Firstly, identify the version of systemd on your system. This is crucial to understanding the validity of systemd-resolved links you're reading on the 'net:
In respect to systemd-resolved, systemd v239 has some material changes. If you see "resolvectl" being mentioned, the document you're reading relates to systemd-resolved as ships with systemd v239 or later. At the time of this writing Raspbian is using v232
No need to install systemd-resolved on Raspbian- it's already there.
HOWEVER: you MUST install
libnss-resolve. It although systemd-resolved is installed, this dependency is NOT:
apt-get install libnss-resolve
Let's find the files we have to work with:
Note: locate command requires mlocate to be installed.
systemd-resolved 's own resolv.conf can't be read directly but must be exposed as a symlink to /etc/resolv.conf
rm -f /etc/resolv.conf
ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
/run/systemd/resolve/resolv.conf is populated with the DNS resolvers specified in /etc/systemd/resolved.conf. Mine looks as follows:
DNS=192.168.3.126 188.8.131.52 2001:4860:4860::8888
You'll note the first resolver is an RFC1918 non-world-routable IP: why use this?!?! Because that's my Pi's router GW connection which has been set to cache DNS queries. So the Pi will try to resolve from the router's cache first and if the query cannot be answered there, systemd-resolveed will reach out across the 'net to a distant DNS server.
So as soon as any host in the subnet resolves a name now, all the Pi's can grab that mapping from the router and it will be cached locally making everything a bit snappier.
Even if the router is rebooted and its' DNS cache is consequently blown-away, the Pi's will retain their DNS caches. Sweet.
Add "resolve" to the front of the list so name resolution is attempted by the stub resolver first:
hosts: resolve dns files mdns4_minimal [NOTFOUND=return]
/etc/dhcpcd.conf: In order to bypass DHCP assigning our DNS servers, we need to tweak this file by removing
domain_name_servers to look as below:
#option domain_name_servers, domain_name, domain_search, host_name
option domain_name, domain_search, host_name
Finally, we need to read the changes we just made to /etc/systemd/resolved.conf:
systemctl restart systemd-resolved
If you look at /etc/resolv.conf now, you'll see the DNS servers you specified.
If your config is righteous, should see a status as below:
sudo systemctl status systemd-resolved
● systemd-resolved.service - Network Name Resolution
Loaded: loaded (/lib/systemd/system/systemd-resolved.service; disabled; vendor preset: enabled)
Active: active (running) since Wed 2019-04-24 16:27:10 BST; 9min ago
Process: 17635 ExecStartPost=/bin/sh -c [ ! -e /run/resolvconf/enable-updates ] || echo "nameserver 127.0.0.53" | /sbin/resolvconf -a systemd-resolved (code=exited, status=0/SUCCESS)
Main PID: 17634 (systemd-resolve)
Status: "Processing requests..."
Lets run some tests:
ping -c1 www.google.com
ping6 -c1 www.google.com
Yes, those ping by name.
Let's resolve directly against the systemd-resolved local caching stub resolver next:
systemd-resolve www.microsoft.com 127.0.0.1
systemd-resolve www.microsoft.co.uk 127.0.0.1
systemd-resolve is pre- systemd v239. Use
resolvectl query instead if systemd version is v239 or later
Results will look as follows:
pi@pi3Bplus-tablet:~ $ systemd-resolve www.microsoft.com 127.0.0.1
-- Information acquired via protocol DNS in 55.6ms.
-- Data is authenticated: no
-- Information acquired via protocol DNS in 3.9ms.
-- Data is authenticated: yes
Now let's look at our cache statistics:
pi@pi3Bplus-tablet:~ $ systemd-resolve --statistics
DNSSEC supported by current servers: no
Current Transactions: 0
Total Transactions: 14
Current Cache Size: 6
Cache Hits: 7
Cache Misses: 7
Another useful command is:
NOTE: Any time you either reboot or
systemctl restart systemd-resolved you'll blow-away the cache which lives in memory. Obviously cold caches are non-performant, so avoid doing this if possible.
Can't over-emphasize the importance of employing caching of DNS queries on a local stub resolver. This will have a material impact on performance.
I think I've covered it all. If you spot any errors or omissions please ping me and I'll make the appropriate updates. HTH folks- T