DNS-over-TLS on the EdgeRouter Lite
Published on Dec 17, 2017
DNS-over-TLS is a fairly recent specificiation described in RFC7858, which enables DNS clients to communicate with servers over a TLS (encrypted) connection instead of requests and responses being sent in plain text. I won’t ramble on about why it’s a good thing that your ISP, government, or neighbour can’t see your DNS requests…
I use an EdgeRouter Lite from Ubiquiti Networks at home, and recently configured it to use DNS-over-TLS for all DNS queries. Here’s how I did it.
Out of the box, the ERL uses
dnsmasq to service DNS requests from local clients. To get
DNS-over-TLS support I switched to using Unbound, an open source DNS resolver
with support for many modern features such as DNSSEC and DNS-over-TLS.
Installing unbound on the ERL is a simple case of SSHing in, and then:
sudo apt-get update sudo apt-get install unbound
And then configuring the ERL to use the new local resolver for DNS requests, turn off dnsmasq, and and tell DHCP clients to send DNS requests to it (obviously substituting network names and subnets as appropriate):
set system name-server 127.0.0.1 set service dhcp-server shared-network-name lan1 subnet 192.168.1.0/24 dns-server 192.168.1.1 set service dhcp-server use-dnsmasq disable set service dns
At this point DNS should still work, but Unbound will still be sending requests out in plain text.
The unbound configuration lives in
/etc/unbound/unbound.conf, here’s a basic example that I use
to enable DNS-over-TLS:
# Unbound configuration file for Debian. # # See the unbound.conf(5) man page. # # See /usr/share/doc/unbound/examples/unbound.conf for a commented # reference config file. server: auto-trust-anchor-file: "/var/lib/unbound/root.key" verbosity: 1 interface: 0.0.0.0 interface: ::0 port: 53 do-ip4: yes do-ip6: yes do-udp: yes do-tcp: yes access-control: 192.168.0.0/16 allow access-control: 127.0.0.0/8 allow access-control: 10.0.0.0/8 allow root-hints: "/var/lib/unbound/root.hints" hide-identity: yes hide-version: yes harden-glue: yes harden-dnssec-stripped: yes cache-min-ttl: 900 cache-max-ttl: 14400 prefetch: yes rrset-roundrobin: yes ssl-upstream: yes use-caps-for-id: yes private-address: 192.168.0.0/16 private-address: 172.16.0.0/12 private-address: 10.0.0.0/8 logfile: "/var/lib/unbound/unbound.log" verbosity: 0 val-log-level: 3 forward-zone: name: "." forward-addr: 220.127.116.11@853
Notice the server directive
ssl-upstream, and that the forward zone specifies the quad9 resolver on its TLS port (853).
To enable Unbound to validate DNSSEC signatures, we need to provide it with some information about the root
nameservers that we trust. First, download the list of root nameservers to the
specified in the unbound config:
wget ftp://FTP.INTERNIC.NET/domain/named.cache -O /var/lib/unbound/root.hints
Then we need to add the root keys to the
auto-trust-anchor-file. The trust anchor at the time of
writing is below, but you can get the latest values from the IANA.
. 172800 IN DNSKEY 257 3 8 AwEAAaz/tAm8yTn4Mfeh5eyI96WSVexTBAvkMgJzkKTOiW1vkIbzxeF3+/4RgWOq7HrxRixHlFlExOLAJr5emLvN7SWXgnLh4+B5xQlNVz8Og8kvArMtNROxVQu . 172800 IN DNSKEY 257 3 8 AwEAAagAIKlVZrpC6Ia7gEzahOR+9W29euxhJhVVLOyQbSEW0O8gcCjFFVQUTf6v58fLjwBd0YI0EzrAcQqBGCzh/RStIoO8g0NfnfL2MTJRkxoXbfDaUeVPQuY
Redirecting unencrypted requests
I have a slew of devices on my network that, over time, I have configured to use 18.104.22.168 as a DNS server. They’re not going to care about the DHCP reply, and I don’t really feel like going around checking every weird and wonderful internet-connected device in the house, so I decided to just intercept requests to 22.214.171.124 and send them to Unbound. A simple NAT rule does the trick:
set service nat rule 1 description "HonestDNS Redirect" set service nat rule 1 destination address 126.96.36.199 set service nat rule 1 destination port 53 set service nat rule 1 inbound-interface eth0 set service nat rule 1 inside-address address 192.168.1.1 set service nat rule 1 inside-address port 53 set service nat rule 1 log disable set service nat rule 1 protocol tcp_udp set service nat rule 1 type destination
You could of course redirect any traffic to port 53, but that would prevent you from explicitly querying any other DNS server. By just intercepting traffic to 188.8.131.52 I’m taking care of the vast majority of my statically configured devices, and can still issue manual queries to other resolvers when needed.
To check that everything is working, you can use
tcpdump on the router to inspect packets on the
WAN interface directed at port 53:
sudo tcpdump -Xi eth0 port 53
You should, hopefully, not see anything.