The DHCP 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 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.

Sections of the type dnsmasq specify per dnsmasq instance the values and options relevant to the overall operation of the dnsmasq instance 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:

root@lede:/# uci show dhcp
dhcp.@dnsmasq[0]=dnsmasq
dhcp.@dnsmasq[0].domainneeded='1'
dhcp.@dnsmasq[0].boguspriv='1'
dhcp.@dnsmasq[0].filterwin2k='0'
dhcp.@dnsmasq[0].localise_queries='1'
dhcp.@dnsmasq[0].rebind_protection='1'
dhcp.@dnsmasq[0].rebind_localhost='1'
dhcp.@dnsmasq[0].local='/lan/'
dhcp.@dnsmasq[0].domain='lan'
dhcp.@dnsmasq[0].expandhosts='1'
dhcp.@dnsmasq[0].nonegcache='0'
dhcp.@dnsmasq[0].authoritative='1'
dhcp.@dnsmasq[0].readethers='1'
dhcp.@dnsmasq[0].leasefile='/tmp/dhcp.leases'
dhcp.@dnsmasq[0].resolvfile='/tmp/resolv.conf.auto'
dhcp.@dnsmasq[0].localservice='1'
...

root@lede:/# cat /etc/config/dhcp
config 'dnsmasq'
	option domainneeded	 1
	option boguspriv	 1
	option filterwin2k	 0
	option localise_queries	 1
	option rebind_protection 1
	option rebind_localhost  1
	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 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.

Name	Type	Required	Default	Description
dhcp_option	list of strings	no	(none)	The ID dhcp_option here must be with written with an underscore. It will be translated to –dhcp-option, with a hyphen, as ultimately used by dnsmasq. Multiple option values can be given for this network-id, with a a space between them and the total string between ””. E.g. '26,1470' or 'option:mtu, 1470' that can assign an MTU per DHCP. Your client must accept MTU by DHCP for this to work. Or “3,192.168.1.1 6,192.168.1.1” to give out gateway and dns server addresses.
dhcp_option_force	list of strings	no	(none)	Exactly the same as dhcp_option (note the underscores), but it will be translated to –dhcp-option-force, meaning that the DHCP option will be sent regardless on whether the client requested it. dhcp_option_force on Trunk but not 17.01.0
dynamicdhcp	boolean	no	1	Dynamically allocate client addresses, if set to 0 only clients present in the ethers files are served
force	boolean	no	0	Forces DHCP serving on the specified interface even if another DHCP server is detected on the same network segment
ignore	boolean	no	0	Specifies whether dnsmasq should ignore this pool if set to 1
dhcpv4	string	no	none	Specifies whether DHCPv4 server should be enabled none or disabled (disabled)
dhcpv6	string	no	none	Specifies whether DHCPv6 server should be enabled (server), relayed (relay) or disabled (disabled)
ra	string	no	none	Specifies whether Router Advertisements should be enabled (server), relayed (relay) or disabled (disabled)
ra_default	integer	no	0	Default router lifetime in the RA message will be set if default route is present and a global IPv6 address (0) or if default route is present but no global IPv6 address (1) or neither of both conditions (2)
ra_management	integer	no	1	RA management mode : no M-Flag but A-Flag (0), both M and A flags (1), M flag but not A flag (2)
ra_offlink	bool	no	0	Announce prefixes as offlink (1) in RAs
ra_preference	string	no	medium	Announce routes with either high (high), medium (medium) or low (low) priority in RAs
ra_mininterval	integer	no	200	Minimum time interval between RAs (in seconds)
ra_maxinterval	integer	no	600	Maximum time interval between RAs (in seconds)
ra_lifetime	integer	no	1800	Advertised router lifetime (in seconds)
ra_useleasetime	bool	no	0	Limit the preferred and valid lifetimes of the prefixes in the RA messages to the configured DHCP leasetime
ra_hoplimit	integer	no	0	Advertised current hop limit (0-255)
ra_reachabletime	integer	no	0	Advertised reachable time (in milliseconds) (0-3600000)
ra_retranstime	integer	no	0	Advertised NS retransmission time (in milliseconds) (0-60000)
ra_mtu	integer	no	none	Maximum advertised MTU
ndp	string	no	none	Specifies whether NDP should be relayed relay or disabled none
ndproxy_routing	bool	no	1	Learn routes from NDP
ndproxy_slave	bool	no	0	Ignore neighbor messages on slave enabled (1) interfaces
master	boolean	no	0	Specifies whether DHCPv6, RA and NDP in relay mode is a master interface or not.
interface	logical interface name	yes	(none)	Specifies the interface associated with this DHCP address pool; must be one of the interfaces defined in /etc/config/network.
leasetime	string	yes	12h	Specifies the lease time of addresses handed out to clients, for example 12h or 30m
limit	integer	yes	150	Specifies the size of the address pool (e.g. with start=100, limit=150, maximum address will be .249)
networkid	string	no	(value of interface)	The dhcp functionality defined in the dhcp section is limited to the interface indicated here through its network-id. In case omitted the system tries to know the network-id via the 'interface' setting in this dhcp section, through consultation of /etc/config/network. Some IDs get assigned dynamically, are not provided by network, but still can be set here.
start	integer	yes	100	Specifies the offset from the network address of the underlying interface to calculate the minimum address that may be leased to clients. It may be greater than 255 to span subnets.
instance	dnsmasq instance	no	(none)	Dnsmasq instance to which the dhcp section is bound; if not specified the section is valid for all dnsmasq instances.
tag	list of tag names	no	(none)	List of tags that dnsmasq needs to match to use this dhcp-range.

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.

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.

Name	Type	Required	Default	Description
dhcp_option	list of strings	no	(none)	Additional options to be added for this network-id. If you specify this, you also need to specify the network-id.
filename	string	yes	(none)	The filename the host should request from the boot server.
networkid	string	no	(none)	The network-id these boot options should apply to. Applies to all clients if left unspecified.
serveraddress	string	yes	(none)	The IP address of the boot server.
servername	string	yes	(none)	The hostname of the boot server.
force	bool	no	(none)	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.
instance	dnsmasq instance	no	(none)	Dnsmasq instance to which the boot section is bound; if not specified the section is valid for all dnsmasq instances.

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.

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

DHCP needs UDP ports 67 and 68 open from your zone to/from the firewall. See configuration 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.

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'

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 parameter. 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 main is bound to the lan interface while the dnsmasq instance guest is bound to the guest interface:

config dnsmasq 'main'
	option domainneeded '1'
	option boguspriv '1'
	option filterwin2k '0'
	option localise_queries '1'
	option rebind_protection '1'
	option rebind_localhost '1'
	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'
	option nonwildcard '1'
	list interface 'lan'

config dnsmasq 'guest'
	option domainneeded '1'
	option boguspriv '1'
	option filterwin2k '0'
	option localise_queries '1'
	option rebind_protection '1'
	option rebind_localhost '1'
	option local '/guest/'
	option domain 'guest'
	option expandhosts '1'
	option nonegcache '0'
	option authoritative '1'
	option readethers '1'
	option leasefile '/tmp/dhcp.leases.guest'
	option resolvfile '/tmp/resolv.conf.guest'
	option strictorder '1'
	option nonwildcard '1'
	list interface 'guest'
	list notinterface 'lo'

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

config dhcp 'guest_private'
	option instance 'guest'
	option interface 'guest'
	option start '100'
	option limit '150'
	option leasetime '12h'
...

The web interface (luci) has not been updated to support multiple dnsmasq instances.

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@lede:~# 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

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

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.

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.