DNS and DHCP examples

This how-to provides most common dnsmasq and odhcpd tuning scenarios adapted for OpenWrt.

LuCI → DHCP and DNS → Static Leases

Add a fixed IPv4 address 192.168.1.22 and name mydesktop for a machine with the MAC address 00:11:22:33:44:55.

uci add dhcp host
uci set dhcp.@host[-1].name="mydesktop"
uci set dhcp.@host[-1].mac="00:11:22:33:44:55"
uci set dhcp.@host[-1].ip="192.168.1.22"
uci commit dhcp
/etc/init.d/dnsmasq restart

Add a fixed IPv4 address 192.168.1.23, IPv6 interface identifier (address suffix) 23 and name mylaptop for a machine with the MAC address 11:22:33:44:55:66 or aa:bb:cc:dd:ee:ff and DUID 000100004fd454041c6f65d26f43.

uci add dhcp host
uci set dhcp.@host[-1].name="mylaptop"
uci add_list dhcp.@host[-1].mac="11:22:33:44:55:66"
uci add_list dhcp.@host[-1].mac="aa:bb:cc:dd:ee:ff"
uci set dhcp.@host[-1].ip="192.168.1.23"
uci set dhcp.@host[-1].duid="000100004fd454041c6f65d26f43"
uci set dhcp.@host[-1].hostid="23"
uci commit dhcp
/etc/init.d/dnsmasq restart
/etc/init.d/odhcpd restart

Reconnect your clients to apply the changes.

Using multiple MACs per host entry is unreliable, add a separate host entry for each MAC if the host has more than one interface connected simultaneously.

Create persistent DNS records for the configured static lease.

uci set dhcp.@host[-1].dns="1"
uci commit dhcp
/etc/init.d/dnsmasq restart

See also: odhcpd leases

Ignore DHCP requests from specific clients.

uci add dhcp host
uci set dhcp.@host[-1].name="mydesktop"
uci set dhcp.@host[-1].mac="00:11:22:33:44:55"
uci set dhcp.@host[-1].ip="ignore"
uci commit dhcp
/etc/init.d/dnsmasq restart

Ignore all DHCP requests except the ones from known clients configured with static leases or /etc/ethers.

uci set dhcp.lan.dynamicdhcp="0"
uci commit dhcp
/etc/init.d/dnsmasq restart

Avoid using this as a security measure since the client can still access the network with a static IP.

Synchronize hostnames with static leases.

while uci -q delete dhcp.@domain[0]; do :; done
uci show dhcp \
| sed -n -r -e "/^dhcp\.@host\[.*\]=.*$/\
s//uci add dhcp domain/p
/^dhcp\.@host\[.*\]\.(name|ip)='(.*)'$/\
s//uci set dhcp.@domain[-1].\1=\2/p" \
| while read -r CMD; do ${CMD}; done
uci commit dhcp
/etc/init.d/dnsmasq restart

Resolve the race condition with netifd service.

uci set dhcp.lan.force="1"
uci commit dhcp
/etc/init.d/dnsmasq restart

Suppress warnings about missing public prefix.

uci set dhcp.odhcpd.loglevel="3"
uci commit dhcp
/etc/init.d/odhcpd restart

DHCP options can be configured under the DHCP pool section via dhcp_option. Use an alternative default gateway, DNS server and NTP server, disable WINS.

uci add_list dhcp.lan.dhcp_option="3,192.168.1.2"
uci add_list dhcp.lan.dhcp_option="6,172.16.60.64"
uci add_list dhcp.lan.dhcp_option="42,172.16.60.64"
uci add_list dhcp.lan.dhcp_option="44"
uci commit dhcp
/etc/init.d/dnsmasq restart

Use the tag classifier to assign individual DHCP options to the tagged hosts. Specify custom DNS and possibly other DHCP options.

uci set dhcp.tag1="tag"
uci set dhcp.tag1.dhcp_option="6,8.8.8.8,8.8.4.4"
uci add dhcp host
uci set dhcp.@host[-1].name="j400"
uci set dhcp.@host[-1].mac="00:21:63:75:aa:17"
uci set dhcp.@host[-1].ip="10.11.12.14"
uci set dhcp.@host[-1].tag="tag1"
uci add dhcp host
uci set dhcp.@host[-1].name="j500"
uci set dhcp.@host[-1].mac="01:22:64:76:bb:18"
uci set dhcp.@host[-1].ip="10.11.12.15"
uci set dhcp.@host[-1].tag="tag1"
uci commit dhcp
/etc/init.d/dnsmasq restart

Use the mac classifier to create a tagged network to assign different DHCP options. Disable default gateway and specify custom DNS.

uci set dhcp.mac1="mac"
uci set dhcp.mac1.mac="00:FF:*:*:*:*"
uci set dhcp.mac1.networkid="vpn"
uci add_list dhcp.mac1.dhcp_option="3"
uci add_list dhcp.mac1.dhcp_option="6,192.168.1.3"
uci commit dhcp
/etc/init.d/dnsmasq restart
  • 10.0.0.0 - network address
  • 255.0.0.0 - network mask
  • 10.22.0.1 - pool start
  • 10.22.0.254 - pool end
  • $((22*2**16+1)) - start offset
  • 253 - limit offset
# ipcalc.sh 10.0.0.0 255.0.0.0 $((22*2**16+1)) 253
IP=10.0.0.0
NETMASK=255.0.0.0
BROADCAST=10.255.255.255
NETWORK=10.0.0.0
PREFIX=8
START=10.22.0.1
END=10.22.0.254
 
uci set dhcp.lan.start="$((22*2**16+1))"
uci set dhcp.lan.limit="253"
uci commit dhcp
/etc/init.d/dnsmasq restart

LuCI → Network → Hostnames

This is an implementation of the --host-record option. Define a custom domain name and the corresponding PTR record - assigns the IPv4 address 192.168.1.23 and IPv6 address fdce::23 to the domain name mylaptop and construct an appropriate reverse records. You can also use this to rebind domain names. It works like an entry in /etc/hosts but more flexible and integrated.

uci add dhcp domain
uci set dhcp.@domain[-1].name="mylaptop"
uci set dhcp.@domain[-1].ip="192.168.1.23"
uci add dhcp domain
uci set dhcp.@domain[-1].name="mylaptop"
uci set dhcp.@domain[-1].ip="fdce::23"
uci commit dhcp
/etc/init.d/dnsmasq restart

Make sure to set up static leases to avoid possible collisions due to race conditions.

This is an implementation of the --address option. Return 10.10.10.1 on query domain home and subdomain *.home.

uci add_list dhcp.@dnsmasq[0].address="/home/10.10.10.1"
uci commit dhcp
/etc/init.d/dnsmasq restart

This is an implementation of the --srv-host option. Define an SRV record for SIP over UDP, with the default port of 5060 on the host pbx.mydomain.com, with a class of 0 and a weight of 10.

uci add dhcp srvhost
uci set	dhcp.@srvhost[-1].srv="_sip._udp.mydomain.com"
uci set	dhcp.@srvhost[-1].target="pbx.mydomain.com"
uci set	dhcp.@srvhost[-1].port="5060"
uci set	dhcp.@srvhost[-1].class="0"
uci set	dhcp.@srvhost[-1].weight="10"
uci commit dhcp
/etc/init.d/dnsmasq restart

This is an implementation of the --cname option. A Canonical Name record specifes that a domain name is an alias for another domain, the “canonical” domain. Specify that the FTP server is on the same host as the web server.

uci add dhcp cname
uci set dhcp.@cname[-1].cname="ftp.example.com"
uci set dhcp.@cname[-1].target="www.example.com"
uci commit dhcp
/etc/init.d/dnsmasq restart

Make sure to set up hostnames since CNAME depends on it.

This is an implementation of the --mx-host option. If you're running the mail server for your domain behind a firewall (and therefore, with split-horizon for your own domain) then you might need to convince that mailer that it's actually authoritative for your domain.

If sendmail tells you “Domain of sender address xxx@yyy.zzz does not exist” this is because it isn't finding an MX record confirming that it's an MX relay for that domain.

Mitigate the issues caused by split-horizon:

uci add dhcp mxhost
uci set	dhcp.@mxhost[-1].domain="yyy.zzz"
uci set	dhcp.@mxhost[-1].relay="my.host.com"
uci set	dhcp.@mxhost[-1].pref="10"
uci commit dhcp
/etc/init.d/dnsmasq restart

Direct BOOTP requests to the TFTP server. Tell the client to load pxelinux.0 from the server at 192.168.1.2, and mount root from /data/netboot/root on the same server.

uci set dhcp.linux="boot"
uci set dhcp.linux.filename="/tftpboot/pxelinux.0"
uci set dhcp.linux.serveraddress="192.168.1.2"
uci set dhcp.linux.servername="fileserver"
uci add_list dhcp.linux.dhcp_option="option:root-path,192.168.1.2:/data/netboot/root"
uci commit dhcp
/etc/init.d/dnsmasq restart

If you need multiple DNS forwarders with different configurations or DHCP server with different sets of lease files.

Running multiple dnsmasq instances as DNS forwarder and/or DHCPv4 server, each having their own configuration and lease list can be configured by creating multiple dnsmasq sections. Typically in such configs each dnsmasq section will be bound to a specific interface by using the interface list; assigning sections like dhcp, host, etc. to a specific dnsmasq instance is done by the instance option. By default dnsmasq adds the loopback interface to the interface list to listen when the --interface option is used; therefore the loopback interface needs to be excluded in one of the dnsmasq instances by using the notinterface list.

These are example settings for multiple dnsmasq instances each having their own dhcp section. dnsmasq instance lan_dns is bound to the lan interface while the dnsmasq instance guest_dns is bound to the guest interface.

# Remove default instances
while uci -q delete dhcp.@dnsmasq[0]; do :; done
while uci -q delete dhcp.@dhcp[0]; do :; done
 
# Use network interface names for DHCP/DNS instance names
for INST in lan guest
do
uci set dhcp.${INST}_dns="dnsmasq"
uci set dhcp.${INST}_dns.domainneeded="1"
uci set dhcp.${INST}_dns.boguspriv="1"
uci set dhcp.${INST}_dns.filterwin2k="0"
uci set dhcp.${INST}_dns.localise_queries="1"
uci set dhcp.${INST}_dns.rebind_protection="1"
uci set dhcp.${INST}_dns.rebind_localhost="1"
uci set dhcp.${INST}_dns.local="/${INST}/"
uci set dhcp.${INST}_dns.domain="${INST}"
uci set dhcp.${INST}_dns.expandhosts="1"
uci set dhcp.${INST}_dns.nonegcache="0"
uci set dhcp.${INST}_dns.authoritative="1"
uci set dhcp.${INST}_dns.readethers="1"
uci set dhcp.${INST}_dns.leasefile="/tmp/dhcp.leases.${INST}"
uci set dhcp.${INST}_dns.resolvfile="/etc/resolv.conf.${INST}"
uci set dhcp.${INST}_dns.nonwildcard="1"
uci add_list dhcp.${INST}_dns.interface="${INST}"
uci add_list dhcp.${INST}_dns.notinterface="loopback"
uci set dhcp.${INST}="dhcp"
uci set dhcp.${INST}.instance="${INST}_dns"
uci set dhcp.${INST}.interface="${INST}"
uci set dhcp.${INST}.start="100"
uci set dhcp.${INST}.limit="150"
uci set dhcp.${INST}.leasetime="12h"
ln -f -s /tmp/resolv.conf.auto /etc/resolv.conf.${INST}
done
uci -q delete dhcp.@dnsmasq[0].notinterface
uci commit dhcp
/etc/init.d/dnsmasq restart

The LuCI web interface has not been updated to support multiple dnsmasq instances.

Log DNS queries for troubleshooting.

uci set dhcp.@dnsmasq[0].logqueries="1"
uci commit dhcp
/etc/init.d/dnsmasq restart

This change turns off DHCP on the specified interface but leaves DNS services available.

uci set dhcp.lan.ignore="1"
uci commit dhcp
/etc/init.d/dnsmasq restart
/etc/init.d/odhcpd restart

This is useful when you just want to hand out addresses to clients, without doing any DNS by dnsmasq.

/etc/init.d/dnsmasq stop
uci set dhcp.@dnsmasq[0].localuse="0"
uci set dhcp.@dnsmasq[0].port="0"
uci commit dhcp
/etc/init.d/dnsmasq start

Remove dnsmasq and use odhcpd for both DHCP and DHCPv6.

opkg update
opkg remove dnsmasq odhcpd-ipv6only
opkg install odhcpd
uci -q delete dhcp.@dnsmasq[0]
uci set dhcp.lan.dhcpv4="server"
uci set dhcp.odhcpd.maindhcp="1"
uci commit dhcp
/etc/init.d/odhcpd restart

Use Unbound for DNS.

opkg update
opkg install unbound-control unbound-daemon
uci set unbound.@unbound[0].dhcp_link="odhcpd"
uci set unbound.@unbound[0].unbound_control="1"
uci commit unbound
/etc/init.d/unbound restart
uci set dhcp.odhcpd.leasetrigger="/usr/lib/unbound/odhcpd.sh"
uci commit dhcp
/etc/init.d/odhcpd restart

Send custom DNS servers with DHCP.

# Configure dnsmasq
uci -q delete dhcp.lan.dhcp_option
uci add_list dhcp.lan.dhcp_option="6,8.8.8.8,8.8.4.4"
uci commit dhcp
/etc/init.d/dnsmasq restart
 
# Configure odhcpd
uci -q delete dhcp.lan.dns
uci add_list dhcp.lan.dns="2001:4860:4860::8888"
uci add_list dhcp.lan.dns="2001:4860:4860::8844"
uci commit dhcp
/etc/init.d/odhcpd restart

Reconnect your clients to apply the changes.

Send ISP DNS servers with DHCP. Fetch the settings dynamically with DHCP client scripts.

cat << "EOF" > /etc/udhcpc.user
DHCP_POOLS="lan"
DHCPC_EVENT="${1}"
DNS_SERVS="${dns}"
case ${DHCPC_EVENT} in
(bound|renew) ;;
(*) exit 0 ;;
esac
for DHCP_POOL in ${DHCP_POOLS}
do DHCP_OPTS="$(uci -q get dhcp.${DHCP_POOL}.dhcp_option)"
for DHCP_OPT in ${DHCP_OPTS}
do if [ "${DHCP_OPT%%,*}" = "6" ]
then uci del_list dhcp.${DHCP_POOL}.dhcp_option="${DHCP_OPT}"
fi
done
uci add_list dhcp.${DHCP_POOL}.dhcp_option="6,${DNS_SERVS// /,}"
done
uci commit dhcp
/etc/init.d/dnsmasq restart
EOF
 
cat << "EOF" > /etc/odhcp6c.user
DHCP_POOLS="lan"
DHCPC_EVENT="${2}"
DNS_SERVS="${RA_DNS} ${RDNSS}"
case ${DHCPC_EVENT} in
(bound|informed|updated|rebound|ra-updated) ;;
(*) exit 0 ;;
esac
for DHCP_POOL in ${DHCP_POOLS}
do uci -q delete dhcp.${DHCP_POOL}.dns
for DNS_SERV in ${DNS_SERVS}
do uci add_list dhcp.${DHCP_POOL}.dns="${DNS_SERV}"
done
done
uci commit dhcp
/etc/init.d/odhcpd restart
EOF

Reconnect your clients to apply the changes.

Answer DNS queries arriving from non-local networks. This can be useful to provide DNS for VPN clients.

uci set dhcp.@dnsmasq[0].localservice="0"
uci commit dhcp
/etc/init.d/dnsmasq restart

Forward DNS queries to specific servers.

uci -q delete dhcp.@dnsmasq[0].server
uci add_list dhcp.@dnsmasq[0].server="8.8.8.8"
uci add_list dhcp.@dnsmasq[0].server="8.8.4.4"
uci commit dhcp
/etc/init.d/dnsmasq restart

Disable resolvfile option limiting upstream resolvers to server option. Also makes local system to not use dnsmasq.

/etc/init.d/dnsmasq stop
uci set dhcp.@dnsmasq[0].noresolv="1"
uci commit dhcp
/etc/init.d/dnsmasq start

Enforce local system to use dnsmasq if it is running with noresolv option. Beware of race condition with Adblock service when using DNS encryption.

uci set dhcp.@dnsmasq[0].localuse="1"
uci commit dhcp
/etc/init.d/dnsmasq restart

Forward DNS queries for a specific domain and all its subdomains to a different server. More specific domains take precedence over less specific domains allowing to combine with unconditional DNS forwarding from above.

uci add_list dhcp.@dnsmasq[0].server="/example.com/192.168.2.1"
uci commit dhcp
/etc/init.d/dnsmasq restart

Simple DNS-based content filtering.

# Blacklist
uci add_list dhcp.@dnsmasq[0].server="/example.com/"
uci add_list dhcp.@dnsmasq[0].server="/example.net/"
uci commit dhcp
/etc/init.d/dnsmasq restart
 
# Whitelist
uci add_list dhcp.@dnsmasq[0].server="/example.com/#"
uci add_list dhcp.@dnsmasq[0].server="/example.net/#"
uci add_list dhcp.@dnsmasq[0].server="/#/"
uci commit dhcp
/etc/init.d/dnsmasq restart

See also: Ad blocking, DNS-based firewall with IP sets

Resolve the race condition with sysntpd service. When running dnsmasq with noresolv and localuse options and using DNS encryption for local system.

# Fetch peer DNS provider
. /lib/functions/network.sh
network_flush_cache
network_find_wan NET_IF
network_find_wan6 NET_IF6
network_get_dnsserver NET_DNS "${NET_IF}"
network_get_dnsserver NET_DNS6 "${NET_IF6}"
 
# Configure fallback DNS provider
if [ -z "${NET_DNS}" ]
then NET_DNS="8.8.8.8 8.8.4.4"
fi
if [ -z "${NET_DNS6}" ]
then NET_DNS6="2001:4860:4860::8888 2001:4860:4860::8844"
fi
 
# Bypass DNS forwarding for NTP provider
uci get system.ntp.server \
| sed -e "s/\s/\n/g" \
| sed -e "s/^[0-9]*\.//" \
| sort -u \
| while read -r NTP_DOMAIN
do for DNS_SERV in ${NET_DNS} ${NET_DNS6}
do uci add_list dhcp.@dnsmasq[0].server="/${NTP_DOMAIN}/${DNS_SERV}"
done
done
uci commit dhcp
/etc/init.d/dnsmasq restart

OpenWrt uses peer DNS as the upstream resolvers for dnsmasq by default. These are typically provided by the ISP upstream DHCP server. You can change it to any other DNS provider or a local DNS server running on another host. Make sure selected resolvers support DNSSEC validation if required. Specify several resolvers to improve fault tolerance.

# Configure DNS provider
uci -q delete network.wan.dns
uci add_list network.wan.dns="8.8.8.8"
uci add_list network.wan.dns="8.8.4.4"
 
# Configure DNSv6 provider
uci -q delete network.wan6.dns
uci add_list network.wan6.dns="2001:4860:4860::8888"
uci add_list network.wan6.dns="2001:4860:4860::8844"
 
# Disable peer DNS
uci set network.wan.peerdns="0"
uci set network.wan6.peerdns="0"
 
# Save and apply
uci commit network
/etc/init.d/network restart

General notes

  • Resolvers from all active interfaces are combined in a single runtime configuration indiscriminately.
  • If the interface is down, its resolvers are not used, so it's reasonable to specify resolvers only on interfaces they are reachable from.
  • Dnsmasq periodically queries all the listed resolvers and then uses the fastest one for a period of time.

Multiple DNS providers

  • The more DNS providers, the higher the fault tolerance of your DNS relative to DoS.
  • Different DNS providers may return different answers to a DNS query due to differences in caching, synchronization, load balancing, content filtering, etc.
  • To distinguish between correct and incorrect answers such as false-negatives, you need to utilize DNSSEC which may negatively impact fault tolerance and performance.

Peer DNS options

  • Keep peer DNS enabled to improve your DNS fault tolerance.
  • Disable peer DNS to prevent DNS leak if you have configured a VPN connection on OpenWrt.
  • Disable peer DNS to actually change your DNS provider and receive more predictable DNS replies.
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  • Last modified: 2021/04/08 19:42
  • by vgaetera