[master] Changes prefix-length-mode default to be "prefer"

Merges in rt45615.

RELNOTES

@@ -256,6 +256,14 @@ dhcp-users@lists.isc.org.
   FQDN option unfortunately broke its ability send back dhcp6.vendor-opts.
   [ISC-Bugs #46427]
 
+- The default value for server (-6) parameter, prefix-length-mode, has been
+  changed from "exact" to "prefer". In "prefer" mode the server will offer
+  the first available prefix with the same length as that requested by the
+  client. If none are found then it will offer the first available prefix of
+  any length.  This is more in line with with RFC 8168 and should improve
+  the out-of-the-box user experience.
+  [ISC-Bugs #45615]
+
 			Changes since 4.3.0 (bug fixes)
 
 - Tidy up several small tickets.

server/dhcpd.c

@@ -77,7 +77,7 @@ option server.ddns-rev-domainname = \"in-addr.arpa.\";";
 int ddns_update_style;
 int dont_use_fsync = 0; /* 0 = default, use fsync, 1 = don't use fsync */
 int server_id_check = 0; /* 0 = default, don't check server id, 1 = do check */
-int prefix_length_mode = PLM_EXACT;
+int prefix_length_mode = PLM_PREFER;
 
 int authoring_byte_order = 0; /* 0 = not set */
 int lease_id_format = TOKEN_OCTAL; /* octal by default */

server/dhcpd.conf.5

@@ -2922,11 +2922,11 @@ available prefix.
 .PP
 2. prefer - The server will offer the first available prefix with the same
 length as the requested length.  If none are found then it will offer the
-first available prefix of any length.
+first available prefix of any length.  This is the default behavior.
 .PP
 3. exact - The server will offer the first available prefix with the same
 length as the requested length.  If none are found, it will return a status
-indicating no prefixes available.  This is the default behavior.
+indicating no prefixes available.
 .PP
 4. minimum - The server will offer the first available prefix with the same
 length as the requested length.  If none are found, it will return the first