What Are DHCP Options?
- Defined in RFCs – standardized fields that DHCP servers can send to clients.
- Extend beyond IP address – include gateways, DNS servers, NTP servers, log servers, and more.
- Standard vs Vendor-Specific – some options are universal, others are designed for special devices (e.g., VoIP phones, PXE boot).
⚠️ Important: Not all client operating systems support all options. Always verify.
Common & Essential Options
These are widely supported across OSes:
- Routers (default gateway)
- Domain name / DNS servers
- NTP servers (time sync)
- Syslog/log servers
- PXE boot parameters (for network booting new machines)
Example:
option domain-name "corp.example";
option domain-name-servers 10.0.0.53, 10.0.1.53;
option ntp-servers 10.0.0.123, 10.0.1.123;
option log-servers 10.0.0.251;Tricky or Inconsistent Options
- Option 252 (WPAD / Proxy Auto-Config):
- Works on Windows (commonly used for PAC files).
- Often ignored by Linux, which prefers DNS-based discovery.
option wpad code 252 = text;
option wpad "http://wpad.corp.example/wpad.dat";⚠️ Don’t rely on this unless you’ve confirmed client behavior.
PXE Boot (Datacenter Use Case)
Used for provisioning servers with network boot.
- Legacy BIOS systems expect pxelinux.0.
- Modern UEFI clients may require .efi boot files.
Example:
next-server 10.0.0.60;
filename "pxelinux.0"; # or "bootx64.efi" for UEFI
option tftp-server-name "10.0.0.60";
option bootfile-name "pxelinux.0";Vendor-Specific Options
- DHCP is extensible – you can define your own options.
- Example: VoIP phones may need SIP server addresses or VLAN IDs.
- Syntax often requires code definitions before usage.
DHCPv4 vs DHCPv6
- Option codes are not identical between IPv4 and IPv6.
- Always check the correct RFCs and client support when working with IPv6.
Best Practices
- Verify with packet captures: Use tcpdump or Wireshark to confirm clients actually receive and apply the options.
- Document non-standard options: Future admins need to know why a custom field exists.
- Test across OS types: Windows, Linux, macOS, phones, and IoT devices may behave differently.
✅ Key Takeaway: DHCP options extend the protocol from simple IP assignment to full client configuration. Use them wisely, verify client support, and always document customisations.