Hostname Resolution Time
Estimate hostname resolution latency across DNS, /etc/hosts and cache lookup layers.
About Hostname Resolution Time
Hostname resolution on Linux is governed by /etc/nsswitch.conf's 'hosts:' line, which typically checks local sources (files, i.e. /etc/hosts) before falling back to dns, with a local resolver cache (systemd-resolved, nscd) short-circuiting repeated lookups. Average resolution time is dominated by the (usually tiny) cache check cost most of the time, plus the full DNS round-trip cost only on the fraction of lookups that miss the cache — meaning even a fast cache with a high miss rate can leave average latency close to full DNS query time.
Formula
Average resolution time
time = cache_check + (miss_rate × dns_query_time)
- T_{cache}
- cache check time
- m
- cache miss rate (0-1)
- T_{dns}
- DNS query time on a miss
Worked example
With the default values below, Hostname Resolution Time returns Avg Resolution Time (ms) of 3.800. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Cache Check Time (ms)minimum 0
- 0.05
- Cache Miss Rate %accepts 0–100
- 15
- DNS Query Time on Miss (ms)minimum 0
- 25
Results
- Avg Resolution Time (ms)
- 3.800
- Best Case — Cache Hit (ms)
- 0.050
- Worst Case — Cache Miss (ms)
- 25.050
- Typical nsswitch.conf Lookup Order
- files → dns (default /etc/nsswitch.conf 'hosts:' line; mdns/resolve may also appear depending on distro)
Step by step
- 1Values used
- 2Average resolution time
- 3Avg Resolution Time (ms)= 3.800
- 4Best Case — Cache Hit (ms)= 0.050
- 5Worst Case — Cache Miss (ms)= 25.050
- 6Typical nsswitch.conf Lookup Order= files → dns (default /etc/nsswitch.conf 'hosts:' line; mdns/resolve may also appear depending on distro)
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How do I see the actual nsswitch.conf lookup order on my system?
`cat /etc/nsswitch.conf | grep ^hosts` shows the configured order (e.g. 'hosts: files dns'), meaning /etc/hosts is checked before falling back to DNS. Some distros insert `mdns4_minimal` or `resolve` (for systemd-resolved) into this chain.
How can I measure real-world hostname resolution time?
`time getent hosts <hostname>` measures a single resolution round-trip end-to-end through the full nsswitch chain; running it twice in a row highlights the cache-hit speedup versus the first (likely cache-miss) lookup.
Why would cache miss rate be high even with a resolver cache running?
A high miss rate typically means the workload queries a large number of distinct, rarely-repeated hostnames (common in microservice service-discovery churn or short-TTL cloud DNS records) faster than entries can be reused before expiring — increasing cache capacity doesn't help if the working set of names genuinely exceeds what gets repeated.