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Stubby for OpenWRT

Stubby Description

Stubby is an application that acts as a local DNS Privacy stub resolver (using DNS-over-TLS). Stubby encrypts DNS queries sent from a client machine to a DNS Privacy resolver increasing end user privacy.

Stubby is useful on an OpenWRT device, because it can sit between the usual DNS resolver (dnsmasq by default) and the upstream DNS resolver and be used to ensure that DNS traffic is encrypted between the OpenWRT device and the resolver.

Stubby is developed by the getdns project.

For more background and FAQ see the About Stubby page.

Installation

Installation of this package can be achieved at the command line using opkg install stubby, or via the LUCI Web Interface. Installing the stubby package will also install the required dependency packages, including the ca-certificates package.

Configuration

The default configuration of the package has been chosen to ensure that stubby should work after installation.

By default, configuration of stubby is integrated with the OpenWRT UCI system using the file /etc/config/stubby. The configuration options available are also documented in that file. If for some reason you wish to configure stubby using the /etc/stubby/stubby.yml file, then you simply need to set option manual '1' in /etc/config/stubby and all other settings in /etc/config/stubby will be ignored.

Stubby port and addresses

The default configuration ensures that stubby listens on port 5453 on the loopback interfaces for IPv4 and IPv6. As such, by default, stubby will respond only to lookups from the OpenWRT device itself.

By setting the listening ports to non-standard values, this allows users to keep the main name server daemon in place (dnsmasq/unbound/etc.) and have that name server forward to stubby.

Upstream resolvers

The default package configuration uses the CloudFlare resolvers, configured for both IPv4 and IPv6.

CloudFlare have not published SPKI pinsets, and even though they are available, they have made no commitment to maintaining them. Using the currently known SPKI pinsets for CloudFlare brings the risk that in the future they may be changed by CloudFlare, and DNS would stop working. The default configuration has those SPKI entries commented out for this reason.

CloudFlare's privacy statement details how they treat data from DNS requests.

More resolvers are available in the upstream stubby example configuration and the DNS Privacy list.

Integration of stubby with dnsmasq

The recommended way to use stubby on an OpenWRT device is to integrate it with a caching resolver. The default caching resolver in OpenWRT is dnsmasq.

Set dnsmasq to send DNS requests to stubby

Since dnsmasq responds to LAN DNS requests on port 53 of the OpenWRT device by default, all that is required is to have dnsmasq forward those requests to stubby which is listening on port 5453 of the OpenWRT device. To achieve this, we need to set the server option in the dnsmasq configuration in the /etc/config/dhcp file to '127.0.0.1#5453'. We also need to tell dnsmasq not to use resolvers found in /etc/resolv.conf by setting the dnsmasq option noresolv to 1 in the same file. This can be achieved by editing the /etc/config/dhcp file directly or executing the following commands at the command line:

uci add_list dhcp.@dnsmasq[-1].server='127.0.0.1#5453'
uci dhcp.@dnsmasq[-1].noresolv=1
uci commit && reload_config

The same outcome can be achieved in the LUCI web interface as follows:

  1. Select the Network->DHCP and DNS menu entry.
  2. In the "General Settings" tab, enter the address 127.0.0.1#5453 as the only entry in the "DNS Forwardings" dialogue.
  3. In the "Resolv and Host files" tab tick the "Ignore resolve file" checkbox.

Disable sending DNS requests to ISP provided DNS servers

The configuration changes in the previous section ensure that DNS queries are sent over TLS encrypted connections once dnsmasq and stubby are started. When the OpenWRT device is first brought up, there is a possibility that DNS queries can go to ISP provided DNS servers ahead of dnsmasq and stubby being active. In order to mitigate this leakage, it's necessary to ensure that upstream resolvers aren't available, and the only DNS resolver used by the system is dnsmasq+stubby.

This requires setting the option peerdns to 0 and the option dns to the loopback address for both the wan and wan6 interfaces in the /etc/config/network file. This can be achieved by editing the /etc/config/network file directly, or by executing the following commands:

uci set network.wan.peerdns='0'
uci set network.wan.dns='127.0.0.1'
uci set network.wan6.peerdns='0'
uci set network.wan6.dns='0::1'
uci commit && reload_config

The same outcome can also be achieved using the LUCI web interface as follows:

  1. Select the Network->Interfaces menu entry.
  2. Click on Edit for the WAN interfaces.
  3. Choose the Advanced Settings tab.
  4. Unselect the "Use DNS servers advertised by peer" checkbox
  5. Enter 127.0.0.1 in the "Use custom DNS servers" dialogue box.
  6. Repeat the above steps for the WAN6 interface, but use the address 0::1 instead of 127.0.0.1.

Enabling DNSSEC

The configuration described above ensures that DNS queries are executed over TLS encrypted links. However, the responses themselves are not validated; DNSSEC provides the ability to validate returned DNS responses, and mitigate against DNS poisoning risks.

With the combination of stubby+dnsmasq there are two possible ways to enable DNSSEC:

  1. Configure stubby to perform DNSSEC validation, and configure dnsmasq to proxy the DNSSEC data to clients.
  2. Configure stubby not to perform DNSSEC validation and configure dnsmasq to require DNSSEC validation.

Either option achieves the same outcome, and there appears to be little reason for choosing one over the other other than that the second option is easier to configure in the LUCI web interface. Both options are detailed below, and both require that the dnsmasq package on the OpenWRT device is replaced with the dnsmasq-full package. That can be achieved by running the following command:

opkg install dnsmasq-full --download-only && opkg remove dnsmasq && opkg install dnsmasq-full --cache . && rm *.ipk

DNSSEC by stubby

Configuring stubby to perform DNSSEC validation requires setting the stubby configuration option dnssec_return_status to '1' in /etc/config/stubby, which can be done by editing the file directly or by executing the commands:

uci set stubby.global.dnssec_return_status=1
uci commit && reload_config

With stubby performing DNSSEC validation, dnsmasq needs to be configured to proxy the DNSSEC data to clients. This requires setting the option proxydnssec to 1 in the dnsmasq configuration in /etc/config/dhcp. That can be achieved by the following commands:

uci set dhcp.@dnsmasq[-1].proxydnssec=1
uci commit && reload_config

DNSSEC by dnsmasq

Configuring dnsmasq to perform DNSSEC validation requires setting the dnsmasq option dnssec to 1 in the /etc/config/dhcp file. In addition, it is advisable to also set the dnsmasq option dnsseccheckunsigned to 1. this can be achieved by editing the file /etc/config/dhcp or by executing the following commands:

uci set dhcp.@dnsmasq[-1].dnssec=1
uci set dhcp.@dnsmasq[-1].dnsseccheckunsigned=1
uci commit && reload_config

The same options can be set in the LUCI web interface as follows:

  1. Select the "Network->DHCP and DNS" menu entry.
  2. Select the "Advanced Settings" tab.
  3. Ensure both the "DNSSEC" and "DNSSEC check unsigned" check boxes are ticked.

Validating DNSSEC operation

Having configured DNSSEC validation using one of the two approaches above, it's important to check it's actually working. The following command can be used:

dig dnssectest.sidn.nl +dnssec +multi @192.168.1.1

This command should return output like the following:

; <<>> DiG 9.11.4-P1-RedHat-9.11.4-5.P1.fc28 <<>> dnssectest.sidn.nl +dnssec +multi @192.168.1.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26579
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 512
;; QUESTION SECTION:
;dnssectest.sidn.nl.	IN A

;; ANSWER SECTION:
dnssectest.sidn.nl.	14399 IN A 213.136.9.12
dnssectest.sidn.nl.	14399 IN RRSIG A 8 3 14400 (
			20181104071058 20181005071058 42033 sidn.nl.
			YAQl3tef36M9EQUOmCneHKCCkxox3csLpfUOql5i/6ND
			zPrQFsNr3g32HPoxOsi+hD2BE5+bEsnARayDSVLyx0qU
			6Hpi2rzQ0zGNZZkCJhCsdp3wnM1BWlMgPrCD0iIsJDok
			+DH5zu+yYufVUdSLQrMqA3MZDFUIqDUqSZuYDF4= )

;; Query time: 77 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Sat Oct 06 20:36:25 BST 2018
;; MSG SIZE  rcvd: 230

The key thing to note is the flags: qr rd ra ad part - the ad flag signifies that DNSSEC validation is working. If that flag is absent DNSSEC validation is not working.

Appendix: stubby configuration options

This section details the options available for use in the /etc/config/stubby file. The global configuration section specifies the configuration parameters for the stubby daemon. One or more resolver sections are used to configure upstream resolvers for the stubby daemon to use.

global section options

option manual

Specify whether to use this file to configure the stubby service. If this is set to '1' stubby will be configured using the file /etc/stubby/stubby.yml. If this is set to '0', configuration options will be taken from this file, and the service will be managed through UCI.

option trigger

This specifies an interface to trigger stubby start up on; stubby startup will be triggered by a procd signal associated with this interface being ready. If this interface is restarted, stubby will also be restarted.

This option can also be set to 'timed', in which case a time, specified by the option triggerdelay, will be waited before starting stubby.

option triggerdelay

If the trigger option specifies an interface, this option sets the time that is waited after the procd signal is received before starting stubby.

If trigger is set to 'timed' then this is the delay before starting stubby. This option is specified in seconds and defaults to the value '2'.

list dns_transport

The dns_transport list specifies the allowed transports. Allowed values are: GETDNS_TRANSPORT_UDP, GETDNS_TRANSPORT_TCP and GETDNS_TRANSPORT_TLS. The transports are tried in the order listed.

option tls_authentication

This option specifies whether TLS authentication is mandatory. A value of '1' mandates TLS authentication, and is the default.

If this is set to '0', and GETDNS_TRANSPORT_TCP or GETDNS_TRANSPORT_UDP appears in the dns_transport list, stubby is allowed to fall back to non-TLS authenticated lookups. You probably don't want this though.

option tls_query_padding_blocksize

This option specifies the block size to pad DNS queries to. You shouldn't need to set this to anything but '128' (the default), as recommended by https://tools.ietf.org/html/draft-ietf-dprive-padding-policy-03

option tls_connection_retries

This option specifies the number of connection failures stubby permits before Stubby backs-off from using an individual upstream resolver. You shouldn't need to change this from the default value of '2'.

option tls_backoff_time

This option specifies the maximum time in seconds Stubby will back-off from using an individual upstream after failures. You shouldn't need to change this from the default value of '3600'.

option timeout

This option specifies the timeout on getting a response to an individual request. This is specified in milliseconds. You shouldn't need to change this from the default value of '5000'.

option dnssec_return_status

This option specifies whether stubby should require DNSSEC validation. Specify to '1' to turn on validation, and '0' to turn it off. By default it is off.

option appdata_dir

This option specifies the location for storing stubby runtime data. In particular, if DNSSEC is turned on, stubby will store its automatically retrieved trust anchor data here. The default value is '/var/lib/stubby'.

option trust_anchors_backoff_time

When Zero configuration DNSSEC failed, because of network unavailability or failure to write to the appdata directory, stubby will backoff trying to refetch the DNSSEC trust-anchor for a specified amount of time expressed in milliseconds (which defaults to two and a half seconds).

option dnssec_trust_anchors

This option sets the location of the file containing the trust anchor data used for DNSSEC validation. If this is not specified, stubby will automatically retrieve a trust anchor at startup. It's unlikely you'll want to manage the trust anchor data manually, so in most cases this is not needed. By default, this is unset.

option edns_client_subnet_private

This option specifies whether to enforce ECS client privacy. The default is '1'. Set to '0' to disable client privacy.

For more details see Section 7.1.2 here.

option idle_timeout

This option specifies the time (in milliseconds) to hold TLS connections open to avoid the overhead of opening a new connection for every query. You should not normally need to change this from the default value (currently '10000').

See here for more details.

option round_robin_upstreams

This option specifies how stubby will use the upstream DNS resolvers. Set to '1' (the default) to instruct stubby to distribute queries across all available name servers - this will use multiple simultaneous connections which can give better performance in most (but not all) cases. Set to '0' to treat the upstream resolvers as an ordered list and use a single upstream resolver until it becomes unavailable, then use the next one.

list listen_address

This list sets the addresses and ports for the stubby daemon to listen for requests on. the default configuration configures stubby to listen on port 5453 on the loopback interface for both IPv4 and IPv6.

option log_level

If set, this option specifies the level of logging from the stubby daemon. By default, this option is not set.

The possible levels are:

'0': EMERG  - System is unusable
'1': ALERT  - Action must be taken immediately
'2': CRIT   - Critical conditions
'3': ERROR  - Error conditions
'4': WARN   - Warning conditions
'5': NOTICE - Normal, but significant, condition
'6': INFO   - Informational message
'7': DEBUG  - Debug-level message

option command_line_arguments

This option specifies additional command line arguments for stubby daemon. By default, this is an empty string.

resolver section options

option address

This option specifies the resolver IP address, and can either be an IPv4 or an IPv6 address.

option tls_auth_name

This option specifies the upstream domain name used for TLS authentication with the supplied server certificate

list spki

This list specifies the SPKI pinset which is verified against the keys in the server cerrtificate. The value takes the form '<digest type>/value>', where the digest type is the hashing algorithm used, and the value is the Base64 encoded hash of the public key. At present, only sha256 is supported for the digest type.

This should ONLY be used if the upstream resolver has committed to maintaining the pinset. CloudFlare have made no such commitment, and so we do not specify the SPKI values in the default configuration, even though they are available.

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