Backends

A server is a URI. Its scheme picks the backend, and its query string says only how to reach the server: the backend's connection settings below, nothing else. What to serve belongs to each reservation: a pydhcp DHCPLease, its options named as pydhcp names them (router, dns, bootfile-name, option-<n>) or by an alias, beside the BOOTP header fields siaddr (next-server), sname and file (the boot file), which are not options: each backend writes them as its server spells the fields.

Scheme	Server	How it applies a reservation	Install
dnsmasq://	dnsmasq	writes --dhcp-hostsfile / --dhcp-optsfile entries, local or over sftp://	nothing locally; pydhcpctl[ssh] for remote paths
kea://, keas://	ISC Kea	reservation-add / reservation-del via the control agent	pydhcpctl[kea]
dhcpd://	ISC dhcpd	an OMAPI host object	pydhcpctl[dhcpd]
windhcp://	Windows DHCP	its PowerShell cmdlets or netsh, locally, over ssh or WinRM	nothing locally; pydhcpctl[ssh] over ssh, pydhcpctl[winrm] for WinRM

What is easy to get wrong

• dnsmasq re-reads --dhcp-hostsfile / --dhcp-optsfile only on SIGHUP and never re-reads its main config. --dhcp-hostsdir / --dhcp-optsdir read a new file by themselves but only ever add: a removed or changed entry is served until SIGHUP. So a directory arms with no reload=, and removing or keeping a client signals pidfile= (or runs reload=, which a remote host needs); a plain file needs reload= every time. Options need optsfile=.
• Kea needs the host_cmds hook loaded and a writable hosts backend; a file-only Kea loads the hook and still refuses.
• dhcpd writes a host added over OMAPI to its leases file, which is what keeps it across a restart: a dhcpd started with a fresh leases file has lost it.
• Windows validates some option values as it stores them: a name server that does not answer is refused. It needs an account with DHCP-administrator rights and, for method=powershell, the DhcpServer module where the shell runs; method=netsh drives netsh dhcp server ... instead. transport= is how the shell is reached (local, ssh, winrm), method= what runs there, and server= the DHCP server it acts on — transport=local with server= is the ordinary RSAT shape. netsh exits 0 after dropping a value it dislikes, so the netsh method reads the state back and fails naming what did not land.

Conditions

A condition serves options only to clients that match a test — the iPXE chainload is the usual one: hand a PXE ROM the iPXE binary, and iPXE itself the script.

Backend	Construct	Created here?	Without the rights
windhcp://	a scope policy per client and condition (plus the class it references), and a base policy for the client; or, with conditions=shared, one policy per condition	yes, through the cmdlets — netsh has none	ConditionMissingError with the PowerShell to run once
dhcpd://	an inline if on the host, or a named group with conditions=group	yes	the recipe is a durable dhcpd.conf group
kea://	a client class of the client's own, plus one serving what it overrides when it does not match	with the class_cmds hook	ConditionMissingError with the client-classes JSON
dnsmasq://	—	no: the tag needs a dhcp-match line in dnsmasq's own config	ConditionUnsupportedError with the lines to add

Extra fragments

A reservation's extras (by phase: add, or remove for teardown), or a subclass overriding extras(), adds text no option models. It is emitted verbatim, after the modelled options:

Backend	A fragment is	Where it lands
dhcpd://	dhcpd config statements	the host object's statements
dnsmasq://	config lines	the options file for the client's tag
kea://	an option-data entry	the reservation's option-data
windhcp://	a PowerShell line, or {"Args": [...], "Ignore": false} for netsh	after the reservation and its options