Source Packet Routing in Networking N. Buraglio
Internet-Draft Energy Sciences Network
Intended status: Standards Track T. Mizrahi
Expires: 6 November 2025 Huawei
T. Tong
China Unicom
L. M. Contreras
Telefonica
F. Gont
SI6 Networks
5 May 2025
Segment Routing IPv6 Security Considerations
draft-ietf-spring-srv6-security-03
Abstract
SRv6 is a traffic engineering, encapsulation and steering mechanism
utilizing IPv6 addresses to identify segments in a pre-defined
policy. This document discusses security considerations in SRv6
networks, including the potential threats and the possible mitigation
methods. The document does not define any new security protocols or
extensions to existing protocols.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://github.com/buraglio/draft-bdmgct-spring-srv6-security.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-spring-srv6-security/.
Discussion of this document takes place on the Source Packet Routing
in Networking Working Group mailing list (mailto:spring@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/
spring/. Subscribe at https://www.ietf.org/mailman/listinfo/spring/.
Source for this draft and an issue tracker can be found at
https://github.com/buraglio/draft-bdmgct-spring-srv6-security.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scope of this Document . . . . . . . . . . . . . . . . . . . 4
3. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
3.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
3.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
4. Threat Terminology . . . . . . . . . . . . . . . . . . . . . 5
5. Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Attack Abstractions . . . . . . . . . . . . . . . . . . . 9
6.2. Modification Attack . . . . . . . . . . . . . . . . . . . 10
6.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 10
6.2.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2.3. Effect . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. Passive Listening . . . . . . . . . . . . . . . . . . . . 11
6.3.1. Overview . . . . . . . . . . . . . . . . . . . . . . 11
6.3.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3.3. Effect . . . . . . . . . . . . . . . . . . . . . . . 12
6.4. Packet Insertion . . . . . . . . . . . . . . . . . . . . 12
6.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 12
6.4.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . 12
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6.4.3. Effect . . . . . . . . . . . . . . . . . . . . . . . 12
6.5. Control and Management Plane Attacks . . . . . . . . . . 12
6.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 12
6.5.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . 13
6.5.3. Effect . . . . . . . . . . . . . . . . . . . . . . . 13
6.6. Other Attacks . . . . . . . . . . . . . . . . . . . . . . 13
6.7. Attacks - Summary . . . . . . . . . . . . . . . . . . . . 14
7. Mitigation Methods . . . . . . . . . . . . . . . . . . . . . 14
7.1. Trusted Domains and Filtering . . . . . . . . . . . . . . 14
7.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . 14
7.1.2. SRH Filtering . . . . . . . . . . . . . . . . . . . . 15
7.1.3. Address Range Filtering . . . . . . . . . . . . . . . 16
7.2. Encapsulation of Packets . . . . . . . . . . . . . . . . 16
7.3. Hashed Message Authentication Code (HMAC) . . . . . . . . 16
8. Implications on Existing Equipment . . . . . . . . . . . . . 17
8.1. Middlebox Filtering Issues . . . . . . . . . . . . . . . 17
8.2. Limited capability hardware . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Topics for Further Consideration . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
Segment Routing (SR) [RFC8402] utilizing an IPv6 data plane is a
source routing model that leverages an IPv6 underlay and an IPv6
extension header called the Segment Routing Header (SRH) [RFC8754] to
signal and control the forwarding and path of packets by imposing an
ordered list of segments that are processed at each hop along the
signaled path. SRv6 is fundamentally bound to the IPv6 protocol and
introduces a new extension header. There are security considerations
which must be noted or addressed in order to operate an SRv6 network
in a reliable and secure manner. Specifically, some primary
properties of SRv6 that affect the security considerations are:
* SRv6 may use the SRH which is a type of Routing Extension Header
defined by [RFC8754]. Security considerations of the SRH are
discussed [RFC8754] section 7, and were based in part on security
considerations of the deprecated routing header 0 as discussed in
[RFC5095] section 5.
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* SRv6 uses the IPv6 data-plane, and therefore security
considerations of IPv6 are applicable to SRv6 as well. Some of
these considerations are discussed in Section 10 of [RFC8200] and
in [RFC9099].
* While SRv6 uses what appear to be typical IPv6 addresses, the
address space is processed differently by segment endpoints. A
typical IPv6 unicast address is comprised of a network prefix,
host identifier. A typical SRv6 segment identifier (SID) is
comprised of a locator, a function identifier, and optionally,
function arguments. The locator must be routable, which enables
both SRv6 capable and incapable devices to participate in
forwarding, either as normal IPv6 unicast or SRv6 segment
endpoints. The capability to operate in environments that may
have gaps in SRv6 support allows the bridging of islands of SRv6
devices with standard IPv6 unicast routing.
This document describes various threats to SRv6 networks and also
presents existing approaches to avoid or mitigate the threats.
2. Scope of this Document
The following IETF RFCs were selected for security assessment as part
of this effort:
* [RFC8402] : "Segment Routing Architecture"
* [RFC8754] : "IPv6 Segment Routing Header (SRH)"
* [RFC8986] : "Segment Routing over IPv6 (SRv6) Network Programming"
* [RFC9020] : "YANG Data Model for Segment Routing"
* [RFC9256] : "Segment Routing Policy Architecture"
* [RFC9491] : "Integration of the Network Service Header (NSH) and
Segment Routing for Service Function Chaining (SFC)"
* [RFC9524] : "Segment Routing Replication for Multipoint Service
Delivery"
We note that SRv6 is under active development and, as such, the above
documents might not cover all protocols employed in an SRv6
deployment.
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3. Conventions and Definitions
3.1. Requirements Language
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.
3.2. Terminology
* HMAC TLV: Hashed Message Authentication Code Type Length Value
[RFC8754]
* SID: Segment Identifier [RFC8402]
* SRH: Segment Routing Header [RFC8754]
* SRv6: Segment Routing over IPv6 [RFC8402]
4. Threat Terminology
This section introduces the threat taxonomy that is used in this
document, based on terminology from the Internet threat model
[RFC3552], as well as some concepts from [RFC9055], [RFC7384],
[RFC7835] and [RFC9416]. Details regarding inter-domain segment
routing (SR) are out of scope for this document.
Internal vs. External: An internal attacker in the context of SRv6
is an attacker who is located within an SR domain. Specifically,
an internal attacker either has access to a node in the SR domain,
or is located such that it can send and receive packets to and
from a node in the SR domain without traversing an SR ingress node
or an SR egress node. External attackers, on the other hand, are
not within the SR domain.
On-path vs. Off-path: On-path attackers are located in a position
that allows interception, modification or dropping of in-flight
packets, as well as insertion (generation) of packets. Off-path
attackers can only attack by insertion of packets.
Data plane vs. control plane vs. Management plane: Attacks can be
classified based on the plane they target: data, control, or
management. The distinction between on-path and off-path
attackers depends on the plane where the attack occurs. For
instance, an attacker might be off-path from a data plane
perspective but on-path from a control plane perspective.
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The following figure depicts an example of an SR domain with five
attacker types, labeled 1-5. For instance, attacker 2 is located
along the path between the SR ingress node and SR endpoint 1, and is
therefore an on-path attacker both in the data plane and in the
control plane. Thus, attacker 2 can listen, insert, delete, modify
or replay data plane and/or control plane packets in transit. Off-
path attackers, such as attackers 4 and 5, can insert packets, and in
some cases can passively listen to some traffic, such as multicast
transmissions. In this example a Path Computation Element as a
Central Controller (PCECC) [RFC9050] is used as part of the control
plane. Thus, attacker 3 is an internal on-path attacker in the
control plane, as it is located along the path between the PCECC and
SR endpoint 1.
1.on-path 2.on-path 3.mgmt. PCE as a Central 4.off-path 5.off-path
external internal plane Controller internal external
attacker attacker on-path (PCECC) attacker attacker
| | | | | |
| | v _____ v ____ _ | __ |
| SR __ | _ __ / +---+ \___/ | \ |
| domain / | \/ \_/ X-----|PCECC| v / v
| \ | | +---+ X \ X
v / v | /
----->X------>O--->X---------->O------->O-------------->O---->
^\ ^ /^\ /^
| \___/\_ /\_ | _/\__/ | \___/\______/ |
| \__/ | | |
| | | |
SR SR SR SR
ingress endpoint 1 endpoint 2 egress
node node
Figure 1: Threat Model Taxonomy
As defined in [RFC8402], SR operates within a "trusted domain".
Therefore, in the current threat model the SR domain defines the
boundary that distinguishes internal from external threats.
Specifically, an attack on one domain that is invoked from within a
different domain is considered an external attack in the context of
the current document.
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5. Effect
One of the important aspects of threat analysis is assessing the
potential effect or outcome of each threat. SRv6 allows for the
forwarding of IPv6 packets via predetermined SR policies, which
determine the paths and the processing of these packets. An attack
on SRv6 may cause packets to traverse arbitrary paths and to be
subject to arbitrary processing by SR endpoints within an SR domain.
This may allow an attacker to perform a number of attacks on the
victim networks and hosts that would be mostly unfeasible for a non-
SRv6 environment.
The threat model in [ANSI-Sec] classifies threats according to their
potential effect, defining six categories. For each of these
categories we briefly discuss its applicability to SRv6 attacks.
* Unauthorized Access: an attack that results in unauthorized access
might be achieved by having an attacker leverage SRv6 to
circumvent security controls as a result of security devices being
unable to enforce security policies. For example, this can occur
if packets are directed through paths where packet filtering
policies are not enforced, or if some security policies are not
enforced in the presence of IPv6 Extension Headers.
* Masquerade: various attacks that result in spoofing or
masquerading are possible in IPv6 networks. However, these
attacks are not specific to SRv6, and are therefore not within the
scope of this document.
* System Integrity: attacks on SRv6 can manipulate the path and the
processing that the packet is subject to, thus compromising the
integrity of the system. Furthermore, an attack that compromises
the control plane and/or the management plane is also a means of
affecting the system integrity. Specific SRv6-targeted attack may
cause one or more of the following outcomes:
- Avoiding a specific node or path: when an SRv6 policy is
manipulated, specific nodes or paths may be bypassed, for
example in order to bypass the billing service or avoid access
controls and security filters.
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- Preferring a specific path: packets can be manipulated so that
they are diverted to a specific path. This can result in
allowing various unauthorized services such as traffic
acceleration. Alternatively, an attacker can divert traffic to
be forwarded through a specific node that the attacker has
access to, thus facilitating more complex on-path attacks such
as passive listening, recon and various man-in-the-middle
attacks.
- Causing header modifications: SRv6 network programming
determines the SR endpoint behavior, including potential header
modifications. Thus, one of the potential outcomes of an
attack is unwanted header modifications.
* Communication Integrity: SRv6 attacks may cause packets to be
forwarded through paths that the attacker controls, which may
facilitate other attacks that compromise the integrity of user
data. Integrity protection of user data, which is implemented in
higher layers, avoids these aspects, and therefore communication
integrity is not within the scope of this document.
* Confidentiality: as in communication integrity, packets forwarded
through unintended paths may traverse nodes controlled by the
attacker. Since eavesdropping of user data can be avoided by
using encryption in higher layers, it is not within the scope of
this document. However, eavesdropping of a network that uses SRv6
allows the attacker to collect information about SR endpoint
addresses, SR policies, and network topologies, is a specific form
of reconnaissance
* Denial of Service: the availability aspects of SRv6 include the
ability of attackers to leverage SRv6 as a means for compromising
the performance of a network or for causing Denial of Service
(DoS), including:
- Resource exhaustion: compromising the availability of the
system can be achieved by sending SRv6-enabled packets to/
through victim nodes in a way that results in a negative
performance impact of the victim systems (e.g., [RFC9098]).
For example, network programming can be used in some cases to
manipulate segment endpoints to perform unnecessary functions
that consume processing resources. Resource exhaustion may in
severe cases cause Denial of Service (DoS).
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- Forwarding loops: an attacker might achieve attack
amplification by increasing the number hops that each packet is
forwarded through and thus increase the load on the network.
For example, a set of SIDs can be inserted in a way that
creates a forwarding loop ([RFC8402], [RFC5095],
[CanSecWest2007]) and thus loads the nodes along the loop.
- Causing packets to be discarded: an attacker may cause a packet
to be forwarded to a point in the network where it can no
longer be forwarded, causing the packet to be discarded.
Section 6 discusses specific implementations of these attacks, and
possible mitigations are discussed in Section 7.
6. Attacks
6.1. Attack Abstractions
Packet manipulation and processing attacks can be implemented by
performing a set of one or more basic operations. These basic
operations (abstractions) are as follows:
* Passive listening: an attacker who reads packets off the network
can collect information about SR endpoint addresses, SR policies
and the network topology. This information can then be used to
deploy other types of attacks.
* Packet replaying: in a replay attack the attacker records one or
more packets and transmits them at a later point in time.
* Packet insertion: an attacker generates and injects a packet to
the network. The generated packet may be maliciously crafted to
include false information, including for example false addresses
and SRv6-related information.
* Packet deletion: by intercepting and removing packets from the
network, an attacker prevents these packets from reaching their
destination. Selective removal of packets may, in some cases,
cause more severe damage than random packet loss.
* Packet modification: the attacker modifies packets during transit.
This section describes attacks that are based on packet manipulation
and processing, as well as attacks performed by other means. While
it is possible for packet manipulation and processing attacks against
all the fields of the IPv6 header and its extension headers, this
document limits itself to the IPv6 header and the SRH.
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6.2. Modification Attack
6.2.1. Overview
An on-path internal attacker can modify a packet while it is in
transit in a way that directly affects the packet's segment list.
A modification attack can be performed in one or more of the
following ways:
* SID list: the SRH can be manipulated by adding or removing SIDs,
or by modifying existing SIDs.
* IPv6 Destination Address (DA): when an SRH is present modifying
the destination address (DA) of the IPv6 header affects the active
segment. However, DA modification can affect the SR policy even
in the absence of an SRH. One example is modifying a DA which is
used as a Binding SID [RFC8402]. Another example is modifying a
DA which represents a compressed segment list
[I-D.ietf-spring-srv6-srh-compression]. SRH compression allows
encoding multiple compressed SIDs within a single 128-bit SID, and
thus modifying the DA can affect one or more hops in the SR
policy.
* Add/remove SRH: an attacker can insert or remove an SRH.
* SRH TLV: adding, removing or modifying TLV fields in the SRH.
It is noted that the SR modification attack is performed by an on-
path attacker who has access to packets in transit, and thus can
implement these attacks directly. However, SR modification is
relatively easy to implement and requires low processing resources by
an attacker, while it facilitates more complex on-path attacks by
averting the traffic to another node that the attacker has access to
and has more processing resources.
An on-path internal attacker can also modify, insert or delete other
extension headers but these are outside the scope of this document.
6.2.2. Scope
An SR modification attack can be performed by on-path attackers. If
filtering is deployed at the domain boundaries as described in
Section 7.1, the ability to implement SR modification attacks is
limited to on-path internal attackers.
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6.2.3. Effect
SR modification attacks, including adding/removing an SRH, modifying
the SID list and modifying the IPv6 DA, can have one or more of the
following outcomes, which are described in Section 5.
* Unauthorized access
* Avoiding a specific node or path
* Preferring a specific path
* Causing header modifications
* Causing packets to be discarded
* Resource exhaustion
* Forwarding loops
Maliciously adding unnecessary TLV fields can cause further resource
exhaustion.
6.3. Passive Listening
6.3.1. Overview
An on-path internal attacker can passively listen to packets and
specifically listen to the SRv6-related information that is conveyed
in the IPv6 header and the SRH. This approach can be used for
reconnaissance, i.e., for collecting segment lists.
6.3.2. Scope
A reconnaisance attack is limited to on-path internal attackers.
If filtering is deployed at the domain boundaries (Section 7.1), it
prevents any leaks of explicit SRv6 routing information through the
boundaries of the administrative domain. In this case external
attackers can only collect SRv6-related data in a malfunctioning
network in which SRv6-related information is leaked through the
boundaries of an SR domain.
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6.3.3. Effect
While the information collected in a reconnaisance attack does not
compromise the confidentiality of the user data, it allows an
attacker to gather information about the network which in turn can be
used to enable other attacks.
6.4. Packet Insertion
6.4.1. Overview
In a packet insertion attack packets are inserted (injected) into the
network with a segment list. The attack can be applied either by
using synthetic packets or by replaying previously recorded packets.
6.4.2. Scope
Packet insertion can be performed by either on-path or off-path
attackers. In the case of a replay attack, recording packets in-
flight requires on-path access and the recorded packets can later be
injected either from an on-path or an off-path location.
If filtering is deployed at the domain boundaries (Section 7.1),
insertion attacks can only be implemented by internal attackers.
6.4.3. Effect
The main effect of this attack is resource exhaustion, which
compromises the availability of the network, as described in
Section 6.2.3.
6.5. Control and Management Plane Attacks
6.5.1. Overview
Depending on the control plane protocols used in a network, it is
possible to use the control plane as a way of compromising the
network. For example, an attacker can advertise SIDs in order to
manipulate the SR policies used in the network. Known IPv6 control
plane attacks (e.g., overclaiming) are applicable to SRv6 as well.
A compromised management plane can also facilitate a wide range of
attacks, including manipulating the SR policies or compromising the
network availability.
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6.5.2. Scope
The control plane and management plane may be either in-band or out-
of-band, and thus the on-path and off-path taxonomy of Section 4 is
not necessarily common between the data plane, control plane and
management plane. As in the data plane, on-path attackers can be
implement a wide range of attacks in order to compromise the control
and/or management plane, including selectively removing legitimate
messages, replaying them or passively listening to them. However,
while an on-path attacker in the data plane is potentially more
harmful than an off-path attacker, effective control and/or
management plane attacks can be implemented off-path rather than by
trying to intercept or modify traffic in-flight, for example by
exchanging malicious control plane messages with legitimate routers,
by spoofing an SDN (Software Defined Network) controller, or by
gaining access to an NMS (Network Management System).
SRv6 domain boundary filtering can be used for mitigating potential
control plane and management plane attacks from external attackers.
Segment routing does not define any specific security mechanisms in
existing control plane or management plane protocols. However,
existing control plane and management plane protocols use
authentication and security mechanisms to validate the authenticity
of information.
6.5.3. Effect
A compromised control plane or management plane can affect the
network in various possible ways. SR policies can be manipulated by
the attacker to avoid specific paths or to prefer specific paths, as
described in Section 6.2.3. Alternatively, the attacker can
compromise the availability, either by defining SR policies that load
the network resources, as described in Section 6.2.3, or by
blackholing some or all of the SR policies. A passive attacker can
use the control plane or management plane messages as a means for
recon, similarly to Section 6.2.3.
6.6. Other Attacks
Various attacks which are not specific to SRv6 can be used to
compromise networks that deploy SRv6. For example, spoofing is not
specific to SRv6, but can be used in a network that uses SRv6. Such
attacks are outside the scope of this document.
Because SRv6 is completely reliant on IPv6 for addressing,
forwarding, and fundamental networking basics, it is potentially
subject to any existing or emerging IPv6 vulnerabilities [RFC9099],
however, this is out of scope for this document.
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6.7. Attacks - Summary
The following table summarizes the attacks that were described in the
previous subsections, and the corresponding effect of each of the
attacks. Details about the effect are described in Section 5.
+=============+==================+===================================+
| Attack | Details | Effect |
+=============+==================+===================================+
|Modification |Modification of: |* Unauthorized access |
| |* SID list |* Avoiding a specific node or path |
| |* IPv6 DA |* Preferring a specific path |
| |Add/remove/modify:|* Causing header modifications |
| |* SRH |* Causing packets to be discarded |
| |* SRH TLV |* Resource exhaustion |
| | |* Forwarding loops |
+-------------+------------------+-----------------------------------+
|Passive |Passively listen |* Reconnaissance |
|listening |to SRv6-related | |
| |information | |
+-------------+------------------+-----------------------------------+
|Packet |Maliciously inject|* Resource exhaustion |
|insertion |packets with a | |
| |segment list | |
+-------------+------------------+-----------------------------------+
|Control and |Manipulate control|* Unauthorized access |
|management |or management |* Avoiding a specific node or path |
|plane attacks|plane in order to |* Preferring a specific path |
| |manipulate SRv6 |* Causing header modifications |
| |functionality |* Causing packets to be discarded |
| | |* Resource exhaustion |
| | |* Forwarding loops |
+-------------+------------------+-----------------------------------+
Figure 2: Attack Summary
7. Mitigation Methods
This section presents methods that can be used to mitigate the
threats and issues that were presented in previous sections. This
section does not introduce new security solutions or protocols.
7.1. Trusted Domains and Filtering
7.1.1. Overview
As specified in [RFC8402]:
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By default, SR operates within a trusted domain. Traffic MUST be
filtered at the domain boundaries.
The use of best practices to reduce the risk of tampering within the
trusted domain is important. Such practices are discussed in
[RFC4381] and are applicable to both SR-MPLS and SRv6.
Following the spirit of [RFC8402], the current document assumes that
SRv6 is deployed within a trusted domain. Traffic MUST be filtered
at the domain boundaries. Thus, most of the attacks described in
this document are limited to within the domain (i.e., internal
attackers).
Such an approach has been commonly referred to as the concept of
"fail-open", a state of which the attributes are frequently described
as containing inherently more risk than fail-closed methodologies.
The reliance of perfectly crafted filters on on all edges of the
trusted domain, noting that if the filters are removed or adjusted in
an erroneous manner, there is a demonstrable risk of inbound or
outbound leaks. It is also important to note that some filtering
implementations have limits on the size, complexity, or protocol
support that can be applied, which may prevent the filter adjustments
or creation required to properly secure the trusted domain for a new
protocol such as SRv6.
Practically speaking, this means successfully enforcing a "Trusted
Domain" may be operationally difficult and error-prone in practice,
and that attacks that are expected to be unfeasible from outside the
trusted domain may actually become feasible when any of the involved
systems fails to enforce the filtering policy that is required to
define the Trusted Domain.
7.1.2. SRH Filtering
Filtering can be performed based on the presence of an SRH. More
generally, [RFC9288] provides recommendations on the filtering of
IPv6 packets containing IPv6 extension headers at transit routers.
However, filtering based on the presence of an SRH is not necessarily
useful for two reasons: 1. The SRH is optional for SID processing as
described in [RFC8754] section 3.1 and 4.1. 2. A packet containing
an SRH may not be destined to the SR domain, it may be simply
transiting the domain.
For these reasons SRH filtering is not necessarily a useful method of
mitigation.
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7.1.3. Address Range Filtering
The IPv6 destination address can be filtered at the SR ingress node
and at all nodes implementing SRv6 SIDs within the SR domain in order
to mitigate external attacks. Section 5.1 of [RFC8754] describes
this in detail and a summary is presented here: 1. At ingress nodes,
any packet entering the SR domain and destined to a SID within the SR
domain is dropped. 2. At every SRv6 enabled node, any packet
destined to a SID instantiated at the node from a source address
outside the SR domain is dropped.
In order to apply such a filtering mechanism the SR domain needs to
have an infrastructure address range for SIDs, and an infrastructure
address range for source addresses, that can be detected and
enforced. Some examples of an infrastructure address range for SIDs
are: 1. ULA addresses 2. The prefix defined in [RFC9602] 3. GUA
addresses
Many operators reserve a /64 block for all loopback addresses and
allocate /128 for each loopback interface. This simplifies the
filtering of permitted source addresses.
Failure to implement address range filtering at ingress nodes is
mitigated with filtering at SRv6 enabled nodes. Failure to implement
both filtering mechanisms could result in a "fail open" scenario,
where some attacks by internal attackers described in this document
may be launched by external attackers.
Filtering on prefixes has been shown to be useful, specifically
[RFC8754]'s description of packet filtering. There are no known
limitations with filtering on infrastructure addresses, and [RFC9099]
expands on the concept with control plane filtering.
7.2. Encapsulation of Packets
Packets steered in an SR domain are often encapsulated in an IPv6
encapsulation. This mechanism allows for encapsulation of both IPv4
and IPv6 packets. Encapsulation of packets at the SR ingress node
and decapsulation at the SR egress node mitigates the ability of
external attackers to attack the domain.
7.3. Hashed Message Authentication Code (HMAC)
The SRH can be secured by an HMAC TLV, as defined in [RFC8754]. The
HMAC is an optional TLV that secures the segment list, the SRH flags,
the SRH Last Entry field and the IPv6 source address. A pre-shared
key is used in the generation and verification of the HMAC.
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Using an HMAC in an SR domain can mitigate some of the SR
Modification Attacks (Section 6.2). For example, the segment list is
protected by the HMAC.
The following aspects of the HMAC should be considered:
* The HMAC TLV is OPTIONAL.
* While it is presumed that unique keys will be employed by each
participating node, in scenarios where the network resorts to
manual configuration of pre-shared keys, the same key might be
reused by multiple systems as an (incorrect) shortcut to keeping
the problem of pre-shared key configuration manageable.
* When the HMAC is used there is a distinction between an attacker
who becomes internal by having physical access, for example by
plugging into an active port of a network device, and an attacker
who has full access to a legitimate network node, including for
example encryption keys if the network is encrypted. The latter
type of attacker is an internal attacker who can perform any of
the attacks that were described in the previous section as
relevant to internal attackers.
* An internal attacker who does not have access to the pre-shared
key can capture legitimate packets, and later replay the SRH and
HMAC from these recorded packets. This allows the attacker to
insert the previously recorded SRH and HMAC into a newly injected
packet. An on-path internal attacker can also replace the SRH of
an in-transit packet with a different SRH that was previously
captured.
These considerations limit the extent to which HMAC TLV can be relied
upon as a security mechanism that could readily mitigate threats
associated with spoofing and tampering protection for the IPv6 SRH.
8. Implications on Existing Equipment
8.1. Middlebox Filtering Issues
When an SRv6 packet is forwarded in the SRv6 domain, its destination
address changes constantly and the real destination address is
hidden. Security devices on SRv6 network may not learn the real
destination address and fail to perform access control on some SRv6
traffic.
The security devices on SRv6 networks need to take care of SRv6
packets. However, SRv6 packets are often encapsulated by an SR
ingress device with an IPv6 encapsulation that has the loopback
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address of the SR ingress device as a source address. As a result,
the address information of SR packets may be asymmetric, resulting in
improper traffic filter problems, which affects the effectiveness of
security devices. For example, along the forwarding path in SRv6
network, the SR-aware firewall will check the association
relationships of the bidirectional VPN traffic packets. And it is
able to retrieve the final destination of an SRv6 packet from the
last entry in the SRH. When the <source, destination> tuple of the
packet from PE1 (Provider Edge 1) to PE2 is <PE1-IP-ADDR, PE2-VPN-
SID>, and the other direction is <PE2-IP-ADDR, PE1-VPN-SID>, the
source address and destination address of the forward and backward
traffic are regarded as different flows. Thus, legitimate traffic
may be blocked by the firewall.
Forwarding SRv6 traffic through devices that are not SRv6-aware might
in some cases lead to unpredictable behavior. Because of the
existence of the SRH, and the additional headers, security
appliances, monitoring systems, and middle boxes could react in
different ways if they do not incorporate support for the supporting
SRv6 mechanisms, such as the IPv6 Segment Routing Header (SRH)
[RFC8754]. Additionally, implementation limitations in the
processing of IPv6 packets with extension headers may result in SRv6
packets being dropped [RFC7872],[RFC9098].
8.2. Limited capability hardware
In some cases, access control list capabilities are a resource shared
with other features across a given hardware platform. Filtering
capabilities should be considered along with other hardware reliant
functions such as VLAN scale, route table size, MAC address table
size, etc. Filtering both at the control and data plane may or may
not require shared resources. For example, some platforms may
require allocating resources from route table size in order to
accommodate larger numbers of access lists. Hardware and software
configurations should be considered when designing the filtering
capabilities for an SRv6 control and data plane.
9. Security Considerations
The security considerations of SRv6 are presented throughout this
document.
10. IANA Considerations
This document has no IANA actions.
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11. Topics for Further Consideration
This section lists topics that will be discussed further before
deciding whether they need to be included in this document, as well
as some placeholders for items that need further work.
* Add tables for attack section
* The following references may be used in the future: RFC9256
[RFC8986]
* SRH compression
* Spoofing
* Path enumeration
* host to host scenario involving a WAN and/or a data center fabric.
* Terms that may be used in a future version: Locator Block, FRR,
uSID
* L4 checksum: [RFC8200] specifies that when the Routing header is
present the L4 checksum is computed by the originating node based
on the IPv6 address of the last element of the Routing header.
When compressed segment lists
[I-D.ietf-spring-srv6-srh-compression] are used, the last element
of the Routing header may be different than the Destination
Address as received by the final destination. Furthermore,
compressed segment lists can be used in the Destination Address
without the presence of a Routing header, and in this case the
IPv6 Destination address can be modified along the path. As a
result, some existing middleboxes which verify the L4 checksum
might miscalculate the checksum. This issue is currently under
discussion in the SPRING WG.
* Segment Routing Header figure: the SRv6 Segment Routing Header
(SRH) is defined in [RFC8754].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Entry | Flags | Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[0] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
...
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Segment List[n] (128 bits IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12. References
12.1. 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/rfc/rfc2119>.
[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/rfc/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/rfc/rfc8402>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/rfc/rfc8754>.
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[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/rfc/rfc8986>.
[RFC9020] Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J.
Tantsura, "YANG Data Model for Segment Routing", RFC 9020,
DOI 10.17487/RFC9020, May 2021,
<https://www.rfc-editor.org/rfc/rfc9020>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/rfc/rfc9256>.
[RFC9491] Guichard, J., Ed. and J. Tantsura, Ed., "Integration of
the Network Service Header (NSH) and Segment Routing for
Service Function Chaining (SFC)", RFC 9491,
DOI 10.17487/RFC9491, November 2023,
<https://www.rfc-editor.org/rfc/rfc9491>.
[RFC9524] Voyer, D., Ed., Filsfils, C., Parekh, R., Bidgoli, H., and
Z. Zhang, "Segment Routing Replication for Multipoint
Service Delivery", RFC 9524, DOI 10.17487/RFC9524,
February 2024, <https://www.rfc-editor.org/rfc/rfc9524>.
12.2. Informative References
[ANSI-Sec] "Operations, Administration, Maintenance, and Provisioning
Security Requirements for the Public Telecommunications
Network: A Baseline of Security Requirements for the
Management Plane", 2003, <https://www.ieee802.org/1/ecsg-
linksec/meetings/July03/3m150075.pdf>.
[CanSecWest2007]
"IPv6 Routing Header Security", 2007, <https://airbus-
seclab.github.io/ipv6/IPv6_RH_security-csw07.pdf>.
[I-D.ietf-spring-srv6-srh-compression]
Cheng, W., Filsfils, C., Li, Z., Decraene, B., and F.
Clad, "Compressed SRv6 Segment List Encoding (CSID)", Work
in Progress, Internet-Draft, draft-ietf-spring-srv6-srh-
compression-23, 6 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
srv6-srh-compression-23>.
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[IANAIPv6SPAR]
"IANA IPv6 Special-Purpose Address Registry", n.d.,
<https://www.iana.org/assignments/iana-ipv6-special-
registry/iana-ipv6-special-registry.xhtml>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/rfc/rfc3552>.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
DOI 10.17487/RFC5095, December 2007,
<https://www.rfc-editor.org/rfc/rfc5095>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/rfc/rfc7384>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <https://www.rfc-editor.org/rfc/rfc7855>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/rfc/rfc7872>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>.
[RFC9055] Grossman, E., Ed., Mizrahi, T., and A. Hacker,
"Deterministic Networking (DetNet) Security
Considerations", RFC 9055, DOI 10.17487/RFC9055, June
2021, <https://www.rfc-editor.org/rfc/rfc9055>.
[RFC9098] Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
G., and W. Liu, "Operational Implications of IPv6 Packets
with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
September 2021, <https://www.rfc-editor.org/rfc/rfc9098>.
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[RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
"Operational Security Considerations for IPv6 Networks",
RFC 9099, DOI 10.17487/RFC9099, August 2021,
<https://www.rfc-editor.org/rfc/rfc9099>.
[RFC9288] Gont, F. and W. Liu, "Recommendations on the Filtering of
IPv6 Packets Containing IPv6 Extension Headers at Transit
Routers", RFC 9288, DOI 10.17487/RFC9288, August 2022,
<https://www.rfc-editor.org/rfc/rfc9288>.
[RFC9416] Gont, F. and I. Arce, "Security Considerations for
Transient Numeric Identifiers Employed in Network
Protocols", BCP 72, RFC 9416, DOI 10.17487/RFC9416, July
2023, <https://www.rfc-editor.org/rfc/rfc9416>.
[STRIDE] "The STRIDE Threat Model", 2018,
<https://msdn.microsoft.com/en-us/library/
ee823878(v=cs.20).aspx>.
Acknowledgments
The authors would like to acknowledge the valuable input and
contributions from Zafar Ali, Andrew Alston, Dale Carder, Bruno
Decraene, Dhruv Dhody, Mike Dopheide, Darren Dukes, Joel Halpern,
Boris Hassanov, Alvaro Retana, Eric Vyncke, and Russ White.
Authors' Addresses
Nick Buraglio
Energy Sciences Network
Email: buraglio@forwardingplane.net
Tal Mizrahi
Huawei
Email: tal.mizrahi.phd@gmail.com
Tian Tong
China Unicom
Email: tongt5@chinaunicom.cn
Luis M. Contreras
Telefonica
Email: luismiguel.contrerasmurillo@telefonica.com
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Fernando Gont
SI6 Networks
Email: fgont@si6networks.com
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