From Ever changing code
This is a source of general information about Domain Name System aka DNS.
The DNS server stores different types of resource records used to resolve names, records like:
- A - Address record - returns a 32-bit IPv4 address, most commonly used to map hostnames to an IP address of the host
- NS - Name server record - an authoritative name server, delegates a DNS zone to use the given authoritative name servers
- CNAME - Canonical name record - the canonical name (or Fully Qualified Domain Name) for an alias; Alias of one name to another: the DNS lookup will continue by retrying the lookup with the new name. Used when multiple services have the single network address, but each service has its own entry in DNS
- MX - mail exchange record; maps a domain name to a list of mail exchange servers (MTA) for that domain
- SRV - Service Locator, multi line record of form of eg in AWS
[priority] [weight] [port] [server host name], multiline must start with
_a new line delimiter
- SOA - Start of [a zone of] authority record - Specifies authoritative information about a DNS zone, including the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
- PTR - Pointer record - pointer to a canonical name. Unlike a CNAME, DNS processing stops and just the name is returned. The most common use is for implementing reverse DNS lookups
ipconfig /displaydns command displays all of the cached DNS entries on a Windows computer system.
dig (domain information groper) and
nslookup (query Internet name servers interactively) are tools that query name servers. Unless a specific name server is specified as a commandline argument they will query the name server(s) found in
/etc/resolv.conf. They simply don't look at alternative sources of host information such as the
/etc/hosts file or other sources specified in
To force all dns queries through dnsmasq on your host, the
/etc/resolv.conf there should point to dnsmasq, i.e. it should look like:
#/etc/resolv.conf on sun nameserver 127.0.0.1
Hosts file is part of Name Service Switch. Configured at
$ cat /etc/nsswitch.conf # /etc/nsswitch.conf # # Example configuration of GNU Name Service Switch functionality. # If you have the `glibc-doc-reference' and `info' packages installed, try: # `info libc "Name Service Switch"' for information about this file. passwd: compat systemd group: compat systemd shadow: compat gshadow: files hosts: files mdns4_minimal [NOTFOUND=return] dns myhostname networks: files protocols: db files services: db files ethers: db files rpc: db files
Example entries in
10.10.11.11 echo-1.service.k8s.acme.cloud # via app-service LoadBalancer 10.10.22.22 k8s.acme.cloud echo-1.ingress.k8s.acme.cloud # via ingress-service (k8s entry point)
can be verified using
getent utility, to get entries from Name Service Switch libraries
$ getent hosts 10.10.11.11 10.10.11.11 echo-1.service.k8s.acme.cloud $ getent hosts echo-1.ingress.k8s.acme.cloud 10.10.22.22 k8s.acme.cloud echo-1.ingress.k8s.acme.cloud
sudo systemctl is-active systemd-resolved.service # -> active # Ubuntu 18.04, 20.04 resolvectl statistics # show statistics, the same output as 'systemd-resolve --statistics' sudo systemd-resolve --statistics # or --reset-statistics - resets resolver statistics sudo systemd-resolve --flush-caches # Flush Ubuntu DNS Cache sudo systemctl restart nscd # Other distros, eg arch Linux # Resolve a name without using local cache sudo systemd-resolve --flush-caches systemd-resolve --statistics | grep 'Current Cache Size' # -> Current Cache Size: 0 dig +short tvp.info @22.214.171.124 systemd-resolve --statistics | grep 'Current Cache Size' # -> Current Cache Size: 0 dig +short tvp.info systemd-resolve --statistics | grep 'Current Cache Size' # -> Current Cache Size: 1 # Display cached dns entries sudo killall -USR1 systemd-resolved # it doesn't stop the service, it tells systemd-resolved to write all the current cache entries to the system log journalctl -u systemd-resolved # list the cached entries from the log ## Oneliner sudo killall -USR1 systemd-resolved; journalctl -u systemd-resolved --since "5s ago"
Netplan is the default network management tool on Ubuntu 18.04, replacing the /etc/resolv.conf and /etc/network/interfaces configuration files that have been used to configure the network in the previous Ubuntu versions.
Back in the days, whenever you wanted to configure DNS resolvers in Linux you would simply open the /etc/resolv.conf file, edit the entries, save the file and you are good to go. This file still exists but it is a symlink controlled by the systemd-resolved service and should not be edited manually.
Note: Info As of 18/05/2020 Network Manager doesn’t respect the Netplan option nameservers: addresses [126.96.36.199,188.8.131.52] option even when you specify dhcp4-overrides: use-dns: false it still uses (and give priority to) the default DHCP DNS servers. This renders any custom DNS servers redundant. The only way around this AFAIK is to specify the Ethernet connection as static.
sudo vi /etc/netplan/enp0s3.yaml network: version: 2 renderer: NetworkManager ethernets: enp0s3: dhcp4: false addresses: [192.168.1.114/24] gateway4: 192.168.1.1 nameservers: addresses: [184.108.40.206, 220.127.116.11] # Using this method you’ll lose the Network Manager GUI and network icon and let Netplan to manage all devices sudo netplan apply systemd-resolve --status | grep 'DNS Servers' -A2
- List of DNS record types Wikipedia