Say you have a local nginx or traefik, routing traffic to multiple Vaadin apps based on a hostname, and you want to resolve *.foo.fake to 127.0.0.1 to test http://app1.foo.fake routes to app1 and http://app2.foo.fake routes to app2. The easiest way is to add entries to /etc/hosts but /etc/hosts does not support wildcard dns entries.

The simplest solution is to add both app1.foo.fake and app2.foo.fake to /etc/hosts, but that can be tedious when there are many such apps.

A better solution is to use dnsmasq:

$ sudo apt install dnsmasq
$ sudo vim /etc/dnsmasq.conf   # add address=/foo.fake/127.0.0.1
$ systemctl reload dnsmasq

Unfortunately, this is not enough:

$ ping foo.fake
ping: foo.fake: Name or service not known
$ host foo.fake
Host foo.fake not found: 3(NXDOMAIN)

The reason is that systemd-resolved is in charge of DNS resolution and it won’t use your local dnsmasq automatically, as can be seen via resolvectl status. Also see Linux DNS.

The solution is to force systemd-resolved to use the local dnsmasq server. Edit /etc/systemd/resolved.conf and set DNS=127.0.0.1. Then, restart systemd-resolved and check that your DNS server is now in effect:

$ sudo systemctl restart systemd-resolved
$ resolvectl status
$ host app1.foo.fake
app1.foo.fake has address 127.0.0.1