How to Manually Clear DHCP Leases on Ubuntu DHCP Server for VM Testing Environments

During intensive VM testing cycles with rapid provisioning and teardown (especially with tools like Puppet, Chef, or Ansible), DHCP leases can accumulate quickly. Unlike Windows systems, Linux VMs (particularly Ubuntu) don't automatically send DHCPRELEASE packets when shut down. With default lease times of 7 days, this leads to IP exhaustion - exactly what you're experiencing.

On Ubuntu systems running dhcpd, leases are stored in:

/var/lib/dhcp/dhcpd.leases

This is a plain text file containing all active leases. Before modifying, create a backup:

sudo cp /var/lib/dhcp/dhcpd.leases /var/lib/dhcp/dhcpd.leases.bak

Method 1: Direct File Editing (For Precise Control)

1. Stop the DHCP server:

sudo service isc-dhcp-server stop

2. Open the leases file with root privileges:

sudo nano /var/lib/dhcp/dhcpd.leases

3. Locate leases by client identifier or MAC address. A lease block looks like:

lease 192.168.1.105 {
  starts 5 2024/02/16 14:32:15;
  ends 5 2024/02/23 14:32:15;
  hardware ethernet 00:0c:29:3f:5e:2d;
  uid "\001\000\014)?^";
  client-hostname "test-vm-047";
}

4. Delete the entire lease block (from 'lease' to closing brace)

5. Restart DHCP server:

sudo service isc-dhcp-server start

Method 2: Using dhcpd Command Line

For individual lease removal without stopping service:

echo "lease 192.168.1.105 { }" | sudo tee -a /var/lib/dhcp/dhcpd.leases
sudo service isc-dhcp-server restart

When dealing with hundreds of test VMs, use this Python script to clean stale leases:

#!/usr/bin/python3
import re
from datetime import datetime, timedelta

LEASE_FILE = '/var/lib/dhcp/dhcpd.leases'
BACKUP_FILE = '/var/lib/dhcp/dhcpd.leases.bak'
MAX_AGE_DAYS = 1  # Delete leases older than this

def clean_leases():
    with open(LEASE_FILE, 'r') as f:
        content = f.read()
    
    # Make backup
    with open(BACKUP_FILE, 'w') as f:
        f.write(content)
    
    leases = re.findall(r'(lease .*?\{.*?\})', content, re.DOTALL)
    current_time = datetime.now()
    
    new_content = []
    for lease in leases:
        match = re.search(r'ends (\d) (\d{4}/\d{2}/\d{2} \d{2}:\d{2}:\d{2});', lease)
        if match:
            weekday = int(match.group(1))
            end_time = datetime.strptime(match.group(2), '%Y/%m/%d %H:%M:%S')
            if (current_time - end_time) < timedelta(days=MAX_AGE_DAYS):
                new_content.append(lease)
    
    with open(LEASE_FILE, 'w') as f:
        f.write('\n'.join(new_content))

if __name__ == '__main__':
    clean_leases()
    print("Lease cleanup completed. Restart DHCP server to apply changes.")

1. Configure shorter lease times in /etc/dhcp/dhcpd.conf for test subnets:

subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.100 192.168.1.200;
    default-lease-time 600;  # 10 minutes
    max-lease-time 3600;     # 1 hour
    option routers 192.168.1.1;
}

2. Force DHCP release in VM shutdown scripts (/etc/init.d/halt):

/sbin/dhclient -r

After cleaning, check available leases with:

dhcp-lease-list

Or parse the lease file directly:

grep -c "lease " /var/lib/dhcp/dhcpd.leases

During intensive VM provisioning tests (particularly with Ubuntu systems), we often encounter DHCP lease retention issues. The default Ubuntu behavior doesn't release DHCP leases upon VM termination, and with a typical 7-day lease duration, this quickly leads to IP exhaustion. Here's how to surgically remove stale leases without disrupting production systems.

On Ubuntu with dhcpd, leases are stored in:

/var/lib/dhcp/dhcpd.leases

This human-readable file contains all active leases with timestamps and client identifiers.

Use this command to find leases from your test VMs:

grep -A 5 "your-test-vm-prefix" /var/lib/dhcp/dhcpd.leases | less

Or for all inactive leases:

grep -B 5 "abandoned" /var/lib/dhcp/dhcpd.leases

1. First, back up the lease file:

sudo cp /var/lib/dhcp/dhcpd.leases /var/lib/dhcp/dhcpd.leases.bak

2. Edit the leases file (use caution - corrupting this can break DHCP):

sudo nano /var/lib/dhcp/dhcpd.leases

3. Find the lease block to remove (starts with 'lease' and ends with '}'). For example:

lease 192.168.1.105 {
  starts 5 2023/11/17 14:02:35;
  ends 5 2023/11/24 14:02:35;
  abandoned;
  hardware ethernet 00:0c:29:4f:8e:39;
  uid "\001\000\014)O\2179";
}

4. Delete the entire lease block (from 'lease' to closing '}')

After editing, restart dhcpd:

sudo service isc-dhcp-server restart
# or for newer systems:
sudo systemctl restart isc-dhcp-server

For frequent testing, create a cleanup script:

#!/bin/bash
# Clean leases for test VMs with 'testvm-' in hostname
sudo sed -i '/lease/,/}/ { /testvm-/d }' /var/lib/dhcp/dhcpd.leases
sudo systemctl restart isc-dhcp-server

For test environments, consider these dhcpd.conf tweaks:

subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.200;
  default-lease-time 3600;  # 1 hour for tests
  max-lease-time 7200;      # 2 hours max
  option domain-name-servers 8.8.8.8;
}