SWITCH Security-Blog

SWITCH-CERT IT-Security Blog



Optimizing Negative Caching Time in DNS

A recent presentation by SIDN (.nl) at the Spring 2016 DNS-OARC workshop reminded me of the importance of Time-To-Live (TTL) values in TLD zones. Specifically, it got me thinking about lowering the negative caching time in .ch/.li from currently 1 hour to 15 minutes.

What is negative caching?
When a resolver receives a response to a query, it caches it for the duration of the TTL specified by the record. For positive responses, the record contains the TTL, but for negative responses (response code NXDOMAIN), there is no answer to the query question. For this case, the response contains the SOA record of the zone in the authority section. Negative caching is specified in RFC 2308 as the minimum of the SOA record’s TTL and the SOA minimum field. For example, the original SOA record of the .ch zone looked as follows:

dig +nocmd +noall +answer @a.nic.ch ch. soa
ch. 3600 IN SOA a.nic.ch. helpdesk.nic.ch. 2016041421 900 600 1123200 3600

The SOA TTL is 3600, and the SOA minimum time is also set to 3600. The minimum of these two values is of course 3600 too. That means the negative caching time for any .ch domain lookup is one hour.

A lower negative caching time is more user-friendly
People who are about to register a new domain name may also look up the name over DNS. However, this means that they just cached the non-existence of the name in the resolver they are using. A domain can be registered in a matter of minutes, and this can prevent them from using the domain name on their network for the duration of the negative caching time.
Continue reading


DNS Zone File Time Value Recommendations

When setting up a zone file for a domain name, the administrator can freely choose what time values he would like to set on the SOA record or regarding the Time To Live (TTL) value on the Resource Records (RR). There are already many useful documents describing recommendations for these time values but most lack the reference to signed zones using DNSSEC because at the time these documents were published, DNSSEC did either not exist or had no relevance. We tried to update the recommendations for these time values so that the none-experts can adapt their template or have a reference. Our recommendations work for both signed and unsigned zones and in the best case it helps improve the stability and resilience of the DNS.

Our recommended DNS example.com zone file in BIND format looks as follow:

$TTL 86400 ; (1 day)
$ORIGIN example.com.
@ IN SOA ns1.example.com. hostmaster.example.com. (
                2014012401 ; serial YYYYMMDDnn
                14400      ; refresh (4 hours)
                1800       ; retry   (30 minutes)
                1209600    ; expire  (2 weeks)
                3600       ; minimum (1 hour)

         172800    IN   NS    ns1
         172800    IN   NS    ns2

                   IN   A
                   IN   AAAA  2001:DB8:BEEF:113::10
www                IN   CNAME example.com.
ftp                IN   CNAME example.com.

ns1      172800    IN   A
         172800    IN   AAAA  2001:DB8:BEEF:2::22
ns2      172800    IN   A
         172800    IN   AAAA  2001:DB8:BEEF:100::22

Please read the following sections for a more detailed explanation.

Continue reading