Addresses allocated like this are not constrained to be in the range given by the --dhcp-range option, but they must be in the same subnet as some valid dhcp-range. For subnets which don't need a pool of dynamically allocated addresses, use the "static" keyword in the --dhcp-range declaration.

It is allowed to use client identifiers (called client DUID in IPv6-land) rather than hardware addresses to identify hosts by prefixing with 'id:'. Thus: --dhcp-host=id:01:02:03:04,..... refers to the host with client identifier 01:02:03:04. It is also allowed to specify the client ID as text, like this: --dhcp-host=id:clientidastext,.....

A single --dhcp-host may contain an IPv4 address or one or more IPv6 addresses, or both. IPv6 addresses must be bracketed by square brackets thus: --dhcp-host=laptop,[1234::56] IPv6 addresses may contain only the host-identifier part: --dhcp-host=laptop,[::56] in which case they act as wildcards in constructed DHCP ranges, with the appropriate network part inserted. For IPv6, an address may include a prefix length: --dhcp-host=laptop,[1234:50/126] which (in this case) specifies four addresses, 1234::50 to 1234::53. This (an the ability to specify multiple addresses) is useful when a host presents either a consistent name or hardware-ID, but varying DUIDs, since it allows dnsmasq to honour the static address allocation but assign a different address for each DUID. This typically occurs when chain netbooting, as each stage of the chain gets in turn allocates an address.

Note that in IPv6 DHCP, the hardware address may not be available, though it normally is for direct-connected clients, or clients using DHCP relays which support RFC 6939.

For DHCPv4, the special option id:* means "ignore any client-id and use MAC addresses only." This is useful when a client presents a client-id sometimes but not others.

If a name appears in /etc/hosts, the associated address can be allocated to a DHCP lease, but only if a --dhcp-host option specifying the name also exists. Only one hostname can be given in a --dhcp-host option, but aliases are possible by using CNAMEs. (See --cname ). Note that /etc/hosts is NOT used when the DNS server side of dnsmasq is disabled by setting the DNS server port to zero.

More than one --dhcp-host can be associated (by name, hardware address or UID) with a host. Which one is used (and therefore which address is allocated by DHCP and appears in the DNS) depends on the subnet on which the host last obtained a DHCP lease: the --dhcp-host with an address within the subnet is used. If more than one address is within the subnet, the result is undefined. A corollary to this is that the name associated with a host using --dhcp-host does not appear in the DNS until the host obtains a DHCP lease.

The special keyword ignore tells dnsmasq never to offer a DHCP lease to a machine. The machine can be specified by hardware address, client ID or hostname. For example: --dhcp-host=00:20:e0:3b:13:af,ignore. This can be useful when there is another DHCP server on the network which should be used by some machines.

The set:<tag> construct sets the tag whenever this --dhcp-host directive is in use. This can be used to selectively send DHCP options just for this host. More than one tag can be set in a --dhcp-host directive (but not in other places where set:<tag> is allowed). When a host matches any --dhcp-host directive (or one implied by /etc/ethers) then the special tag known is set. This allows dnsmasq to be configured to ignore requests from unknown machines using --dhcp-ignore=tag:!known. If the host matches only a --dhcp-host directive which cannot be used because it specifies an address on a different subnet, the tag known-othernet is set.

The tag:<tag> construct filters which dhcp-host directives are used. More than one tag can be provided. In this case, the request must match all tags. Tagged directives are used in preference to untagged ones. Note that one of <hwaddr>, <client_id> or <hostname> still must be specified (can be a wildcard).

Ethernet addresses (but not client-ids) may have wildcard bytes, so for example --dhcp-host=00:20:e0:3b:13:*,ignore will cause dnsmasq to ignore the given range of hardware addresses. Note that the "*" will need to be escaped or quoted on a command line, but not in the configuration file.

Hardware addresses normally match any network (ARP) type, but it is possible to restrict them to a single ARP type by preceding them with the ARP-type (in HEX) and "-". so --dhcp-host=06-00:20:e0:3b:13:af,1.2.3.4 will only match a Token-Ring hardware address, since the ARP-address type for token ring is 6.

As a special case, in DHCPv4, it is possible to include more than one hardware address. eg: --dhcp-host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.2. This allows an IP address to be associated with multiple hardware addresses, and gives dnsmasq permission to abandon a DHCP lease to one of the hardware addresses when another one asks for a lease. Beware that this is a dangerous thing to do: it will only work reliably if only one of the hardware addresses is active at any time and there is no way for dnsmasq to enforce this. It is, for instance, useful to allocate a stable IP address to a laptop which has both wired and wireless interfaces.