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If these settings are correct, move to checking configuration of the server and networking functions. To run diagnostics, it is necessary to know a MAC address of a client's device and have an opportunity to initiate providing a client with an address via DHCP.
Checking a DHCP server
To check DHCP server operation, analyse its logs.
The three main problems can be seen from logs:
- Discover is absent.
- Discover is present, Offer is absent.
- Discover and offer are present, Request is absent.
1. Discover from a client's device is absent in logs.
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In the file /etc/default/isc-dhcp-server all listened interfaces should be listed, for example:INTERFACES="eth0 eth1"
2. Discover is present in DHCP server logs, but Offer is absent.
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- one of the subnetworks used is not specified
- classes are specified incorrectly
- pool vacant addresses exhaustion
- failover problems
2.1 One of the subnetworks used is not specified
Subnetworks containing addresses of listened interfaces and a DHCP-relay (if used) should be specified in the file /etc/dhcp/dhcpd.conf. Even if addresses from these subnetworks are not allocated by the server, for example:
subnet 192.168.1.0 netmask 255.255.255.0 {}
2.2 Pool vacant addresses exhaustion
One of the common causes is a case when a server runs out of vacant addresses. To check it, use a guide: ISC DHCP server pool range usage monitoring
3 Discover and Offer are present in DHCP server logs, but Request is not.
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