DHCP Options

---

DHCP Lease Time and Available Leases

• client can renew the lease when at least half the lease’s period has elapsed (T1 timer)
  • if original DHCP server does not respond to the request to renew the lease,
    • client attempts to rebind the same lease configuration with any available DHCP server
    • by default, this happens after 87.5% of the lease duration is up (T2 timer)
    • if this fails, client releases the IP address and continues to broadcast to discover a new server
• long lease time means the client does not have to renew the lease ofter,
  • but DHCP server’s available pool of IP addresses is not replenished frequently
• if IP addresses are in short supply, a short lease period enables the DHCP server to allocate addresses previously assigned to hosts that are now not active on the network

Info

Windows client can be forced to release a lease by issuing a command like ipconfig

• in Linux, the utility dhclient is often used for this task
• but modern distributions may use NetworkManager or systemd-networkd

DHCP Options

• When DHCP server offers a configuration to a client:
  • supplies an IP address and subnet mask
  • typically also applies other IP-related settings, called DHCP options
• each option is identified by a tag byte or decimal value between 0-255
  • cant use 0 or 255
• common options:
  • default gateway (IP address of router)
  • IP address(es) of DNS servers
  • DNS suffix (domain name)
  • time synchronization (NTP)
  • file transfer (TFTP)
  • VoIP proxy
• can set default (global) options on a server-wide basis
  • can be overridden by setting scope-specific options