systemd

Bug #1449001
Comment #0

Comment 0 for bug 1449001

Revision history for this message

Malcolm Scott (malcscott) wrote on 2015-04-27:

systemd-resolved will fall back to Google public DNS (8.8.8.8, etc.) in the absence of other configured DNS servers.

systemd-resolved is not enabled by default in Ubuntu 15.04, but it is installed by default and will behave in this way if enabled by the user.

$ cat /etc/systemd/resolved.conf
(...)
# Entries in this file show the compile time defaults.
(...)
#FallbackDNS=8.8.8.8 8.8.4.4 2001:4860:4860::8888 2001:4860:4860::8844

This raises privacy concerns since in the event of accidental misconfiguration DNS queries will be sent unencrypted across the internet, and potentially also security concerns given systemd-resolved does not perform DNSSEC validation and is not particularly well hardened against malicious responses e.g. from a MITM (http://www.openwall.com/lists/oss-security/2014/11/12/5).

I believe that it would be better to fail safe if no DNS server is configured -- i.e. have DNS lookups fail; it's better that the user is aware of their misconfiguration, rather than silently sending their queries to Google. The user can intentionally opt to use Google public DNS if they wish.

Steps to reproduce:
1. Remove existing DNS configuration (from /etc/network/interfaces, /etc/resolv.conf, /etc/resolvconf/resolv.conf.d/*)
2. Reboot, or otherwise clear relevant state
3. sudo service systemd-resolved start
4. Note that Google's servers are listed in /run/systemd/resolve/resolv.conf
5. If systemd-resolved is enabled in /etc/nsswitch.conf (it isn't by default), observe that DNS lookups probably still work, and queries are being sent to one of Google's servers

Possible workaround/bugfix: ship a resolved.conf which clears the FallbackDNS parameter.

This issue has been discussed in the Debian BTS (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=761658). My interpretation of the Debian package maintainer's position is that a user concerned with the privacy implications shouldn't let systemd get into a state where it uses the fallback DNS servers (quoting Marco d'Itri: "Short summary: have a resolv.conf file or use DHCP"). I would argue that it's safest not to have fallback DNS servers configured at all by default.

systemd-resolved will fall back to Google public DNS (8.8.8.8, etc.) in the absence of other configured DNS servers.

systemd-resolved is not enabled by default in Ubuntu 15.04, but it is installed by default and will behave in this way if enabled by the user.

$ cat /etc/systemd/resolved.conf 
(...)
# Entries in this file show the compile time defaults.
(...)
#FallbackDNS=8.8.8.8 8.8.4.4 2001:4860:4860::8888 2001:4860:4860::8844

I believe that it would be better to fail safe if no DNS server is configured -- i.e. have DNS lookups fail; it's better that the user is aware of their misconfiguration, rather than silently sending their queries to Google.  The user can intentionally opt to use Google public DNS if they wish.

Possible workaround/bugfix: ship a resolved.conf which clears the FallbackDNS parameter.

This issue has been discussed in the Debian BTS (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=761658).  My interpretation of the Debian package maintainer's position is that a user concerned with the privacy implications shouldn't let systemd get into a state where it uses the fallback DNS servers (quoting Marco d'Itri: "Short summary: have a resolv.conf file or use DHCP").  I would argue that it's safest not to have fallback DNS servers configured at all by default.