/etc/config/dhcp ，文件包含了DNS和DHCP服务的配置信息(DHCP和DNS服务都是通过 dnsmasq 实现的）。
在默认配置中，该文件包含一个 公共部分 来指定DNS和守护进程相关的选项以及一个或多个 DHCP池 ，以定义在网络接口上服务的DHCP。
下面定义了“DHCP”配置文件可能存在的节点类型。可能并非所有类型都出现在文件中，实际上，很多类型只适用于特殊配置。常见的节点类型有 普通选项 ， DHCP池 和 静态租赁 。
dnsmasq 类型的节点包含了全局设置相关的键值，该设置生效于所有dnsmasq实例。下表列出了所有可用键值、各键的默认值以及其相应的 dnsmasq 命令行样式。详情见dnsmasq手册页。
root@lede:/# uci show dhcp dhcp.@dnsmasq=dnsmasq dhcp.@dnsmasq.domainneeded='1' dhcp.@dnsmasq.boguspriv='1' dhcp.@dnsmasq.filterwin2k='0' dhcp.@dnsmasq.localise_queries='1' dhcp.@dnsmasq.rebind_protection='1' dhcp.@dnsmasq.rebind_localhost='1' dhcp.@dnsmasq.local='/lan/' dhcp.@dnsmasq.domain='lan' dhcp.@dnsmasq.expandhosts='1' dhcp.@dnsmasq.nonegcache='0' dhcp.@dnsmasq.authoritative='1' dhcp.@dnsmasq.readethers='1' dhcp.@dnsmasq.leasefile='/tmp/dhcp.leases' dhcp.@dnsmasq.resolvfile='/tmp/resolv.conf.auto' dhcp.@dnsmasq.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' ...
domainenable dnsmasq to serve entries in
/etc/hostsas well as the DHCP client's names as if they were entered into the lan DNS domain.
expandhostsmake sure that requests for these local host names (and the reverse lookup) never get forwarded to the upstream DNS servers.
authoritativemakes the router the only DHCP server on this network; clients get their IP lease a lot faster this way.
leasefilestores the leases in a file, so that they can be picked up again if dnsmasq is restarted.
resolvfiletells dnsmasq to use this file to find upstream name servers; it gets created by the WAN DHCP client or the PPP client.
| ||boolean|| ||Add the local domain as search directive in resolv.conf.|
| ||boolean|| || Add A, AAAA, and PTR records for this router only on DHCP served LAN.
enhanced function available on Trunk with option
| ||integer|| || Add A, AAAA, and PTR records for this router only on DHCP served LAN. 0 - Disable. 1 - Hostname on Primary Address. 2 - Hostname on All Addresses. 3 - FDQN on All Addresses. 4 -
| ||integer|| || Labels WAN interfaces like
| ||list of file paths||(none)|| ||Additional host files to read for serving DNS responses|
| ||boolean|| || ||Force dnsmasq into authoritative mode. This speeds up DHCP leasing. Used if this is the only server on the network|
| ||list of IP addresses||(none)|| ||IP addresses to convert into NXDOMAIN responses (to counteract “helpful” upstream DNS servers that never return NXDOMAIN).|
| ||boolean|| || || Reject reverse lookups to private IP ranges where no corresponding entry exists in
| ||boolean|| || When set to
| ||integer|| || ||Size of dnsmasq query cache.|
| ||boolean|| || || Enable DBus messaging for dnsmasq.
Standard builds of dnsmasq do not include DBus support.
| ||string||(none)|| Specifies BOOTP options, in most cases just the file name. You can also use: “
| ||file path||(none)|| ||Specify an external file with per host DHCP options|
| ||integer|| || ||Maximum number of DHCP leases|
| ||integer|| || ||Maximum number of concurrent connections|
| ||domain name||(none)|| ||DNS domain handed out to DHCP clients|
| ||boolean|| || ||Tells dnsmasq never to forward queries for plain names, without dots or domain parts, to upstream nameservers. If the name is not known from /etc/hosts or DHCP then a “not found” answer is returned|
| ||boolean|| || || Validate DNS replies and cache DNSSEC data.
Requires the dnsmasq-full package.
| ||boolean|| || || Check the zones of unsigned replies to ensure that unsigned replies are allowed in those zones. This protects against an attacker forging unsigned replies for signed DNS zones, but is slower and requires that the nameservers upstream of dnsmasq are DNSSEC-capable.
Requires the dnsmasq-full package.
Caution: If you use this option on a device that doesn't have a hardware clock, dns resolution may break after a reboot of the device due to an incorrect system time.
| ||integer|| || ||Specify the largest EDNS.0 UDP packet which is supported by the DNS forwarder|
| ||boolean|| || ||Enable the builtin TFTP server|
| ||boolean|| || || Add the local domain part to names found in
| ||boolean|| || ||Do not forward requests that cannot be answered by public name servers|
| ||boolean|| || || Do not resolve unqualifed local hostnames. Needs
| ||list of interface names||(all interfaces)|| || List of interfaces to listen on. If unspecified, dnsmasq will listen to all interfaces except those listed in
| ||file path||(none)|| ||Store DHCP leases in this file|
| ||string||(none)|| || Look up DNS entries for this domain from
| ||boolean|| || || Choose IP address to match the incoming interface if multiple addresses are assigned to a host name in
| ||boolean|| || ||Accept DNS queries only from hosts whose address is on a local subnet, ie a subnet for which an interface exists on the server.|
| ||boolean|| || ||Log the results of DNS queries, dump cache on SIGUSR1|
| ||boolean|| || ||Don't daemonize the dnsmasq process|
| ||boolean|| || || Don't read DNS names from
| ||boolean|| || ||Disable caching of negative “no such domain” responses|
| ||boolean|| || || Don't read upstream servers from
| ||list of interface names||(none)|| ||Interfaces dnsmasq should not listen on.|
| ||boolean|| || ||Bind only configured interface addresses, instead of the wildcard address.|
| ||port number|| || || Listening port for DNS queries, disables DNS server functionality if set to
| ||integer||(none)|| ||Use a fixed port for outbound DNS queries|
| ||boolean|| || || Read static lease entries from
| ||boolean|| || ||Enables DNS rebind attack protection by discarding upstream RFC1918 responses|
| ||boolean|| || ||Allows upstream 127.0.0.0/8 responses, required for DNS based blacklist services, only takes effect if rebind protection is enabled|
| ||list of domain names||(none)|| ||List of domains to allow RFC1918 responses for, only takes effect if rebind protection is enabled|
| ||file path|| || ||Specifies an alternative resolv file|
| ||list of strings||(none)|| ||List of DNS servers to forward requests to. See the dnsmasq man page for syntax details.|
| ||list of strings||(none)|| ||List of network range with a DNS server to forward reverse DNS requests to. See the dnsmasq man page for syntax details.|
| ||boolean|| || || Obey order of DNS servers in
| ||directory path||(none)|| ||Specifies the TFTP root directory|
| ||integer|| || ||Dnsmasq picks random ports as source for outbound queries. When this option is given, the ports used will always be larger than or equal to the specified minport value (min valid value 1024). Useful for systems behind firewalls.|
| ||integer|| || ||Dnsmasq picks random ports as source for outbound queries. When this option is given, the ports used will always be smaller than or equal to the specified maxport value (max valid value 65535). Useful for systems behind firewalls.|
| ||boolean|| || ||By default dnsmasq checks if an IPv4 address is in use before allocating it to a host by sending ICMP echo request (aka ping) to the address in question. This parameter allows to disable this check.|
| ||boolean|| || ||By default, when dnsmasq has more than one upstream server available, it will send queries to just one server. Setting this parameter forces dnsmasq to send all queries to all available servers. The reply from the server which answers first will be returned to the original requeser.|
| ||boolean|| || ||Suppress logging of the routine operation of DHCP. Errors and problems will still be logged|
| ||boolean|| || ||Dnsmasq is designed to choose IP addresses for DHCP clients using a hash of the client's MAC address. This normally allows a client's address to remain stable long-term, even if the client sometimes allows its DHCP lease to expire. In this default mode IP addresses are distributed pseudo-randomly over the entire available address range. There are sometimes circumstances (typically server deployment) where it is more convenient to have IP addresses allocated sequentially, starting from the lowest available address, and setting this parameter enables this mode. Note that in the sequential mode, clients which allow a lease to expire are much more likely to move IP address; for this reason it should not be generally used.|
| ||[0,1,base64,text]|| || || Add the MAC address of the requester to DNS queries which are forwarded upstream; this may be used to do DNS filtering by the upstream server.
The MAC address can only be added if the requester is on the same subnet as the dnsmasq server. Note that the mechanism used to achieve this (an EDNS0 option) is not yet standardised, so this should be considered experimental. Also note that exposing MAC addresses in this way may have security and privacy implications.
| ||boolean|| || ||Enables extra DHCP logging; logs all the options sent to the DHCP clients and the tags used to determine them|
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
lanspecifies the interface that is served by this DHCP pool
100is the offset from the network address, in the default configuration this would mean start leasing addresses from
150is the maximum number of addresses that may be leased, in the default configuration this would mean leasing addresses up to
12hspecifies the time to live for handed out leases, twelve hours in this example
serverdefines the mode for IPv6 configuration (RA & DHCPv6)
Below is a listing of legal options for
| ||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.|
| ||list of strings||no||(none)|| Exactly the same as
| ||boolean||no|| || Dynamically allocate client addresses, if set to
| ||boolean||no|| ||Forces DHCP serving on the specified interface even if another DHCP server is detected on the same network segment|
| ||boolean||no|| || Specifies whether dnsmasq should ignore this pool if set to
| ||string||no|| || Specifies whether DHCPv4 server should be enabled
| ||string||no|| || Specifies whether DHCPv6 server should be enabled (
| ||string||no|| || Specifies whether Router Advertisements should be enabled (
| ||integer||no|| || Default router lifetime in the RA message will be set if default route is present and a global IPv6 address (
| ||integer||no|| || RA management mode : no M-Flag but A-Flag (
| ||bool||no|| || Announce prefixes as offlink (
| ||string||no|| || Announce routes with either high (
| ||integer||no|| ||Minimum time interval between RAs (in seconds)|
| ||integer||no|| ||Maximum time interval between RAs (in seconds)|
| ||integer||no|| ||Advertised router lifetime (in seconds)|
| ||bool||no|| ||Limit the preferred and valid lifetimes of the prefixes in the RA messages to the configured DHCP leasetime|
| ||integer||no|| || Advertised current hop limit
| ||integer||no|| || Advertised reachable time (in milliseconds)
| ||integer||no|| || Advertised NS retransmission time (in milliseconds)
| ||integer||no|| ||Maximum advertised MTU|
| ||string||no|| || Specifies whether NDP should be relayed
| ||bool||no|| ||Learn routes from NDP|
| ||bool||no|| || Ignore neighbor messages on slave enabled (
| ||boolean||no||0||Specifies whether DHCPv6, RA and NDP in relay mode is a master interface or not.|
| ||logical interface name||yes||(none)|| Specifies the interface associated with this DHCP address pool; must be one of the interfaces defined in
| ||string||yes|| || Specifies the lease time of addresses handed out to clients, for example
| ||integer||yes|| ||Specifies the size of the address pool (e.g. with start=100, limit=150, maximum address will be .249)|
| ||string||no|| (value of ||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.|
| ||integer||yes|| ||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.|
| ||dnsmasq instance||no||(none)||Dnsmasq instance to which the dhcp section is bound; if not specified the section is valid for all dnsmasq instances.|
| ||list of tag names||no||(none)||List of tags that dnsmasq needs to match to use this dhcp-range.|
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.
| ||string||yes||(none)||'ignore' or the IP address to be used for this host.|
| ||string||no||(none)||The hardware address(es) of this host, separated by commas.|
| ||string||no||(none)||The IPv6 interface identifier (address suffix) as hexadecimal number (max. 8 chars)|
| ||string||no||(none)||The DHCPv6-DUID of this host.|
| ||string||no||(none)||Optional hostname to assign.|
| ||string||no||(none)||Set the given tag for matching hosts.|
| ||boolean||no|| ||Add static forward and reverse DNS entries for this host.|
| ||boolean||no|| ||Force broadcast DHCP response.|
| ||string||no||(none)||Host-specific lease time, e.g. 2m, 3h, 5d. Note: introduced by r48801 in trunk|
| ||dnsmasq instance||no||(none)||Dnsmasq instance to which the host section is bound; if not specified the section is valid for all dnsmasq instances.|
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
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.
| ||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.|
| ||string||yes||(none)||The filename the host should request from the boot server.|
| ||string||no||(none)||The network-id these boot options should apply to. Applies to all clients if left unspecified.|
| ||string||yes||(none)||The IP address of the boot server.|
| ||string||yes||(none)||The hostname of the boot server.|
| ||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.|
| ||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:
option classifier 'value'
option networkid 'network-id'
list dhcp_option 'DHCP-option'
classifier can be one of these values:
| ||Hardware address of the client|
| ||String sent by the client representing the vendor of the client. dnsmasq performs a substring match on the vendor class string using this value.|
| ||String sent by the client representing the user of the client. dnsmasq performs a substring match on the user class string using this value.|
| ||Matches the circuit ID as sent by the relay agent, as defined in RFC3046.|
| ||Matches the remote ID as sent by the relay agent, as defined in RFC3046.|
| ||Matches the subscriber ID as sent by the relay agent, as defined in RFC3993.|
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
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
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 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
00:11:22:33:44:55 (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.
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'
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
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.
config dhcp lan option interface lan option start 1441793 option limit 253
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 '6,22.214.171.124,126.96.36.199' # 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 "6,188.8.131.52,6,184.108.40.206" 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.