Dynamic Host Configuration Protocol (DHCP)
Last Updated :
02 Jul, 2025
Dynamic Host Configuration Protocol is a network protocol used to automate the process of assigning IP addresses and other network configuration parameters to devices (such as computers, smartphones and printers) on a network.
Instead of manually configuring each device with an IP address, DHCP allows devices to connect to a network and receive all necessary network information, like IP address, subnet mask, default gateway and DNS server addresses, automatically from a DHCP server.
Components of DHCP
The main components of DHCP include:
- DHCP Server: DHCP Server is a server that holds IP Addresses and other information related to configuration.
- DHCP Client: It is a device that receives configuration information from the server. It can be a mobile, laptop, computer or any other electronic device that requires a connection.
- DHCP Relay: DHCP relays basically work as a communication channel between DHCP Client and Server.
- IP Address Pool: It is the pool or container of IP Addresses possessed by the DHCP Server. It has a range of addresses that can be allocated to devices.
- Subnets: Subnets are smaller portions of the IP network partitioned to keep networks under control.
- Lease: It is simply the time that how long the information received from the server is valid, in case of expiration of the lease, the tenant must have to re-assign the lease.
- DNS Servers: DHCP servers can also provide DNS (Domain Name System) server information to DHCP clients, allowing them to resolve domain names to IP addresses.
- Default Gateway: DHCP servers can also provide information about the default gateway, which is the device that packets are sent to when the destination is outside the local network.
- Options: DHCP servers can provide additional configuration options to clients, such as the subnet mask, domain name and time server information.
- Renewal: DHCP clients can request to renew their lease before it expires to ensure that they continue to have a valid IP address and configuration information.
- Failover: DHCP servers can be configured for failover, where two servers work together to provide redundancy and ensure that clients can always obtain an IP address and configuration information, even if one server goes down.
- Dynamic Updates: DHCP servers can also be configured to dynamically update DNS records with the IP address of DHCP clients, allowing for easier management of network resources.
- Audit Logging: DHCP servers can keep audit logs of all DHCP transactions, providing administrators with visibility into which devices are using which IP addresses and when leases are being assigned or renewed.
DHCP- Hardware Length: This is an 8-bit field defining the length of the physical address in bytes. e.g. for Ethernet the value is 6.
- Hop count: This is an 8-bit field defining the maximum number of hops the packet can travel.
- Transaction ID: This is a 4-byte field carrying an integer. The transaction identification is set by the client and is used to match a reply with the request. The server returns the same value in its reply.
- Number of Seconds: This is a 16-bit field that indicates the number of seconds elapsed since the time the client started to boot.
- Flag: This is a 16-bit field in which only the leftmost bit is used and the rest of the bit should be set to 0. A leftmost bit specifies a forced broadcast reply from the server.
- Client IP Address: This is a 4-byte field that contains the client IP address . If the client does not have this information this field has a value of 0.
- Your IP Address: This is a 4-byte field that contains the client IP address. It is filled by the server at the request of the client.
- Server IP Address: This is a 4-byte field containing the server IP address. It is filled by the server in a reply message.
- Gateway IP Address: This is a 4-byte field containing the IP address of a routers. IT is filled by the server in a reply message.
- Client Hardware Address: This is the physical address of the client .Although the server can retrieve this address from the frame sent by the client it is more efficient if the address is supplied explicitly by the client in the request message.
- Server Name: This is a 64-byte field that is optionally filled by the server in a reply packet. It contains a null-terminated string consisting of the domain name of the server. If the server does not want to fill this filed with data, the server must fill it with all 0s.
- Boot Filename: This is a 128-byte field that can be optionally filled by the server in a reply packet. It contains a null- terminated string consisting of the full pathname of the boot file.
- Options: This is a 64-byte field with a dual purpose. IT can carry either additional information or some specific vendor information. The field is used only in a reply message.
Working of DHCP
DHCP works on the Application layer of the UDP Protocol. The main task of DHCP is to dynamically assigns IP Addresses to the Clients and allocate information on TCP/IP configuration to Clients.
Working of DHCPThe DHCP port number for the server is 67 and for the client is 68. It is a client-server protocol that uses UDP services. An IP address is assigned from a pool of addresses. In DHCP, the client and the server exchange mainly 4 DHCP messages in order to make a connection, also called the DORA process, but there are 8 DHCP messages in the process.
The 8 DHCP Messages
1. DHCP Discover Message: This is the first message generated in the communication process between the server and the client. This message is generated by the Client host in order to discover if there is any DHCP server/servers are present in a network or not. This message is broadcasted to all devices present in a network to find the DHCP server. This message is 342 or 576 bytes long.
DHCP Discover MessageAs shown in the figure, the source MAC address (client PC) is 08002B2EAF2A, the destination MAC address(server) is FFFFFFFFFFFF, the source IP address is 0.0.0.0(because the PC has had no IP address till now) and the destination IP address is 255.255.255.255 (IP address used for broadcasting). As they discover message is broadcast to find out the DHCP server or servers in the network therefore broadcast IP address and MAC address is used.
2. DHCP Offers A Message: The server will respond to the host in this message specifying the unleased IP address and other TCP configuration information. This message is broadcasted by the server. The size of the message is 342 bytes. If there is more than one DHCP server present in the network then the client host will accept the first DHCP OFFER message it receives. Also, a server ID is specified in the packet in order to identify the server.
DHCP Offer MessageNow, for the offer message, the source IP address is 172.16.32.12 (server's IP address in the example), the destination IP address is 255.255.255.255 (broadcast IP address), the source MAC address is 00AA00123456, the destination MAC address is 00:11:22:33:44:55 (client's MAC address). Here, the offer message is broadcast by the DHCP server therefore destination IP address is the broadcast IP address and destination MAC address is 00:11:22:33:44:55 (client's MAC address)and the source IP address is the server IP address and the MAC address is the server MAC address.
Also, the server has provided the offered IP address 192.16.32.51 and a lease time of 72 hours(after this time the entry of the host will be erased from the server automatically). Also, the client identifier is the PC MAC address (08002B2EAF2A) for all the messages.
3. DHCP Request Message: When a client receives an offer message, it responds by broadcasting a DHCP request message. The client will produce a gratuitous ARP in order to find if there is any other host present in the network with the same IP address. If there is no reply from another host, then there is no host with the same TCP configuration in the network and the message is broadcasted to the server showing the acceptance of the IP address. A Client ID is also added to this message.
DHCP Request MessageNow, the request message is broadcast by the client PC therefore source IP address is 0.0.0.0(as the client has no IP right now) and destination IP address is 255.255.255.255 (the broadcast IP address) and the source MAC address is 08002B2EAF2A (PC MAC address) and destination MAC address is FFFFFFFFFFFF.
Note - This message is broadcast after the ARP request broadcast by the PC to find out whether any other host is not using that offered IP. If there is no reply, then the client host broadcast the DHCP request message for the server showing the acceptance of the IP address and Other TCP/IP Configuration.
4. DHCP Acknowledgment Message: In response to the request message received, the server will make an entry with a specified client ID and bind the IP address offered with lease time. Now, the client will have the IP address provided by the server.
DHCP AcknowledgementNow the server will make an entry of the client host with the offered IP address and lease time. This IP address will not be provided by the server to any other host. The destination MAC address is 00:11:22:33:44:55 (client's MAC address) and the destination IP address is 255.255.255.255 and the source IP address is 172.16.32.12 and the source MAC address is 00AA00123456 (server MAC address).
5. DHCP Negative Acknowledgment Message: Whenever a DHCP server receives a request for an IP address that is invalid according to the scopes that are configured, it sends a DHCP NACK message to the client. E.g. when the server has no IP address unused or the pool is empty, then this message is sent by the server to the client.
6. DHCP Decline: If the DHCP client determines the offered configuration parameters are different or invalid, it sends a DHCP decline message to the server. When there is a reply to the gratuitous ARP by any host to the client, the client sends a DHCP decline message to the server showing the offered IP address is already in use.
7. DHCP Release: A DHCP client sends a DHCP release packet to the server to release the IP address and cancel any remaining lease time.
8. DHCP Inform: If a client address has obtained an IP address manually then the client uses DHCP information to obtain other local configuration parameters, such as domain name. In reply to the DHCP inform message, the DHCP server generates a DHCP ack message with a local configuration suitable for the client without allocating a new IP address. This DHCP ack message is unicast to the client.
Security Measures for Using DHCP
To make sure your DHCP servers are safe, consider these DHCP security issues:
- Limited IP Addresses : A DHCP server can only offer a set number of IP addresses. This means attackers could flood the server with requests, causing essential devices to lose their connection.
- Fake DHCP Servers : Attackers might set up fake DHCP servers to give out fake IP addresses to devices on your network.
- DNS Access : When users get an IP address from DHCP, they also get DNS server details. This could potentially allow them to access more data than they should.
Protection Against DHCP Starvation Attack
A DHCP starvation attack happens when a hacker floods a DHCP server with requests for IP addresses. This overwhelms the server, making it unable to assign addresses to legitimate users. The hacker can then block access for authorized users and potentially set up a fake DHCP server to intercept and manipulate network traffic, which could lead to a man-in-the-middle attack.
Advantages of DHCP
- Centralized Management of IP Addresses: DHCP simplifies network administration by centralizing IP address allocation, ensuring efficient management.
- Centralized and Automated TCP/IP Configuration: DHCP automates the assignment of TCP/IP settings to devices, reducing manual configuration and errors.
- Ease of Adding New Clients to a Network: New devices are automatically assigned an IP and network configuration, simplifying network expansion.
- Reuse of IP Addresses: DHCP efficiently reuses IP addresses through its leasing system, minimizing the need for additional addresses.
- Efficient Handling of IP Address Changes for Mobile Devices: DHCP dynamically assigns IP addresses to portable devices that frequently move between networks, ensuring seamless connectivity.
- Simple Reconfiguration of IP Address Space: DHCP allows for easy reconfiguration of the IP address space on the server without requiring changes on individual clients.
- Centralized Configuration Management: DHCP provides a centralized approach to manage and configure network IP addresses, simplifying network administration.
- Easy Handling of New Users and IP Address Reuse: DHCP makes it easy to handle new users and efficiently reuse IP addresses, optimizing network resources.
Disadvantages of DHCP
- IP Conflict Can Occur: DHCP can sometimes cause IP conflicts when multiple devices are mistakenly assigned the same IP address.
- Clients Accept Any Server: Clients can connect to any DHCP server and if an unauthorized server is nearby, it may send invalid configuration data to the client.
- Client Cannot Access the Network in Absence of a DHCP Server: Without a DHCP server, clients will not be able to obtain an IP address and thus cannot access the network.
- Machine Name Is Not Changed: When a new IP address is assigned, the machine name does not automatically change, potentially causing identification issues on the network.
For more information on protocols refer to the article - DHCP VS DNS , UDP
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Heuristic VirusA heuristic virus is a type of computer virus that replicates by guessing. It does this by analyzing what it perceives as the user's behavior and creating an infection that tries to mirror it. The aim of viruses like these is to make the user feel as if their data are being collected, so they could
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How to Prevent Backdoor Attacks?Cyber security or security of the web deals with the security mechanism of the cyber world. Cyber security is extremely necessary as it is important that computer networks have strong cyber security mechanisms set up to prevent any form of attack that may lead to compromise of computer network secur
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Sniffing
What are Active Sniffing Attacks?Active Sniffing Attacks in Ethical Hacking can lead to using the intercepted data for malicious purposes. The most common use of sniffing is to steal passwords, credit card numbers, and other sensitive data. Sniffers are software programs that let you intercept network traffic in a convert way witho
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What is Protocol Analyzer?The network protocol is a set of rules that regulates the communication and exchange of data over the internet or network. Both the sender and receiver should follow the same protocols in order to communicate and exchange the data. Whenever we access a website or exchange some data with another devi
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What is MAC Spoofing Attack?MAC Spoofing is a type of attack used to exploit flaws in the authentication mechanism implemented by wired and wireless networking hardware. In layman’s terms, MAC spoofing is when someone or something intercepts, manipulate or otherwise tampers with the control messages exchanged between a network
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How to Prevent MAC Flooding?A flood of MAC addresses is a serious concern for ethical hackers because it can leave your system vulnerable to attack. In order to prevent this from happening, consider some of the following precautions: Use secure passwords.Install and use antivirus software.Update your operating system on a regu
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What is Port Stealing?Cyber security is the security of the web and related services related to the web. Cyber security is the way to prevent many cyber attacks from taking place. Cyber security ensures that awareness of the cyber-attacks along with the different forms of cyber attacks are well-known to people so that th
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Dynamic Host Configuration Protocol (DHCP)Dynamic Host Configuration Protocol is a network protocol used to automate the process of assigning IP addresses and other network configuration parameters to devices (such as computers, smartphones and printers) on a network. Instead of manually configuring each device with an IP address, DHCP allo
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DHCP Starvation AttackDHCP (Dynamic Host Configuration Protocol) is used to assign IP addresses to machines within any network automatically. It is also known as zeroconf protocol, as network administrators don't need to assign IP addresses to machines manually. To assign IP addresses, DHCP makes use of DORA packets whic
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What is Rogue DHCP Server Attack?Rogue DHCP server attacks are gaining popularity but can be mitigated. The hacker sets up a rogue DHCP server and creates an IP address conflict by broadcasting a duplicate IP address. Hackers infiltrate a network by attacking the wireless router, which they do with ARP poisoning in order to inject
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What is ARP Spoofing Attack?Spoofing is a type of attack in which hackers gain access to the victim's system by gaining the trust of the victim (target user) to spread the malicious code of the malware and steal data such as passwords and PINs stored in the system.In Spoofing, psychologically manipulating the victim is the mai
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How to Prevent DNS Poisoning and Spoofing?The domain name system, or DNS, is a naming system for computers, services, and other internet resources that is hierarchical in nature. It's essentially the internet's phone book. Each domain name has a corresponding set of ten or so numbers that make up the domain name's IP address. Simple, user-f
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DNS Spoofing or DNS Cache poisoningPrerequisite - Domain Name Server Before Discussing DNS Spoofing, First, discuss what is DNS.A Domain Name System (DNS) converts a human-readable name (such as www.geeksforgeeks.org) to a numeric IP address. The DNS system responds to one or more IP-address by which your computer connects to a websi
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How to Detect Sniffer in Your Network?Sniffers are software or hardware devices that can be used for wiretapping over a computer network, such as LAN or WAN. They are used to collect data by listening and capturing IP packets. These devices usually have the ability of recording data which includes email, chat, web browsing, and informat
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Mitigation of DHCP Starvation AttackIn a DHCP starvation attack, an attacker creates spoofed DHCP requests with the goal of consuming all available IP addresses that a DHCP server can allocate. This attack targets DHCP servers. The attack could deny service to authorized network users. In other words, a malicious cyberattack that targ
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Social Engineering
What is Social Engineering? Working, Types, Prevention and ImpactSocial Engineering is an umbrella term for multiple malicious activities done by cyber criminals over the internet through human interaction. It doesn't involve the use of technical hacking techniques. Attackers use psychology and manipulation to trick users into performing actions that could compro
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What is Insider Attack?Cyber attacks on organizations are increasing both in number and magnitude. Often carried out by attackers with intentions to harm an organization, gain financial benefits, pursue political motives, spread terror, etc., these attacks always cost organizations in one form or another. Cyber security i
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What is an Impersonation Attack?An impersonation attack is a threat where hackers impersonate trusted people or organizations—such as your boss, bank, or a well-known service used by companies, to the trick victims so that they can give sensitive information, funds, or access to systems. Unlike classical malware-based attacks (e.g
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What are Tailgating?Tailgating is a term that refers to the practice of parking along the side of a road, usually by an open field or parking lot. Tailgaters are not only prohibited from entering the highway itself, but also have no right to drive on it at all. The term “tailgater†comes from tail-end Charlie, which me
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How Hackers Use Social Engineering to Get Passwords on Facebook?Social engineering is a powerful method for hackers to exploit computer systems without the use of malware or computer hacking tools. To carry out a successful social engineering hack, ethical hackers need to understand how to effectively use social media and human interaction to obtain sensitive in
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Pretexting in Social EngineeringPretexting is a social engineering technique that's used by hackers, spammers, and pranksters to assume and exploit the trust of an individual. It's been defined as the act of taking on an identity (usually those of a trusted person, such as a customer service representative) for the purpose of gain
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Credit Card FraudsThis era belongs to technology where technology becomes a basic part of our lives whether in business or home which requires connectivity with the internet and it is a big challenge to secure these units from being a sufferer of cyber-crime. Wireless credit card processing is a tremendously new serv
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Active Social Engineering Defense (ASED)A proactive tactic called Active Social Engineering Defense (ASED) is used to lessen the risk of being the target of social engineers. We must first comprehend social engineering in order better to comprehend active social engineering defense (ASED). Social Engineering Attack:Without the use of malw
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Cyber Crime - Identity TheftIdentity Theft also called Identity Fraud is a crime that is being committed by a huge number nowadays. Identity theft happens when someone steals your personal information to commit fraud. This theft is committed in many ways by gathering personal information such as transactional information of an
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Penetration Testing - Software EngineeringIn this guide, we'll explore the fundamentals of penetration testing, its importance in cybersecurity, and how it fits into the software development lifecycle (SDLC). From network security to web application security, we'll be going into various aspects of pen testing, equipping you with the knowled
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Denial-of-Service
Distributed Denial of Service DDoS attackImagine a scenario where you are visiting some websites and one of them seems to be a little slow. You might blame their servers for improving their scalability as they might be experiencing a lot of user traffic on their site. Most of the sites already take this issue into account beforehand. Chanc
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What are Bandwidth Attacks?Bandwidth attacks in ethical hacking target a network's bandwidth, and by extension, its available computing resources. A hacker can overload a network by sending a high number of packets to the computer or server at once. The processing power and memory in the system is limited and once it reaches
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HTTP Flood AttackCyber crimes are increasing with the growing demand for the internet. With an increasing number of cybercrime cases reported each year, awareness of cyberattacks is very important. Cyber crimes/cyberattacks are done by hackers who unethically want to breach user data and steal sensitive information
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ICMP Flood DDoS AttackIn today's world, Denial of Service (DoS) and Distributed Denial of Service (DDoS) attacks have become a major threat to present computer networks. DDoS is a kind of attack in which an attacker targets the victim's network resources such as bandwidth or memory so that the victim may stop responding
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Ping Flood AttackA Ping Flood Attack, or ICMP Attack (Internet Control Message Protocol Attack), is a type of offensive cyberattack utilized to attack the targeted device—computer, server, or network—with a flood of false requests. It is a Denial of Service (DoS) attack, as it is intended to disrupt normal operation
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What is a Permanent DoS (PDoS) Attack?In today's modern world, we often come across the term Cybercrime or Cyberattacks very frequently. With the increasing demand for the internet, cybercrime occurrence has taken a great spike. Cyber crimes are crimes involving cyber technology/computer networks. Cyberattacks are extremely dangerous as
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What is Phlashing?Phlashing is a type of Denial of Service (DoS) Attack, also known as a Permanent Denial of Service Attack (PDoS) which is very severe because there is no way to undo this attack's consequences as it damages/corrupts the system to such an extreme level that there is no other way except to replace or
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