DHCP

• Dynamic Host Configuration Protocol
• automatically assigns address on a network
• A network has limited number of addresses
• unused addresses need to be released
• IPv6 version is DHCPv6
• It uses UDP protocol as its transport protocol
• https://superuser.com/questions/1457452/why-does-dhcp-work-even-when-udp-67-is-blocked

DHCP Server

• The DHCP server, automatically assigns IP addresses to the connected devices so they can exchange information
• It also configures network configurations like:
  • subnet mask
  • default gateway
  • DNS server, and more
• A device makes a request for an IP address if it wants to gain access to a network that’s utilizing DHCP
• Generally Router can be DHCP server for home network.
• It is also possible to install DHCP server in a computer but it is better idea to have Windows Server or Ubuntu Server OS for better support
• https://www.cloudns.net/blog/dhcp-server/

Checking your DHCP Server

• Generally your router is the DHCP server
• MacOS

# "server_identifier" is the IP of DHCP Server
# "domain_name_server" is the IP of DNS Server
ipconfig getpacket <network-interface>

• Windows

ipconfig /all

Working of DHCP

• https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol
• Client broadcasts DHCPDISCOVER to find the DHCP server
• DHCP server responds by DHCPOFFER
  • reserves an IP address for the client and makes a lease offer
  • response contains info like
    • Client ID (MAC Address)
    • IP address which is offered
    • subnet mask
    • lease duration
    • IP address of the DHCP server
• A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer
• Client receives the response and then broadcast DHCPREQUEST message requesting the offered address
  • The client must send the server identification option in the DHCPREQUEST message, indicating the server whose offer the client has selected
  • When other DHCP servers receive this message, they withdraw any offers that they have made to the client and return their offered IP address to the pool of available addresses
• DHCP server acknowledges the request and reply with DHCPACK
  • Contains info like lease duration, IP address and any other configuration requested

sequenceDiagram
    participant Client as Client
    participant Server as DHCP Server 1
    participant Server2 as DHCP Server 2
    
    Client->>+Server: DHCPDISCOVER <br/>[broadcast]
    Client->>+Server2: DHCPDISCOVER <br/>[broadcast]
    Server-->>-Client: DHCPOFFER (Server 1)
    Server2-->>-Client: DHCPOFFER (Server 2)
    
    Client->>+Server: DHCPREQUEST (Server 1) <br/>[broadcast] 
    Client->>+Server2: DHCPREQUEST (Server 1) -- REJECTED<br/>[broadcast]
    Server-->>-Client: DHCPACK (Server 1)

DHCP Server Configuration

• IP allocation
  • Automatic allocation
    • This one will automatically assign an IP per client permanently.
    • The IP address will be designated for just one device, so if, in the future, many new devices get connected, the server could run out of IP addresses to give.
  • Dynamic allocation
    • This is the most common configuration. The server auto-assigns IP addresses to clients, but there is a time period.
    • After the time expires, the client needs to ask for a new IP address again. This will prevent the running out of IPs.
  • Manual allocation
    • Manually the network administrator will assign the IP address to the client.
• Lease Duration Times
  • Refers to how long a device can use an IP address assigned by a DHCP server without needing to renew it
  • Shorter Lease time is suitable for dynamic networks, while longer ones provide stability

Connecting to the Internet

• Every host need these 4 things to connect to internet:
  • IP Address
  • Subnet mask
  • Default Gateway
  • DNS Server IP
• DHCP provides these 4 things

Manual assignment

• It is possible to assign static IP manually instead of DHCP
• If two devices were assigned the same IP address, Data would not be delivered reliably or properly, and might not be delivered at all