Ubuntu – systemd-resolved, resolvconf.service, resolvconf and openresolv. Why, which and how


I'm using a VPN client which adds two name servers to /etc/resolv.conf. All my connections are managed by Network-Manager.

I have to use this VPN client for my work VPN but after Ubuntu went to systemd-resolved in 16.10 I am having problems with my connection and DNS. Looks like systemd-resolved changes /etc/resolv.conf back to default name servers for some reason which makes internal pages not resolve. I looked into this some more and ended up replacing resolvconf with openresolv. That helped a lot, but still systemd-resolved resets /etc/resolv.conf after the VPN has been up for a while.

It could be just as the connection is up or after a few minutes or sometimes not at all. I then disabled systemd-resolved and the systemd resolvconf.service and only run openresolv. It all works well it seems.

However, this is all very confusing. Is there a reason for using systemd-resolved with one of the others? It was enabled in Ubuntu 16.10 so I thought there must be a reason for it but it seems to cause a fight over /etc/resolv.conf.

It would be great if I could just run operesolv and get this explained. I have done quite a bit of reading on it but I still do not understand why /etc/resolv.conf is managed like it is, only that when I use systemd for it I can't use my VPN client.

Best Answer

I managed to change the script that handles these configuration items in OpenVPN in Ubuntu (tested on 18.04). Here is a patch for that:

--- /etc/openvpn/update-resolv-conf.orig    2019-03-13 19:14:16.163914424 +0400
+++ /etc/openvpn/update-resolv-conf 2019-03-13 19:29:30.380420708 +0400
@@ -15,7 +15,7 @@
 #     foreign_option_3='dhcp-option DOMAIN be.bnc.ch'

-[ -x /sbin/resolvconf ] || exit 0
+[ -x /usr/bin/systemd-resolve ] || exit 0
 [ "$script_type" ] || exit 0
 [ "$dev" ] || exit 0

@@ -43,16 +43,16 @@
-   [ "$SRCHS" ] && R="search $SRCHS
+   for SRCH in $SRCHS ; do
+       R="${R}--set-domain=$SRCH "
+   done
    for NS in $NMSRVRS ; do
-           R="${R}nameserver $NS
+       R="${R}--set-dns=$NS "
-   echo -n "$R" | /sbin/resolvconf -a "${dev}.openvpn"
+   /usr/bin/systemd-resolve -i ${dev} ${R}
-   /sbin/resolvconf -d "${dev}.openvpn"
+   echo "Doing nothing, interface disappears."

You will need to add the following items to your OpenVPN configuration file:

script-security 2
up /etc/openvpn/update-resolv-conf
down /etc/openvpn/update-resolv-conf
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