I recently decided to setup Unbound as a validating, recursive, caching DNS resolver on a router running OpenWRT Chaos Calmer (15.05.1). The setup includes forwarding to Dnsmasq for local names. This page documents the process.

This post is a copy of the Unbound HOWTO on the OpenWRT wiki that I wrote.

Why Unbound?

By default, OpenWRT uses Dnsmasq for DNS forwarding (and DHCP serving). This works well for most cases. One notable issue is that it requires a separate recursive DNS resolver, usually provided by an ISP or public DNS provider, to resolve requests. This can be a problem due to performance, hijacking, trustworthiness, or several other reasons. Running a recursive resolver, such as unbound, is a solution.

Prerequisites

The following steps assume that OpenWRT has been installed on a device and configured as desired, including the network configuration. If not, consult the Newcomer’s Guide for instructions.

The later steps require accessing the device using a terminal. See First Login.

Installation and Configuration

The installation and configuration instructions below are written in the form of a shell script for precision and clarity to a technical audience. The script can be saved and executed, although it is recommended to run commands and make edits individually both for better understanding and because the script is written to favor readability and clarity of instruction at the cost of thorough error handling and robustness.

Note that the choice of port 53535 is arbitrary. Similar tutorials often use 5353 or 5355 (which can conflict with MDNS).

#!/bin/sh
# Steps to configure unbound on OpenWRT with dnsmasq for dynamic DNS
# Note:  Clarity of instruction is favored over script speed or robustness.
#        It is not idempotent.

# Show commands as executed, error out on failure or undefined variables
set -eux

# Note the local domain (Network -> DHCP & DNS -> General Settings)
lan_domain=$(uci get 'dhcp.@dnsmasq[0].domain')

# Note the LAN network address (Network -> Interfaces -> LAN -> IPv4 address)
lan_address=$(uci get network.lan.ipaddr)

# Update the package list (System -> Software -> Update lists)
opkg update

# Install unbound (System -> Software -> Find package: unbound -> Install)
opkg install unbound # Ignore error that it can't listen on port 53

# Move dnsmasq to port 53535 where it will still serve local DNS from DHCP
# Network -> DHCP & DNS -> Advanced Settings -> DNS server port to 53535
uci set 'dhcp.@dnsmasq[0].port=53535'

# Configure dnsmasq to send a DNS Server DHCP option with its LAN IP
# since it does not do this by default when port is configured.
uci add_list "dhcp.lan.dhcp_option=option:dns-server,$lan_address"

# Save & Apply (will restart dnsmasq, DNS unreachable until unbound is up)
uci commit

# Allow unbound to query dnsmasq on the loopback address
# by adding 'do-not-query-localhost: no' to server section
sed -i '/^server:/a\	do-not-query-localhost: no' /etc/unbound/unbound.conf

# Convert the network address to a Reverse DNS domain
# https://en.wikipedia.org/wiki/Reverse_DNS_lookup
case $(uci get network.lan.netmask) in
    255.255.255.0) ip_to_rdns='\3.\2.\1.in-addr.arpa' ;;
    255.255.0.0) ip_to_rdns='\2.\1.in-addr.arpa' ;;
    255.0.0.0) ip_to_rdns='\1.in-addr.arpa' ;;
    *) echo 'More complex rDNS configuration required.' >&2 ; exit 1 ;;
esac
lan_rdns_domain=$(echo "$lan_address" | \
    sed -E "s/^([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+)$/$ip_to_rdns/")

# Check if the local addresses are in a private address range (very common)
case "$lan_address" in
    0.*) ip_to_priv_rdns='0.in-addr.arpa.' ;;
    10.*) ip_to_priv_rdns='10.in-addr.arpa.' ;;
    169.254.*) ip_to_priv_rdns='254.169.in-addr.arpa.' ;;
    172.1[6-9].*|172.2[0-9].*|172.3[0-1].*) ip_to_priv_rdns='\2.172.in-addr.arpa.' ;;
    192.0.2.*) ip_to_priv_rdns='2.0.192.in-addr.arpa.' ;;
    192.168.*) ip_to_priv_rdns='168.192.in-addr.arpa.' ;;
    198.51.100.*) ip_to_priv_rdns='100.51.198.in-addr.arpa.' ;;
    203.0.113.*) ip_to_priv_rdns='113.0.203.in-addr.arpa.' ;;
esac
if [ -n "${ip_to_priv_rdns-}" ] ; then
    # Disable default "does not exist" reply for private address ranges
    # by adding 'local-zone "$lan_domain" nodefault' to server section
    # Note that this must be on RFC 1918/5735/5737 boundary,
    # this is only equal to $lan_rdns_domain when netmask covers whole range.
    lan_priv_rdns_domain=$(echo "$lan_address" | \
        sed -E "s/^([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+)$/$ip_to_priv_rdns/")
    sed -i "/^server:/a\	local-zone: \"$lan_priv_rdns_domain\" nodefault"  \
        /etc/unbound/unbound.conf
fi

# Ignore DNSSEC chain of trust for the local domain
# by adding 'domain-insecure: "$lan_domain"' to server section
sed -i "/^server:/a\	domain-insecure: \"$lan_domain\"" /etc/unbound/unbound.conf

# Ignore DNSSEC chain of trust for the local reverse domain
# by adding 'domain-insecure: "$lan_rdns_domain"' to server section
sed -i "/^server:/a\	domain-insecure: \"$lan_rdns_domain\"" /etc/unbound/unbound.conf

# Add a stub zone to dnsmasq for the local domain to the unbound configuration
cat >> /etc/unbound/unbound.conf <<DNS_STUB_ZONE
stub-zone:
	name: "$lan_domain"
	stub-addr: 127.0.0.1@53535
DNS_STUB_ZONE

# Add a stub zone to dnsmasq for the local reverse domain to unbound.conf
cat >> /etc/unbound/unbound.conf <<RDNS_STUB_ZONE
stub-zone:
	name: "$lan_rdns_domain"
	stub-addr: 127.0.0.1@53535
RDNS_STUB_ZONE

# Optionally enable DNS Rebinding protection by uncommenting private-address
# configuration and adding 'private-domain: "$lan_domain"' to server section
sed -E -i \
    -e 's/(# )?private-address:/private-address:/' \
    -e "/^server:/a\	private-domain: \"$lan_domain\"" \
    /etc/unbound/unbound.conf

# Restart (or start) unbound (System -> Startup -> unbound -> Restart)
/etc/init.d/unbound restart

The resulting configuration (with defaults and comments removed) should look something like:

server:
	do-not-query-localhost: no
	domain-insecure: "0.168.192.in-addr.arpa"
	domain-insecure: "example.local"
	local-zone: "168.192.in-addr.arpa." nodefault
	private-address: 10.0.0.0/8
	private-address: 169.254.0.0/16
	private-address: 172.16.0.0/12
	private-address: 192.168.0.0/16
	private-address: fd00::/8
	private-address: fe80::/10
	private-domain: "example.local"
stub-zone:
	name: "example.local"
	stub-addr: 127.0.0.1@53535
stub-zone:
	name: "0.168.192.in-addr.arpa"
	stub-addr: 127.0.0.1@53535

The above script and configuration are also available as a Gist.

Troubleshooting

After completing the above steps, DNS should be working for both local and global addresses. If it is not, here are some suggested troubleshooting steps:

Resolution can be attempted from the OpenWRT system by running nslookup openwrt.org 127.0.0.1 and nslookup openwrt.org 127.0.0.1:53535. Unfortunately, the nslookup output does not distinguish between no response and a negative response, which significantly reduces its usefulness for debugging. A much more powerful lookup tool is DiG from the bind-dig package. To use it run dig openwrt.org @127.0.0.1, add -p 53535 to query the Dnsmasq port, or add -x with an IP in place of the domain to do a reverse lookup.

No Response

If Unbound is not responding to any request, try restarting the service with /etc/init.d/unbound restart and checking the system log for errors logread | tail.

Negative Response for Local Only

If the local domain or addresses result in negative responses, check that they are resolved correctly by Dnsmasq on port 53535. If so, check that the domain appears in domain-insecure, local-zone (which may be a suffix and must match a predefined zone), and as a name in stub-zone.

Failures for DNSSEC-Secured Domains

If domains which use DNSSEC fail to resolve while other domains work, check that the system time is correct. Time skew can cause validation failures. If the time is incorrect, check the NTP client configuration.

Further Additions

IPv6

It is relatively straightforward to extend the above configuration for IPv6. Forward resolution (from local domain to IPv6 address) does not require any additional changes to Unbound, although it may require configuration changes to Dnsmasq. See IPv6 DNS.

To configure reverse DNS for IPv6:

  • Determine the rDNS domain from the IPv6 address prefix by reversing the nibbles and appending “.ip6.arpa”.
  • Add domain-insecure: $lan6_rdns_domain.
  • Add local-zone: $lan6_rdns_domain nodefault if it is in a private range (be sure to use a preconfigured range.
  • Add a stub-zone with name: "$lan6_rdns_domain" as above.

The difficulty of adding IPv6 is that Dnsmasq is compiled without DHCPv6 support and does not resolve its own name due to #17457 so the value of forwarding IPv6 reverse requests is currently rather limited. IPv6 address configuration is also more variable and more difficult to detect.