DHCP option 43

The DHCP option that carries vendor-specific information, such as the controller address a wireless access point needs.

RFC 2132 leaves its contents to the vendor named in option 60, so one option number carries several incompatible formats. That is why an access point can get an address, a gateway and DNS servers from DHCP and still never find its controller.

Option 43 is an opaque object of at least one octet, interpreted by vendor-specific code; RFC 2132 recommends, without requiring, that several items be encoded as sub-options in the same code, length and value form as DHCP options themselves. The vendor is the one named in option 60, the vendor class identifier, and RFC 3925 points out that option 43 itself carries no vendor identifier.

Vendors chose differently. RUCKUS spells the controller address in ASCII under sub-option 6 or 3, Extreme puts a text list in sub-option 1, other vendors document four-byte binary addresses, and FortiAP reads option 138 by default. The same bytes are a valid answer in one format and nonsense in another, and RFC 2132 tells servers to ignore what they cannot interpret and clients to try to carry on without it, so the failure is silent.

RFC 2131 returns class-specific options only on an exact match of the client's vendor class identifier, and the client asks for option 43 in its parameter request list. Options 124 and 125 name the vendor with an IANA enterprise number, and option 138 is a plain list of CAPWAP controller addresses. RFC 5417 adds that the DHCP exchange is usually neither integrity protected nor authenticated, so in security-sensitive networks it should not be the only way an access point picks its controller.

Also known as: option 43, vendor-specific information, vendor-encapsulated-options, vendor specific option

Sources

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