[systemd-devel] Where does resolved takes its data from?

[Wojtek Swiatek]

Hello everyone,

I decided to clean up my DNS resolving mess and fully go the
systemd-resolved way = on every machine:
- have /etc/resolv.conf linked to /run/systemd/resolve/stub-resolv.conf
- have the resolver stub running on 127.0.0.53
- provide internal upstream and fallback servers in
/etc/systemd/resolved.conf
- hope for the best

"every machine" above are actually nspawn containers with their own IP
addresses (provided and resolved by the host, via dnsmasq)
I removed any other resolvers (if they were present), everything is under
networkd control.

My first step was to have a broken machine (DNS wise), with a fully
commented out /etc/systemd/resolved.conf (as it is by default), expecting
not to be able to resolve anything and go from there.

To my surprise google.com resolved fine. OK, this must be an invisible
default pointing to 8.8.8.8 or something like that (as the fallbacks are
still commented out).

But I also tried to resolve an internal name, known only by the host and
secondary internal servers (which would be the upstream servers mentioned
above, when actually configured in /etc/systemd/resolved.conf).

I have no idea how resolved managed to get information from other DNS
servers (whihc could be either the host, which runs dnsmasq on 0.0.0.0:53,
or the secondaries which run bind on their_IP:53)).
Where could that resolution come from?

The situation on the machines, showing that resolved is the only one
resolver:

root at dev ~# lsof -i -P -n
COMMAND    PID            USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
systemd-n   51 systemd-network   18u  IPv6   56615      0t0  UDP
[fe80::685a:94ff:fecc:37ce]:546
systemd-n   51 systemd-network   20u  IPv4   59162      0t0  UDP
10.200.0.50:68
rsyslogd    56          syslog    8u  IPv4   66478      0t0  UDP *:57004
salt-mini   68            root   21u  IPv4  829402      0t0  TCP
10.200.0.50:46988->52.210.137.123:4505 (ESTABLISHED)
systemd-r 2519 systemd-resolve   12u  IPv4 1272287      0t0  UDP
127.0.0.53:53
systemd-r 2519 systemd-resolve   13u  IPv4 1272288      0t0  TCP
127.0.0.53:53 (LISTEN)


root at dev ~# ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 Sep04 ?        00:00:00 /lib/systemd/systemd
root        18     1  0 Sep04 ?        00:00:00
/lib/systemd/systemd-journald
systemd+    51     1  0 Sep04 ?        00:00:00
/lib/systemd/systemd-networkd
root        52     1  0 Sep04 ?        00:00:00 /usr/bin/python3
/usr/bin/networkd-dispatcher
root        53     1  0 Sep04 ?        00:00:00 /lib/systemd/systemd-logind
root        54     1  0 Sep04 ?        00:00:00 /usr/sbin/cron -f
message+    55     1  0 Sep04 ?        00:00:00 /usr/bin/dbus-daemon
--system --address=systemd: --nofork --nopidfile --systemd-activation
--syslog-only
syslog      56     1  0 Sep04 ?        00:00:00 /usr/sbin/rsyslogd -n
root        62     1  0 Sep04 ?        00:00:00 /usr/bin/python
/usr/bin/salt-minion
root        63     1  0 Sep04 console  00:00:00 /sbin/agetty -o -p -- \u
--noclear --keep-baud console 115200,38400,9600 vt220
root        68    62  0 Sep04 ?        00:00:34 /usr/bin/python
/usr/bin/salt-minion
root        71    68  0 Sep04 ?        00:00:00 /usr/bin/python
/usr/bin/salt-minion
root       873     1  0 Sep04 pts/0    00:00:00 /usr/bin/fish
root       875     1  0 Sep04 ?        00:00:00 /lib/systemd/systemd --user
root       876   875  0 Sep04 ?        00:00:00 (sd-pam)
systemd+  2519     1  0 07:42 ?        00:00:00
/lib/systemd/systemd-resolved
root      3352   873  0 08:06 pts/0    00:00:00 ps -ef
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