Add a comment explaining a mirror zone glitch

Explain why in a certain edge case mirror zone data may not be used for
resolution purposes despite being available.

diff --git a/lib/dns/zt.c b/lib/dns/zt.c
index 62b0e4a053ac99e8477f8bf07d8fa9b95f529fda..7576396830bce48819078106a2eca22fd196aa48 100644 (file)
--- a/lib/dns/zt.c
+++ b/lib/dns/zt.c
@@ -179,6 +179,16 @@ dns_zt_find(dns_zt_t *zt, const dns_name_t *name, unsigned int options,
                 * a mirror zone which is expired or not yet loaded, treat it
                 * as non-existent.  This will trigger a fallback to recursion
                 * instead of returning a SERVFAIL.
+                *
+                * Note that currently only the deepest match in the zone table
+                * is checked.  Consider a server configured with two mirror
+                * zones: "bar" and its child, "foo.bar".  If zone data is
+                * available for "bar" but not for "foo.bar", a query with
+                * QNAME equal to or below "foo.bar" will cause ISC_R_NOTFOUND
+                * to be returned, not DNS_R_PARTIALMATCH, despite zone data
+                * being available for "bar".  This is considered to be an edge
+                * case, handling which more appropriately is possible, but
+                * arguably not worth the added complexity.
                 */
                if ((options & DNS_ZTFIND_MIRROR) != 0 &&
                    dns_zone_gettype(dummy) == dns_zone_mirror &&