About the DHCP Option Generator
DHCP options carry everything a host needs beyond its address — gateway, DNS servers, search domains, NTP, and the boot parameters used for network installation. Each is identified by a numeric code, and most DHCP server configuration formats expect you to know which number you want.
This generator produces the option block for the syntax you are using, which removes the two most common failures: naming the wrong code, and encoding a value in the wrong type. An option expecting a list of IP addresses will not accept a hostname, and a server that silently ignores a malformed option produces clients that come up with an address and no route.
Options 66 and 67, used for network boot, are the ones most often confused with their DHCPv6 and PXE equivalents. If a machine gets an address but never loads a boot image, the option encoding is the first thing to check.
Frequently asked questions
What are the most commonly used DHCP options?
Option 3 is the default gateway, option 6 is the DNS server list, option 15 is the domain name, option 51 is the lease time, and option 119 is the domain search list. For network boot, option 66 names the TFTP server and option 67 the boot filename. Options 1 through 3 and 6 cover the vast majority of ordinary configurations.
Why is my client ignoring an option I set?
Usually because the client never asked for it. A client sends a parameter request list naming the options it wants, and a well-behaved server sends only those. Many servers have a directive to force an option regardless. The other common cause is scope precedence — an option set at host level overrides subnet level, which overrides global.
What is the difference between option 15 and option 119?
Option 15 supplies a single domain name, historically used as the one suffix appended to unqualified hostnames. Option 119 supplies an ordered search list of several domains and is what modern resolvers prefer. Setting only option 15 in an environment with multiple internal domains means short names resolve in one of them and fail in the rest.
How should lease time be chosen?
Short enough that addresses return to the pool at a useful rate, long enough that renewal traffic is not constant. A day is typical for a stable office network. Guest and conference networks want minutes to an hour, because devices leave without releasing. If a pool is near exhaustion, cutting the lease time recovers addresses faster than any other single change.
Can DHCP options set static routes?
Yes, through option 121, classless static routes, which is the correct modern choice. The older option 33 cannot express a subnet mask and so cannot represent anything but classful routes. Some clients also read a vendor-specific option 249 that mirrors 121, which is why configurations often set both.
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