The dnsmasq and dhcpd configuration is located in /etc/config/dhcp and controls both DNS and DHCP server options on the device (both DHCP and DNS services are implemented using the same OpenWrt program, dnsmasq).

In the default configuration this file contains one common section to specify DNS and daemon related options and one or more DHCP pools to define DHCP serving on network interfaces.

Possible section types of the dhcp configuration file are defined below. Not all types may appear in the file and most of them are only needed for special configurations. The common ones are the Common Options, the DHCP Pools and Static Leases.

The config section type dnsmasq determines values and options relevant to the overall operation of dnsmasq and the DHCP options on all interfaces served. The following table lists all available options, their default value, as well as the corresponding dnsmasq command line option. See the dnsmasq man page for further details.

These are the default settings for the common options:

config 'dnsmasq'
	option domainneeded	 1
	option boguspriv	 1
	option filterwin2k	 0
	option localise_queries	 1
	option rebind_protection 1
	option rebind_localhost  0
	option local        	 '/lan/'
	option domain	         'lan'
	option expandhosts	 1
	option nonegcache	 0
	option authoritative	 1
	option readethers        1
	option leasefile	 '/tmp/dhcp.leases'
	option resolvfile	 '/tmp/resolv.conf.auto'

• Options local and domain enable dnsmasq to serve entries in /etc/hosts as well as the DHCP client's names as if they were entered into the lan DNS domain.
• Options domainneeded, boguspriv, localise_queries, and expandhosts make sure that requests for these local host names (and the reverse lookup) never get forwarded to the upstream DNS servers.
• Option authoritative makes the router the only DHCP server on this network; clients get their IP lease a lot faster this way.
• Option leasefile stores the leases in a file, so that they can be picked up again if dnsmasq is restarted.
• Option resolvfile tells dnsmasq to use this file to find upstream name servers; it gets created by the WAN DHCP client or the PPP client.
• Options "enable_tftp" and "tftp_root" turn on the TFTP server and serve files from tftp_root. You may need to set the server's IP on the client. On the client, change it by setting "serverip" (e.g. "setenv serverip 192.168.1.10").

Sections of the type dhcp specify per interface lease pools and settings for serving DHCP requests. Typically there is at least one section of this type present in the /etc/config/dhcp file to cover the lan interface.

You can disable a lease pool for a specific interface by specifying the ignore option in the corresponding section.

A minimal example of a dhcp section is listed below:

config 'dhcp' 'lan'
	option 'interface'   'lan'
	option 'start'       '100'
	option 'limit'	     '150'
	option 'leasetime'   '12h'
        option ra server
        option dhcpv6 server

• lan specifies the OpenWrt interface that is served by this DHCP pool
• 100 is the offset from the network address, in the default configuration this would mean start leasing addresses from 192.168.1.100
• 150 is the maximum number of addresses that may be leased, in the default configuration this would mean leasing addresses up to 192.168.1.250
• 12h specifies the time to live for handed out leases, twelve hours in this example
• server defines the mode for IPv6 configuration (RA & DHCPv6)

Below is a listing of legal options for dhcp sections.

Notes:

• Although called 'interface', this is the network name, i.e. lan, wan, wifi etc. (section names in /etc/config/network), NOT the interface name used internally, like eth0, eth1, wlan0 etc. (the 'ifname' IDs in /etc/config/network).
• Although called 'networkid', this is the interface name used internally, i.e. eth0, eth1, wlan0 etc., not the network name (lan, wan, wifi etc.).

This departs from 'ifname' and 'network' as used in /etc/config/network and in /etc/config/wireless, so double check!

You can assign fixed IP addresses to hosts on your network, based on their MAC (hardware) address.

The configuration options in this section are used to construct a -G option for dnsmasq.

config host
        option ip       '192.168.1.2'
        option mac      '00:11:22:33:44:55'
        option name     'mypc'

This adds the fixed IP address 192.168.1.2 and the name "mypc" for a machine with the (Ethernet) hardware address 00:11:22:33:44:55.

config host
        option ip       '192.168.1.3'
        option mac      '11:22:33:44:55:66 aa:bb:cc:dd:ee:ff'
        option name     'mylaptop'

This adds the fixed IP address 192.168.1.3 and the name "mylaptop" for a machine with the (Ethernet) hardware address 11:22:33:44:55:66 or aa:bb:cc:dd:ee:ff. Note that this is unreliable if more than one of the listed mac addresses is on the network simultaneously. It's useful for cases such as a laptop with both wireless and wired interfaces, provided that only one will be active at a given time.

As well as adding host sections, you can also enable the dnsmasq section option readethers, and add entries to the /etc/ethers file.

You can specify an alternative default Gateway

config 'dhcp' 'lan'
        option 'interface' 'lan'
        option 'start' '100'
        option 'limit' '150'
        option 'leasetime' '12h'
        list 'dhcp_option' '3,192.168.1.2'

use the list 'dhcp_option' '3,192.168.1.2' to set the default gateway. A list of options can be found here here

Some hosts support booting over the network (PXE booting). DHCP/BOOTP is used to tell the host which file to boot and the server to load it from. Each client can only receive one set of filename and server address options. If different hosts should boot different files, or boot from different servers, you can use network-ids to map options to each client.

Usually, you need to set additional DHCP options (through dhcp_option) for further stages of the boot process. See the dnsmasq man page for details on the syntax of the O option.

The configuration options in this section are used to construct a -M option for dnsmasq.

*Note*: odhcp currently lacks support root-path specification. If you need this functionality, disable odhcpd and use dnsmasq instead.

config boot linux
        option filename         '/tftpboot/pxelinux.0'
        option serveraddress    '192.168.1.2'
        option servername       'fileserver'
        list   dhcp_option      'option:root-path,192.168.1.2:/data/netboot/root'

This tells 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.

DHCP can provide the client with numerous options, such as the domain name, NTP servers, network booting options, etc. While some settings are applicable to all hosts in a network segment, other are more specific and apply only to a group of hosts, or even only a single one. dnsmasq offers to group DHCP options and their values by a network-id, an alphanumeric identifier, and sending options only to hosts which have been tagged with that network-id.

In OpenWrt, you can tag hosts by the DHCP range they're in (section dhcp), or a number of options the client might send with their DHCP request. In each of these sections, you can use the dhcp_option list to add DHCP options to be sent to hosts with this network-id.

Each classifying section has two configuration options: the value of the DHCP option used to distinguish clients, and the network-id that these clients should be tagged with. Here's a template:

config classifier option classifier 'value' option networkid 'network-id' list dhcp_option 'DHCP-option'

The placeholder classifier can be one of these values:

An example using the 'mac' classifier to create a tagged network for openvpn would look like this in the config file:

config mac 'opnvpn'
        option mac  '00:FF:*:*:*:*'
        option networkid   'opnvpn'
        list   dhcp_option '3'

And like this in UCI

dhcp.opnvpn=mac
dhcp.opnvpn.mac=00:FF:*:*:*:*
dhcp.opnvpn.networkid=opnvpn
dhcp.opnvpn.dhcp_option=3

DHCP-option adds a DHCP option for this network-id. See the dnsmsq man page for a complete explanation of the syntax of the -O option.

force is a bool option. It forces dhcp-option to always be sent, even if the client does not ask for it in the parameter request list. This is sometimes needed, for example when sending options to PXELinux.

It is possible to mix the traditional /etc/dnsmasq.conf configuration file with the options found in /etc/config/dhcp.

The dnsmasq.conf file does not exist by default but will be processed by dnsmasq on startup if it is present. Note that options in /etc/config/dhcp take precendence over dnsmasq.conf since they are translated to command line arguments.

You can have dnsmasq execute a script on every action:

dhcp-script=/sbin/action.sh

DNS needs TCP and UDP port 53 open on the firewall. DHCP needs UDP ports 67 and 68 open from your zone to/from the firewall. See http://wiki.openwrt.org/doc/recipes/guest-wlan and http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html (viz "–dhcp-alternate-port") for more information.

Define a static lease for a host with MAC addresses 00:a0:24:5a:33:69 and 00:11:22:33:44:55 (handy when you use both wired and wireless connection on the same computer/laptop - of course, you can use just one MAC address) and assign the IP address 192.168.1.230 and the hostname example-host to it. We call this MAC address hot swap, since IP address stay same, but MAC address changes.

config 'host'
	option 'name' 'example-host'
	option 'ip'   '192.168.1.230'
	option 'mac'  '00:a0:24:5a:33:69 00:11:22:33:44:55'

Windows 7 has introduced a new Microsoft-enhanced feature. It won't assign IP address obtained from a DHCP server to an interface, if the IP was used before for another interface, even if that other interface is NOT active currently (i.e. cable disconnected). This behaviour is unique and was not reported for older Windows versions, Mac OS nor Linux.

If you try configure MAC address hot swap on your router, Windows 7 clients will end up in an infinite DORA loop.

Solution:

1. Create a bridge from the wireless and ethernet interfaces on your client
   • it's trivial: google it
   • you will have to add the MAC address of the bridge to /etc/config/dhcp

     • config 'host'
       	option 'name' 'example-host'
       	option 'ip'   '192.168.1.230'
       	option 'mac'  '00:a0:24:5a:33:69 00:11:22:33:44:55 02:a0:24:5a:33:69 02:11:22:33:44:55'

   • Since the bridge will probably take and alter your ethernet MAC address, you will lose SLAAC on wifi interface, making your laptop IPv6-disabled when only wireless is up.
2. Another solution is IPv6 friendly, you don't need to create a bridge, nor add MAC address to dnsmasq config file, but it involves user interaction:
   • When you plug the ethernet cable in, disable wireless interface in control panel (power off wireless won't do it).
   • When you unplug ethernet cable, enable wireless and disable ethernet.

Notes:

• http://answers.microsoft.com/en-us/windows/forum/windows_7-networking/windows-7-refuses-dhcp-addresses-if-they-were/1b72b289-0f58-492f-afb8-e76c80a81f00
• force is a bool option that will force dhcp-option to always be sent, even if the client does not ask for it in the parameter request list. This is sometimes needed, for example when sending options to PXELinux.

If you want to distribute IPv4 addresses to known clients only (static leases), use:

config dhcp 'lan'
        ...
        option dynamicdhcp 0

With this, dnsmasq will consider static leases defined in "config host" blocks and in /etc/ethers, and refuse to hand out any IPv4 address to unknown clients.

Note that you shouldn't use this as a security feature to prevent unwanted clients from connecting. A client can simply configure a static IP in the right range to have access to the network.

Define a custom domain name and the corresponding PTR record - assigns the IP address 192.168.1.140 to the domain name typhoon and construct an appropriate reverse record 140.1.168.192.in-addr.arpa. It works like an entry in /etc/hosts but more flexible and integrated.

Note that this currently only works for IPv4 addresses and that this functionality is not present in release prior to 8.09.2 .

Note that reverse records are not properly generated at present. (Barrier Breaker 14.07-RC2)

config 'domain'
	option 'name' 'typhoon'
	option 'ip'   '192.168.1.140'

another example: redirect www.facebook.com

config 'domain'
	option 'name' 'www.facebook.com'
	option 'ip'   '1.2.3.4'
        #the request to www.facebook.com will end to 1.2.3.4

To 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 one would use:

config 'srvhost'
	option srv  '_sip._udp.mydomain.com'
	option target 'pbx.mydomain.com'
	option port 5060
	option class 0
	option weight 10

A Canonical Name record specifes that a domain name is an alias for another domain, the "canonical" domain. To specify that the web server also doubles as the FTP server, one might use:

config 'cname'
	option cname 'ftp.example.com'
	option target 'www.example.com'

Note that it is necessary to use fully qualified domain names.

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.

Adding:

config 'mxhost'
	option domain 'yyy.zzz'
	option relay 'my.host.com'
	option pref 10

will mitigate the issues caused by split-horizon.

Direct BOOTP requests to the TFTP server at the IP address 192.168.1.2 and use /tftpboot/pxelinux.0 as boot file name.

config 'boot'
	option 'filename'      'pxelinux.0'
	option 'servername'    'data'
	option 'serveraddress' '192.168.1.2'

Multiple DHCP options can be configured under a single dhcp_option object. In this case, option 66 (tftp-server) and option 150 (multiple tftp servers) were used for a Cisco Callmanager deployment.

config 'dhcp' 'lan'
        option 'interface' 'lan'
        option 'start' '62'
        option 'limit' '192'
        option 'leasetime' '600h'
        list 'dhcp_option' '66,172.16.60.64'
        list 'dhcp_option' '150,172.16.60.64'

If you need multiple DNS forwarders with different configurations or DHCP server with different sets of lease files, have a look at this patch. Multiple dnsmasq "named" instances can be configured:

config 'dnsmasq' 'hotspot'
        option nonwildcard '1' # Tell dnsmasq to bind specific address(es)
        option resolvfile '/tmp/resolv.conf.hotspot'
        ...

Your configs are usally active for all instances, but you can limit them to single instances by:

config 'dhcp' 'lan'
        option 'interface' 'lan'
        option 'dnsmasq_config' 'hotspot'

config 'host'
        option 'name' 'chef'
        option 'mac' '00:00:00:00:00:00'
        option 'ip' '192.168.1.66'
        option 'dnsmasq_config' 'hotspot'

The web interface (luci) has not been updated for this patch yet.

In DHCP pool limit setting, the start and limit values do *not* refer to the "last digit", they're relative offsets to the network address.

• the network address of 10.0.0.1 / 255.0.0.0 is 10.0.0.0
• the 10.22.0.1 start address is 22 x /16 subnets away: (2^16) * 22 = 1441792
• 10.0.0.0 + 1441792 + 1 = 10.22.0.1 → start = 1441793
• 10.22.0.254 - 10.22.0.1 = 253 → limit = 253

config dhcp lan
  option interface lan
  option start 1441793
  option limit 253

Test:

root@OpenWrt:~# ipcalc.sh 10.0.0.1 255.0.0.0 1441793 253
IP=10.0.0.1
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
root@OpenWrt:~#

Assign different dhcp-options to a single MAC address:

uci batch <<'EOF'
add dhcp mac
set dhcp.@mac[-1].mac=00:11:22:33:44:55
set dhcp.@mac[-1].networkid=someone
add_list dhcp.@mac[-1].dhcp_option=6,192.168.1.3,192.168.1.2,192.168.1.1
add_list dhcp.@mac[-1].dhcp_option=3,192.168.1.2
add_list dhcp.@mac[-1].dhcp_option=44,192.168.1.3
commit dhcp
EOF
uci commit dhcp
/etc/init.d/dnsmasq reload

Where 6=DNS, 3=Default Gateway, 44=WINS

Assign different dhcp-options to multiple hosts:

config host
    option name 'j400'
    option mac '00:21:63:75:aa:17'
    option ip '10.11.12.14'
    option tag 'vpn'  # assign tag "vpn" to this host
 
config host
    option name 'j500'
    option mac '01:22:64:76:bb:18'
    option ip '10.11.12.15'
    option tag 'vpn'  # assign tag "vpn" to this host
 
config tag 'vpn'  # match tag "vpn"
    list dhcp_option '6,8.8.8.8,8.8.4.4'  # assign arbritary extra dhcp options to this tag
    option force '1'              #dhcp-option will always be sent, even if the client does not ask for it in the parameter request list. This is sometimes needed, for example when sending options to PXELinux.

Generally, specifying a dhcp option without any value, would disable that option. so for example you can use:

list dhcp_option '3'

to disable sending a default gateway to a specific client

Convert to procd: etc/init.d/dnsmasq restart

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

config dnsmasq
       ...
       option port 0
       option domain ''

The second option prevents dnsmasq from giving out a domain name and DNS search list to clients: this is useless without DNS resolving.

Of course, you will want to hand out the address of a DNS resolver to clients:

config dhcp lan
       option interface lan
       ...
       list dhcp_option "6,80.67.188.188,6,80.67.169.12"
       list dns         "2001:913::8"
       list dns         "2001:910:800::12"       

The `dhcp_option` entry is meant for dnsmasq, while the more elegant `dns` entries are understood by odhcpd. By default, odhcpd is only used for IPv6, but if you also use odhcpd for IPv4, you can just use `dns` entries for everything.

dnsmasq can be used to provide clients with a DNS server but not with DHCP (for example, if DHCP is already supplied by a separate server).

First, dnsmasq must be turned on for the internal interface:

• Network > Interfaces
  • click the desired internal interface to select it
    • DHCP Server
      • click the "Setup DHCP Server" button to enable dnsmasq on this interface – this will enable both DHCP and DNS

Now that dnsmasq is enabled, the DHCP portion of dnsmasq needs to be turned off.

• Network > Interfaces
  • click the desired internal interface to select it
    • DHCP Server
      • Ignore interface: Enable this option
    • Save & Apply

This change will turn off just DHCP but leave DNS services available on the specified interface.

config dnsmasq
        option domainneeded '1'
        option localise_queries '1'
        option local '/lan/'
        option domain 'lan'
        option expandhosts '1'
        option authoritative '1'
        option readethers '1'
        option leasefile '/tmp/dhcp.leases'
        option resolvfile '/tmp/resolv.conf.auto'
        list server '/subdomain.example.com/192.0.2.1'
          #be careful that some options should be absent (or set to False)
          #to allow the forwarding towards the "so defined" private networks
          #http://en.wikipedia.org/wiki/Private_network
          # likely 'rebind_protection'
        list server '/example.com/208.67.222.222'
        option rebind_protection '0'

1. Install dnsmasq using your local package manager

2. Edit /etc/dnsmasq.conf

# Tells dnsmasq to forward anything with the domain of remote.local to dns (example) server 10.25.11.2:
server=/remote.local/10.25.11.2

# Listen to requests only coming from the local machine:
listen-address=127.0.0.1

# Do not cache anything # A decent dns server will already cache for your local network:
cache-size=0

3. Edit /etc/resolv.conf

# Local LAN Domain:
domain ion.lan

# local dnsmasq server:
nameserver 127.0.0.1

# Your main dns server (dnsmasq will forward all requests to this example server):
nameserver 10.20.1.1

4. Start dnsmasq

5. Test – ping a local server and remote server using the FQDN

All dns requests will be forwarded to 10.20.1.1 except any matching *.remote.local. server.remote.local will be forwarded to 10.25.11.2

Credit: http://pyther.net/2010/12/dns-conditional-forwarding-dnsmasq/

cat /etc/config/dhcp 

config dnsmasq
        option localise_queries '1'
        option rebind_protection '0'
        option authoritative '1'
        option leasefile '/tmp/dhcp.leases'
        option localservice '1'
        option dnssec '0'
        option cachesize '0'
        option domain 'example.local'
        option readethers '1'
        option logqueries '1'
        option fliterwin2k '0'
        #Define your Domain and Domain Controllers IP address here.       
        option local '/example.local/192.168.1.X'**
        list server '/0.openwrt.pool.ntp.org/8.8.8.8'
        list server '/1.openwrt.pool.ntp.org/8.8.8.8'
        list server '/2.openwrt.pool.ntp.org/8.8.8.8'
        list server '/3.openwrt.pool.ntp.org/8.8.8.8'
        option resolvfile '/etc/resolv.conf'
        option boguspriv '1'

config dhcp 'lan'
        option interface 'lan'
        option start '100'
        option limit '150'
        option leasetime '12h'

Almost completed, Now on to the finalization of the /etc/resolv.conf Traditionally /etc/resolv.conf is populated via symlink based on interface settings which get inserted via script into /tmp/resolv.conf. We're going to disable this symlink because without doing so it would override our static settings.

You'll want to remove /etc/resolv.conf That will remove the resolv.conf symlink. Then we will add the ip address of the secondary DNS and external resolving address inside the /etc/resolv.conf file finally establishing conditional forwarding, something that should be specified for easy configuration via the GUI.

rm /etc/resolv.conf
echo "domain example.local">>/etc/resolv.conf
echo "nameserver 127.0.0.1">>/etc/resolv.conf
echo "nameserver 208.67.220.220">>/etc/resolv.conf

 cat /etc/resolv.conf
#Define your Domain Below  & Public DNS you desire. 

domain example.local
nameserver 127.0.0.1
nameserver 208.67.220.220

Sometimes when an interface is on the edge of the capacity (especially wifi over longer distances) a dhcp request could be not replied in time and therefore the dhcp client will not be able to receive proper network settings. A possible workaround is using static IPs or very long dhcp leases (more than 12h). This is particularly important when one has several wifi repeaters that use dhcp and are distant from each other or not easily accessible.