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49 lines
2.0 KiB
Markdown
49 lines
2.0 KiB
Markdown
---
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title: WindowsDHCPScopeAlmostExhausted
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---
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## Meaning
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The enabled DHCP scope `scope` has fewer free addresses than the configured
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share of its address pool for 15 minutes. The warning fires below
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`dhcpScopeFreeWarningThreshold` (10 percent by default) and the critical alert
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below `dhcpScopeFreeCriticalThreshold` (2 percent); both fire below the
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critical threshold, so inhibit the warning in Alertmanager. The share divides
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`windows_dhcp_scope_addresses_free` by the sum of free and in-use addresses,
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and the alert only counts scopes whose `windows_dhcp_scope_state` is enabled.
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On a failover pair the free and in-use counts are the totals of both servers.
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The alert needs the `dhcp` collector.
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## Impact
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Once the scope is exhausted, new clients receive no address and cannot use
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the network. Existing clients keep their lease until it expires and then lose
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their address too.
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## Diagnosis
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1. Compare the free and in-use addresses of the scope over the last days to
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see whether the exhaustion is a trend or a sudden burst, and inspect
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`windows_dhcp_scope_reserved_address` and
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`windows_dhcp_scope_pending_offers` for reservations and offers in flight.
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2. On the server, read the scope and its leases:
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```powershell
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Get-DhcpServerv4ScopeStatistics -ScopeId <scope>
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Get-DhcpServerv4Scope -ScopeId <scope> | Select-Object ScopeId, SubnetMask, StartRange, EndRange, LeaseDuration, State
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Get-DhcpServerv4Lease -ScopeId <scope> | Group-Object AddressState
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Get-DhcpServerv4ExclusionRange -ScopeId <scope>
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```
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3. Look for a long lease duration on a network with many short-lived clients,
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large exclusion ranges, a flood of leases from one device that changes its
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MAC address, or a failover partner that holds the free addresses.
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## Mitigation
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Shorten the lease duration for networks with transient clients, remove
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exclusion ranges that are not needed, add a range through a superscope or a
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larger subnet, or reclaim bad leases. On a failover pair, check the split of
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the addresses between the partners. Confirm the free share stays above the
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threshold.
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