Application Layer: DHCP

Define

• The first client/server application program that is used after a host is boosted

A network protocol that automatically assigns IP addresses and other network configuration settings (such as DNS servers, subnet masks, and default gateways) to devices on a network.

This allows devices to communicate on the network without requiring manual configuration, making network management easier and more efficient.

Four params required to connect to the Internet

1. Host IP
2. IP of a name server (DNS server)
3. Subnet mask
4. IP of the default gateway

🟢 NOTE

DHCP uses UDP, instead of TCP.

• DHCP communication is time-sensitive and not require the reliability mechanisms.
• Using TCP would introduce unnecessary overhead.
• UTP is a connectionless protocol, making it faster for use in DHCP.

special PIv4 address

• 0.0.0.0: reserved for communication when a host needs to send an IPv4 packet but it does not know its own address yet
• 255.255.255.255: reserved for limited broadcast

workflow

1) DHCP discovery: client → server

• Meaning: broadcast a message to all devices present in a network to find the DHCP server
• transaction_id: set by the client and used to match a reply with the request
• src IP:
  • 0.0.0.0 (used when we don’t know our IP address)
  • port 68
• dest IP:
  • 255.255.255.255 (used when doing limited broadcast: broadcasting messages to all hosts on the same local network or subnet)
  • port 67

2) DHCP offer: server → client

• Meaning: server will respond to the host in this message specifying the unleased IP address and other TCP configuration information.
• Behaviors:
  • Server: lock the offered IPs
  • Client: accept the first offer if more than one DHCP server present and respond in the network
• src:
  • SIP:67
• dest:
  • 255.255.255.255:68

🟢 NOTE

The well-known port is reserved.

3) DHCP request: client → server

• Behaviors:
  • Client:
    1. notify the DHCP server whether it accepts the proposed IP configuration or not
    2. Let others know the IP address so that other servers can release to lock for the offer
• src: 0.0.0.0:68
• dest: 255.255.255.255:67 (to let others know the IP address so that other servers can release to lock)

4) DHCP ack: server → client

• Meaning: confirm to the DHCP client that it can use the offered IP configuration
• src: ISP:68
• dest: 255.255.255.255:67