DHCP Server not Leasing IP to LAN PC

1. Check LAN PC Connectivity

• Verify the cabling between the LAN PC, CPE, and switch.
• Check the NIC status on both the client and server.

2. Verify DHCP Server Configuration

• Ensure the subnet is assigned to the correct interface.
• Check default/max lease times, router, and gateway configuration.
• Verify if a static IP is assigned to the client’s MAC address.
• Review the DHCP leases page to see if other LAN PCs have been assigned IPs.
• Check if the DHCP pool is full due to maximum assigned clients.

3. Verify DHCP Server Process on the CPE

Log in to the CPE console and run the following command:

ps ax | grep dhcpd

Expected output if the DHCP process is running:

        bash-5.1# ps ax | grep dhcpd
        187346 ?        Ssl    0:32 dhcpd -cf /var/register/system/dhcp/dhcpd.conf -lf /var/register/system/dhcp/dhcpd.leases -q

4. Capture DHCP Requests on CPE using tcpdump

Run the following command to check if the DHCP server is receiving requests:

tcpdump -i <interface name> port bootps

A successful capture should show a sequence of Discover, Offer, Request, and ACK messages.

        bash-5.1# tcpdump -i ETH01 port bootps -nvvv
        09:51:04.076908 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request
        09:51:05.141651 IP 172.5.0.1.67 > 172.5.0.101.68: BOOTP/DHCP, Reply (Offer)
        09:51:05.143519 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request
        09:51:05.146570 IP 172.5.0.1.67 > 172.5.0.101.68: BOOTP/DHCP, Reply (ACK)

5. Verify and Debug Client-side DHCP Requests

If the LAN PC is running Linux, check if the DHCP client is active:

ps ax | grep dhclient

To restart DHCP client with verbose mode for debugging:

dhclient -i <interface name> -v

Expected output for a successful request:

        root@CPE5_L1:/home/router# dhclient -i eth1 -v
        Internet Systems Consortium DHCP Client 4.3.5
        Listening on LPF/eth1/02:00:ac:01:05:01
        Sending DHCPDISCOVER on eth1
        DHCPOFFER from 172.5.0.1
        DHCPREQUEST for 172.5.0.101
        DHCPACK from 172.5.0.1