banshee/active-directory-collection/active-directory/ssh/dhcp-failover

DHCP failover, by SSH

Returns, for each failover relationship of the DHCP Server of a Windows, its state as this server sees it, its partner and its mode. Two watches are proposed, both checked: a relationship out of its normal state, and a relationship that is no longer there.

Versions

Windows Server 2012 and later with the DHCP Server role and its module DhcpServer, and Windows PowerShell 3.0 or later, as the prerequisites name them. Generic: no folder of version. That every release answers alike is assumed, not yet checked on a real DHCP Server.

How it works

The collector runs one PowerShell command that reads the IPv4 failover relationships of the local DHCP Server:

powershell -NoProfile -NonInteractive -Command "$ErrorActionPreference = 'Stop'; [Console]::OutputEncoding = New-Object System.Text.UTF8Encoding; ConvertTo-Json -Compress -Depth 4 -InputObject @{relationships = @(Get-DhcpServerv4Failover | ForEach-Object { @{name = $_.Name; state = [string]$_.State; partner = $_.PartnerServer; mode = [string]$_.Mode} })}"

Get-DhcpServerv4Failover, without a name, returns every failover relationship of the server (Microsoft, DhcpServer module). Its objects are those of the WMI class DhcpServerv4Failover (namespace root\Microsoft\Windows\DHCP), whose properties Microsoft documents: Name, PartnerServer, Mode (HotStandby or LoadBalance) and State.
A relationship is named by its name. Its state is returned as Microsoft writes it: NoState, Normal, Init, CommunicationInterrupted, PartnerDown, PotentialConflict, Startup, ResolutionInterrupted, ConflictDone, Recover, RecoverWait, RecoverDone.
No failover relationship stops the collector as misconfigured: a server without one has nothing to watch here. An answer that is not the expected JSON stops it as misconfigured too.

Nothing is written to the Windows: the command only reads.

Prerequisites

A Windows Server 2012 or later with the DHCP Server role and its PowerShell module (DhcpServer, installed with the management tools of the role), with Windows PowerShell 3.0 or later.
The OpenSSH server of Windows, installed and started, its default shell left at cmd.exe, the Windows firewall open to the poller on TCP 22, and the fingerprint of its host key set in protocol:ssh:fingerprint unless the Security settings trust the first SSH fingerprint presented: the steps are those of the windows/ssh/services collector of banshee/windows-collection.
An account for monitoring, without administrator rights, set in protocol:ssh:user, with its password in protocol:ssh:password or its key in protocol:ssh:private-key, member of the group "DHCP Users", which the role creates (of the domain on a domain controller, local elsewhere): Microsoft documents it as read-only access to the DHCP Server. Never "DHCP Administrators".

Outputs

Name	Label	Unit	Per instance
`state`	State	—	yes, per relationship
`partner`	Partner	—	yes, per relationship
`mode`	Mode	—	yes, per relationship

Watches

Name	Title	Kind	Defaults	Checked by default
`not-normal`	Failover not normal	expected value, on `state`	critical when the state is anything but `Normal` for 3 runs	yes
`missing`	Failover relationship missing	expected instances, on `state`	no relationship named: it raises nothing until names are set on the probe	no

A relationship goes through Startup and the Recover states on its way back to Normal: the three runs let it.

Limits

Each server sees the relationship from its side: watch both partners.
Not yet checked on a real DHCP Server: the execution; that "DHCP Users" is enough for the cmdlet, as it is for the DHCP console; that State and Mode come out of the cmdlet as the words Microsoft lists.