Fix DNS secondary nameserver refesh failure retry period

Fixes #1979

CHANGELOG.md

@@ -22,6 +22,7 @@ Other:
 
 * Set systemd journald log retention to 10 days (from no limit) to reduce disk usage.
 * Fixed log processing for submission lines that have a sasl_sender or other extra information.
+* Fix DNS secondary nameserver refesh failure retry period.
 
 Version 55 (October 18, 2021)
 -----------------------------

management/dns_update.py

@@ -484,7 +484,7 @@ def write_nsd_zone(domain, zonefile, records, env, force):
 	# @ the PRIMARY_HOSTNAME. Hopefully that's legit.
 	#
 	# For the refresh through TTL fields, a good reference is:
-	# http://www.peerwisdom.org/2013/05/15/dns-understanding-the-soa-record/
+	# https://www.ripe.net/publications/docs/ripe-203
 	#
 	# A hash of the available DNSSEC keys are added in a comment so that when
 	# the keys change we force a re-generation of the zone which triggers
@@ -497,7 +497,7 @@ def write_nsd_zone(domain, zonefile, records, env, force):
 @ IN SOA ns1.{primary_domain}. hostmaster.{primary_domain}. (
            __SERIAL__     ; serial number
            7200     ; Refresh (secondary nameserver update interval)
-           86400    ; Retry (when refresh fails, how often to try again)
+           3600     ; Retry (when refresh fails, how often to try again, should be lower than the refresh)
            1209600  ; Expire (when refresh fails, how long secondary nameserver will keep records around anyway)
            86400    ; Negative TTL (how long negative responses are cached)
            )