DHCP Troubleshooting | Network Educator

DHCP Troubleshooting

  • The first step of fixing DHCP related problems is to make sure that there is no problem with the client, use ipconfig command to verify connectivity. If an address conflict occurred you will by warned of this by system tray warning popup as well as address conflict event in system log.
  • Dhcploc.exe can be used to locate DHCP servers including rogue servers, this utility is part of Windows support tools. For AD authorized servers only, use command netsh dhcp show server.
  • The repair button on the remote connection information screen performs these functions:
    • Broadcast DHCP Request message to renew the lease, if this computer is a DHCP client
    • Flush the arp cache, the same thing as arp -d
    • Flush NetBIOS cache, same as nbtstat -R
    • Flush DNS cache, same as ipconfig /flushdns
    • Register computer with WINS server, same as nbtstat -RR
    • Register computer with DNS server, same as ipconfig /registerdns
  • If the computer fails to connect to DHCP server make sure the network medium is up and the DHCP server is operational. Make sure the scope is active and that it still has leases available for its clients.
  • DHCP server knows from which scope to assign address by looking at the address of the 1542 compliant router added to the discovery packet sent out by the client computer (no extra IP added means local subnet)
  • If a client get an IP address from DHCP server, but it is from the wrong scope, verify with dhcploc utility presence of competing DHCP servers. Make sure all authorized servers are leasing from non-overlapping ranges. A single DHCP server can have multiple scopes active on it, scope not native to DHCP server’s subnet are used for remote clients. DHCP matches remote clients to their scope when RFC-1542 compliant router or DHCP relay agent is properly configured. The DHCP Request message contains field named ‘Giaaddr’ which contains originating subnet, when it is empty client is assumed local and assigned address from local scope.
  • For a server to hand out addresses it must be on the same subnet as its clients and DHCP service must be bound to the connection, this is checked from advanced tab in server properties.
  • Make sure scope is active and that scope’s network ID matches that of DHCP server. Also, through it sounds trivial, make sure DHCP server has some addresses available for a lease. To accommodate more users you can simply shorten the lease duration. Don’t forget static addresses exclusions and reserved addresses
  • If the problem lies within the DHCP database, you will need to reconcile the DHCP data for one or all scopes. The data is stored in detailed and summary form on DHCP server, when reconciling the data in these two forms is compared.
  • You can also use the jetpack utility to perform database compaction or use netsh dhcp server set databaserestoreflag 1
  • When the administrator needs to renew IP addresses on few computers he can issue command ipconfig /renew on each one of them, in the case there are more computers, it is easier to just re-boot them using shutdown /i command line utility (show nice GUI interface).
  • To get a mac address only quickly and of any computer, including remote PCs, use getmac /s /v [server name] command line utility