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Managing DNS clients

View or clear your local DNS cache

As applications make DNS queries to obtain the IP addresses of remote resources, your operating system will start to maintain a cache of previous responses. These cached responses are used on subsequent lookups in order to reduce network overhead and speed up the process of loading the applicable resources.

Clearing your DNS cache is an operating system dependent operation:

Windows 10

Open PowerShell as an administrator and run clear-dnsclientcache.

macOS 10.11+:

Run sudo killall -HUP mDNSResponder from Terminal1.

Linux

As an open source operating system with that comes in a variety of flavors, Linux users may find that some research is necessary to determine how DNS is managed in their distribution of choice and whether the system maintains a cache that can be cleared.

Linux DNS caches may be incorporated into a resolver (systemd-resolved), provided by a standalone service (nscd), or built into a name server (bind9) running on localhost.

A few of the most common options are listed below, along with the relevant command to restart the service and/or clear the cache directly.

Service Description Command
systemd-resolved DNS Resolver (distributed with systemd) sudo systemd-resolve --flush-caches
nscd DNS Cache sudo systemctl restart nscd
dnsmasq Name Server sudo systemctl restart dnsmasq
bind9 Name Server sudo systemctl restart bind9
dns-clean DNS Resolver (previously used by Ubuntu) /etc/init.d/dns-clean restart

Perform DNS lookups manually

At times it can be helpful to perform DNS queries manually. Tools like dig allow us to query a nameserver and ask it to provide records about a particular host or domain. These tools provide us with an enormous amount of flexibility in interacting with DNS.

Installing Dig

The dig command balances flexibility with ease of use, making it a popular tool for troubleshooting issues with DNS or performing security-related research on a domain. The utility is installed by default on macOS and some Linux distributions.

Instructions are provided below if dig isn't available on your system.

Linux

For Debian/Ubuntu based Linux including Arch, Mint, and Raspberry Pi OS, dig is part of the dnsutils package and is installed with sudo apt install dnsutils.

For Fedora/RedHat based Linux including CentOS, dig is part of the bind-utils package and can be installed with sudo dnf install bind-utils -- use sudo yum install bind-utils if dnf is not available.

Windows

Windows users may install dig by downloading ISC BIND 9 and installing with the Tools Only option2. Similar functionality is also provided by the PowerShell Resolve-DnsName command.

Common Usage

dig <domain name>: Request records for the given domain name. By default dig will send a query for A records (contain IPv4 addresses) to the default name server for the system.

dig <type> <domain name>: Override the type in order to obtain cname (DNS aliases for the given name), mx (mail servers), or ns name servers for the given host or domain rather than the default A record.

dig @<name server IP> <domain name>: Override the system's name server (often helpful to determine if other resolvers are returning different records).

Example

pi@titan.local:~ $ dig @1.1.1.1 cname uw.edu

; <<>> DiG 9.10.6 <<>> @1.1.1.1 cname uw.edu
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50713
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1452
;; QUESTION SECTION:
;uw.edu.                IN  CNAME

;; AUTHORITY SECTION:
uw.edu.         600 IN  SOA hanna.cac.washington.edu. domainmaster.cac.washington.edu. 2020012403 10800 1800 3600000 600

;; Query time: 40 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Sun Jan 26 23:32:59 PST 2020
;; MSG SIZE  rcvd: 105

Resolving DNS with PowerShell

PowerShell for Windows provides a powerful, scriptable DNS client that can be called via Resolve-DnsName. This tool replicates many of the features provided by dig, such as overriding the query type with the -type <type> option or the target server with -server <name server IP>.

Run man Resolve-DnsName from PowerShell or view the online documentation to learn more about this command.

  1. Note that we are using sudo in order to perform this operation with root privileges. ↩

  2. Detailed instructions provided at https://help.dyn.com/how-to-use-binds-dig-tool/. ↩

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