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DNS
In DNS, a fully qualified domain name (FQDN) includes the hostname and its full domain, such as “example.com.” When a hostname is not fully qualified, the resolver appends search domains to the hostname to attempt to resolve it. The ~. wildcard indicates that the resolver should search all domains for such hostnames
Setup
Add the IP address of the domain server to /run/systemd/resolve/resolv.conf: nameserver 'IP address'
Example:
nameserver 9.9.9.9
Pre Debian 12 Bookworm
resolvectl is a multi-call binary. When invoked as “resolvconf” (generally achieved by means of a symbolic link of this name to the resolvectl binary)
systemd: resolvectl is not active by default
Example command: resolvectl status
Domains=~.
“Domains=~.” represents a wildcard domain search. When used in the Domains= option within the [Resolve] section of systemd-resolved configuration, ~. signifies that the resolver should search all domains for hostnames that are not fully qualified.
In DNS, a fully qualified domain name (FQDN) includes the hostname and its full domain, such as “example.com.” When a hostname is not fully qualified, the resolver appends search domains to the hostname to attempt to resolve it. The ~. wildcard indicates that the resolver should search all domains for such hostnames.
Clear the DNS cache
Only applicable if a DNS cache is running which is not the case by the default installation of Debian
What is running, on Debian 10, is /etc/init.d/networking. Run /etc/init.d/networking status to see if it is running. Options: force-reload, reload, restart, start, stop
To clear the web browser or router cache, if any, restart the web browser or router.
Lookup programs
Name | Remark |
---|---|
bind9-dnsutils | Contains: dig, nslookup, nsupdate |
dig | Part of bind9-dnsutils |
djbdns-utils | Contains: dnsfilter, dnsip, dnsipq, dnsmx, dnsname, dnsq, dnsqr, dnstrace, dnstxt, random-ip |
dnsutils | Obsolete. Install bind9-dnsutils |
hosts | |
nslookup | Part of bind9-dnsutils. Set timeout=t does not work. Use -timeout=t where t is a time in seconds greater then 0. This is not accurate. -timeout=1 takes about 5 seconds if the DNS server is not available. Whereas resolving takes less than a second when the DNS server is available. Some versions of nslookup will always bypass any local caches and will instead read /etc/resolv.conf and automatically contact the nameserver(s) found in there directly (unless the command line includes a specific nameserver to query) |
ping | will always use the currently configured resolver, which may or may not include a caching feature |
Source for some text
Public servers
A full list showing the Netherlands
IP | Owner | Remark |
---|---|---|
1.1.1.1 | Cloudflare | |
2.2.2.2 | France Telecom | No DNS |
3.3.3.3 | Amazon Technologies Inc. | No DNS |
4.4.4.4 | Level 3 Parent, LLC | No DNS |
5.5.5.5 | Telefonica Germany GmbH & Co.OHG | No DNS |
6.6.6.6 | Headquarters, USAISC (HEADQU-3) | No DNS |
7.7.7.7 | DoD Network Information Center (DNIC) | No DNS |
8.8.8.8 | ||
9.9.9.9 | Quad nine | |
193.110.81.92) | zero.dns0.eu | |
NextDNS |
dnsmasq
Clear the dnsmasq dns cache
service dnsmasq restart
TXT field
To check the TXT fields of a DNS entry you can do
dig -t txt example.com | grep TXT nslookup -q=TXT example.com
DNS server
How to configure bind as an authoritative only dns server on ubuntu 14-04
An other possibility: knot
Dynamic DNS setting
- Driver= (for the driver name of the network interface)
- Gateway=
- MACAddress=
- Name= (for the network interface)
- Path= (for the path of the network interface's .device unit in the systemd unit hierarchy)
Create files in /etc/systemd/resolved.conf.d, one per location.
Headers of sections can be
- [Match]
- [Network]
- [Resolve] allows you to configure global DNS resolution options, such as the DNS servers to be used by default, the fallback DNS servers, and other related settings
Example to set the DNS server depending on the gateway IP address:
[Match] Gateway=192.168.1.1 [Network] DNS=9.9.9.9
Restart the DNS service: systemctl restart systemd-resolved.service
[Resolve] DNS=9.9.9.9 Domains=~.
Slow DNS
Work in progress
/etc/systemd/resolved.conf /etc/systemd/network/ tcpdump -i wlan0 udp -w udp_traffic.pcap (log UDP traffic on wlan0 to the udp_traffic.pcap file)
/etc/systemd/resolved.conf.d/dns_servers.conf
[Resolve] DNS=192.168.1.1 Domains=~. DNSSEC=no
/lib/systemd/system/systemd-resolved.service.d/resolvconf.conf
resolvectl service restart Resolve call failed: All attempts to contact name servers or networks failed
- Ensure, with dig or nslookup, that the DNS servers configured in the network configuration like files /etc/systemd/resolved.conf are responsive and reliable
- Check the status of the DNS cache: systemd-resolve --statistics
- Flush the DNS cache: systemd-resolve --flush-caches
- Check /etc/network/interfaces
- Disable DNS Security Extensions (DNSSEC) to check if DNSSEC cause delays in DNS resolution. Add DNSSEC=no under the [Resolve] section in the /etc/network/interfaces file
- Consider adding fallback DNS servers. Add to the resolved configuration file FallbackDNS= with the IP addresses of additional DNS servers
- systemctl restart systemd-resolved.service to activate the changes
- A large number of domains in your search domains list can increase DNS lookup times
Show the gateway the computer is connected to: ip route | grep default
Show all connection details: ip route
default via 10.10.10.1 dev wlan0 10.10.10.0/22 dev wlan0 proto kernel scope link src 10.10.10.110
- Check if network-manager is installed
- Check if /lib/systemd/systemd-resolved is installed
- Add /lib/systemd to the path of the root user
To get more information from resolved add
[Service] Environment=SYSTEMD_LOG_LEVEL=debug
to override.conf of systemd-resolved via
systemctl edit systemd-resolved
then open a new terminal window and login as root
journalctl -u systemd-resolved -f
go back to the previous terminal window
systemctl restart systemd-resolved
When done debugging undo the change to override.conf with
systemctl revert systemd-resolved systemctl restart systemd-resolved
systemctl service-log-level systemd-resolved debug
is not persistent. Results in: “Unknown operation service-log-level”
resolvectl query webevaluation.nl
results in
webevaluation.nl: 46.235.42.123 -- link: wlan0 -- Information acquired via protocol DNS in 180.4ms. -- Data is authenticated: yes
Checking the loopback interface with
lsof -i @127.0.0.53:53
should result in something like
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME systemd-r 7251 systemd-resolve 17u IPv4 463540 0t0 UDP 127.0.0.53:domain systemd-r 7251 systemd-resolve 18u IPv4 463541 0t0 TCP 127.0.0.53:domain (LISTEN)
Files:
ll /run/systemd/resolve
results in
total 8,0K -rw-r--r-- 1 systemd-resolve systemd-resolve 708 dec 31 20:26 stub-resolv.conf -rw-r--r-- 1 systemd-resolve systemd-resolve 584 dec 31 20:26 resolv.conf
resolvectl status
should result in something like
Global LLMNR setting: yes MulticastDNS setting: yes DNSOverTLS setting: no DNSSEC setting: allow-downgrade DNSSEC supported: yes Current DNS Server: 9.9.9.9 DNS Servers: 9.9.9.9 DNS Domain: ~. DNSSEC NTA: 10.in-addr.arpa 16.172.in-addr.arpa 168.192.in-addr.arpa 17.172.in-addr.arpa 18.172.in-addr.arpa 19.172.in-addr.arpa 20.172.in-addr.arpa 21.172.in-addr.arpa 22.172.in-addr.arpa 23.172.in-addr.arpa 24.172.in-addr.arpa 25.172.in-addr.arpa 26.172.in-addr.arpa 27.172.in-addr.arpa 28.172.in-addr.arpa 29.172.in-addr.arpa 30.172.in-addr.arpa 31.172.in-addr.arpa corp d.f.ip6.arpa home internal intranet lan local private test Link 3 (wlan0) Current Scopes: LLMNR/IPv4 LLMNR/IPv6
resolvctl
results in something like
Global LLMNR setting: yes MulticastDNS setting: yes DNSOverTLS setting: no DNSSEC setting: allow-downgrade DNSSEC supported: yes Current DNS Server: 9.9.9.9 DNS Servers: 9.9.9.9 DNS Domain: ~. DNSSEC NTA: 10.in-addr.arpa 16.172.in-addr.arpa 168.192.in-addr.arpa 17.172.in-addr.arpa 18.172.in-addr.arpa 19.172.in-addr.arpa 20.172.in-addr.arpa 21.172.in-addr.arpa 22.172.in-addr.arpa 23.172.in-addr.arpa 24.172.in-addr.arpa 25.172.in-addr.arpa 26.172.in-addr.arpa 27.172.in-addr.arpa 28.172.in-addr.arpa 29.172.in-addr.arpa 30.172.in-addr.arpa 31.172.in-addr.arpa corp d.f.ip6.arpa home internal intranet lan local private test Link 3 (wlan0) Current Scopes: LLMNR/IPv4 LLMNR/IPv6 DefaultRoute setting: no LLMNR setting: yes MulticastDNS setting: no DNSOverTLS setting: no DNSSEC setting: allow-downgrade DNSSEC supported: yes Link 2 (eth0) Current Scopes: none DefaultRoute setting: no LLMNR setting: yes MulticastDNS setting: no DNSOverTLS setting: no DNSSEC setting: allow-downgrade DNSSEC supported: yes
resolvectl statistics
should result in something like
DNSSEC supported by current servers: yes Transactions Current Transactions: 0 Total Transactions: 28 Cache Current Cache Size: 0 Cache Hits: 0 Cache Misses: 32 DNSSEC Verdicts Secure: 24 Insecure: 7 Bogus: 0 Indeterminate: 0
Issues
No DNS resolution
Check if /run/systemd/resolve/resolv.conf exists
If not, run aptitude install systemd-resolved and check /run/systemd/resolve/stub-resolv.conf
Useful links
What is a DNS pointer
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