Cache Configuration

For most home users, the best reason for using a local DNS server is caching DNS entries and speeding up web browsing. Listing 8 shows an example configuration for a simple, yet powerful, DNS cache.

# /etc/unbound/unbound.conf.d/cache_options.conf
prefetch: yes # Fetch things before they expire from cache.
prefetch-key: yes # Fetch DNSSEC keys early in the validation process.
cache-min-ttl: 1200 # Seconds it takes for items in cache to die at minimum.

Domain names have an official time to live assigned by the manager of that domain. This time to live is the time that recursive DNS servers are supposed to keep the DNS entries in their caches before deleting them. Imagine that your Unbound server resolved richard-falken.com, which at the time of this writing has a TTL of 86400 seconds. It would remember the DNS entry for richard-falken.com for 24 hours.

Some domains have very short TTLs. The cache-min-ttl directive in the example defines the minimum time a cached DNS entry will be allowed to live in the cache. If Unbound comes across a domain with a TTL shorter than 1200 seconds, the official TTL will be ignored, and 1200 seconds will be used instead. Beware that DNS entries that are conserved in the cache for too long may become stalled and outdated, which could be counterproductive and lead to problems. Use this directive wisely.

prefetch-key instructs Unbound to fetch DNSSEC keys earlier than usual in the DNSSEC validation process. It saves time at the expense of CPU load.

prefetch instructs the server to try to resolve cached entries that are about to expire from the cache in order to keep the cache fresh. This option might increase bandwidth consumption by about 10 percent, but response times will be better.

Privacy

Listing 9 has some privacy options. The most important is qname-minimisation. Enabling this option makes the queries sent by Unbound to other DNS servers more compact and less prone to leak private information.

# /etc/unbound/unbound.conf.d/privacy_options.conf
hide-identity: yes # If enabled id.server and hostname.bind queries are refused.
hide-version: yes # If enabled version.server and version.bind queries are refused.
qname-minimisation: yes # Send minimum amount of information to upstream servers to enhance privacy.

hide-identity and hide-version are less relevant in LAN scenarios, since they prevent the DNS server from replying to internal special queries that attempt to obtain information, such as the hostname of the server or the software version.

Query Times

You may want to check the query times for a server provided by your ISP, another open DNS server on the Internet, or any server you configure on your own LAN.

Proper DNS benchmarking is difficult. A quick and dirty way to check the connection latency for a DNS server is using icmp echo requests with the ping utility:

$ ping -c 4 $address_of_server

The time stat for each reply indicates the time it took to get a "pong" response from the server. The bigger the time, the longer it takes for the server to reply to you when you ask it to.

The dig utility (Figure 3) performs name resolution, and it is very useful for retrieving DNS records and checking query times.

$ dig @$address_of_dns_server example.org

Figure 3: This command uses dig to test how quickly server 8.8.4.4 can give an answer.

If you are serious about benchmarking a DNS server, you will need heavier tools for the job. Google's namebench [11] is a popular option.