Helper script for OpenVPN to directly update the DNS settings of a link through systemd-resolved via...
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This is a helper script designed to integrate OpenVPN with the
systemd-resolvedservice via DBus instead of trying to override
/etc/resolv.conf, or manipulate
Since systemd-229, the
systemd-resolvedservice has an API available via DBus which allows directly setting the DNS configuration for a link. This script makes use of
busctlfrom systemd to send DBus messages to
systemd-resolvedto update the DNS for the link created by OpenVPN.
This script may not be compatible with recent versions of NetworkManager. It seems that NetworkManager overrides the
upcommand to use its own helper script (nm-openvpn-service-openvpn-helper). This script only supports
DNSSECoverrides). 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.
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
Make sure that you have
systemd-resolvedenabled and running. First, make sure that
systemd-resolved.serviceis 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.conffile to use
systemd-resolvedas a stub resolver (or both):
/etc/nsswitch.conffile to look up DNS via the
resolveservice (you may need to install the NSS library which connects libnss to
# 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.
systemd-resolvedservice (since systemd-231) also listens on
lointerface, 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.conffile. This set up can be installed by linking to
ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
The NSS interface for
systemd-resolvedmay 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 28 makes use of
authselectto manage the NSS settings on the system. Directly editing
nsswitch.confis not recommended as it may be overwritten at any time if
authselectis run. Proper overrides may not yet be possible - see pbrezina/authselect for details. However, like Ubuntu, the Stub Resolver method is recommended here too.
Finally, update your OpenVPN configuration file and set the
downoptions to point to the script, and
down-preto 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 /etc/openvpn/scripts/update-systemd-resolved up-restart down /etc/openvpn/scripts/update-systemd-resolved down-pre
It is recommended to use
up-restartin your configuration to ensure that
upate-systemd-resolvedis 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-tunis enabled). If you do not have
persist-tunset, or you use
ping-timeout, you most likely will not need this.
down-preoptions here will not work as expected where the
openvpndaemon drops privileges after establishing the connection (i.e. when using the
groupoptions). This is because only the
rootuser will have the privileges required to talk to
systemd-resolved.serviceover DBus. The
openvpn-plugin-down-root.soplug-in does provide support for enabling the
downscript to be run as the
rootuser, but this has been known to be unreliable.
Ultimately this shouldn't affect normal operation as
systemd-resolved.servicewill 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-prejust make this step explicit before the device is torn down rather than implicit on the change in environment.
Alternatively if you don't want to edit your client configuration, you can add the following options to your
openvpn \ --script-security 2 \ --setenv PATH '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin' \ --up /etc/openvpn/scripts/update-systemd-resolved --up-restart \ --down /etc/openvpn/scripts/update-systemd-resolved --down-pre
Or, you can add the following argument to the command-line arguments of
openvpn, which will use the
openvpn \ --config /etc/openvpn/scripts/update-systemd-resolved.conf
update-systemd-resolvedworks by processing the
dhcp-optioncommands set in OpenVPN, either through the server, or the client, configuration:
| Option | Examples | Notes | DBus Call | |--:|---|---|---| |
::1| This sets the DNS servers for the link and can take any IPv4 or IPv6 address. | SetLinkDNS | |
::1| This sets the DNS servers for the link and can take only IPv6 addresses. | SetLinkDNS | |
example.com| The primary domain for this host. If set multiple times, the first provided is used as the primary search domain for bare hostnames. Any subsequent
DOMAINoptions will be added as the equivalent of
DOMAIN-SEARCHoptions. All requests for this domain as well will be routed to the
DNSservers provided on this link. | SetLinkDomains | |
example.com| Secondary domains which will be used to search for bare hostnames (after any
DOMAIN, if set) and in the order provided. All requests for this domain will be routed to the
DNSservers provided on this link. | SetLinkDomains | |
example.com| All requests for these domains will be routed to the
DNSservers provided on this link. They will not be used to search for bare hostnames, only routed. A
.(single period) will instruct
systemd-resolvedto route the entire DNS name-space through to the
DNSservers configured for this connection (unless a more specific route has been offered by another connection for a selected name/name-space). This is useful if you wish to prevent DNS leakage. | SetLinkDomains | |
default| Control of DNSSEC should be enabled (
yes) or disabled (
allow-downgradeto switch off DNSSEC only if the server doesn't support it, for any queries over this link only, or use the system default (
default). | SetLinkDNSSEC |
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.
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.example.officewill be attempted first. The domains
example.comare also added as an additional search domain, so if
mail.example.lanwill be tried next, followed by
example.orgwill also be routed through to the four DNS servers listed, but they will not be appended (i.e.
mail.example.netwill not be attempted, nor
mail.example.comdo not exist).
Finally, DNSSEC has been enabled for this link (and this link only).
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.
systemd-resolvedwill 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-ROUTEoptions. 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-ROUTEwill 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:
If you want to prevent DNS queries leaking over untrusted networks (for example, over public WiFi hotspots), then you need to tell
systemd-resolvedto 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.
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-resolvedas you need to configure the network settings of other links to send all queries by default to your nominated DNS server (e.g. over
wlp2s0for your Ethernet or Wireless network cards). This needs to be configured under the
[Network]section of your
.networkfile for your interface in
/etc/systemd/network. For example:
[Network] DHCP=yes DNS=220.127.116.11 DNS=18.104.22.168 Domains=.
When you connect, all domains except those explicitly listed using the
DOMAIN-ROUTEoptions of your VPN link will be sent to the DNS server of your nominated link.
Note that these two options are mutually exclusive, as if you establish a VPN link with
.while you have also configured it inside a
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.
There are a number of known issues relating to some third-party servers and services:
~.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.
$ systemd-resolve eu-central-1.console.aws.amazon.com eu-central-1.console.aws.amazon.com: resolve call failed: DNSSEC validation failed: no-signature # or $ systemd-resolve 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
no(or maybe just
allow-downgrade) in your VPN configuration.
dhcp-option DNSSEC allow-downgrade
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-testsfrom 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.
Jonathan Wright [email protected]