Compress zone transfers properly

After change 5995, zone transfers were using a small
compression context that only had space for the first
few dozen names in each message. They now use a large
compression context with enough space for every name.

CHANGES

@@ -1,3 +1,9 @@
+6032.	[bug]		After change 5995, zone transfers were using a small
+			compression context that only had space for the first
+			few dozen names in each message. They now use a large
+			compression context with enough space for every name.
+			[GL #3706]
+
 6031.	[bug]		Move the "final reference detached" log message
 			from dns_zone unit to the DEBUG(1) log level.
 			[GL #3707]

doc/notes/notes-current.rst

@@ -52,7 +52,6 @@ Bug Fixes
   10 to 100 to accomodate for some browsers that send more that 10
   headers by default. :gl:`#3670`
 
-
 - Copy TLS identifier when setting up primaries for catalog member
   zones. :gl:`#3638`
 
@@ -64,6 +63,10 @@ Bug Fixes
   :iscman:`named-checkzone` tool from superfluously logging this message
   in non-debug mode. :gl:`#3707`
 
+- The new name compression code in BIND 9.19.7 was not compressing
+  names in zone transfers that should have been compressed, so zone
+  transfers were larger than before. :gl:`#3706`
+
 Known Issues
 ~~~~~~~~~~~~
 

lib/dns/include/dns/compress.h

@@ -58,9 +58,10 @@ ISC_LANG_BEGINDECLS
  * few dozen names in the hash set. (We can't actually use every slot because
  * space is reserved for performance reasons.) For large messages, the number
  * of names is limited by the minimum size of an RR (owner, type, class, ttl,
- * length) which is 16 bytes when the owner has a new 3-character label
- * before the compressed zone name. Divide the maximum compression offset
- * 0x3FFF by 16 and you get roughly 1024.
+ * length) which is 12 bytes - call it 16 bytes to make space for a new label.
+ * Divide the maximum compression offset 0x4000 by 16 and you get 0x400 == 1024.
+ * In practice, the root zone (for example) uses less than 200 distinct names
+ * per message.
  */
 enum {
 	DNS_COMPRESS_SMALLBITS = 6,

lib/ns/xfrout.c

@@ -1527,7 +1527,8 @@ sendstream(xfrout_ctx_t *xfr) {
 
 	if (is_tcp) {
 		isc_region_t used;
-		dns_compress_init(&cctx, xfr->mctx, DNS_COMPRESS_CASE);
+		dns_compress_init(&cctx, xfr->mctx,
+				  DNS_COMPRESS_CASE | DNS_COMPRESS_LARGE);
 		cleanup_cctx = true;
 		CHECK(dns_message_renderbegin(msg, &cctx, &xfr->txbuf));
 		CHECK(dns_message_rendersection(msg, DNS_SECTION_QUESTION, 0));