TC bit reaches a client that has no truncation to recover
+/// from, and a client that retries over TCP is answered with the same truncated
+/// bytes again.
 pub fn classify(response: []const u8, negative_ttl_max: u32) ?Class {
     const p = packet.parse(response) catch return null;
+    if (p.header.flags.tc) return null;
     const rcode = p.header.flags.rcode;
 
     if (rcode == .no_error and p.header.ancount > 0) {
@@ -643,6 +650,27 @@ test "classify refuses a zero ttl and unparsable bytes" {
     try testing.expectEqual(@as(?Class, null), classify(response[0 .. response.len - 1], 3600));
 }
 
+/// The TC bit is bit 9 of the flags word, which is the second 16-bit word of
+/// the header.
+fn setTruncated(bytes: []u8) []u8 {
+    const flags = std.mem.readInt(u16, bytes[2..4], .big);
+    std.mem.writeInt(u16, bytes[2..4], flags | 0x0200, .big);
+    return bytes;
+}
+
+test "classify refuses a truncated response whatever its rcode" {
+    var buf: [512]u8 = undefined;
+    const answer = try buildAnswer(&buf, &.{300});
+    try testing.expectEqual(@as(u32, 300), classify(answer, 3600).?.ttl_seconds);
+    try testing.expectEqual(@as(?Class, null), classify(setTruncated(answer), 3600));
+
+    // The negative path reads the same bit: a truncated NXDOMAIN is a partial
+    // message too.
+    var nx_buf: [nxdomain_bytes.len]u8 = undefined;
+    @memcpy(&nx_buf, nxdomain_bytes);
+    try testing.expectEqual(@as(?Class, null), classify(setTruncated(&nx_buf), 3600));
+}
+
 test "classify caches NXDOMAIN with the SOA minimum" {
     const class = classify(nxdomain_bytes, 3600).?;
     try testing.expectEqual(@as(u32, 600), class.ttl_seconds);