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Dhcp

Chapter 11: DHCP - Complete Deep Dive

Section titled “Chapter 11: DHCP - Complete Deep Dive”

DHCP automates IP address assignment. This comprehensive chapter covers everything about DHCP.

11.1 Introduction to DHCP

Section titled “11.1 Introduction to DHCP”

DHCP (Dynamic Host Configuration Protocol) automatically assigns network configuration to devices.

+------------------------------------------------------------------+
|                   DHCP Overview                                        |
+------------------------------------------------------------------+


Why DHCP?
+------------------------------------------------------------------+


Manual Configuration Problems:
+------------------------------------------------------------------+


1. Time-consuming for many devices
2. IP conflicts possible
3. Difficult to change network settings
4. No centralized tracking


DHCP Solutions:
+------------------------------------------------------------------+


1. Centralized management
2. Automatic IP assignment
3. Easy to change network settings
4. Prevents IP conflicts
5. Efficient use of addresses


What DHCP Provides:
+------------------------------------------------------------------+


1. IP Address
2. Subnet Mask
3. Default Gateway (Router)
4. DNS Servers (Primary and Secondary)
5. Domain Name
6. Lease Time
7. Additional Options (NTP, WINS, etc.)


+------------------------------------------------------------------+

11.2 DHCP Process (DORA)

Section titled “11.2 DHCP Process (DORA)”

DHCP uses a four-step process called DORA.

+------------------------------------------------------------------+
|                   DORA Process Deep Dive                               |
+------------------------------------------------------------------+


DHCP Four-Way Handshake:
+------------------------------------------------------------------+


Step 1: DISCOVER (Client -> Server)
+------------------------------------------------------------------+


Client broadcasts: "Is there a DHCP server?"


Packet details:
+------------------------------------------------------------------+
| Field          | Value                           |
|----------------|--------------------------------|
| Source IP      | 0.0.0.0                        |
| Source Port    | 68                              |
| Dest IP        | 255.255.255.255                |
| Dest Port      | 67                              |
| Transaction ID | Random number                   |
| Message Type   | DHCP DISCOVER                  |
+------------------------------------------------------------------+


Step 2: OFFER (Server -> Client)
+------------------------------------------------------------------+


Server responds with available IP


Packet details:
+------------------------------------------------------------------+
| Field          | Value                           |
|----------------|--------------------------------|
| Your IP        | 192.168.1.100 (offered)      |
| Subnet Mask    | 255.255.255.0                  |
| Router         | 192.168.1.1                    |
| DNS            | 8.8.8.8                        |
| Lease Time     | 86400 seconds                  |
| Message Type   | DHCP OFFER                     |
+------------------------------------------------------------------+


Step 3: REQUEST (Client -> Server)
+------------------------------------------------------------------+


Client requests the offered IP


Packet details:
+------------------------------------------------------------------+
| Field          | Value                           |
|----------------|--------------------------------|
| Requested IP   | 192.168.1.100                 |
| Message Type   | DHCP REQUEST                   |
+------------------------------------------------------------------+


Step 4: ACK (Server -> Client)
+------------------------------------------------------------------+


Server confirms the lease


Packet details:
+------------------------------------------------------------------+
| Field          | Value                           |
|----------------|--------------------------------|
| Your IP        | 192.168.1.100                 |
| Lease Time     | 86400 seconds                  |
| Message Type   | DHCP ACK                       |
+------------------------------------------------------------------+


Complete Flow Diagram:
+------------------------------------------------------------------+


    Client                           Server
      |                                |
      |------- DHCP DISCOVER ------> |
      |    (broadcast)                |
      |                               |
      |<------ DHCP OFFER ----------- |
      |    (offer IP)                |
      |                               |
      |------- DHCP REQUEST --------> |
      |    (request IP)             |
      |                               |
      |<------ DHCP ACK ------------- |
      |    (confirmation)           |
      |                               |
      |====== CONFIGURED ========== |


DHCP Nak (Negative Acknowledgment):
+------------------------------------------------------------------+


If server cannot fulfill request:
+------------------------------------------------------------------+


Server -> Client: DHCP NAK
+------------------------------------------------------------------+


Example: Client requested IP that server no longer has


Client then starts over with DISCOVER


+------------------------------------------------------------------+

11.3 DHCP Message Types

Section titled “11.3 DHCP Message Types”
+------------------------------------------------------------------+
|                   DHCP Message Types                                   |
+------------------------------------------------------------------+


DHCP Messages:
+------------------------------------------------------------------+


| Type | Value | Direction   | Description                    |
|------|-------|-------------|--------------------------------|
| DHCPDISCOVER | 1 | Client->Server | Find available servers     |
| DHCPOFFER    | 2 | Server->Client | Offer available IP        |
| DHCPREQUEST   | 3 | Client->Server | Request offered IP       |
| DHCPDECLINE  | 4 | Client->Server | IP already in use       |
| DHCPACK      | 5 | Server->Client | Confirm lease            |
| DHCPNAK      | 6 | Server->Client | Request rejected         |
| DHCPRELEASE  | 7 | Client->Server | Release IP early        |
| DHCPINFORM   | 8 | Client->Server | Client has IP, need config|


DHCP Packet Format:
+------------------------------------------------------------------+


+------------------------------------------------------------------+
| Field             | Size      | Description                       |
|-------------------|-----------|-----------------------------------|
| Operation         | 1 byte   | 1=request, 2=reply             |
| Hardware Type     | 1 byte   | 1=Ethernet                       |
| Hardware Length  | 1 byte   | 6 for Ethernet                   |
| Hops             | 1 byte   | Relay agent hops                 |
| Transaction ID    | 4 bytes  | Matches requests/responses       |
| Seconds          | 2 bytes   | Time since request started       |
| Flags            | 2 bytes   | Broadcast/unicast flags         |
| Client IP        | 4 bytes  | Client IP (if known)            |
| Your IP          | 4 bytes  | IP being offered                |
| Server IP        | 4 bytes  | Server IP                       |
| Gateway IP       | 4 bytes  | Relay agent IP                  |
| Client Hardware  | 16 bytes | MAC address                     |
| Server Hostname  | 64 bytes | Server name                      |
| Boot Filename    | 128 bytes| Boot file name                   |
| Options          | Variable  | DHCP options                     |
+------------------------------------------------------------------+


DHCP Options (Common):
+------------------------------------------------------------------+


| Option | Code | Description                    | Example            |
|--------|------|--------------------------------|---------------------|
| Subnet Mask | 1  | Network mask             | 255.255.255.0     |
| Router      | 3  | Default gateway         | 192.168.1.1       |
| DNS         | 6  | Name servers            | 8.8.8.8           |
| Host Name   | 12 | Client hostname         | client-pc          |
| Domain Name | 15 | Domain suffix           | example.com       |
| Broadcast   | 28 | Broadcast address       | 192.168.1.255    |
| NTP Server  | 42 | Time servers            | time.example.com   |
| Lease Time  | 51 | Lease duration          | 86400 seconds     |
| Message Type| 53 | DHCP message type       | 1-8                |
| Server ID   | 54 | DHCP server identifier | 192.168.1.1       |
| Parameter Req| 55 | Parameter request list | 1,3,6,15,28      |
| Renewal T1  | 58 | Renewal time            | 43200 seconds     |
| Rebinding T2| 59 | Rebinding time         | 75600 seconds     |


+------------------------------------------------------------------+

11.4 DHCP Lease Process

Section titled “11.4 DHCP Lease Process”
+------------------------------------------------------------------+
|                   DHCP Lease Lifecycle                                |
+------------------------------------------------------------------+


Lease Timeline:
+------------------------------------------------------------------+


T0: IP Assigned
     |
     | 50% of lease time (T1)
     v
T1: Renewal Attempt
     |
     | If ACK: Lease renewed
     | If no response: Continue using IP
     |
     | 87.5% of lease time (T2)
     v
T2: Rebinding
     |
     | Broadcast to ANY server
     | If ACK: Continue
     | If NAK or no response: Start DORA
     |
     | 100% of lease time
     v
T3: Lease Expired
     |
     | Must stop using IP
     | Start DORA process


Lease Renewal States:
+------------------------------------------------------------------+


1. BOUND:
   +------------------------------------------------------------------+
   | IP address successfully assigned                                |
   | Can use IP for duration of lease                              |
   +------------------------------------------------------------------+


2. RENEWING:
   +------------------------------------------------------------------+
   | T1 reached (50% of lease)                                    |
   | Unicast to server to renew                                   |
   | Can continue using IP while waiting                          |
   +------------------------------------------------------------------+


3. REBINDING:
   +------------------------------------------------------------------+
   | T2 reached (87.5% of lease)                                 |
   | Broadcast to any DHCP server                                  |
   | Last attempt before expiration                                |
   +------------------------------------------------------------------+


4. REBOOTING:
   +------------------------------------------------------------------+
   | Lease expired or client restarted                             |
   | Broadcast REQUEST for previously assigned IP                  |
   | If NAK, start DORA                                          |
   +------------------------------------------------------------------+


Default Lease Times:
+------------------------------------------------------------------+


| Service          | Default Lease | Maximum Lease |
|-----------------|---------------|---------------|
| Home router     | 24 hours     | 24 hours     |
| Enterprise      | 8 hours      | 24 hours     |
| Server          | 24 hours     | 7 days       |
| Mobile device   | 24 hours     | 7 days       |


+------------------------------------------------------------------+

11.5 DHCP Configuration

Section titled “11.5 DHCP Configuration”
+------------------------------------------------------------------+
|                   DHCP Server Configuration                           |
+------------------------------------------------------------------+


ISC DHCP Server (dhcpd):
+------------------------------------------------------------------+


# /etc/dhcpd.conf


# Global settings
default-lease-time 86400;        # 1 day
max-lease-time 172800;          # 2 days
authoritative;                  # This is authoritative server


# DNS settings
option domain-name "example.com";
option domain-name-servers 8.8.8.8, 8.8.4.4;
option ntp-servers time.example.com;


# Subnet declaration
subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.50 192.168.1.200;    # Address pool
    option routers 192.168.1.1;            # Default gateway
    option subnet-mask 255.255.255.0;
    option broadcast-address 192.168.1.255;
    option ntp-servers 192.168.1.1;


    # Reserved addresses
    host printer {
        hardware ethernet 00:1a:2b:3c:4d:5e;
        fixed-address 192.168.1.50;
    }
}


# Another subnet
subnet 10.0.0.0 netmask 255.255.0.0 {
    range 10.0.1.10 10.0.1.250;
    option routers 10.0.0.1;
    option domain-name-servers 10.0.0.53;
}


# Failover configuration
failover peer "dhcp-failover" {
    primary;
    address 192.168.1.1;
    port 519;
    peer address 192.168.1.2;
    max-response-delay 30;
    max-unacked-updates 10;
    mclt 1800;
    split 128;
    load balance max seconds 3;
}


Start DHCP Server:
+------------------------------------------------------------------+


# Arch Linux
sudo systemctl enable dhcpd4
sudo systemctl start dhcpd4


# Check status
sudo systemctl status dhcpd4


DHCP Client Configuration:
+------------------------------------------------------------------+


systemd-networkd:
+------------------------------------------------------------------+


# /etc/systemd/network/eth0.network
[Match]
Name=eth0


[Network]
DHCP=ipv4


[DHCP]
UseDNS=yes
UseRoutes=yes
UseGateway=yes


dhcpcd:
+------------------------------------------------------------------+


# Install
sudo pacman -S dhcpcd


# Enable
sudo systemctl enable dhcpcd
sudo systemctl start dhcpcd


# For specific interface
sudo systemctl enable dhcpcd@eth0
sudo systemctl start dhcpcd@eth0


View DHCP Lease:
+------------------------------------------------------------------+


# Current leases
cat /var/lib/dhcp/dhcpd.leases


# New format
cat /var/lib/dhcp/dhcpd.leases~


# Client leases
cat /var/lib/dhcpcd/eth0.lease


+------------------------------------------------------------------+

11.6 DHCP Relay

Section titled “11.6 DHCP Relay”
+------------------------------------------------------------------+
|                   DHCP Relay Agent                                   |
+------------------------------------------------------------------+


Problem:
+------------------------------------------------------------------+


DHCP broadcast doesn't cross routers by default
+------------------------------------------------------------------+


Solution: DHCP Relay Agent (IP Helper)
+------------------------------------------------------------------+


Router receives broadcast and forwards to DHCP server as unicast


Configuration (Cisco):
+------------------------------------------------------------------+


interface GigabitEthernet0/0
 ip helper-address 192.168.1.100


Multiple DHCP servers:
+------------------------------------------------------------------+


interface GigabitEthernet0/0
 ip helper-address 192.168.1.100
 ip helper-address 192.168.1.101


Linux DHCP Relay (relayd):
+------------------------------------------------------------------+


# Install
sudo pacman -S dhcp


# Run relay
sudo dhcrelay -4 -i eth0 192.168.1.100


# With multiple servers
sudo dhcrelay -4 -i eth0 192.168.1.100 192.168.1.101


+------------------------------------------------------------------+

11.7 DHCP Security

Section titled “11.7 DHCP Security”
+------------------------------------------------------------------+
|                   DHCP Security                                       |
+------------------------------------------------------------------+


Attacks:
+------------------------------------------------------------------+


1. Rogue DHCP Server:
+------------------------------------------------------------------+


Attack: Malicious DHCP server responds to requests
Result: Users get wrong IP, gateway, DNS


Defense:
+------------------------------------------------------------------+
| - Port security on switches                                      |
| - DHCP snooping (Cisco)                                          |
| - 802.1X authentication                                         |
+------------------------------------------------------------------+


2. DHCP Starvation:
+------------------------------------------------------------------+


Attack: Request all available IPs
Result: Legitimate users can't get IPs


Defense:
+------------------------------------------------------------------+
| - Rate limiting                                                  |
| - Port security                                                  |
| - DHCP snooping                                                 |
+------------------------------------------------------------------+


DHCP Snooping (Cisco):
+------------------------------------------------------------------+


# Enable on switch
ip dhcp snooping


# On specific VLAN
ip dhcp snooping vlan 10,20


# Configure trusted ports
interface GigabitEthernet0/1
 ip dhcp snooping trust


# Verify
show ip dhcp snooping


+------------------------------------------------------------------+

Summary

Section titled “Summary”

In this chapter, you learned:

  • DHCP fundamentals and why use it
  • DORA process in detail
  • DHCP message types
  • Lease lifecycle
  • Server and client configuration
  • DHCP relay
  • DHCP security

Next Chapter

Section titled “Next Chapter”

Chapter 12: NAT & PAT

Last Updated: February 2026