RFC 4704

The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option

📅 published October 2006, from ietf

RFC 4704 🎀 The DHCPv6 Client Fully Qualified Domain Name (FQDN) Option

okay bestie, RFC 4704 is giving standards track main character energy 💅 It's from the Network Working Group, written by B. Volz at Cisco Systems, Inc., dated October 2006. Its title block doesn't list anything it obsoletes or updates ✨

Status of This Memo ✨

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice 🌈

Abstract 🦋

This doc specifies a brand new DHCPv6 option 🎀 It lets a DHCPv6 client share its Fully Qualified Domain Name (FQDN), and it also settles who's responsible for updating the DNS resource records (RRs) tied to the client's address assignments.

Table of Contents 📋

1. Introduction ....................................................3
2. Terminology .....................................................3
3. Models of Operation .............................................3
4. The DHCPv6 Client FQDN Option ...................................4
   4.1. The Flags Field ............................................5
   4.2. The Domain Name Field ......................................6
5. DHCPv6 Client Behavior ..........................................7
   5.1. Client Desires to Update AAAA RRs ..........................7
   5.2. Client Desires Server to Do DNS Updates ....................7
   5.3. Client Desires No Server DNS Updates .......................7
   5.4. Domain Name and DNS Update Issues ..........................8
6. DHCPv6 Server Behavior ..........................................9
   6.1. When to Perform DNS Updates ................................9
7. DNS RR TTLs ....................................................10
8. DNS Update Conflicts ...........................................11
9. IANA Considerations ............................................11
10. Security Considerations .......................................12
11. Acknowledgements ..............................................12
12. References ....................................................13
   12.1. Normative References .....................................13
   12.2. Informative References ...................................13

1. Introduction ✨

okay so DNS ([2], [3]) keeps track of (among other things) which Fully Qualified Domain Names (FQDNs) [10] map to which IPv6 addresses on hosts 🌸 That info lives in two kinds of Resource Records (RRs): AAAA and PTR [12]. The DNS update specification [4] explains how DNS info can get updated over a network 💜

The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [5] is how a host (a DHCPv6 client) can grab certain configuration info, along with its stateful IPv6 address(es) 🧚 This document adds a new DHCPv6 option, the Client FQDN option. Clients and servers use it to swap info about the client's fully qualified domain name, and about who's in charge of updating the DNS with the matching AAAA and PTR RRs 💅

2. Terminology 📖

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [1].

Familiarity with the DNS Update protocol [4] and with DHCPv6 and its terminology, as defined in [5], is assumed 🫶

3. Models of Operation 🍓

When a DHCPv6 client gets an address, a site's administrator might want the AAAA RR for the client's FQDN and the PTR RR for that address to get updated 👑 So two separate DNS update transactions may happen. Getting an address through DHCPv6 involves two entities: a DHCPv6 client and a DHCPv6 server. In theory, each one could do none, one, or both of the DNS update transactions. In practice though, not every combo makes sense 🙈 The DHCPv6 Client FQDN option is mainly meant for these two cases:

DHCPv6 client updates the AAAA RR; DHCPv6 server updates the PTR RR.
DHCPv6 server updates both the AAAA and the PTR RRs.

The only difference between these two cases is whether the FQDN-to-IPv6-address mapping gets updated by the DHCPv6 client or by the DHCPv6 server. The IPv6-address-to-FQDN mapping gets updated by the DHCPv6 server in both cases ✨

These two cases matter, while the others are unlikely, and it comes down to authority over the DNS domain names 🔥 A DHCPv6 client may be given authority over mapping its own AAAA RRs. Or that authority may be kept with a server, so the client can't list arbitrary addresses or attach its addresses to arbitrary domain names. In every case, the only sensible place for authority over the PTR RRs tied to an address is the DHCPv6 server that hands out that address 💖

Note: A third case is supported in which the client requests that the server perform no updates. However, this case is presumed to be rare because of the authority issues.

In any case, whether a site lets all, some, or none of its DHCPv6 servers and clients do DNS updates into the zones it controls is totally up to local administrative policy 🦋 This document doesn't require any specific administrative policy, and it doesn't propose one. The range of possible policies is super broad. Some sites might give credentials to only the DHCPv6 servers that the DNS servers will accept. Others might configure each individual DHCPv6 client with credentials that let it modify its own domain name 🌈 Compliant implementations MAY support some or all of these possibilities. Also, this specification only covers DHCPv6 client and server processes. It doesn't cover other processes that initiate DNS updates 🎀

This document describes a new DHCPv6 option that a client can use to send all or part of its domain name to a DHCPv6 server. Site-specific policy decides whether DHCPv6 servers use the names clients offer, and what DHCPv6 servers do when clients don't supply domain names 🧚

4. The DHCPv6 Client FQDN Option ✨

To update the IPv6-address-to-FQDN mapping, a DHCPv6 server needs to know the FQDN of the client for the addresses in the client's IA_NA bindings 💅 So this document defines a new DHCPv6 option called "Client FQDN" so the client can send its FQDN to the server. The Client FQDN option also carries Flags that DHCPv6 clients and servers use to negotiate who does which updates 🫶

The code for this option is 39. Its minimum length is 1 octet.

The format of the DHCPv6 Client FQDN option is shown below:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          OPTION_FQDN          |         option-len            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   flags       |                                               |
       +-+-+-+-+-+-+-+-+                                               |
       .                                                               .
       .                          domain-name                          .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         option-code      OPTION_CLIENT_FQDN (39)

         option-len       1 + length of domain name

         flags            flag bits used between client and server to
                          negotiate who performs which updates

         domain-name      the partial or fully qualified domain name
                          (with length option-len - 1)

The Client FQDN option MUST only appear in a message's options field and applies to all addresses for all IA_NA bindings in the transaction 🌸

4.1. The Flags Field 🩷

The format of the Flags field is:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |  MBZ    |N|O|S|
       +-+-+-+-+-+-+-+-+

The "S" bit tells whether the server SHOULD or SHOULD NOT perform the AAAA RR (FQDN-to-address) DNS updates 💜 A client sets the bit to 0 to say the server SHOULD NOT perform the updates, and to 1 to say the server SHOULD perform them. The bit's state in the server's reply shows what the server will do. If it's 1, the server has taken responsibility for AAAA RR updates for the FQDN 🎀

The "O" bit shows whether the server has overridden the client's preference for the "S" bit. A client MUST set this bit to 0. A server MUST set this bit to 1 if the "S" bit in its reply to the client doesn't match the "S" bit it got from the client 🙈

The "N" bit shows whether the server SHOULD NOT perform any DNS updates 🌈 A client sets this bit to 0 to ask the server to SHOULD perform updates (the PTR RR and possibly the AAAA RR depending on the "S" bit), or to 1 to ask the server to SHOULD NOT perform any DNS updates. A server sets the "N" bit to say whether it SHALL (0) or SHALL NOT (1) perform DNS updates. If the "N" bit is 1, the "S" bit MUST be 0 🔥

The remaining bits in the Flags field are reserved for future assignment. DHCPv6 clients and servers that send the Client FQDN option MUST clear the MBZ bits, and they MUST ignore these bits 🦋

4.2. The Domain Name Field 🍓

The Domain Name part of the option carries all or part of a DHCPv6 client's FQDN. The data in the Domain Name field MUST be encoded as described in Section 8 of [5]. To tell whether the FQDN changed between message exchanges, the client and server MUST NOT alter the Domain Name field contents unless the FQDN has actually changed ✨

A client MAY be configured with a fully qualified domain name, or with a partial name that isn't fully qualified. If a client only knows part of its name, it MAY send a name that isn't fully qualified. That tells the server it knows part of the name but doesn't necessarily know the zone the name belongs in 💖

To send a fully qualified domain name, the Domain Name field is set to the DNS-encoded domain name, including the terminating zero-length label. To send a partial name, the Domain Name field is set to the DNS-encoded domain name without the terminating zero-length label 🌸

A client MAY also leave the Domain Name field empty if it wants the server to provide a name 🧚

Servers SHOULD send the complete fully qualified domain name in Client FQDN options 👑

5. DHCPv6 Client Behavior 💅

The following describes the behavior of a DHCPv6 client that implements the Client FQDN option 🫶

A client MUST only include the Client FQDN option in SOLICIT, REQUEST, RENEW, or REBIND messages 🌈

A client that sends the Client FQDN option MUST also include the option in the Option Request option if it expects the server to include the Client FQDN option in any responses ✨

to be continued 🦋

5.1. Client Desires to Update AAAA RRs 💖

Okay so, if a client owns and maintains its own FQDN and wants to be the one keeping the FQDN-to-IPv6-address mapping up to date for that name and its address(es), the client MUST include the Client FQDN option in the SOLICIT with Rapid Commit, REQUEST, RENEW, and REBIND messages it originates. It MAY include the Client FQDN option in its SOLICIT messages too. And the "S", "O", and "N" bits in the Flags field MUST be 0. Zero bits, zero drama 🫶

Once the client's DHCPv6 configuration is done (it gets a REPLY message and passes a final check on the parameters in that message), the client MAY send an update for its AAAA RRs (the ones tied to its FQDN), unless the server has set the "S" bit to 1. If the "S" bit is 1, the DHCPv6 client SHOULD NOT start an update for the name in the server's returned Client FQDN option Domain Name field. But a DHCPv6 client that was explicitly configured with an FQDN MAY ignore the "S" bit if the name the server sent back matches the name it was configured with. Main character energy for sure 👑

5.2. Client Desires Server to Do DNS Updates 🌸

A client can hand off the job of updating the FQDN-to-IPv6-address mapping for its FQDN and address(es) to the server, and that's totally a valid vibe ✨ To let the server know, the client SHOULD include the Client FQDN option in its SOLICIT with Rapid Commit, REQUEST, RENEW, and REBIND messages, and it MAY include the Client FQDN option in its SOLICIT. The "S" bit in the Flags field MUST be 1, and the "O" and "N" bits MUST be 0. Server, take the wheel 💅

5.3. Client Desires No Server DNS Updates 🦋

Some clients just want the server to do zero DNS updates on their behalf, and honestly, respect 🙈 To say that, the client SHOULD include the Client FQDN option in its SOLICIT with Rapid Commit, REQUEST, RENEW, and REBIND messages, and it MAY include the Client FQDN option in its SOLICIT. The "N" bit in the Flags field MUST be 1, and the "S" and "O" bits MUST be 0.

Once the client's DHCPv6 configuration is done (same deal: a REPLY message and a successful final check on the parameters), the client MAY send its own DNS updates as long as the server's "N" bit is 1. If the server's "N" bit is 0, the server MAY do the PTR RR updates. It MAY also do the AAAA RR updates if the "S" bit is 1. 🎀

5.4. Domain Name and DNS Update Issues 🧚

Here's the tea 🍓 The DHCPv6 server might be set up to complete or replace a domain name the client sends, so the client MAY find it useful to send the Client FQDN option in its SOLICIT messages. If the server sends back different Domain Name data in its ADVERTISE message, the client could use that data for its own AAAA RR update later, or put it into the Client FQDN option it sends in later messages. There's no rule saying the client has to send the exact same Client FQDN option data in its SOLICIT, REQUEST, RENEW, or REBIND messages.

Also, if a client already sent the Client FQDN option to its server and then its notion of its domain name changes, it MAY send the new name in a Client FQDN option the next time it talks to the server. This MAY cause the DHCPv6 server to update the name tied to the PTR records and, if the server updated the AAAA record for that client, to delete that record and try an update for the client's current domain name. Plot twist energy 😭

A client that hands off the job of updating the FQDN-to-IPv6-address mapping to a server gets no signal at all (good or bad) about whether the server actually pulled it off 💀 The client MAY use a DNS query to check whether the mapping is current. But depending on how loaded the DHCPv6 and DNS servers are and how long DNS propagation takes, the client can only infer success. If the info isn't up to date in DNS, the authoritative servers might not have finished the updates or zone transfers yet, or caching resolvers might still be catching up on their caches. Patience, bestie ⏳

If a client releases an address before its valid lifetime runs out and the client is responsible for its AAAA RR, the client SHOULD delete the AAAA RR tied to that address before it sends a RELEASE message. Same vibe if a client is responsible for its AAAA RRs but can't renew the lifetimes for an address: it SHOULD try to delete the AAAA RR before the lifetime on that address stops being valid. And a DHCPv6 client that couldn't delete an AAAA RR it added SHOULD try to tell its administrator, maybe by writing a log message 📝

A client SHOULD NOT perform DNS updates to AAAA RRs for its non-Global Unicast addresses [7] or temporary addresses [6]. Not it 🙅‍♀️

6. DHCPv6 Server Behavior ✨

This section is the server's side of the story, and it only kicks in when the client's message includes the Client FQDN option 🎀

Servers MUST only include a Client FQDN option in ADVERTISE and REPLY messages if the client included a Client FQDN option and the Client FQDN option is requested by the Option Request option in the client's message that the server is responding to.

The server checks its configuration and the Flag bits in the client's Client FQDN option to figure out how to respond:

The server sets the "S", "O", and "N" bits in its copy of the option it sends back to the client to 0.
If the client's "N" bit is 1 and the server's configuration lets it honor the client's request for no server-initiated DNS updates, the server sets the "N" bit to 1.
Otherwise, if the client's "S" bit is 1 and the server's configuration lets it honor the client's request for the server to initiate AAAA RR DNS updates, the server sets the "S" bit to 1. If the server's "S" bit doesn't match the client's "S" bit, the server sets the "O" bit to 1.

The server MAY use the name the client supplied in its Client FQDN option, or it MAY modify that name or swap in a different one. The server SHOULD send its own idea of the complete FQDN for the client in the Domain Name field. The server MAY also just copy the Domain Name field from the Client FQDN option the client sent 💅

6.1. When to Perform DNS Updates 🦋

The server SHOULD NOT perform any DNS updates if the "N" bit is 1 in the Flags field of the Client FQDN option in the REPLY messages (to be) sent to the client. But the server SHOULD delete any RRs it previously added through DNS updates for the client.

The server MAY perform the PTR RR DNS update (unless the "N" bit is 1).

The server MAY perform the AAAA RR DNS update if the "S" bit is 1 in the Flags field of the Client FQDN option in the REPLY message (to be) sent to the client.

The server MAY perform these updates even if the client's message didn't carry the Client FQDN option. The server MUST NOT initiate DNS updates when responding with an ADVERTISE message to the client. Hard boundary, no exceptions 🚫

The server MAY complete its DNS updates (PTR RR, or PTR and AAAA RR) before or after it sends the REPLY message to the client.

If the server's AAAA RR DNS update doesn't finish until after the server has already replied to the DHCPv6 client, the server's interaction with the DNS server MAY cause the DHCPv6 server to change the domain name it associates with the client. This can happen, for example, if the server detects and resolves a domain-name conflict [8]. In that case, the domain name the server returns to the DHCPv6 client would change between two DHCPv6 exchanges. Chaotic but make it make sense 🌈

If the server already did DNS updates for the client and the client's information hasn't changed, the server MAY skip doing additional DNS updates.

When a server gets a RELEASE or DECLINE for an address, notices that the valid lifetime on an address it bound to a client has expired, or ends a binding on an address before the binding's expiration time (for instance, by sending a REPLY with a zero valid lifetime for an address), the server SHOULD delete any PTR RR it associated with that address through DNS update. Plus, if the server took responsibility for AAAA RRs, the server SHOULD also delete the AAAA RR 🧹

7. DNS RR TTLs 💜

RRs tied to DHCP clients can be way more volatile than statically configured RRs 🌪️ DHCP clients and servers that do dynamic updates are wise to set resource record TTLs that reflect this volatility, so that DNS queries are less likely to return records pointing at DHCP IP address assignments that have already expired or been released.

The coupling among primary, secondary, and caching DNS servers is 'loose', and that's a core part of how the DNS was designed 🧶 That looseness means there's no way to prevent every situation where a resolver returns a record for a DHCP-assigned IP address that has expired or been released. In real deployments this rarely, if ever, causes a big problem. Most DHCP-managed clients don't get looked up by name in the DNS very often, and the deployment of IXFR [13] and NOTIFY [14] can shrink the delay between updates and when they show up at secondary servers.

Here are some basic guidelines for implementers 🎀 In general, the TTLs for RRs added because of DHCP IP address assignment activity SHOULD be less than the initial lifetime. The RR TTL on a DNS record added SHOULD NOT exceed 1/3 of the lifetime, but SHOULD NOT be less than 10 minutes. Everyone has different needs, so DHCP servers and clients SHOULD let administrators configure TTLs and upper and lower bounds on the TTL values, either as an absolute time interval or as a percentage of the lease lifetime 🫶

Clients and servers MAY update the TTL of the records as the lifetime is about to expire. There's no requirement that they do, though, since that just adds load to the DNS system for probably very little benefit 🤷‍♀️

8. DNS Update Conflicts ⚡

This document doesn't cover how a DHCPv6 client or server prevents name conflicts 🙅‍♀️ It only covers how a DHCPv6 client and server negotiate the fully qualified domain name and who does the DNS updates.

Implementers of this work will need to think about how name conflicts get prevented. If a DNS updater needs a security token to successfully perform DNS updates on a specific name, name conflicts can only happen if multiple updaters are given a security token for that name. Or, if the fully qualified domains are based on the specific address bound to a client, conflicts won't happen. Or, a name conflict resolution technique as described in "Resolving Name Conflicts" [8]) SHOULD be used.

9. IANA Considerations 🌟

The IANA has assigned DHCPv6 option code 39 for the Client FQDN option. Iconic ✨

10. Security Considerations 🔐

Okay, bestie, this is the serious part 😭 Unauthenticated DNS updates can cause total chaos, whether someone is being sneaky on purpose or somebody just misconfigured something by accident. Admins, be wary of letting unsecured DNS updates happen in zones that are exposed to the global Internet. 🚨 Both DHCPv6 clients and servers SHOULD use some form of update request origin authentication procedure (for example, Secure DNS Dynamic Update [11]) when they're doing DNS updates. 🔑

Whether a DHCPv6 client is responsible for updating an FQDN-to-IPv6-address mapping, or whether that job belongs to the DHCPv6 server, is a site-local decision 🏠 The choice probably depends on the security model used with the DNS update protocol. For example, only a client may have enough credentials to update the FQDN-to-IPv6-address mapping for its own FQDN. 💖

Whether a DHCPv6 server is always responsible for updating the FQDN-to-IPv6-address mapping (on top of updating the IPv6-to-FQDN mapping), no matter what an individual DHCPv6 client wants, is also a site-local matter 🌸 Again, the choice probably comes down to the security model being used with DNS updates. When a DHCPv6 server does DNS updates on behalf of a client, the DHCPv6 server SHOULD be sure of the DNS name it's using for the client, and sure of the client's identity. 🫶

Depending on whether the Authentication option is present and what type of authentication is used with it, a DHCPv6 server may not have much confidence in who its clients are 🙈 There are lots of ways for a DHCPv6 server to come up with a DNS name for a client, but only in certain circumstances will the server actually know for certain who the client is. It's giving "trust but verify" 💅

It is critical to implement proper conflict resolution, and the security considerations of conflict resolution apply [8]. 💀 Don't skip this one, fr fr.

11. Acknowledgements 🎀

Many thanks to Mark Stapp and Yakov Rekhter, since this document is based on the DHCPv4 Client FQDN option [9] 🙌 Also huge thanks to Ralph Droms, Ted Lemon, Josh Littlefield, Kim Kinnear, Pekka Savola, and Mark Stapp for reviewing and commenting. The real MVPs 👑

12. References 📚

12.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[2] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.

[3] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.

[4] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.

[5] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

[6] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001.

[7] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.

[8] Stapp, M. and B. Volz, "Resolution of Fully Qualified Domain Name (FQDN) Conflicts among Dynamic Host Configuration Protocol (DHCP) Clients", RFC 4703, October 2006.

12.2. Informative References

[9] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host Configuration Protocol (DHCP) Client Fully Qualified Domain Name (FQDN) Option", RFC 4702, October 2006.

[10] Marine, A., Reynolds, J., and G. Malkin, "FYI on Questions and Answers - Answers to Commonly asked "New Internet User" Questions", FYI 4, RFC 1594, March 1994.

[11] Wellington, B., "Secure Domain Name System (DNS) Dynamic Update", RFC 3007, November 2000.

[12] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003.

[13] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August 1996.

[14] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996.

Author's Address 🦋

Bernard Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA  01719
USA

Phone: +1 978 936 0382
EMail: volz@cisco.com

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