Internet Engineering Task Force (IETF) M. Boucadair

Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 8973 Orange
Category: Standards Track T. Reddy.K
ISSN: 2070-1721 McAfee
January 2021

DDoS Open Threat Signaling (DOTS) Agent Discovery

Abstract

This document specifies mechanisms to configure DDoS Open Threat
Signaling (DOTS) clients with their DOTS servers. The discovery
procedure also covers the DOTS signal channel Call Home. It can be
useful to know the appropriate DOTS server for a given location in
order to engage mitigation actions. This is true even in cases where
the DOTS client cannot localize the attack: cases where it only knows
that some resources are under attack and that help is needed.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8973.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Why Multiple Discovery Mechanisms?
4. DOTS Discovery Procedure
5. DHCP Options for DOTS Agent Discovery
5.1. DHCPv6 DOTS Options
5.1.1. Format of DOTS Reference Identifier Option
5.1.2. Format of DOTS Address Option
5.1.3. DHCPv6 Client Behavior
5.2. DHCPv4 DOTS Options
5.2.1. Format of DOTS Reference Identifier Option
5.2.2. Format of DOTS Address Option
5.2.3. DHCPv4 Client Behavior
6. Discovery Using Service Resolution
7. DNS Service Discovery
8. Security Considerations
8.1. DHCP
8.2. Service Resolution
8.3. DNS Service Discovery
9. IANA Considerations
9.1. Service Name and Transport Protocol Port Number Registry
9.2. DHCPv6 Options
9.3. DHCPv4 Options
9.4. Application Service & Application Protocol Tags
9.4.1. DOTS Application Service Tag Registration
9.4.2. DOTS Call Home Application Service Tag Registration
9.4.3. signal.udp Application Protocol Tag Registration
9.4.4. signal.tcp Application Protocol Tag Registration
9.4.5. data.tcp Application Protocol Tag Registration
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

DDoS Open Threat Signaling (DOTS) [RFC8811] specifies an architecture
in which a DOTS client can inform a DOTS server that the network is
under a potential attack and that appropriate mitigation actions are
required. Indeed, because the lack of a common method to coordinate
a real-time response among involved actors and network domains
inhibits the effectiveness of DDoS attack mitigation, the DOTS signal
channel protocol [RFC8782] is meant to carry requests for DDoS attack
mitigation. With this approach, DOTS can reduce the impact of an
attack and lead to more efficient defensive actions in various
deployment scenarios, such as those discussed in [DOTS-USE-CASES].
Moreover, DOTS clients can instruct a DOTS server to install named
filtering rules by means of the DOTS data channel protocol [RFC8783].

The basic high-level DOTS architecture is illustrated in Figure 1.

+-----------+ +-------------+
| Mitigator | ~~~~~~~~~~ | DOTS Server |
+-----------+ +------+------+
|
|
|
+---------------+ +------+------+
| Attack Target | ~~~~~~ | DOTS Client |
+---------------+ +-------------+

Figure 1: Basic DOTS Architecture

[RFC8811] specifies that the DOTS client may be provided with a list
of DOTS servers, each associated with one or more IP addresses.
These addresses may or may not be of the same address family. The
DOTS client establishes one or more DOTS sessions by connecting to
the provided DOTS server addresses.

This document specifies methods for DOTS clients to discover their
DOTS server(s). The rationale for specifying multiple discovery
mechanisms is discussed in Section 3.

The discovery methods can also be used by a DOTS server to locate a
DOTS client in the context of DOTS signal channel Call Home
[DOTS-SIG-CALL-HOME]. The basic high-level DOTS Call Home
architecture is illustrated in Figure 2.

+---------------+ +-------------+
| Alert | ~~~~~~ | Call Home |
| | | DOTS Client |
+---------------+ +------+------+
|
|
|
+---------------+ +------+------+
| Attack | ~~~~~~ | Call Home |
| Source(s) | | DOTS Server |
+---------------+ +-------------+

Figure 2: Basic DOTS Signal Channel Call Home Functional Architecture

A DOTS agent may be used to establish base DOTS channels, DOTS Call
Home, or both. This specification accommodates all these deployment
cases.

Considerations for the selection of DOTS server(s) by multihomed DOTS
clients are out of this document's scope; readers should refer to
[DOTS-MULTIHOMING] for more details.

This document assumes that security credentials to authenticate DOTS
server(s) are pre-provisioned to a DOTS client using a mechanism such
as (but not limited to) those discussed in [RFC8572] or
[BTSRP-KEYINFR]. DOTS clients use those credentials for
authentication purposes following the rules documented in [RFC8782].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The reader should be familiar with the terms defined in [RFC3958].

This document makes use of the following terms:

DHCP: refers to both DHCPv4 [RFC2131] and DHCPv6 [RFC8415].

DOTS client: refers to a DOTS-aware software module responsible for
requesting attack response coordination with other DOTS-aware
elements.

DOTS server: is a DOTS-aware software module handling and responding
to messages from DOTS clients. The DOTS server enables mitigation
on behalf of the DOTS client, if requested, by communicating the
DOTS client's request to the mitigator and returning selected
mitigator feedback to the requesting DOTS client.

Call Home DOTS client or server: refers to a DOTS client or server
deployed in a Call Home scenario (Figure 2).

DOTS agent: is any DOTS-aware software module capable of
participating in a DOTS channel.

Peer DOTS agent: refers to the peer DOTS server (base DOTS
operation) or to a peer Call Home DOTS client (for DOTS signal
channel Call Home).

3. Why Multiple Discovery Mechanisms?

Analysis of the various use cases sketched in [DOTS-USE-CASES]
reveals that it is unlikely that one single discovery method can be
suitable for all the sample deployments. Concretely:

* Many of the use cases discussed in [DOTS-USE-CASES] do involve
Customer Premises Equipment (CPE). Multiple CPEs connected to
distinct network providers may even be considered. It is
intuitive to leverage existing mechanisms, such as discovery using
service resolution or DHCP, to provision the CPE acting as a DOTS
client with the DOTS server(s).

* Resolving a DOTS server domain name offered by an upstream transit
provider provisioned to a DOTS client into IP address(es) requires
the use of the appropriate DNS resolvers; otherwise, resolving
those names will fail. The use of protocols, such as DHCP, does
allow associating provisioned DOTS server domain names with a list
of DNS servers to be used for name resolution. Furthermore, DHCP
allows for directly providing IP addresses, therefore, avoiding
the need for extra lookup delays.

* Some of the use cases may allow DOTS clients to have direct
communications with upstream DOTS servers, that is, no DOTS
gateway is involved. Leveraging existing protocol behaviors that
do not require specific features on the node embedding the DOTS
client may ease DOTS deployment. Typically, the use of
Straightforward-Naming Authority Pointer (S-NAPTR) lookups
[RFC3958] allows the DOTS server administrators to provision the
preferred DOTS transport protocol between the DOTS client and the
DOTS server and allows the DOTS client to discover this
preference.

* The upstream network provider is not the DDoS mitigation provider
for some of these use cases. It is safe to assume that, for such
deployments, the DOTS server(s) domain name is provided during the
service subscription (i.e., manual/local configuration).

* Multiple DOTS clients may be enabled within a network (e.g., an
enterprise network). Dynamic discovery needs to be deterministic
from an operational standpoint.

* Some of the use cases may involve a DOTS gateway that is
responsible for selecting the appropriate DOTS server(s) to relay
requests received from DOTS clients.

Consequently, this document describes a unified discovery logic
(Section 4) that involves the following mechanisms:

* dynamic discovery using DHCP (Section 5)

* a resolution mechanism based on S-NAPTR resource records in the
DNS (Section 6)

* DNS Service Discovery (Section 7)

4. DOTS Discovery Procedure

Operators will need a consistent set of ways in which DOTS clients
can discover this information and a consistent priority among these
options. If some devices prefer manual configuration over dynamic
discovery while others prefer dynamic discovery over manual
configuration, the result will be a process where the operator must
find devices that are using the wrong DOTS server(s), determine how
to ensure the devices are configured properly, and then reconfigure
the device through the preferred method.

All DOTS clients MUST support at least one of the three mechanisms
below to determine a DOTS server list. All DOTS clients SHOULD
implement all three, or as many as are practical for any specific
device, of the following ways to discover DOTS servers in order to
facilitate the deployment of DOTS in large-scale environments. For
example, a CPE will support the first two mechanisms, a host within a
LAN will support the last two mechanisms, or an application server
will support a local configuration. More examples are discussed in
Section 3. DOTS clients will prefer information received from the
discovery methods in the order listed below.

1. Explicit Configuration:

Local/Manual Configuration: A DOTS client will learn the DOTS
server(s) by means of local or manual DOTS configuration
(i.e., DOTS servers configured at the system level).
Configuration discovered from a DOTS client application is
considered a local configuration.

An implementation may give the user an opportunity (e.g., by
means of configuration file options or menu items) to specify
DOTS server(s) for each address family. These may be
specified either as a list of IP addresses or the DNS name of
a DOTS server. When only DOTS server IP addresses are
configured, a reference identifier must also be configured for
authentication purposes.

Automatic Configuration (e.g., DHCP): The DOTS client attempts
to discover DOTS server(s) names and/or addresses from DHCP,
as described in Section 5.

2. Service Resolution: The DOTS client attempts to discover DOTS
server name(s) using service resolution, as specified in
Section 6.

3. DNS-SD: DNS-based Service Discovery. The DOTS client attempts to
discover DOTS server name(s) using DNS service discovery, as
specified in Section 7.

Some of these mechanisms imply the use of DNS to resolve the IP
address(es) of the DOTS server, while others imply an IP address of
the relevant DOTS server is obtained directly. Implementation
options may vary on a per-device basis, as some devices may not have
DNS capabilities and/or suitable DNS configuration.

On hosts with more than one interface or address family (IPv4/IPv6),
the DOTS server discovery procedure has to be performed for each
interface-/address-family combination. A DOTS client may choose to
perform the discovery procedure only for a desired interface/address
combination if the client does not wish to discover a DOTS server for
all interface-/address-family combinations.

This procedure is also followed by a Call Home DOTS server to
discover its Call Home DOTS client in the context of
[DOTS-SIG-CALL-HOME].

The discovery method is performed upon the bootstrapping of a DOTS
agent and is reiterated by the DOTS agent upon the following events:

* expiry of a validity timer (e.g., DHCP lease, DHCP information
refresh time, or DNS TTL) associated with a discovered DOTS agent

* expiry of the certificate of a peer DOTS agent currently in use

* attachment to a new network

5. DHCP Options for DOTS Agent Discovery

As reported in Section 1.7.2 of [RFC6125]:

| Some certification authorities issue server certificates based on
| IP addresses, but preliminary evidence indicates that such
| certificates are a very small percentage (less than 1%) of issued
| certificates.

In order to allow for PKIX-based authentication between a DOTS client
and server while accommodating the current best practices for issuing
certificates, this document allows DOTS agents to retrieve the names
of their peer DOTS agents. These names can be used for two purposes:
(1) to retrieve the list of IP addresses of a peer DOTS agent or (2)
to be presented as a reference identifier for authentication
purposes.

Defining the option to include a list of IP addresses would avoid
depending on an underlying name resolution, but that design requires
also supplying a name for PKIX-based authentication purposes.

Given that DOTS gateways can be involved in a DOTS session, a peer
DOTS agent can be reachable using a link-local address. Such
addresses can also be discovered using the options defined in
Section 5.1.

The list of the IP addresses returned by DHCP servers is typically
used to feed the DOTS server selection procedure, including when DOTS
agents are provided with primary and backup IP addresses of their
peer DOTS agents. An example of the DOTS server selection procedure
is specified in Section 4.3 of [RFC8782].

The design assumes that the same peer DOTS agent is used for
establishing both signal and data channels. For more customized
configurations (e.g., transport-specific configuration and distinct
DOTS servers for the signal and data channels), an operator can
supply only a DOTS reference identifier that will be then passed to
the procedure described in Section 6.

The design allows terminating the base DOTS channels and DOTS Call
Home on the same or distinct peer DOTS agents. If distinct peer DOTS
agents are deployed, the DHCP option can return, for example, a list
of IP addresses to a requesting DOTS agent. This list includes the
IP address to be used for the base DOTS channels and the IP address
for the DOTS Call Home. The DOTS client (or Call Home DOTS server)
will then use the address selection procedure specified in
Section 4.3 of [RFC8782] to identify the IP address of the peer DOTS
server (or Call Home DOTS client).

For example, let's consider that the DOTS server is reachable at
2001:db8:122:300::1, while the Call Home DOTS client is reachable at
2001:db8:122:300::2. The DHCP server will then return one DOTS
reference identifier and a list that includes both
2001:db8:122:300::1 and 2001:db8:122:300::2 to a requesting DHCP
client. That list is passed to the DOTS client (or Call Home DOTS
server), which will try to establish connections to the addresses of
that list and destination port number 4646 (or the Call Home port
number). As a result, the DOTS client (or Call Home DOTS server)
will select 2001:db8:122:300::1 (or 2001:db8:122:300::2) as a DOTS
server (or Call Home DOTS client).

5.1. DHCPv6 DOTS Options

5.1.1. Format of DOTS Reference Identifier Option

The DHCPv6 DOTS Reference Identifier option is used to configure the
name of the DOTS server (or the name of the Call Home DOTS client).
The format of this option is shown in Figure 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_V6_DOTS_RI | Option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| dots-agent-name (FQDN) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: DHCPv6 DOTS Reference Identifier Option

The fields of the option shown in Figure 3 are as follows:

Option-code: OPTION_V6_DOTS_RI (141, see Section 9.2).
Option-length: Length of the dots-agent-name field in octets.
dots-agent-name: A fully qualified domain name of the peer DOTS
agent. This field is formatted as specified in Section 10 of
[RFC8415].

An example of the dots-agent-name encoding is shown in Figure 4.
This example conveys the FQDN "dots.example.com", and the resulting
Option-length field is 18.

+------+------+------+------+------+------+------+------+------+
| 0x04 | d | o | t | s | 0x07 | e | x | a |
+------+------+------+------+------+------+------+------+------+
| m | p | l | e | 0x03 | c | o | m | 0x00 |
+------+------+------+------+------+------+------+------+------+

Figure 4: An Example of the dots-agent-name Encoding

5.1.2. Format of DOTS Address Option

The DHCPv6 DOTS Address option can be used to configure a list of
IPv6 addresses of a DOTS server (or a Call Home DOTS client). The
format of this option is shown in Figure 5.

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_V6_DOTS_ADDRESS | Option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DOTS ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DOTS ipv6-address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: DHCPv6 DOTS Address Option

The fields of the option shown in Figure 5 are as follows:

Option-code: OPTION_V6_DOTS_ADDRESS (142, see Section 9.2).
Option-length: Length of the DOTS ipv6-address fields in octets.
This MUST be a multiple of 16.
DOTS ipv6-address: Includes one or more IPv6 addresses [RFC4291] of
the peer DOTS agent to be used by a DOTS agent for establishing a
DOTS session. The addresses are listed in the order of preference
for use by the DOTS agent.

Note that IPv4-mapped IPv6 addresses (Section 2.5.5.2 of [RFC4291])
may be included in this option when there is no DHCPv4 server able to
advertise the DHCPv4 DOTS options (Section 5.2) and when only IPv4
connectivity is possible to the peer DOTS agent.

5.1.3. DHCPv6 Client Behavior

DHCP clients MAY request options OPTION_V6_DOTS_RI and
OPTION_V6_DOTS_ADDRESS, as defined in Sections 18.2.1, 18.2.2,
18.2.4, 18.2.5, 18.2.6, and 21.7 of [RFC8415]. As a convenience to
the reader, it is mentioned here that the DHCP client includes the
requested option codes in the Option Request option.

If the DHCP client receives more than one instance of option
OPTION_V6_DOTS_RI (or OPTION_V6_DOTS_ADDRESS), it MUST use only the
first instance of that option.

The DHCP client MUST silently discard multicast and host loopback
addresses [RFC6890] conveyed in OPTION_V6_DOTS_ADDRESS.

If the DHCP client receives and validates both OPTION_V6_DOTS_RI and
OPTION_V6_DOTS_ADDRESS, the content of OPTION_V6_DOTS_RI is used as
the reference identifier for authentication purposes (e.g., PKIX
[RFC6125]), while the valid addresses included in
OPTION_V6_DOTS_ADDRESS are used to reach the peer DOTS agent. In
other words, the name conveyed in OPTION_V6_DOTS_RI MUST NOT be
passed to an underlying resolution library in the presence of a valid
OPTION_V6_DOTS_ADDRESS in a response.

If the DHCP client receives OPTION_V6_DOTS_RI only, but
OPTION_V6_DOTS_RI contains more than one name, the DHCP client MUST
use only the first name. Once the name is validated (Section 10 of
[RFC8415]), the name is passed to a name resolution library.
Moreover, that name is also used as a reference identifier for
authentication purposes.

If the DHCP client receives OPTION_V6_DOTS_ADDRESS only, the
address(es) included in OPTION_V6_DOTS_ADDRESS are used to reach the
peer DOTS agent. In addition, these addresses can be used as
identifiers for authentication.

5.2. DHCPv4 DOTS Options

5.2.1. Format of DOTS Reference Identifier Option

The DHCPv4 [RFC2132] DOTS Reference Identifier option is used to
configure a name of the peer DOTS agent. The format of this option
is illustrated in Figure 6.

Code Length Peer DOTS agent name
+-----+-----+-----+-----+-----+-----+-----+--
| 147 | n | s1 | s2 | s3 | s4 | s5 | ...
+-----+-----+-----+-----+-----+-----+-----+--

Figure 6: DHCPv4 DOTS Reference Identifier Option

The values s1, s2, s3, etc. represent the domain name labels in the
domain name encoding.

The fields of the option shown in Figure 6 are as follows:

Code: OPTION_V4_DOTS_RI (147, see Section 9.3).
Length: Includes the length of the "Peer DOTS agent name" field in
octets.
Peer DOTS agent name: The domain name of the peer DOTS agent. This
field is formatted as specified in Section 10 of [RFC8415].

5.2.2. Format of DOTS Address Option

The DHCPv4 DOTS Address option can be used to configure a list of
IPv4 addresses of a peer DOTS agent. The format of this option is
illustrated in Figure 7.

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code=148 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DOTS IPv4 Address |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
| | |
| DOTS IPv4 Address | |
| | optional
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
. ... . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

Figure 7: DHCPv4 DOTS Address Option

The fields of the option shown in Figure 7 are as follows:

Code: OPTION_V4_DOTS_ADDRESS (148, see Section 9.3).
Length: Set to 4*N, where N is the number of IPv4 addresses included
in the option.
DOTS IPv4 Address(es): Contains one or more IPv4 addresses of the
peer DOTS agent to be used by a DOTS agent. The addresses are
listed in the order of preference for use by the DOTS agent.

OPTION_V4_DOTS_ADDRESS is a concatenation-requiring option. As such,
the mechanism specified in [RFC3396] MUST be used if
OPTION_V4_DOTS_ADDRESS exceeds the maximum DHCPv4 option size of 255
octets.

5.2.3. DHCPv4 Client Behavior

To discover a peer DOTS agent, the DHCPv4 client MUST include both
OPTION_V4_DOTS_RI and OPTION_V4_DOTS_ADDRESS in a Parameter Request
List option [RFC2132].

If the DHCP client receives more than one instance of
OPTION_V4_DOTS_RI option, it MUST use only the first instance of that
option.

The DHCP client MUST silently discard multicast and host loopback
addresses [RFC6890] conveyed in OPTION_V4_DOTS_ADDRESS.

If the DHCP client receives and validates both OPTION_V4_DOTS_RI and
OPTION_V4_DOTS_ADDRESS, the content of OPTION_V4_DOTS_RI is used as
the reference identifier for authentication purposes (e.g., PKIX
[RFC6125]), while the valid addresses included in
OPTION_V4_DOTS_ADDRESS are used to reach the peer DOTS agent. In
other words, the name conveyed in OPTION_V4_DOTS_RI MUST NOT be
passed to an underlying resolution library in the presence of valid
OPTION_V4_DOTS_ADDRESS in a response.

If the DHCP client receives OPTION_V4_DOTS_RI only, but
OPTION_V4_DOTS_RI option contains more than one name, as
distinguished by the presence of multiple root labels, the DHCP
client MUST use only the first name. Once the name is validated
(Section 10 of [RFC8415]), the name is passed to a name resolution
library. Moreover, that name is also used as a reference identifier
for authentication purposes.

If the DHCP client receives OPTION_V4_DOTS_ADDRESS only, the
address(es) included in OPTION_V4_DOTS_ADDRESS are used to reach the
peer DOTS server. In addition, these addresses can be used as
identifiers for authentication.

6. Discovery Using Service Resolution

This mechanism is performed in two steps:

1. A DNS domain name is retrieved for each combination of interface
and address family. A DOTS agent has to determine the domain in
which it is located relying on dynamic means, such as DHCP
(Section 5). Implementations may allow the user to specify a
default name that is used if no specific name has been
configured.

2. Retrieved DNS domain names are then used for S-NAPTR lookups
[RFC3958]. Further DNS lookups may be necessary to determine the
peer DOTS agent IP address(es).

Once the DOTS agent has retrieved its DNS domain or discovered the
peer DOTS agent name that needs to be resolved, an S-NAPTR lookup
with the appropriate application service and the desired protocol tag
is made to obtain information necessary to connect to the
authoritative peer DOTS agent within the given domain.

This specification defines "DOTS" and "DOTS-CALL-HOME" as application
service tags (Sections 9.4.1 and 9.4.2). It also defines
"signal.udp" (Section 9.4.3), "signal.tcp" (Section 9.4.4), and
"data.tcp" (Section 9.4.5) as application protocol tags. An example
is provided in Figure 8.

In the example below, for domain "example.net", the resolution
algorithm will result in IP address, port, tag, and protocol tuples
listed in Table 1.

example.net.
IN NAPTR 100 10 "" DOTS:signal.udp "" signal.example.net.
IN NAPTR 200 10 "" DOTS:signal.tcp "" signal.example.net.
IN NAPTR 300 10 "" DOTS:data.tcp "" data.example.net.

signal.example.net.
IN NAPTR 100 10 "s" DOTS:signal.udp "" _dots-signal._udp.example.net.
IN NAPTR 200 10 "s" DOTS:signal.tcp "" _dots-signal._tcp.example.net.

data.example.net.
IN NAPTR 100 10 "s" DOTS:data.tcp "" _dots-data._tcp.example.net.
IN NAPTR 200 10 "a" DOTS:data.tcp "" b.example.net.

_dots-signal._udp.example.net.
IN SRV 0 0 5000 a.example.net.

_dots-signal._tcp.example.net.
IN SRV 0 0 5001 a.example.net.

_dots-data._tcp.example.net.
IN SRV 0 0 5002 a.example.net.

a.example.net.
IN AAAA 2001:db8::1

b.example.net.
IN AAAA 2001:db8::2

Figure 8: Example of Discovery of DOTS Servers Using Service
Resolution

+=======+==========+=============+======+========+
| Order | Protocol | IP address | Port | Tag |
+=======+==========+=============+======+========+
| 1 | UDP | 2001:db8::1 | 5000 | Signal |
+-------+----------+-------------+------+--------+
| 2 | TCP | 2001:db8::1 | 5001 | Signal |
+-------+----------+-------------+------+--------+
| 3 | TCP | 2001:db8::1 | 5002 | Data |
+-------+----------+-------------+------+--------+
| 4 | TCP | 2001:db8::2 | 443 | Data |
+-------+----------+-------------+------+--------+

Table 1: Resolution Results

An example is provided in Figure 9 for the Call Home case. In this
example, the resolution algorithm will result in IP address, port,
and protocol tuples listed in Table 2 for domain "example.net".

example.net.
IN NAPTR 100 10 "" DOTS-CALL-HOME:signal.udp "" signal.example.net.
IN NAPTR 200 10 "" DOTS-CALL-HOME:signal.tcp "" signal.example.net.

signal.example.net.
IN NAPTR 100 10 "s" DOTS-CALL-HOME:signal.udp ""
_dots-call-home._udp.example.net.
IN NAPTR 200 10 "s" DOTS-CALL-HOME:signal.tcp ""
_dots-call-home._tcp.example.net.

_dots-call-home._udp.example.net.
IN SRV 0 0 6000 b.example.net.

_dots-call-home._tcp.example.net.
IN SRV 0 0 6001 b.example.net.

b.example.net.
IN AAAA 2001:db8::2

Figure 9: Example of Discovery of DOTS Call Home Client Using Service
Resolution

+=======+==========+=============+======+
| Order | Protocol | IP address | Port |
+=======+==========+=============+======+
| 1 | UDP | 2001:db8::2 | 6000 |
+-------+----------+-------------+------+
| 2 | TCP | 2001:db8::2 | 6001 |
+-------+----------+-------------+------+

Table 2: Resolution Results (Call Home)

Note that customized port numbers are used for the DOTS signal
channel, DOTS data channel, and DOTS signal channel Call Home in the
examples shown in Figures 8 and 9 for illustration purposes. If
default port numbers are used in a deployment, the discovery
procedure will return 4646 (DOTS signal channel) and 443 (DOTS data
channel) as DOTS service port numbers.

If no DOTS-specific S-NAPTR records can be retrieved, the discovery
procedure fails for this domain name (and the corresponding interface
and IP protocol version). If more domain names are known, the
discovery procedure MAY perform the corresponding S-NAPTR lookups
immediately. However, before retrying a lookup that has failed, a
DOTS client MUST wait a time period that is appropriate for the
encountered error (e.g., NXDOMAIN, timeout, etc.).

7. DNS Service Discovery

DNS-based Service Discovery (DNS-SD) [RFC6763] provides generic
solutions for discovering services. DNS-SD defines a set of naming
rules for certain DNS record types that they use for advertising and
discovering services.

Section 4.1 of [RFC6763] specifies that a service instance name in
DNS-SD has the following structure:

<Instance> . <Service> . <Domain>

The <Domain> portion specifies the DNS subdomain where the service
instance is registered. It is a conventional domain name, such as
"example.com".

The <Service> portion of the DOTS service instance name MUST be
"_dots-signal._udp", "_dots-signal._tcp", "_dots-data._tcp", "_dots-
call-home._udp", or "_dots-call-home._tcp".

This document does not define any keys; the TXT record of a DNS-SD
service is thus empty (Section 6 of [RFC6763]).

Figure 10 depicts an excerpt of the DNS zone configuration file
listing record examples to discover two DOTS signal channel servers.
In this example, only UDP is supported as transport for the
establishment of the DOTS signal channel.

_dots-signal._udp.example.net. PTR a._dots-signal._udp.example.net.
_dots-signal._udp.example.net. PTR b._dots-signal._udp.example.net.
a._dots-signal._udp.example.net. SRV 0 0 4646 a.example.net.
b._dots-signal._udp.example.net. SRV 0 0 4646 b.example.net.
a._dots-signal._udp.example.net. TXT ""
b._dots-signal._udp.example.net. TXT ""

Figure 10: An Example of DNS-SD Records for the UDP DOTS Signal
Channel Involving Two Servers with the Same Priority

8. Security Considerations

DOTS-related security considerations are discussed in Section 5 of
[RFC8811]. As a reminder, DOTS agents must authenticate each other
using (D)TLS before a DOTS session is considered valid according to
the [RFC8782].

An attacker may block some protocol messages (e.g., DHCP) to force
the client to use a discovery mechanism with a lower priority. The
security implications of such attack are those inherent to the
fallback discovery mechanism discussed in the following subsections.

The results of the discovery procedure are a function of the
interface/address family. Contacting a discovered DOTS server via an
interface to which it is not bound may exacerbate the delay required
to establish a DOTS channel. Moreover, such behavior may reveal that
a DOTS service is enabled by a DOTS client domain and exposes the
identity of the DOTS service provider (which can be inferred from the
name and the destination IP address) to external networks.

Security considerations related to how security credentials to
authenticate DOTS server(s) are provisioned to a DOTS client are
those inherent to the mechanism used for that purpose (for example,
see [RFC8572]).

8.1. DHCP

The security considerations in [RFC2131] and [RFC8415] are to be
considered. In particular, issues related to rogue DHCP servers and
means to mitigate many of these attacks are discussed in Section 22
of [RFC8415].

An attacker can get a domain name, get a domain-validated public
certificate from a certification authority (CA), and host a DOTS
agent. An active attacker can then spoof DHCP responses to include
the attacker's DOTS agent. Such an attacker can also launch other
attacks, as discussed in Section 22 of [RFC8415]. In addition to the
mitigations listed in Section 22 of [RFC8415], a DOTS agent may be
preconfigured with a list of trusted DOTS domain names. If such a
list is preconfigured, a DOTS agent will accept a DHCP-discovered
name if it matches a name in that list. Also, the DOTS agent has to
check that the "DNS-ID" identifier type within subjectAltName in the
server certificate matches a preconfigured name. If the DOTS agent
is instructed to trust subdomains of the names in that list as well
(e.g., "*.example.com"), a DOTS agent will accept a DHCP-discovered
name that matches a name in the preconfigured list (e.g., "dots-
1.example.com" or "dots-2.example.com").

Relying on an underlying resolution library to resolve a supplied
reference identifier has similar security issues as those discussed
in Section 8.2 (e.g., an active attacker may modify DNS messages used
to resolve the supplied reference identifier and point the client to
an attacker server).

Supplying both an IP address and the reference identifier makes it
easier to use a mis-issued certificate.

8.2. Service Resolution

The primary attack against the methods described in Section 6 is one
that would lead to impersonation of a peer DOTS agent. An attacker
could attempt to compromise the S-NAPTR resolution.

The DOTS client (or a Call Home DOTS server) constructs one reference
identifier for the DOTS server (or a Call Home DOTS client) based on
the domain name that is used for S-NAPTR lookup: DNS-ID. If the
reference identifier is found (as described in Section 6 of
[RFC6125]) in the PKIX certificate's subjectAltName extension, the
DOTS client should accept the certificate for the server.

DNS Security Extensions (DNSSEC) [RFC4033] uses cryptographic keys
and digital signatures to provide authentication of DNS data. The
information that is retrieved from the S-NAPTR lookup and that is
validated using DNSSEC is thereby proved to be the authoritative
data.

8.3. DNS Service Discovery

Since DNS-SD is a specification for how to name and use records in
the existing DNS system, it has no specific additional security
requirements over and above those that already apply to DNS queries
and DNS updates. For DNS queries, DNSSEC SHOULD be used where the
authenticity of information is important. For DNS updates, secure
updates [RFC2136] [RFC3007] SHOULD generally be used to control which
clients have permission to update DNS records.

Note that means such as DNS over TLS (DoT) [RFC7858] or DNS over
HTTPS (DoH) [RFC8484] can be used to prevent eavesdroppers from
accessing DNS messages.

9. IANA Considerations

9.1. Service Name and Transport Protocol Port Number Registry

IANA has allocated the following service names from the registry
available at: <https://www.iana.org/assignments/service-names-port-
numbers/>.

Service Name: dots-data
Port Number: N/A
Transport Protocol(s): TCP
Description: DOTS Data Channel Protocol. The service
name is used to construct the SRV service
name "_dots-data._tcp" for discovering DOTS
servers used to establish DOTS data channel.
Assignee: IESG: [email protected]
Contact: IETF Chair: [email protected]
Reference: [RFC8973]

Service Name: dots-call-home
Transport Protocol(s): TCP/UDP
Description: DOTS Signal Channel Call Home Protocol. The
service name is used to construct the SRV
service names "_dots-call-home._udp" and
"_dots-call-home._tcp" for discovering Call
Home DOTS clients used to establish DOTS
signal channel Call Home.
Assignee: IESG: [email protected]
Contact: IETF Chair: [email protected]
Reference: [RFC8973]

IANA has updated the following entry from the registry available at:
<https://www.iana.org/assignments/service-names-port-numbers/>.

Port Number: 4646
Transport Protocol(s): TCP/UDP
Description: DOTS Signal Channel Protocol. The service
name is used to construct the SRV service
names "_dots-signal._udp" and "_dots-
signal._tcp" for discovering DOTS servers
used to establish DOTS signal channel.
Assignee: IESG: [email protected]
Contact: IETF Chair: [email protected]
Reference: [RFC8782][RFC8973]

9.2. DHCPv6 Options

IANA has assigned the following new DHCPv6 Option Codes in the
registry maintained in <https://www.iana.org/assignments/
dhcpv6-parameters/>.

+=======+========================+============+==================+
| Value | Description | Client ORO | Singleton Option |
+=======+========================+============+==================+
| 141 | OPTION_V6_DOTS_RI | Yes | Yes |
+-------+------------------------+------------+------------------+
| 142 | OPTION_V6_DOTS_ADDRESS | Yes | Yes |
+-------+------------------------+------------+------------------+

Table 3: DHCPv6 Options

9.3. DHCPv4 Options

IANA has assigned the following new DHCPv4 Option Codes in the
registry maintained in <https://www.iana.org/assignments/bootp-dhcp-
parameters/>.

+========================+=====+=========+==============+===========+
| Name | Tag | Data | Meaning | Reference |
| | | Length | | |
+========================+=====+=========+==============+===========+
| OPTION_V4_DOTS_RI | 147 | N | The name | [RFC8973] |
| | | | of the | |
| | | | peer DOTS | |
| | | | agent. | |
+------------------------+-----+---------+--------------+-----------+
| OPTION_V4_DOTS_ADDRESS | 148 | N (the | N/4 IPv4 | [RFC8973] |
| | | minimal | addresses | |
| | | length | of peer | |
| | | is 4) | DOTS | |
| | | | agent(s). | |
+------------------------+-----+---------+--------------+-----------+

Table 4: DHCPv4 Options

9.4. Application Service & Application Protocol Tags

IANA has made the following allocations from the registries available
at <https://www.iana.org/assignments/s-naptr-parameters/> for
application service tags and application protocol tags.

9.4.1. DOTS Application Service Tag Registration

Application Service Tag: DOTS
Intended Usage: See Section 6
Security Considerations: See Section 8
Interoperability Considerations: None
Relevant Publications: RFC 8973

9.4.2. DOTS Call Home Application Service Tag Registration

Application Service Tag: DOTS-CALL-HOME
Intended Usage: See Section 6
Security Considerations: See Section 8
Interoperability Considerations: None
Relevant Publications: RFC 8973

9.4.3. signal.udp Application Protocol Tag Registration

Application Protocol Tag: signal.udp
Intended Usage: See Section 6
Security Considerations: See Section 8
Interoperability Considerations: None
Relevant Publications: RFC 8973

9.4.4. signal.tcp Application Protocol Tag Registration

Application Protocol Tag: signal.tcp
Intended Usage: See Section 6
Security Considerations: See Section 8
Interoperability Considerations: None
Relevant Publications: RFC 8973

9.4.5. data.tcp Application Protocol Tag Registration

Application Protocol Tag: data.tcp
Intended Usage: See Section 6
Security Considerations: See Section 8
Interoperability Considerations: None
Relevant Publications: RFC 8973

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.

[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.

[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
DOI 10.17487/RFC3396, November 2002,
<https://www.rfc-editor.org/info/rfc3396>.

[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
January 2005, <https://www.rfc-editor.org/info/rfc3958>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.

[RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC6890, April 2013,
<https://www.rfc-editor.org/info/rfc6890>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.

10.2. Informative References

[BTSRP-KEYINFR]
Pritikin, M., Richardson, M. C., Eckert, T., Behringer, M.
H., and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", Work in Progress, Internet-
Draft, draft-ietf-anima-bootstrapping-keyinfra-45, 11
November 2020, <https://tools.ietf.org/html/draft-ietf-
anima-bootstrapping-keyinfra-45>.

[DOTS-MULTIHOMING]
Boucadair, M., Reddy, T., and W. Pan, "Multi-homing
Deployment Considerations for Distributed-Denial-of-
Service Open Threat Signaling (DOTS)", Work in Progress,
Internet-Draft, draft-ietf-dots-multihoming-05, 23
November 2020, <https://tools.ietf.org/html/draft-ietf-
dots-multihoming-05>.

[DOTS-SIG-CALL-HOME]
Reddy, T., Boucadair, M., and J. Shallow, "Distributed
Denial-of-Service Open Threat Signaling (DOTS) Signal
Channel Call Home", Work in Progress, Internet-Draft,
draft-ietf-dots-signal-call-home-13, 11 January 2021,
<https://tools.ietf.org/html/draft-ietf-dots-signal-call-
home-13>.

[DOTS-USE-CASES]
Dobbins, R., Migault, D., Moskowitz, R., Teague, N., Xia,
L., and K. Nishizuka, "Use cases for DDoS Open Threat
Signaling", Work in Progress, Internet-Draft, draft-ietf-
dots-use-cases-25, 5 July 2020,
<https://tools.ietf.org/html/draft-ietf-dots-use-cases-
25>.

[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.

[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<https://www.rfc-editor.org/info/rfc3007>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.

[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/info/rfc8572>.

[RFC8782] Reddy.K, T., Ed., Boucadair, M., Ed., Patil, P.,
Mortensen, A., and N. Teague, "Distributed Denial-of-
Service Open Threat Signaling (DOTS) Signal Channel
Specification", RFC 8782, DOI 10.17487/RFC8782, May 2020,
<https://www.rfc-editor.org/info/rfc8782>.

[RFC8783] Boucadair, M., Ed. and T. Reddy.K, Ed., "Distributed
Denial-of-Service Open Threat Signaling (DOTS) Data
Channel Specification", RFC 8783, DOI 10.17487/RFC8783,
May 2020, <https://www.rfc-editor.org/info/rfc8783>.

[RFC8811] Mortensen, A., Ed., Reddy.K, T., Ed., Andreasen, F.,
Teague, N., and R. Compton, "DDoS Open Threat Signaling
(DOTS) Architecture", RFC 8811, DOI 10.17487/RFC8811,
August 2020, <https://www.rfc-editor.org/info/rfc8811>.

Acknowledgements

Thanks to Brian Carpenter for the review of the Bootstrapping Remote
Secure Key Infrastructure (BRSKI) text used in previous draft
versions of the specification.

Many thanks to Russ White for the review, comments, and text
contribution.

Thanks to Dan Wing, Pei Wei, Valery Smyslov, and Jon Shallow for the
review and comments.

Thanks to Bernie Volz for the review of the DHCP section.

Many thanks to Benjamin Kaduk for the detailed AD review.

Thanks to Zhen Cao, Kyle Rose, Nagendra Nainar, and Peter Yee for the
directorate reviews.

Thanks to Barry Leiba, Martin Duke, Roman Danyliw, Éric Vyncke, and
Magnus Westerlund for the IESG review.

Contributors

Prashanth Patil
Cisco Systems, Inc.

Email: [email protected]

Authors' Addresses

Mohamed Boucadair
Orange
35000 Rennes
France

Email: [email protected]

Tirumaleswar Reddy.K
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore 560071
Karnataka
India

Email: [email protected]