update-systemd-resolved

This is a helper script designed to integrate OpenVPN with the systemd-resolved service via DBus instead of trying to override /etc/resolv.conf, or manipulate systemd-networkd configuration files.

Since systemd-229, the systemd-resolved service has an API available via DBus which allows directly setting the DNS configuration for a link. This script makes use of busctl from systemd to send DBus messages to systemd-resolved to update the DNS for the link created by OpenVPN.

NetworkManager

This script may not be compatible with recent versions of NetworkManager. It seems that NetworkManager overrides the up command to use its own helper script (nm-openvpn-service-openvpn-helper). The script that ships with NetworkManager only supports DNS and DOMAIN options (not DNS6, DOMAIN-SEARCH and DOMAIN-ROUTE, nor DNSSEC overrides). It will also set the main network interface to route ~. DNS queries (i.e the whole name-space) to the LAN or ISP DNS servers, making it difficult to override using DOMAIN - see DNS Leakage below.

Installation

If you are using a distribution of Linux with uses the Arch User Repository, the simplest way to install is by using the openvpn-update-systemd-resolved AUR package as this will take care of any updates through your package manager.

Alternatively, the package can be manually installed by running the following:

git clone https://github.com/jonathanio/update-systemd-resolved.git
cd update-systemd-resolved
make

How to Enable

Make sure that you have systemd-resolved enabled and running. First, make sure that systemd-resolved.service is enabled and started:

systemctl enable systemd-resolved.service
systemctl start systemd-resolved.service

Next, you can either configure the system libraries to talk to it using NSS, or you can override the resolv.conf file to use systemd-resolved as a stub resolver (or both):

NSS and nssswitch.conf

Update your /etc/nsswitch.conf file to look up DNS via the resolve service (you may need to install the NSS library which connects libnss to systemd-resolved):

# Use /etc/resolv.conf first, then fall back to systemd-resolved
hosts: files dns resolve myhostname
# Use systemd-resolved first, then fall back to /etc/resolv.conf
hosts: files resolve dns myhostname
# Don't use /etc/resolv.conf at all
hosts: files resolve myhostname

The changes will be applied as soon as the file is saved.

Stub Resolver

The systemd-resolved service (since systemd-231) also listens on 127.0.0.53 via the lo interface, providing a stub resolver which any client can call to request DNS, whether or not it uses the system libraries to resolve DNS, and you no longer have to worry about trying to manage your /etc/resolv.conf file. This set up can be installed by linking to stub-resolv.conf:

ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

Issues with Ubuntu and Fedora

Ubuntu

The NSS interface for systemd-resolved may be deprecated and has already been flagged for deprecation in Ubuntu (see LP#1685045 for details). In this case, you should use the Stub Resolver method now.

Fedora

Fedora 28 makes use of authselect to manage the NSS settings on the system. Directly editing nsswitch.conf is not recommended as it may be overwritten at any time if authselect is run. Proper overrides may not yet be possible - see pbrezina/authselect for details. However, like Ubuntu, the Stub Resolver method is recommended here too.

OpenVPN Configuration

Finally, update your OpenVPN configuration file and set the up and down options to point to the script, and down-pre to ensure that the script is run before the device is closed:

script-security 2
setenv PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
up /usr/bin/update-systemd-resolved
up-restart
down /usr/bin/update-systemd-resolved
down-pre

up-restart

It is recommended to use up-restart in your configuration to ensure that upate-systemd-resolved is run on restarts - where the connection is re-established but the TUN/TAP device remained open (for example, where the original connection has timed out and persist-tun is enabled). If you do not have persist-tun set, or you use ping-exit instead of ping-timeout, you most likely will not need this.

down/pre-down with user/group

The down and down-pre options here will not work as expected where the openvpn daemon drops privileges after establishing the connection (i.e. when using the user and group options). This is because only the root user will have the privileges required to talk to systemd-resolved.service over DBus. The openvpn-plugin-down-root.so plug-in does provide support for enabling the down script to be run as the root user, but this has been known to be unreliable.

Ultimately this shouldn't affect normal operation as systemd-resolved.service will remove all settings associated with the link (and therefore naturally update /etc/resolv.conf, if you have it symlinked) when the TUN or TAP device is closed. The option for down and down-pre just make this step explicit before the device is torn down rather than implicit on the change in environment.

Command Line Settings

Alternatively if you don't want to edit your client configuration, you can add the following options to your openvpn command:

openvpn \
  --script-security 2 \
  --setenv PATH '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin' \
  --up /usr/bin/update-systemd-resolved --up-restart \
  --down /usr/bin/update-systemd-resolved --down-pre

Or, you can add the following argument to the command-line arguments of openvpn, which will use the update-systemd-resolve.conf file instead:

openvpn \
  --config /usr/bin/update-systemd-resolved.conf

Usage

update-systemd-resolved works by processing the dhcp-option commands set in OpenVPN, either through the server, or the client, configuration:

Note: There are no local or system options to be configured. All configuration for this script is handled through OpenVPN, including, for example, the name of the interface to be configured.

Example

push "dhcp-option DNS 10.62.3.2"
push "dhcp-option DNS 10.62.3.3"
push "dhcp-option DNS6 2001:db8::a3:c15c:b56e:619a"
push "dhcp-option DNS6 2001:db8::a3:ffec:f61c:2e06"
push "dhcp-option DOMAIN example.office"
push "dhcp-option DOMAIN example.lan"
push "dhcp-option DOMAIN-SEARCH example.com"
push "dhcp-option DOMAIN-ROUTE example.net"
push "dhcp-option DOMAIN-ROUTE example.org"
push "dhcp-option DNSSEC yes"

This, added to the OpenVPN server's configuration file will set two IPv4 DNS servers and two IPv6 and will set the primary domain for the link to be example.office. Therefore if you try to look up the bare address mail then mail.example.office will be attempted first. The domains example.lan and example.com are also added as an additional search domain, so if mail.example.office fails, then mail.example.lan will be tried next, followed by mail.example.com.

Requests for example.net and example.org will also be routed through to the four DNS servers listed, but they will not be appended (i.e. mail.example.net will not be attempted, nor mail.example.org, if mail.example.office or mail.example.com do not exist).

Finally, DNSSEC has been enabled for this link (and this link only).

DNS Leakage

DNS Leakage is something to be careful of when using any VPN or untrusted network, and it can heavily depend on how you configure your normal DNS settings as well as how you configure the DNS on your VPN connection.

By default, systemd-resolved will send all DNS queries to at least one DNS server on every link configured with DNS servers. The first to reply back with a valid query is the one returned to the client, and the last to return back a failure (assuming all other queries also failed) will also be returned to the client.

The changes in this handling come in when you start using the DOMAIN, DOMAIN-SEARCH and DOMAIN-ROUTE options. The three differ in how domains are treated for searching bare domains, but all three work exactly the same when it comes to how it routes domains to specific DNS servers.

Any domain added using DOMAIN, DOMAIN-SEARCH, or DOMAIN-ROUTE will be added explicitly to the VPN link and therefore any queries for domain suffixes which match these will be routed through this link, and only this link. Any other domains which do not match these will revert back to distributing the queries across all links.

There are two ways to override this:

Preventing Leakage in on untrusted networks

If you want to prevent DNS queries leaking over untrusted networks (for example, over public WiFi hotspots), then you need to tell systemd-resolved to send all DNS queries over the VPN link. To do this, add the following to your server or client VPN configurations respectively:

# Server Configuration
push "dhcp-option DOMAIN-ROUTE ."

# Client Configuration
dhcp-option DOMAIN-ROUTE .

All DNS queries (which do not match a more explicit entry on another link) will now be routed over the VPN only.

Preventing Leakage to Corporate networks

In an alternate situation, you may want to have DNS queries specifically routed over the VPN for corporate or private network access, but you don't want your general DNS queries to be visible to anyone who has access to the logs of the corporate DNS servers.

This option cannot be directly managed by update-systemd-resolved as you need to configure the network settings of other links to send all queries by default to your nominated DNS server (e.g. over ens0 or wlp2s0 for your Ethernet or Wireless network cards). This needs to be configured under the [Network] section of your .network file for your interface in /etc/systemd/network. For example:

[Network]
DHCP=yes
DNS=8.8.8.8
DNS=8.8.4.4
Domains=.

When you connect, all domains except those explicitly listed using the DOMAIN, DOMAIN-SEARCH, or DOMAIN-ROUTE options of your VPN link will be sent to the DNS server of your nominated link.

Concurrent Configuration

Note that these two options are mutually exclusive, as if you establish a VPN link with DOMAIN-ROUTE set to . while you have also configured it inside a .network file via systemd-networkd, then you will have two links responsible for routing all queries, and so both links will get all requests.

How to manage the DNS settings of other links while the VPN is operational is outside the scope of this script at this time.

Known Issues

There are a number of known issues relating to some third-party servers and services:

NetworkManager

There is currently a regression with versions of NetworkManager 1.2.6 or later (see LP#1671606 and LP#1688018) which means that it will automatically set all normal network interfaces with ~. for DNS routing. This means that even if you set dhcp-option DOMAIN-ROUTE . for your VPN connection, you will still leak DNS queries over potentially insecure networks.

If you are concerned by potentially leaking DNS on systems which use NetworkManager, you may need to configure an additional script into NetworkManager which change the domain routing settings on all non-VPN interfaces.

DNSSEC Issues

$ resolvectl eu-central-1.console.aws.amazon.com
eu-central-1.console.aws.amazon.com: resolve call failed: DNSSEC validation failed: no-signature
# or
$ resolvectl eu-central-1.console.aws.amazon.com
eu-central-1.console.aws.amazon.com: resolve call failed: DNSSEC validation failed: incompatible-server

If you are seeing failed queries in your logs due to DNSSEC issues, support may be partially or fully enabled and you are now working with a server which does not support this extension. You may therefore need to set DNSSEC to no (or maybe just allow-downgrade) in your VPN configuration.

dhcp-option DNSSEC allow-downgrade

How to help

If you can help with any of these areas, or have bug fixes, please fork and raise a Pull Request for me.

I have built a basic test framework around the script which can be used to monitor and validate the calls made by the script based on the environment variables available to it at run-time. Please add a test for any new features you may wish to add, or update any which are wrong, and test your code by running ./run-tests from the root of the repository. There are no dependencies on run-tests - it runs 100% bash and doesn't call out to any other program or language.

TravisCI is enabled on this repository: Click the link at the top of this README to see the current state of the code and its tests.

Licence

GPL

Author

Jonathan Wright [email protected]