Discovering Location-to-Service Translation (LoST) Servers Using the Dynamic Host Configuration Protocol (DHCP)
Extracted elements (14)
DHCP is used to deliver only a domain name (not a direct IP address) to the LoST client; the domain name is then fed into the DNS-based U-NAPTR resolution mechanism from RFC 5222/RFC 4848. This indirection allows the access network to point clients toward a local LoST server without hardcoding addresses.
Placement of LoST servers closer to the end host (e.g., in the access network) is desirable because in disaster situations with intermittent connectivity, a local LoST server improves resiliency of emergency service communication.
The domain name returned by the DHCP option is used as input to the DNS-based U-NAPTR LoST discovery procedure specified in RFC 5222, which reuses the URI-enabled NAPTR specification from RFC 4848. Clients must implement this downstream DNS resolution step.
A DHCPv4 client MAY request a LoST server domain name in a Parameter Request List option, as described in RFC 2131.
A DHCPv6 client MAY request a LoST server domain name in an Options Request Option (ORO), as described in RFC 3315.
Each DHCP option (DHCPv4 and DHCPv6) contains a single domain name and MUST contain precisely one root label, ensuring the encoded name is a single FQDN.
The high-order two bits of every length octet in the domain name encoding MUST be zero, and the remaining six bits limit each label to 63 octets or less. The total domain name length is restricted to 255 octets or less.
The DHCPv4 option OPTION_V4_LOST (code 137) carries a DNS FQDN to be used by a LoST client to locate a LoST server. The option format is: 1-byte code (137), 1-byte length (n), followed by n octets of RFC 1035-encoded domain name labels.
The DHCPv6 option OPTION_V6_LOST (code 51) carries a DNS FQDN for LoST server discovery. The format is a 2-byte option code, 2-byte option-length, followed by the LoST Server Domain Name in RFC 1035 encoding.
IANA assigned DHCPv4 option code 137 (OPTION_V4_LOST) for the LoST Protocol server option, as described in Section 4.
IANA assigned DHCPv6 option code 51 (OPTION_V6_LOST) for the LoST Protocol server domain name option, as described in Section 5.
If an adversary modifies or spoofs the DHCP server response, a LoST client could be directed to a rogue LoST server or given an invalid address, threatening the integrity of emergency service resolution. These threats are documented in RFC 5069, and the security considerations of RFC 2131, RFC 2132, and RFC 3315 apply.
The DHCPv4 LoST option encodes the server domain name as: Code=137 (1 byte), Len=n (1 byte), followed by RFC 1035 DNS name encoding (variable, up to 255 octets). Each label is a 1-octet length followed by that many octets; a zero-length byte terminates the name.
The DHCPv6 LoST option is a standard DHCPv6 option with OPTION_V6_LOST=51 (2 bytes), option-length (2 bytes), and a variable-length LoST Server Domain Name field encoded per RFC 1035 Section 3.1.