DHCP and DNS examples
See also: DHCP and DNS configuration, DNS encryption, DNS hijacking
Introduction
This how-to provides most common dnsmasq and odhcpd tuning scenarios adapted for OpenWrt.
Instructions
Static leases
LuCI → DHCP and DNS → Static Leases
Add a fixed IPv4 address 192.168.1.22
and name mylaptop
for a machine with the MAC address 11:22:33:44:55:66
.
uci add dhcp host uci set dhcp.@host[-1].name="mylaptop" uci set dhcp.@host[-1].mac="11:22:33:44:55:66" uci set dhcp.@host[-1].ip="192.168.1.22" uci set dhcp.@host[-1].dns="1" uci commit dhcp service dnsmasq restart
Reconnect your clients to apply the changes.
Add a fixed IPv6 interface identifier aka address suffix 23
and name mylaptop
for a machine with the DUID 000100004fd454041c6f65d26f43
.
uci add dhcp host uci set dhcp.@host[-1].name="mylaptop" uci set dhcp.@host[-1].duid="000100004fd454041c6f65d26f43" uci set dhcp.@host[-1].hostid="23" uci set dhcp.@host[-1].dns="1" uci commit dhcp service odhcpd restart
Reconnect your clients to apply the changes.
This is an implementation of the --dhcp-host
option.
You can combine IPv4 and IPv6 reservations to a single host entry.
Using multiple MACs per host entry is possible but unreliable.
Add a separate host entry for each MAC if the host has more than one interface connected simultaneously.
See also: odhcpd leases
MAC filtering
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 service 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 service dnsmasq restart
Avoid using this as a security measure since the client can still access the network with a static IP.
Race conditions with netifd
Resolve the race condition with netifd service and skip check for competing DHCP servers.
uci set dhcp.lan.force="1" uci commit dhcp service dnsmasq restart
Missing public prefix
Suppress warnings about missing GUA prefix.
uci set dhcp.odhcpd.loglevel="3" uci commit dhcp service odhcpd restart
Providing IPv6 default route with DHCP
Announce IPv6 default route for clients using the ULA prefix.
uci set dhcp.lan.ra_default="1" uci commit dhcp service odhcpd restart
DHCP options
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 service dnsmasq restart
Client classifying and individual options
Use the tag
classifier to create a tagged group.
Assign individual DHCP options to hosts tagged with tag1
.
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 service dnsmasq restart
Use the mac
classifier to create a tagged group.
Assign different DHCP options to hosts with matching MACs.
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 service dnsmasq restart
Use vendor-specific DHCP option to disable NetBios over TCP for Windows Clients
See also: Vendor-Specific Option Code 0x01 - Microsoft Disable NetBIOS Option
If you want to disable NetBIOS over TCP on Windows clients, it's possible with the following vendor-specific DHCP option:
- DHCP Option 43 (Vendor Specific Information)
- Vendor-specific Option Code (1 byte): 0x01 (Microsoft Disable NetBios Option)
- Vendor-specific Option Length (1 byte): 0x04
- Vendor-specific Option Data (4 bytes): See table below
Value | Effect |
0x00000000 | Enable NetBIOS over TCP/IP |
0x00000001 | Ignore setting |
0x00000002 | Disable NetBIOS over TCP/IP |
It needs to be pushed to clients who have the “MSFT 5.0” Vendor class identifier in their DHCP requests.
This can be achieved with the following configuration snippet:
uci set dhcp.msft="vendorclass" uci set dhcp.msft.networkid="msft" uci set dhcp.msft.vendorclass="MSFT" uci add_list dhcp.msft.dhcp_option="vendor:MSFT,1,2i" uci commit dhcp service dnsmasq restart
DHCP pool for a large network
10.0.0.0
- network address255.0.0.0
- network mask10.22.0.1
- pool start10.22.0.254
- pool end22*2**16+1
- pool offset253
- pool limit
# 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 service dnsmasq restart
Hostnames
LuCI → Network → Hostnames
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.
The init service merges all entries to an additional hosts file used with the --addn-hosts
option.
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 service dnsmasq restart
Be sure to set up static leases to avoid possible collisions due to race conditions.
A and AAAA RR
This is an implementation of the --address
option.
Return 10.10.10.1
for the domain home
and all its subdomains.
uci add_list dhcp.@dnsmasq[0].address="/home/10.10.10.1" uci commit dhcp service dnsmasq restart
SRV RR
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 service dnsmasq restart
CNAME RR
This is an implementation of the --cname
option.
A Canonical Name record specifies 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 service dnsmasq restart
Be sure to set up hostnames since CNAME depends on it.
MX RR
This is an implementation of the --mx-host
option.
Mitigate the issues caused by split DNS for your own domain if you're running the mail server for your domain behind a firewall.
Convince that mailer that it's actually authoritative for your domain, otherwise sendmail may not find an MX record to confirm that the domain is an MX relay and complain about non-existent domain of sender address.
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 service dnsmasq restart
TFTP boot
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 service dnsmasq restart
Multi-Arch TFTP boot
For PXE boot, each client needs a specific binary for its architecture e.g. PC BIOS, UEFI x86 32bit, UEFI x86 64bit, ARM, etc. You can match on the DHCP “Vendor Class Identifier” option (60) specified by the client to send back the right filename.
uci set dhcp.@dnsmasq[0].logdhcp='1' uci set dhcp.@dnsmasq[0].enable_tftp='1' uci set dhcp.@dnsmasq[0].tftp_root='/srv/tftp' uci add dhcp match uci set dhcp.@match[-1].networkid='bios' uci set dhcp.@match[-1].match='60,PXEClient:Arch:00000' uci add dhcp match uci set dhcp.@match[-1].networkid='efi32' uci set dhcp.@match[-1].match='60,PXEClient:Arch:00006' uci add dhcp match uci set dhcp.@match[-1].networkid='efi64' uci set dhcp.@match[-1].match='60,PXEClient:Arch:00007' uci add dhcp match uci set dhcp.@match[-1].networkid='efi64' uci set dhcp.@match[-1].match='60,PXEClient:Arch:00009' uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:bios,bios/lpxelinux.0' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:efi32,efi32/syslinux.efi' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:efi64,efi64/syslinux.efi' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci commit dhcp service dnsmasq reload
Chainloading iPXE
If you are configuring chainloading for iPXE, you can match on the DHCP “User Class” option (77) to send an iPXE script to the client, and on the client-arch option to choose the iPXE binary (for other values of client-arch, see RFC or this forum thread).
uci set dhcp.@dnsmasq[0].logdhcp='1' uci set dhcp.@dnsmasq[0].enable_tftp='1' uci set dhcp.@dnsmasq[0].tftp_root='/srv/tftp' uci add dhcp match uci set dhcp.@match[-1].networkid='bios' uci set dhcp.@match[-1].match='option:client-arch,0' uci add dhcp match uci set dhcp.@match[-1].networkid='efi64' uci set dhcp.@match[-1].match='option:client-arch,7' uci add dhcp match uci set dhcp.@match[-1].networkid='efi64' uci set dhcp.@match[-1].match='option:client-arch,9' uci add dhcp match uci set dhcp.@match[-1].networkid='ipxe' uci set dhcp.@match[-1].match='77,"iPXE"' uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:bios,undionly.kpxe' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:efi64,ipxe.efi' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci add dhcp boot uci set dhcp.@boot[-1].filename='tag:ipxe,boot_script.ipxe' uci set dhcp.@boot[-1].serveraddress="$(uci get network.lan.ipaddr)" uci set dhcp.@boot[-1].servername="$(uci get system.@system[0].hostname)" uci commit dhcp service dnsmasq reload
Multiple DHCP/DNS server/forwarder instances
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 INST="lan guest" for INST in ${INST} do uci set dhcp.${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.${INST}_dns.resolvfile="/tmp/resolv.conf.d/resolv.conf.auto" uci set dhcp.${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" done uci -q delete dhcp.@dnsmasq[0].notinterface uci commit dhcp service dnsmasq restart
The LuCI web interface has not been updated to support multiple dnsmasq instances.
Logging DNS queries
LuCI → Network → DHCP and DNS → General Settings → Log queries
Log DNS queries for troubleshooting.
uci set dhcp.@dnsmasq[0].logqueries="1" uci commit dhcp service dnsmasq restart
See also: Reading logs
Disabling DHCP role
This change turns off DHCP on the specified interface but leaves DNS services available.
uci set dhcp.lan.ignore="1" uci commit dhcp service dnsmasq restart service odhcpd restart
Disabling IPv6 DNS for odhcpd
Stop advertising IPv6 DNS with DHCPv6/RA.
uci set dhcp.lan.dns_service="0" uci set dhcp.lan.ra_dns="0" uci commit dhcp service odhcpd restart
Disabling DNS role
This is useful when you just want to hand out addresses to clients, without doing any DNS by dnsmasq.
service dnsmasq stop uci set dhcp.@dnsmasq[0].localuse="0" uci set dhcp.@dnsmasq[0].port="0" uci commit dhcp service dnsmasq start
Replacing dnsmasq with odhcpd and Unbound
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 service odhcpd restart
Use Unbound for DNS.
opkg update opkg install unbound-control unbound-daemon uci set unbound.@unbound[0].add_local_fqdn="3" uci set unbound.@unbound[0].add_wan_fqdn="1" uci set unbound.@unbound[0].dhcp_link="odhcpd" uci set unbound.@unbound[0].dhcp4_slaac6="1" uci set unbound.@unbound[0].unbound_control="1" uci commit unbound service unbound restart uci set dhcp.odhcpd.leasefile="/var/lib/odhcpd/dhcp.leases" uci set dhcp.odhcpd.leasetrigger="/usr/lib/unbound/odhcpd.sh" uci commit dhcp service odhcpd restart
See also: Unbound official documentation
Providing custom DNS with DHCP
LuCI → Network → Interfaces → LAN → Edit → DHCP Server
- Advanced Settings → DHCP-Options
- IPv6 Settings → Announced IPv6 DNS servers
Announce 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 service 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 service odhcpd restart
Reconnect your clients to apply the changes.
See also: ISP DNS with DHCP, IPv6 ISP DNS with DHCPv6
Providing DNS for non-local networks
Answer DNS queries arriving from non-local networks. Reply to VPN clients using point-to-point topology.
uci set dhcp.@dnsmasq[0].localservice="0" uci commit dhcp service dnsmasq restart
Disabling DNS cache
Disable DNS cache.
uci set dhcp.@dnsmasq[0].cachesize="0" uci commit dhcp service dnsmasq restart
DNS forwarding
LuCI → Network → DHCP and DNS → General Settings → DNS forwardings
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 service dnsmasq restart
This can be combined with selective DNS forwarding.
Split DNS
LuCI → Network → DHCP and DNS → Resolv and Hosts Files → Ignore resolve file
Ignore resolvfile
option and limit upstream resolvers to server
option.
This effectively enables split DNS and makes the local system not to use dnsmasq.
service dnsmasq stop uci set dhcp.@dnsmasq[0].noresolv="1" uci commit dhcp service dnsmasq start
Enforcing dnsmasq for local system
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 service dnsmasq restart
Selective DNS forwarding
LuCI → Network → DHCP and DNS → General Settings → DNS forwardings
Forward DNS queries for a specific domain and all its subdomains to a different server. More specific domains take precedence over less specific domains.
uci add_list dhcp.@dnsmasq[0].server="/example.com/192.168.2.1" uci commit dhcp service dnsmasq restart
This can be combined with unconditional DNS forwarding.
DNS filtering
LuCI → Network → DHCP and DNS → General Settings → DNS forwardings
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 service 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 service dnsmasq restart
See also: Ad blocking, DNS-based firewall with IP sets
Race conditions with sysntpd
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 or use a fallback DNS provider.
Bypass DNS forwarding for NTP provider.
. /lib/functions/network.sh network_flush_cache for IPV in 4 6 do eval network_find_wan${IPV%4} NET_IF network_get_dnsserver NET_DNS "${NET_IF}" case ${IPV} in (4) NET_DNS="${NET_DNS:-8.8.8.8 8.8.4.4}" ;; (6) NET_DNS="${NET_DNS:-2001:4860:4860::8888 2001:4860:4860::8844}" ;; esac for NET_DNS in ${NET_DNS} do uci get system.ntp.server \ | sed -e "s/\s/\n/g" \ | sed -r -e "/\.pool\.ntp\.org$/s|^[0-9]*\.||;s|.*|\ del_list dhcp.@dnsmasq[0].server='/\0/${NET_DNS}'\n\ add_list dhcp.@dnsmasq[0].server='/\0/${NET_DNS}'|" \ | uci -q batch done done uci commit dhcp service dnsmasq restart
Upstream DNS provider
LuCI → Network → Interfaces → WAN & WAN6 → Edit
- Use DNS servers advertised by peer
- Use custom DNS servers
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. Use resolvers supporting DNSSEC validation if necessary. 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 IPv6 DNS 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 service 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 leaks if you have configured a VPN connection on OpenWrt.
- Disable peer DNS to actually change your DNS provider and receive more predictable DNS replies.