Source Address Validation Improvement J. Bi
Internet-Draft J. Wu
Intended status: Standards Track G. Yao
Expires: July 13, 2015 Tsinghua Univ.
F. Baker
Cisco
January 9, 2015
SAVI Solution for DHCP
draft-ietf-savi-dhcp-31
Abstract
This document specifies the procedure for creating a binding between
a DHCPv4/DHCPv6-assigned IP address and a binding anchor on a Source
Address Validation Improvements (SAVI) device. The bindings set up
by this procedure are used to filter packets with forged source IP
addresses. This mechanism complements BCP 38 ingress filtering,
providing finer-grained source IP address validation.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 13, 2015.
Copyright Notice
Copyright (c) 2015 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Deployment Scenario and Configuration . . . . . . . . . . . . 7
4.1. Elements and Scenario . . . . . . . . . . . . . . . . . . 8
4.2. SAVI Binding Type Attributes . . . . . . . . . . . . . . 9
4.2.1. Trust Attribute . . . . . . . . . . . . . . . . . . . 9
4.2.2. DHCP-Trust Attribute . . . . . . . . . . . . . . . . 10
4.2.3. DHCP-Snooping Attribute . . . . . . . . . . . . . . . 10
4.2.4. Data-Snooping Attribute . . . . . . . . . . . . . . . 11
4.2.5. Validating Attribute . . . . . . . . . . . . . . . . 11
4.2.6. Table of Mutual Exclusions . . . . . . . . . . . . . 12
4.3. Perimeter . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.1. SAVI-DHCP Perimeter Overview . . . . . . . . . . . . 12
4.3.2. SAVI-DHCP Perimeter Configuration Guideline . . . . . 13
4.3.3. On the Placement of the DHCP Server and Relay . . . . 14
4.3.4. An Alternative Deployment . . . . . . . . . . . . . . 14
4.3.5. Considerations regarding Binding Anchors . . . . . . 15
4.4. Other Device Configuration . . . . . . . . . . . . . . . 16
5. Binding State Table (BST) . . . . . . . . . . . . . . . . . . 16
6. DHCP Snooping Process . . . . . . . . . . . . . . . . . . . . 17
6.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Binding States Description . . . . . . . . . . . . . . . 18
6.3. Events . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3.1. Timer Expiration Event . . . . . . . . . . . . . . . 18
6.3.2. Control Message Arriving Events . . . . . . . . . . . 18
6.4. The State Machine of DHCP Snooping Process . . . . . . . 20
6.4.1. Initial State: NO_BIND - No binding has been set up . 20
6.4.2. Initial State: INIT_BIND - A potential binding has
been set up . . . . . . . . . . . . . . . . . . . . . 22
6.4.3. Initial State: BOUND - The binding has been set up . 25
6.4.4. Table of State Machine . . . . . . . . . . . . . . . 27
7. Data Snooping Process . . . . . . . . . . . . . . . . . . . . 29
7.1. Scenario . . . . . . . . . . . . . . . . . . . . . . . . 29
7.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 30
7.3. Additional Binding States Description . . . . . . . . . . 30
7.4. Events . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.5. Initial State: state NO_BIND - No binding has been set up 32
7.5.1. Event: EVE_DATA_UNMATCH: A data packet without
matched binding is received . . . . . . . . . . . . . 32
7.5.2. Events not observed in NO_BIND . . . . . . . . . . . 33
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7.6. Initial State: state DETECTION - The address in the entry
is under local duplication detection . . . . . . . . . . 33
7.6.1. Event: EVE_ENTRY_EXPIRE . . . . . . . . . . . . . . . 33
7.6.2. Event: EVE_DATA_CONFLICT: ARP Reply/Neighbor
Advertisement(NA) message received from unexpected
system . . . . . . . . . . . . . . . . . . . . . . . 34
7.6.3. Events not observed in DETECTION . . . . . . . . . . 34
7.7. Initial State: state RECOVERY - The SAVI device is
querying the assignment and lease time of the address in
the entry through DHCP Leasequery . . . . . . . . . . . . 34
7.7.1. Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE
or LEASEQUERY-REPLY is received . . . . . . . . . . . 34
7.7.2. Event: EVE_ENTRY_EXPIRE . . . . . . . . . . . . . . . 36
7.7.3. Events not observed in RECOVERY . . . . . . . . . . . 36
7.8. Initial State: state BOUND - The binding has been set up 36
7.9. Table of State Machine . . . . . . . . . . . . . . . . . 36
8. Filtering Specification . . . . . . . . . . . . . . . . . . . 38
8.1. Data Packet Filtering . . . . . . . . . . . . . . . . . . 38
8.2. Control Packet Filtering . . . . . . . . . . . . . . . . 38
9. State Restoration . . . . . . . . . . . . . . . . . . . . . . 39
9.1. Attribute Configuration Restoration . . . . . . . . . . . 39
9.2. Binding State Restoration . . . . . . . . . . . . . . . . 39
10. Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 40
11. Security Considerations . . . . . . . . . . . . . . . . . . . 40
11.1. Security Problems about the Data Snooping Process . . . 40
11.2. Client departure issues . . . . . . . . . . . . . . . . 41
11.3. Duplicate Bindings to the Same Address . . . . . . . . . 42
11.4. Compatibility with DNA (Detecting Network Attachment) . 42
11.5. Binding Number Limitation . . . . . . . . . . . . . . . 43
11.6. Privacy Considerations . . . . . . . . . . . . . . . . . 43
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
13. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
14.1. Normative References . . . . . . . . . . . . . . . . . . 44
14.2. Informative References . . . . . . . . . . . . . . . . . 45
1. Introduction
This document describes a fine-grained source IPv4 or IPv6 source
address validation mechanism. This mechanism creates bindings
between IP addresses assigned to network interfaces by DHCP and
suitable binding anchors (Section 4.3.5). As discussed in Section 3
and [RFC7039], a "binding anchor" is an attribute that is immutable
or difficult to change that may be used to identify the system an IP
address has been assigned to; common examples include a MAC address
found on an Ethernet switch port or WiFi security association. The
bindings are used to identify and filter packets originated by these
interfaces using forged source IP addresses. In this way, this
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mechanism can prevent hosts from using IP addresses assigned to the
other attachment points or invalid in the network. This behavior is
referred to as "spoofing", and is key to amplification attacks, in
which a set of systems send messages to another set of systems
claiming to be from a third set of systems, and sending the replies
to systems that don't expect them. If [RFC2827] protects a network
from a neighboring network by providing prefix granularity source IP
address validity, this mechanism protects a network, including a
Local Area Network, from itself by providing address granularity
source IP validity when DHCP/DHCPv6 is used to assign IPv4/IPv6
addresses. Both provide a certain level of traceability, in that
packet drops indicate the presence of a system that is producing
packets with spoofed IP addresses.
SAVI-DHCP snoops DHCP address assignments to set up bindings between
IP addresses assigned by DHCP and corresponding binding anchors. It
includes the DHCPv4 and v6 snooping process (Section 6), the Data
Snooping process (Section 7), as well as a number of other technical
details. The Data Snooping process is a data-triggered procedure
that snoops the header of data packet to set up bindings. It is
designed to avoid a permanent block of valid addresses in the case
that DHCP snooping is insufficient to set up all the valid bindings.
This mechanism is designed for the stateful DHCP scenario [RFC2131],
[RFC3315]. Stateless DHCP [RFC3736] is out of scope for this
document, as it has nothing to do with IP address allocation. The
appropriate SAVI method must be used in those cases. For hosts using
Stateless Auto-Configuration to allocate addresses, SAVI-FCFS
[RFC6620] should be enabled. Besides, this mechanism is primarily
designed for pure DHCP scenarios in which only addresses assigned
through DHCP are allowed. However, it does not block link-local
addresses, as they are not assigned using DHCP. It is RECOMMENDED
that the administration enable a SAVI solution for link-local
addresses, e.g., SAVI-FCFS [RFC6620].
This mechanism works for DHCPv4-only, DHCPv6-only, or both DHCPv4 and
DHCPv6. However, the DHCP address assignment mechanism in IPv4/IPv6
transition scenarios, e.g., [RFC7341], are beyond the scope of this
document.
2. Requirements Language
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 RFC 2119 [RFC2119].
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3. Terminology
Binding anchor: A "binding anchor" is defined to be a physical and/or
link-layer property of an attached device, as in [RFC7039]. A list
of sample binding anchors can be found in Section 3.2 of that
document. To the degree possible, a binding anchor associates an IP
address with something unspoofable that identifies a single client
system or one of its interfaces. See Section 4.3.5 for more detail.
Attribute: A configurable property of each binding anchor (port, MAC
Address, or other information) that indicates the actions to be
performed on packets received from the attached network device.
DHCP address: An IP address assigned via DHCP.
SAVI-DHCP: The name of this SAVI function for DHCP-assigned
addresses.
SAVI device: A network device on which SAVI-DHCP is enabled.
Non-SAVI device: A network device on which SAVI-DHCP is not enabled.
DHCP Client-Server message: A message that is sent from a DHCP client
to a DHCP server or DHCP servers. Such a message is of one of the
following types:
o DHCPv4 Discover: DHCPDISCOVER [RFC2131]
o DHCPv4 Request: DHCPREQUEST generated during SELECTING state
[RFC2131]
o DHCPv4 Renew: DHCPREQUEST generated during RENEWING state
[RFC2131]
o DHCPv4 Rebind: DHCPREQUEST generated during REBINDING state
[RFC2131]
o DHCPv4 Reboot: DHCPREQUEST generated during INIT-REBOOT state
[RFC2131]
o Note: DHCPv4 Request/Renew/Rebind/Reboot messages can be
identified based on the Table 4 of [RFC2131]
o DHCPv4 Decline: DHCPDECLINE [RFC2131]
o DHCPv4 Release: DHCPRELEASE [RFC2131]
o DHCPv4 Inform: DHCPINFORM [RFC2131]
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o DHCPv6 Request: REQUEST [RFC3315]
o DHCPv6 Solicit: SOLICIT [RFC3315]
o DHCPv6 Confirm: CONFIRM [RFC3315]
o DHCPv6 Decline: DECLINE [RFC3315]
o DHCPv6 Release: RELEASE [RFC3315]
o DHCPv6 Rebind: REBIND [RFC3315]
o DHCPv6 Renew: RENEW [RFC3315]
o DHCPv6 Information-Request: INFORMATION-REQUEST [RFC3315]
DHCP Server-to-Client message: A message that is sent from a DHCP
server to a DHCP client. Such a message is of one of the following
types:
o DHCPv4 ACK: DHCPACK [RFC2131]
o DHCPv4 NAK: DHCPNAK [RFC2131]
o DHCPv4 Offer: DHCPOFFER [RFC2131]
o DHCPv6 Reply: REPLY [RFC3315]
o DHCPv6 Advertise: ADVERTISE [RFC3315]
o DHCPv6 Reconfigure: RECONFIGURE [RFC3315]
Lease time: The lease time in IPv4 [RFC2131] or the valid lifetime in
IPv6 [RFC3315].
Binding entry: A rule that associates an IP address with a binding
anchor.
Binding State Table (BST): The data structure that contains the
binding entries.
Binding entry limit: The maximum number of binding entries that may
be associated with a binding anchor. Limiting the number of binding
entries per binding anchor prevents a malicious or malfunctioning
node from overloading the binding table on a SAVI device.
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Direct attachment: Ideally, a SAVI device is an access device that
hosts are attached to directly. In such a case, the hosts are direct
attachments (e.g., they attach directly) to the SAVI device.
Indirect attachment: A SAVI device MAY be an aggregation device that
other access devices are attached to, and which hosts in turn attach
to. In such a case, the hosts are indirect attachments (e.g., they
attach indirectly) to the SAVI device.
Unprotected link: Unprotected links are links that connect to hosts
or networks of hosts that the device may not see DHCP messages to,
and are therefore outside the SAVI perimeter.
Unprotected device: An unprotected device is a device associated with
an unprotected link. One example might be the gateway router of a
network.
Protected link: Protected links are links that connect to hosts that
the device will invariably see DHCP messages to, and are therefore
within the SAVI perimeter.
Protected device: A protected device is a device associated with a
protected link. One example might be a desktop switch in the
network, or a host.
Cut Vertex: A cut vertex is any vertex whose removal increases the
number of connected components. This is a concept in graph theory.
This term is used in Section 6.1 to accurately specify the required
deployment location of SAVI devices when they only perform the DHCP
snooping process.
Identity Association (IA): "A collection of addresses assigned to a
client." [RFC3315]
Detection message: a Neighbor Solicitation or ARP message intended to
detect a duplicate address by the Data Snooping Process.
DHCP_DEFAULT_LEASE: default lifetime for DHCPv6 address when the
binding is triggered by a DHCPv6 Confirm message but a DHCPv6 lease
query exchange [RFC5007] cannot be performed by the SAVI device to
fetch the lease.
4. Deployment Scenario and Configuration
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4.1. Elements and Scenario
The essential elements in a SAVI-DHCP deployment scenario include a
DHCP server (which may or may not be assigned an address using DHCP,
and therefore may or may not be protected), zero or more protected
DHCP clients, and one or more SAVI devices. It may also include DHCP
relays, when the DHCP server is not co-located with a set of clients,
and zero or more protected Non-SAVI devices. Outside the perimeter,
via unprotected links, there may be many unprotected devices.
+--------+ +------------+ +----------+
|DHCP |-----| Non-SAVI |----|Bogus DHCP|
|Server A| | Device 1 | |Server |
+--------+ +-----|------+ +----------+
|unprotected link
. . . . . . . . . . .|. . . . . . . . . . . . . .
. | .
. Protection +---|------+ .
. Perimeter | SAVI |--------------+ .
. | Device C| | .
. +---|------+ | .
. | | .
. +----------+ +---|------+ +------|---+ .
protected . | SAVI | | Non SAVI| | SAVI | .
link +----.-| Device A|----| Device 3|-------| Device B| .
| . +----|--|--+ +----------+ +-|---|----+ .
| . | +----------+ . . . . . | | .
| '. . . . . . . | . . | | .
| | . | . +--------+ | .
+----|-----+ +--|---+ . +----|-+ . +--|---+ . +---|----+ .
| Non-SAVI | |Client| . |DHCP | . |Client| . |DHCP | .
| Device 2 | |A | . |Relay | . |B | . |Server B| .
+----------+ +------+ . +------+ . +------+ . +--------+ .
. . . . . . . . . . . .
Figure 1: SAVI-DHCP Scenario
Figure 1 shows a deployment scenario that contains these elements.
Note that a physical device can instantiate multiple elements, e.g.,
a switch can be both a SAVI device and a DHCP relay, or in a cloud
computing environment, a physical host may contain a virtual switch
plus some number of virtual hosts. In such cases, the links are
logical links rather than physical links.
Networks are not usually isolated. As a result, traffic from other
networks, including transit traffic as specified in [RFC6620] (e.g.,
traffic from another SAVI switch or a router) may enter a SAVI-DHCP
network through the unprotected links. Since SAVI solutions are
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limited to validating traffic generated from a local link, SAVI-DHCP
does not set up bindings for addresses assigned in other networks and
cannot validate them. Traffic from unprotected links should be
checked by an unprotected system or [RFC2827] mechanisms. The
generation and deployment of such a mechanism is beyond the scope of
this document.
Traffic from protected links is, however, locally generated, and
should be checked by SAVI-DHCP if possible. In the event that there
is an intervening protected non-SAVI device between the host and the
SAVI device, however, use of the physical attachment point alone as a
binding anchor is insufficiently secure, as the several devices on a
port or other point of attachment can spoof each other. Hence,
additional information such as a MAC address SHOULD be used to
disambiguate them.
4.2. SAVI Binding Type Attributes
As illustrated in Figure 1, an system attached to a SAVI device can
be a DHCP client, a DHCP relay/server, a SAVI device, or a non-SAVI
device. Different actions are performed on traffic originated from
different elements. To distinguish among their requirements, several
properties are associated with their point of attachment on the SAVI
device.
When a binding association is uninstantiated, e.g., when no host is
attached to the SAVI device using a given port or other binding
anchor, the binding port attributes take default values unless
overridden by configuration. By default, a SAVI switch does not
filter DHCP messages, nor does it attempt to validate source
addresses. This is because a SAVI switch that depends on DHCP cannot
tell, a priori, which ports have valid DHCP servers attached, or
which have routers or other equipment that would validly appear to
use an arbitrary set of source addresses.
4.2.1. Trust Attribute
The "Trust Attribute" is a Boolean value. If TRUE, it indicates that
the packets from the corresponding attached device need not have
their source addresses validated. Examples of a trusted binding
anchor would be a port to another SAVI device, or to an IP router, as
shown in Figure 1. In both cases, traffic using many source IP
addresses will be seen. By default, the Trust attribute is FALSE,
indicating that any device found on that port will seek an address
using DHCP and be limited to using such addresses.
SAVI devices will not set up bindings for points of attachment with
the Trust attribute set TRUE; DHCP messages and data packets from
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attached devices with this attribute will not be checked. If the
DHCP Server-to-Client messages from attached devices with this
attribute can trigger the state transitions specified in Section 6
and Section 7, the corresponding processes in Section 6 and Section 7
will handle these messages.
4.2.2. DHCP-Trust Attribute
The "DHCP-Trust Attribute" is similarly a Boolean attribute. It
indicates whether the attached device is permitted to initiate DHCP
Server-to-Client messages. In Figure 1, the points of attachment of
the DHCP Server and the DHCP Relay would have this attribute set
TRUE, and ports that are trusted would have it set TRUE.
If the DHCP-Trust Attribute is TRUE, SAVI devices will forward DHCP
Server-to-Client messages from the points of attachment with this
attribute. If the DHCP Server-to-Client messages can trigger the
state transitions, the binding setup processes specified in Section 6
and Section 7 will handle them. By default, the DHCP-Trust attribute
is FALSE, indicating that the attached system is not a DHCP server.
A DHCPv6 implementor can refer to [I-D.ietf-opsec-dhcpv6-shield] for
more details.
4.2.3. DHCP-Snooping Attribute
The "DHCP-Snooping Attribute" is similarly a Boolean attribute. It
indicates whether bindings will be set up based on DHCP snooping.
If this attribute is TRUE, DHCP Client-Server messages to points of
attachment with this attribute will trigger creation of bindings
based on the DHCP snooping procedure described in Section 6. If it
is FALSE, either the Trust attribute must be TRUE (so that bindings
become irrelevant) or another SAVI mechanism such as SAVI-FCFS must
be used on the point of attachment.
The DHCP-Snooping attribute is configured on the DHCP Client's point
of attachment. This attribute can be also used on the attachments to
protected Non-SAVI devices that are used by DHCP clients. In
Figure 1, the attachment from the Client A to the SAVI Device A, the
attachment from the Client B to the SAVI Device B, and the attachment
from the Non-SAVI Device 2 to the SAVI Device A can be configured
with this attribute.
Whenever this attribute is set TRUE on a point of attachment, the
"Validating Attribute" MUST also be set TRUE.
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4.2.4. Data-Snooping Attribute
The "Data-Snooping Attribute" is a Boolean attribute. It indicates
whether data packets from the corresponding point of attachment may
trigger the binding setup procedure.
Data packets from points of attachment with this attribute may
trigger the setup of bindings. SAVI devices will set up bindings on
points of attachment with this attribute based on the data-triggered
process described in Section 7.
If the DHCP-Snooping attribute is configured on a point of
attachment, the bindings on this attachment are set up based on DHCP
message snooping. However, in some scenarios, a DHCP client may use
a DHCP address without the DHCP address assignment procedure being
performed on its current attachment. For such attached devices, the
Data-Snooping process, which is described in Section 7, is necessary.
This attribute is configured on such attachments. The usage of this
attribute is further discussed in Section 7.
Whenever this attribute is set on an attachment, the "Validating
Attribute" MUST be set on the same attachment. Since some networks
require DHCP deployment and others avoid it, there is no obvious
universal default value for the Data-Snooping Attribute. However,
note that deployment of SLAAC (and therefore SAVI-FCFS) is generally
configuration-free, while the deployment of DHCP involves at minimum
the deployment of a server. Hence, the Data-Snooping Attribute
should default to FALSE, and a mechanism should be implemented to
conveniently set it to TRUE on all points of attachment for which the
Trust attribute is FALSE.
4.2.5. Validating Attribute
The "Validating Attribute" is a Boolean attribute. It indicates
whether packets from the corresponding attachment will have their IP
source addresses validated based on binding entries on the
attachment.
If it is TRUE, packets coming from attachments with this attribute
will be checked based on binding entries on the attachment as
specified in Section 8. If it is FALSE, they will not. Since the
binding table is used in common with other SAVI algorithms, it merely
signifies whether the check will be done, not whether it will be done
for SAVI-DHCP originated bindings.
This attribute is by default the inverse of the Trust attribute;
source addresses on untrusted links are validated by default. It MAY
be set FALSE by the administration.
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The expected use case is when SAVI is used to monitor but not block
unvalidated transmissions. The network manager, in that case, may
set the DHCP-Snooping and/or Data-Snooping attribute TRUE but the
VALIDATING attribute FALSE.
4.2.6. Table of Mutual Exclusions
Different types of attributes may indicate mutually exclusive actions
on a packet. Mutually exclusive attributes MUST NOT be set TRUE on
the same attachment. The compatibility of different attributes is
listed in Figure 2. Note that although Trust and DHCP-Trust are
compatible, there is no need to configure DHCP-Trust to TRUE on an
attachment with Trust attribute TRUE.
+----------+----------+----------+----------+----------+----------+
| | | | DHCP- | Data- | |
| | Trust |DHCP-Trust| Snooping | Snooping |Validating|
+----------+----------+----------+----------+----------+----------+
| | | | mutually | mutually | mutually |
| Trust | - |compatible| exclusive| exclusive| exclusive|
+----------+----------+----------+----------+----------+----------+
| | | | | | |
|DHCP-Trust|compatible| - |compatible|compatible|compatible|
+----------+----------+----------+----------+----------+----------+
|DHCP- |mutually | | | | |
|Snooping |exclusive |compatible| - |compatible|compatible|
+----------+----------+----------+----------+----------+----------+
|Data- |mutually | | | | |
|Snooping |exclusive |compatible|compatible| - |compatible|
+----------+----------+----------+----------+----------+----------+
| |mutually | | | | |
|Validating|exclusive |compatible|compatible|compatible| - |
+----------+----------+----------+----------+----------+----------+
Figure 2: Table of Mutual Exclusions
4.3. Perimeter
4.3.1. SAVI-DHCP Perimeter Overview
SAVI devices form a perimeter separating trusted and untrusted
regions of a network, as SAVI-FCFS does ( Section 2.5 of [RFC6620]).
The perimeter is primarily designed for scalability. It has two
implications.
o SAVI devices only need to establish bindings for directly attached
clients, or clients indirectly attached through a non-SAVI
protected device, rather than all of the clients in the network.
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o Each SAVI device only need to validate traffic from clients
attached to it, without checking all the traffic passing by.
Consider the example in Figure 1. The protection perimeter is formed
by SAVI Devices A, B and C. In this case, SAVI device B does not
create a binding for client A. However, because SAVI device A filters
spoofed traffic from client A, SAVI device B can avoid receiving
spoofed traffic from client A.
The perimeter in SAVI-DHCP is not only a perimeter for data packets,
but also a perimeter for DHCP messages. The placement of the DHCP
Relay and DHCP Server, which are not involved in [RFC6620], is
related to the construction of the perimeter. The requirement on the
placement and configuration of DHCP Relay and DHCP Server are
discussed in Section 4.3.3.
4.3.2. SAVI-DHCP Perimeter Configuration Guideline
A perimeter separating trusted and untrusted regions of the network
is formed as follows:
(1) Configure the Validating and DHCP-Snooping attributes TRUE on
the direct attachments of all DHCP clients.
(2) Configure the Validating and DHCP-Snooping attributes TRUE on
the indirect attachments of all DHCP clients (i.e., DHCP clients
on protected links).
(3) Configure the Trust attribute TRUE on the attachments to other
SAVI devices.
(4) If a Non-SAVI device, or a number of connected Non-SAVI devices,
are attached only to SAVI devices, set the Trust attribute TRUE
on their attachments.
(5) Configure the DHCP-Trust attribute TRUE on the direct
attachments to trusted DHCP relays and servers.
In this way, the points of attachments with the Validating attribute
TRUE (and generally together with attachments of unprotected devices)
on SAVI devices can form a perimeter separating DHCP clients and
trusted devices. Data packet checks are only performed on the
perimeter. The perimeter is also a perimeter for DHCP messages. The
DHCP-Trust attribute is only TRUE on the inside links of the
perimeter. Only DHCP server-to-client messages originated within the
perimeter are trusted.
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4.3.3. On the Placement of the DHCP Server and Relay
As a result of the configuration guideline, SAVI devices only trust
DHCP Server-to-Client messages originated inside the perimeter.
Thus, the trusted DHCP Relays and DHCP Servers must be placed within
the perimeter. DHCP server-to-client messages will be filtered on
the perimeter. Server-to-relay messages will not be filtered, as
they are within the perimeter. In this way, DHCP server-to-client
messages from bogus DHCP servers are filtered on the perimeter,
having entered through untrusted points of attachment. The SAVI
devices are protected from forged DHCP messages.
DHCP server-to-client messages arriving at the perimeter from outside
the perimeter are not trusted. There is no distinction between a
DHCP server owned and operated by the correct administration but
outside the SAVI perimeter and a bogus DHCP server. For example, in
Figure 1, DHCP server A is valid, but it is attached to Non-SAVI
device 1. A bogus DHCP server is also attached Non-SAVI device 1.
While one could imagine a scenario in which the valid one had a
statistically configured port number and MAC address, and therefore a
binding, by default SAVI-DHCP cannot distinguish whether a message
received from the port of Non-SAVI device 1 is from DHCP server A or
the bogus DHCP server. If the DHCP server A is contained in the
perimeter, Non-SAVI device 1 will also be contained in the perimeter.
Thus, the DHCP server A cannot be contained within the perimeter
apart from manual configuration of the binding anchor.
Another consideration on the placement is that if the DHCP server/
relay is not inside the perimeter, the SAVI devices may not be able
to set up bindings correctly, because the SAVI devices may not be on
the path between the clients and the server/relay, or the DHCP
messages are encapsulated (e.g., Relay-reply and Relay-forward).
4.3.4. An Alternative Deployment
In common deployment practice, the traffic from the unprotected
network is treated as trustworthy, which is to say that it is not
filtered. In such a case, the Trust attribute can be set TRUE on the
unprotected link. If Non-SAVI devices, or a number of connected Non-
SAVI devices, are only attached to SAVI devices and unprotected
devices, their attachment to SAVI devices can have the Trust
attribute set TRUE. Then an unclosed perimeter will be formed, as
illustrated in Figure 3.
To configure such a perimeter, at minimum the DHCP messages from
unprotected networks MUST be ensured to be trustworthy. Achieving
this is beyond the scope of this document.
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| . . Protection |
| | | Perimeter |
| | | |
| Unprotected | | Unprotected |
| Link | | Link |
| | | |
| | | |
| +--------+ +--------+ +--------+ |
| |SAVI |----|Non-SAVI|----|SAVI | |
| |Device | |Device | |Device | |
| +--------+ +--------+ +--------+ |
| | | |
\__________________________________________________/
| |
| |
+--------+ +--------+
|DHCP | |DHCP |
|Client | |Client |
+--------+ +--------+
Figure 3: Alternative Perimeter Configuration
4.3.5. Considerations regarding Binding Anchors
The strength of this binding-based mechanism depends on the strength
of the binding anchor. The sample binding anchors in [RFC7039] have
the property that they associate an IP address with a direct physical
or secure virtual interface such as a switch port, a subscriber
association, or a security association. In addition, especially in
the case that a protected non-SAVI device such as a desktop switch or
a hub is between the client device and the SAVI switch, they MAY be
extended to also include a MAC address or other link-layer attribute.
In short, a binding anchor is intended to associate an IP address
with something unspoofable that identifies a single client system or
one of its interfaces; this may be a physical or virtual interface or
that plus disambiguating link-layer information.
If the binding anchor is spoofable, such as a plain MAC address, or
non-exclusive, such as a switch port extended using a non-SAVI
device, an attacker can use a forged binding anchor to evade
validation. Indeed, using a binding anchor that can be easily
spoofed can lead to worse outcomes than allowing IP spoofing traffic.
Thus, a SAVI device MUST use a non-spoofable and exclusive binding
anchor.
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4.4. Other Device Configuration
In addition to a possible binding anchor configuration specified in
Section 4.2, an implementation has the following configuration
requirements:
(1) Address configuration. For DHCPv4: the client of a SAVI device
MUST have an IPv4 address. For DHCPv6: the client of a SAVI
device MUST have a link-local address; when the DHCPv6 server is
not on the same link as the SAVI device, the SAVI device MUST
also have a global IPv6 address.
(2) DHCP server address configuration: a SAVI device MUST store the
list of the DHCP server addresses that it could contact during a
Lease query process.
5. Binding State Table (BST)
The Binding State Table, which may be implemented centrally in the
switch or distributed among its ports, is used to contain the
bindings between the IP addresses assigned to the attachments and the
corresponding binding anchors of the attachments. Note that in this
description, there is a binding entry for each IPv4 or IPv6 address
associated with each binding anchor, and there may be several of each
such address, especially if the port is extended using a protected
non-SAVI device. Each binding entry, has 5 fields:
o Binding Anchor(Anchor): the binding anchor, i.e., a physical and/
or link-layer property of the attachment.
o IP Address(Address): the IPv4 or IPv6 address assigned to the
attachment by DHCP.
o State: the state of the binding. Possible values of this field
are listed in Section 6.2 and Section 7.3.
o Lifetime: the remaining seconds of the binding. Internally, this
MAY be stored as the timestamp value at which the lifetime
expires.
o TID: the Transaction ID (TID) ( [RFC2131] [RFC3315]) of the
corresponding DHCP transaction. TID field is used to associate
DHCP Server-to-Client messages with corresponding binding entries.
The IA is not present in the BST for three reasons:
o The lease of each address in one IA is assigned separately.
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o When the binding is set up based on data-snooping, the IA cannot
be recovered from the lease query protocol.
o DHCPv4 does not define an IA.
An instance of this table is shown in Figure 4.
+---------+----------+----------+-----------+-------+
| Anchor | Address | State | Lifetime |TID |
+---------+----------+----------+-----------+-------+
| Port_1 | IP_1 | BOUND | 65535 |TID_1 |
+---------+----------+----------+-----------+-------+
| Port_1 | IP_2 | BOUND | 10000 |TID_2 |
+---------+----------+----------+-----------+-------+
| Port_2 | IP_3 |INIT_BIND | 1 |TID_3 |
+---------+----------+----------+-----------+-------+
Figure 4: Instance of BST
6. DHCP Snooping Process
This section specifies the process of setting up bindings based on
DHCP snooping. This process is illustrated using a state machine.
6.1. Rationale
The rationale of the DHCP Snooping Process is that if a DHCP client
is legitimately using a DHCP-assigned address, the DHCP address
assignment procedure that assigns the IP address to the client must
have been performed on the client's point of attachment. This basis
works when the SAVI device is always on the path(s) from the DHCP
client to the DHCP server(s)/relay(s). Without considering the
movement of DHCP clients, the SAVI device should be the cut vertex
whose removal will separate the DHCP client and the remaining network
containing the DHCP server(s)/ and relay(s). For most of the
networks whose topologies are simple, it is possible to deploy this
SAVI function at proper devices to meet this requirement.
However, if there are multiple paths from a DHCP client to the DHCP
server and the SAVI device is only on one of them, there is an
obvious failure case: the SAVI device may not be able to snoop the
DHCP procedure. Host movement may also make this requirement
difficult to meet. For example, when a DHCP client moves from one
attachment to another attachment in the same network, it may fail to
reinitialize its interface or send a Confirm message because of
incomplete protocol implementation. Thus, there can be scenarios in
which only performing this DHCP snooping process is insufficient to
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set up bindings for all the valid DHCP addresses. These exceptions
and the solutions are discussed in Section 7.
6.2. Binding States Description
Following binding states are present in this process and the
corresponding state machine:
NO_BIND: No binding has been set up.
INIT_BIND: A potential binding has been set up.
BOUND: The binding has been set up.
6.3. Events
This section describes events in this process and the corresponding
state machine.
6.3.1. Timer Expiration Event
EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires.
6.3.2. Control Message Arriving Events
EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
received.
EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received.
EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received.
EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received.
EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received.
EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with Rapid Commit
option is received.
EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received.
EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
received.
EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
received.
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EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY (refer to
section 4.3.3 of [RFC5007]) is received.
Note: the events listed here do not cover all the DHCP messages in
section 3. The messages which do not really determine address usage
(DHCPv4 Discover, DHCPv4 Inform, DHCPv6 Solicit without Rapid Commit,
DHCPv6 Information-Request, DHCPv4 Offer, DHCPv6 Advertise, DHCPv6
Reconfigure), and which are not necessary to snoop (DHCPv4 NAK, refer
to section 6.4.2.1), are not included.
Moreover, only if a DHCP message can pass the following checks, the
corresponding event is regarded as a valid event:
o Attribute check: the DHCP Server-to-Client messages and
LEASEQUERY_REPLY should be from attachments with DHCP-Trust
attribute; the DHCP Client-Server messages should be from
attachments with DHCP-Snooping attribute.
o Destination check: the DHCP Server-to-Client messages should be
destined to attachments with DHCP-Snooping attribute. This check
is performed to ensure the binding is set up on the SAVI device
which is nearest to the destination client.
o Binding anchor check: the DHCP Client-Server messages which may
trigger modification or removal of an existing binding entry must
have a matching binding anchor with the corresponding entry.
o TID check: the DHCP Server-to-Client/Client-Server messages which
may cause modification on existing binding entries must have
matched TID with the corresponding entry. Note that this check is
not performed on Lease query and Lease query-reply messages as
they are exchanged between the SAVI devices and the DHCP servers.
Besides, this check is not performed on DHCP Renew/Rebind
messages.
o Binding limitation check: the DHCP messages must not cause new
binding setup on an attachment whose binding entry limitation has
been reached. (refer to Section 11.5).
o Address check: the source address of the DHCP messages should pass
the check specified in Section 8.2.
On receiving a DHCP message without triggering a valid event, the
state will not change, and the actions will not be performed. Note
that if a message does not trigger a valid event but it can pass the
checks in Section 8.2, it MUST be forwarded.
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6.4. The State Machine of DHCP Snooping Process
This section specifies state transitions and their corresponding
actions.
6.4.1. Initial State: NO_BIND - No binding has been set up
6.4.1.1. Event: EVE_DHCP_REQUEST - A DHCPv4 Request or a DHCPv6 Request
message is received
The SAVI device MUST forward the message.
The SAVI device will generate an entry in the BST. The Binding
anchor field is set to the binding anchor of the attachment from
which the message is received. The State field is set to INIT_BIND.
The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. The TID
field is set to the TID of the message. If the message is DHCPv4
Request or DHCPv4 Reboot, the Address field can be set to the address
to request, i.e., the 'requested IP address'. An example of the
entry is illustrated in Figure 5.
+---------+-------+---------+-----------------------+-------+
| Anchor |Address| State | Lifetime |TID |
+---------+-------+---------+-----------------------+-------+
| Port_1 | |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |
+---------+-------+---------+-----------------------+-------+
Figure 5: Binding entry in BST on Request/Rapid Commit/Reboot
triggered initialization
Resulting state: INIT_BIND - A potential binding has been set up
6.4.1.2. Event: EVE_DHCP_REBOOT - A DHCPv4 Reboot message is received
The SAVI device MUST forward the message.
The SAVI device will generate an entry in the BST. The Binding
anchor field is set to the binding anchor of the attachment from
which the message is received. The State field is set to INIT_BIND.
The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. The TID
field is set to the TID of the message. If the message is DHCPv4
Request or DHCPv4 Reboot, the Address field can be set to the address
to request, i.e., the 'requested IP address'. An example of the
entry is illustrated in Figure 5.
Resulting state: INIT_BIND - A potential binding has been set up
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6.4.1.3. Event: EVE_DHCP_SOLICIT_RC - A DHCPv6 Solicitation message
with Rapid Commit option is received
The SAVI device MUST forward the message.
The SAVI device will generate an entry in the BST. The Binding
anchor field is set to the binding anchor of the attachment from
which the message is received. The State field is set to INIT_BIND.
The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME. The TID
field is set to the TID of the message. If the message is DHCPv4
Request or DHCPv4 Reboot, the Address field can be set to the address
to request, i.e., the 'requested IP address'. An example of the
entry is illustrated in Figure 5.
Resulting state: INIT_BIND - A potential binding has been set up
6.4.1.4. Event: EVE_DHCP_CONFIRM - A DHCPv6 Confirm message is received
The SAVI device MUST forward the message.
The SAVI device will generate corresponding entries in the BST for
each address in each Identity Association (IA) option of the Confirm
message. The Binding anchor field is set to the binding anchor of
the attachment from which the message is received. The State field
is set to INIT_BIND. The Lifetime field is set to be
MAX_DHCP_RESPONSE_TIME. The TID field is set to the TID of the
message. The Address field is set to the address(es) to confirm. An
example of the entries is illustrated in Figure 6.
+---------+--------+---------+-----------------------+-------+
| Anchor | Address| State | Lifetime |TID |
+---------+--------+---------+-----------------------+-------+
| Port_1 | Addr1 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |
+---------+--------+---------+-----------------------+-------+
| Port_1 | Addr2 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |
+---------+--------+---------+-----------------------+-------+
Figure 6: Binding entry in BST on Confirm triggered initialization
Resulting state: INIT_BIND - A potential binding has been set up
6.4.1.5. Events that cannot happen in the NO_BIND state
o EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires
o EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received
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o EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is
received
o EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received
o EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
received
o EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
received
o EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY is
received
These cannot happen because they are each something that happens
AFTER a binding has been created.
6.4.2. Initial State: INIT_BIND - A potential binding has been set up
6.4.2.1. Event: EVE_DHCP_REPLY - A DHCPv4 ACK or a DHCPv6 Reply message
is received
The message MUST be forwarded to the corresponding client.
If the message is DHCPv4 ACK, the Address field of the corresponding
entry (i.e., the binding entry whose TID is the same of the message)
is set to the address in the message(i.e., 'yiaddr' in DHCPv4 ACK).
The Lifetime field is set to the sum of the lease time in ACK message
and MAX_DHCP_RESPONSE_TIME. The State field is changed to BOUND.
If the message is DHCPv6 Reply, there are following cases:
1. If the status code is not "Success", no modification on
corresponding entries will be made. Corresponding entries will
expire automatically if no "Success" Reply is received during the
lifetime. The entries are not removed immediately due to the
client may be able to use the addresses whenever a "Success"
Reply is received ("If the client receives any Reply messages
that do not indicate a NotOnLink status, the client can use the
addresses in the IA and ignore any messages that indicate a
NotOnLink status." [RFC3315]).
2. If the status code is "Success", the SAVI device checks the IA
options in the Reply message.
A. If there are IA options in the Reply message, the SAVI device
checks each IA option. When the first assigned address is
found, the Address field of the binding entry with matched
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TID is set to the address. The Lifetime field is set to the
sum of the lease time in Reply message and
MAX_DHCP_RESPONSE_TIME. The State field is changed to BOUND.
If there are more than one address assigned in the message,
new binding entries are set up for the remaining address
assigned in the IA options. An example of the entries is
illustrated in Figure 8. SAVI devices do not specially
process IA options with NoAddrsAvail status, because there
should be no address contained in such IA options.
B. Otherwise, the DHCP Reply message is in response to a Confirm
message. The state of the binding entries with matched TID
is changed to BOUND. Because [RFC3315] does not require
lease time of addresses to be contained in the Reply message,
the SAVI device SHOULD send a LEASEQUERY [RFC5007] message
querying by IP address to All_DHCP_Servers multicast address
[RFC3315] or a list of configured DHCP server addresses. The
Lease query message is generated for each IP address if
multiple addresses are confirmed. The Lifetime of
corresponding entries is set to 2*MAX_LEASEQUERY_DELAY. If
there is no response message after MAX_LEASEQUERY_DELAY, send
the LEASEQUERY message again. An example of the entries is
illustrated in Figure 7. If the SAVI device does not send
the LEASEQUERY message, a pre-configured lifetime
DHCP_DEFAULT_LEASE MUST be set on the corresponding entry.
(Note: it is RECOMMENDED to use T1 configured on DHCP servers
as the DHCP_DEFAULT_LEASE.)
Note: the SAVI devices do not check if the assigned addresses are
duplicated because in SAVI-DHCP scenarios, the DHCP servers are the
only source of valid addresses. However, the DHCP servers should be
configured to make sure no duplicated addresses are assigned.
+---------+----------+-------+------------------------+-------+
| Anchor | Address | State | Lifetime |TID |
+---------+----------+-------+------------------------+-------+
| Port_1 | Addr1 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID |
+---------+----------+-------+------------------------+-------+
| Port_1 | Addr2 | BOUND | 2*MAX_LEASEQUERY_DELAY |TID |
+---------+----------+-------+------------------------+-------+
Figure 7: From INIT_BIND to BOUND on DHCP Reply in response to
Confirm
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+---------+----------+-------+------------------------+-------+
| Anchor | Address | State | Lifetime |TID |
+---------+----------+-------+------------------------+-------+
| Port_1 | Addr1 | BOUND |Lease time+ |TID |
| | | |MAX_DHCP_RESPONSE_TIME | |
+---------+----------+-------+------------------------+-------+
| Port_1 | Addr2 | BOUND |Lease time+ |TID |
| | | |MAX_DHCP_RESPONSE_TIME | |
+---------+----------+-------+------------------------+-------+
Figure 8: From INIT_BIND to BOUND on DHCP Reply in response to
Request
Resulting state: BOUND - The binding has been set up
6.4.2.2. Event: EVE_ENTRY_EXPIRE - The lifetime of a binding entry
expires
The entry MUST be deleted from BST.
Resulting state: An entry that has been deleted from the BST may be
considered to be in the state "NO_BIND" - No binding has been set up.
6.4.2.3. Events that are ignored in INIT_BIND
If no DHCP Server-to-Client messages which assign addresses or
confirm addresses are received, corresponding entries will expire
automatically. Thus, other DHCP Server-to-Client messages (e.g.,
DHCPv4 NAK) are not specially processed.
As a result, the following events, should they occur, are ignored
until either a DHCPv4 ACK or a DHCPv6 Reply message is received or
the lifetime of the binding entry expires.
o EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
received
o EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received
o EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received
o EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received
o EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is
received
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o EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with Rapid
Commit option is received
o EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
received
o EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
received
o EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY is
received
In each case, the message MUST be forwarded.
Resulting state: INIT_BIND - A potential binding has been set up
6.4.3. Initial State: BOUND - The binding has been set up
6.4.3.1. Event: EVE_ENTRY_EXPIRE - The lifetime of a binding entry
expires
The entry MUST be deleted from BST.
Resulting state: An entry that has been deleted from the BST may be
considered to be in the state "NO_BIND" - No binding has been set up.
6.4.3.2. Event: EVE_DHCP_DECLINE - A DHCPv4 Decline or a DHCPv6 Decline
message is received
The message MUST be forwarded.
The SAVI device first gets all the addresses ("Requested IP address"
in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, addresses in all the
IA options of DHCPv6 Decline/Release) to decline/release in the
message. Then the corresponding entries MUST be removed.
Resulting state in each relevant BST entry: An entry that has been
deleted from the BST may be considered to be in the state "NO_BIND" -
No binding has been set up.
6.4.3.3. Event: EVE_DHCP_RELEASE - A DHCPv4 Release or a DHCPv6 Release
message is received
The message MUST be forwarded.
The SAVI device first gets all the addresses ("Requested IP address"
in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, addresses in all the
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IA options of DHCPv6 Decline/Release) to decline/release in the
message. Then the corresponding entries MUST be removed.
Resulting state in each relevant BST entry: An entry that has been
deleted from the BST may be considered to be in the state "NO_BIND" -
No binding has been set up.
6.4.3.4. Event: EVE_DHCP_REBIND - A DHCPv4 Rebind or a DHCPv6 Rebind
message is received
The message MUST be forwarded.
In such case, a new TID will be used by the client. The TID field of
the corresponding entries MUST be set to the new TID. Note that TID
check will not be performed on such messages.
Resulting state: BOUND: The binding has been set up
6.4.3.5. Event: EVE_DHCP_RENEW - A DHCPv4 Renew or a DHCPv6 Renew
message is received
The message MUST be forwarded.
In such case, a new TID will be used by the client. The TID field of
the corresponding entries MUST be set to the new TID. Note that TID
check will not be performed on such messages.
Resulting state: BOUND: The binding has been set up
6.4.3.6. Event: EVE_DHCP_REPLY - A DHCPv4 ACK or a DHCPv6 Reply message
is received
The message MUST be forwarded.
The DHCP Reply messages received in current states should be in
response to DHCP Renew/Rebind.
If the message is DHCPv4 ACK, the SAVI device updates the binding
entry with matched TID, with the Lifetime field set to be the sum of
the new lease time and MAX_DHCP_RESPONSE_TIME, leaving the entry in
the state BOUND.
If the message is DHCPv6 Reply, the SAVI device checks each IA
Address option in each IA option. For each:
1. If the IA entry in the REPLY message has the status "NoBinding",
there is no address in the option, and no operation on an address
is performed.
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2. If the valid lifetime of an IA address option is 0, the binding
entry with matched TID and address is removed, leaving it
effectively in the state NO_BIND.
3. Otherwise, set the Lifetime field of the binding entry with
matched TID and address to be the sum of the new valid lifetime
and MAX_DHCP_RESPONSE_TIME, leaving the entry in the state BOUND.
Resulting state: NO_BIND or BOUND, as specified.
6.4.3.7. Event: EVE_DHCP_LEASEQUERY - A successful DHCPv6
LEASEQUERY_REPLY is received
The message MUST be forwarded.
The message should be in response to the Lease query message sent in
Section 6.4.2. The related binding entry can be determined based on
the address in the IA Address option in the Lease query-reply
message. The Lifetime field of the corresponding binding entry is
set to the sum of the lease time in the LEASEQUERY_REPLY message and
MAX_DHCP_RESPONSE_TIME.
Resulting state: BOUND: The binding has been set up
6.4.3.8. Events not processed in the state BOUND
The following events are ignored if received while the indicated
entry is in the state BOUND. Any required action will be the result
of the next message in the client/server exchange.
o EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
received
o EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received
o EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received
o EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with Rapid
Commit option is received
6.4.4. Table of State Machine
The main state transits are listed as follows. Note that not all the
details are specified in the table and the diagram.
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State Event Action Next State
NO_BIND RQ/RC/CF/RE Generate entry INIT_BIND
INIT_BIND RPL Record lease time BOUND
(send lease query if no lease)
INIT_BIND EVE_ENTRY_EXPIRE Remove entry NO_BIND
BOUND RLS/DCL Remove entry NO_BIND
BOUND EVE_ENTRY_EXPIRE Remove entry NO_BIND
BOUND RPL Set new lifetime BOUND
BOUND LQR Record lease time BOUND
Figure 9: Table of Transit
RQ: EVE_DHCP_REQUEST
CF: EVE_DHCP_CONFIRM
RC: EVE_DHCP_SOLICIT_RC
RE: EVE_DHCP_REBOOT
RPL: EVE_DHCP_REPLY
DCL: EVE_DHCP_DECLINE
RLS: EVE_DHCP_RELEASE
LQR: EVE_DHCP_LEASEQUERY
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+-------------+
| |
/---------| NO_BIND |<----------\
| ------>| | |
| | +-------------+ |EVE_DHCP_RELEASE
EVE_DHCP_REQUEST | | |EVE_DHCP_DECLINE
EVE_DHCP_CONFIRM | |EVE_ENTRY_EXPIRE |EVE_ENTRY_EXPIRE
EVE_DHCP_SOLICIT_RC| | |
EVE_DHCP_REBOOT | | |
| | |
| | |
v | |
+-------------+ +------------+
| | EVE_DHCP_REPLY | |
| INIT_BIND ------------------------>| BOUND |<-\
| | | | |
+-------------+ +------------+ |
| |
\--------/
EVE_DHCP_REPLY
EVE_DHCP_LEASEQUERY
Figure 10: Diagram of Transit
7. Data Snooping Process
7.1. Scenario
The rationale of the DHCP Snooping Process specified in Section 6 is
that if a DHCP client's use of a DHCP address is legitimate, the
corresponding DHCP address assignment procedure must have been
finished on the attachment of the DHCP client. This is the case
stands when the SAVI device is persistently on the path(s) from the
DHCP client to the DHCP server(s)/relay(s). However, there are two
case when this does not work:
o Multiple paths: there is more than one feasible link-layer paths
from the client to the DHCP server/relay, and the SAVI device is
not on everyone of them. The client may get its address through
one of the paths not passing by the SAVI device, but packets from
the client can travel through paths that pass through the SAVI
device. Because the SAVI device could not snoop the DHCP packet
exchange procedure, the DHCP snooping procedure cannot set up the
corresponding binding.
o Dynamic path: there is only one feasible link-layer path from the
client to the DHCP server/relay, but the path is dynamic due to
topology change (for example, some link turns broken due to
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failure or as planned) or link-layer path change. This situation
also covers the local-link movement of clients without address
confirm/re-configuration process. For example, a host changes its
attached switch port in a very short time. In such cases, the
DHCP snooping process will not set up the corresponding binding.
Data Snooping Process prevents permanently blocking legitimate
traffic in case of these two exceptions. This process is performed
on attachments with the Data-Snooping attribute. Data packets
without matching binding entry may trigger this process to set up
bindings.
Snooping data traffic introduces considerable burden on the processor
and ASIC-to-Processor bandwidth of SAVI devices. Because of the
overhead of this process, the implementation of this process is a
conditional SHOULD. This function SHOULD be enabled unless the
implementation is known to be used in the scenarios without the above
exceptions. For example, if the implementation is to be used in
networks with tree topology and without host local-link movement,
there is no need to implement this process in such scenarios.
This process is not intended to set up a binding whenever a data
packet without matched binding entry is received. Instead, unmatched
data packets trigger this process probabilistically and generally a
number of unmatched packets will be discarded before the binding is
set up.
7.2. Rationale
This process makes use of NS/ARP and DHCP LEASEQUERY to set up
bindings. If an address is not used by another client in the
network, and the address has been assigned in the network, the
address can be bound with the binding anchor of the attachment from
which the unmatched packet is received.
The security issues about this process is discussed is Section 11.1.
7.3. Additional Binding States Description
In addition to NO_BIND and BOUND from Section 6.2, two new states
used in this process are listed here. The INIT_BIND state is not
used, as it is entered by observing a DHCP message.
DETECTION: The address in the entry is under local duplication
detection.
RECOVERY: The SAVI device is querying the assignment and lease time
of the address in the entry through DHCP Lease query.
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7.4. Events
In addition to EVE_DATA_LEASEQUERY and EVE_DHCP_REBIND, these
additional events are described here. If an event will trigger the
creation of a new binding entry, the binding entry limit on the
binding anchor MUST NOT be exceeded.
EVE_DATA_UNMATCH: A data packet without matched binding is received.
EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message
against an address in DETECTION state is received from a host other
than the one for which the entry was added.
EVE_DATA_LEASEQUERY:
o IPv4: A DHCPLEASEACTIVE message with IP Address Lease Time option
is received.
o IPv6: A successful LEASEQUERY-REPLY is received.
The triggering packet should pass the following checks to trigger a
valid event:
o Attribute check: the data packet should be from attachments with
Data-Snooping attribute; the DHCPLEASEACTIVE/LEASEQUERY_REPLY
messages should be from attachments with DHCP-Snooping attribute.
o Binding limitation check: the DHCP messages must not cause new
binding setup on an attachment whose binding entry limitation has
been reached. (refer to Section 11.5).
o Address check: For EVE_DATA_LEASEQUERY, the source address of the
DHCP Lease query messages must pass the check specified in
Section 8.2. For EVE_DATA_CONFLICT, the source address and target
address of the ARP or NA messages must pass the check specified in
Section 8.2.
o Interval check: the interval between two successive
EVE_DATA_UNMATCH events triggered by an attachment MUST be no
smaller than DATA_SNOOPING_INTERVAL.
o TID check: the DHCPLEASEACTIVE/LEASEQUERY-REPLY messages must have
matched TID with the corresponding entry.
o Prefix check: the source address of the data packet should be of a
valid local prefix, as specified in section 7 of [RFC7039].
EVE_ENTRY_EXPIRE: A timer expires.
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7.5. Initial State: state NO_BIND - No binding has been set up
7.5.1. Event: EVE_DATA_UNMATCH: A data packet without matched binding
is received
Make a probabilistic determination whether to act on this event. The
probability may be configured or calculated based on the state of the
SAVI device. This probability should be low enough to mitigate the
damage from DoS attack against this process.
Create a new entry in the BST. Set the Binding Anchor field to the
corresponding binding anchor of the attachment. Set the Address
field to the source address of the packet. Set the State field to
DETECTION. Set the Lifetime of the created entry to
2*DETECTION_TIMEOUT.
Check if the address has a local conflict (it violates an address
being used by another node):
(1) IPv4 address: send an Address Resolution Protocol (ARP) Request
[RFC0826] or an ARP probe [RFC5227] on the address; if there is
no response message after DETECTION_TIMEOUT, send another ARP
Request or ARP probe;
(2) IPv6 address: send a Duplicate Address Detection message
[RFC4861] targeting the address; ideally, only the host on that
point of attachment responds with a Neighbor Advertisement; if
more than one Neighbor Advertisement is observed, the BST entry
should be removed.
As Duplicate Address Detection is an unreliable process (either the
packet to or from the other system may be lost in transit), if there
is no response, it should be repeated, as described in [RFC6620].
The packet that triggers this event SHOULD be discarded.
This local conflict process SHOULD be performed. If it is not
performed, the state of the entry is set to RECOVERY, the lifetime is
set to 2*MAX_LEASEQUERY_DELAY, and the lease query process specified
in the following section will be performed directly.
An example of the entry is illustrated in Figure 11.
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+---------+-------+---------+-----------------------+-------+
| Anchor |Address| State | Lifetime |TID |
+---------+-------+---------+-----------------------+-------+
| Port_1 | Addr1 |DETECTION|2*DETECTION_TIMEOUT | |
+---------+-------+---------+-----------------------+-------+
Figure 11: Binding entry in BST on data triggered initialization
Resulting state: DETECTION - The address in the entry is under local
duplication detection
7.5.2. Events not observed in NO_BIND
EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message
received from unexpected system
EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received
EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or LEASEQUERY-REPLY is
received
EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received
EVE_ENTRY_EXPIRE
7.6. Initial State: state DETECTION - The address in the entry is under
local duplication detection
7.6.1. Event: EVE_ENTRY_EXPIRE
(1) IPv4 address: Send a DHCPLEASEQUERY [RFC4388] message querying
by IP address to each DHCPv4 server with IP Address Lease Time
option (option 51). A list of authorized DHCP servers are kept
by the SAVI device. The list should be pre-configured or
discovered by sending DHCPv4 Discover messages and parsing the
replied DHCPv4 Offer messages. Change the state of the
corresponding entry to RECOVERY. Change the lifetime of the
entry to be 2*MAX_LEASEQUERY_DELAY. The TID field is set to the
TID used in the DHCPLEASEQUERY message. If there is no response
message after MAX_LEASEQUERY_DELAY, send a DHCPLEASEQUERY to each
DHCPv4 server again.
(2) IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP
address to All_DHCP_Relay_Agents_and_Servers multicast address or
a list of pre-configured DHCPv6 server addresses. Change the
state of the corresponding entry to RECOVERY. Change the
lifetime of the entry to be 2*MAX_LEASEQUERY_DELAY. The TID
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field is set to the TID used in the LEASEQUERY message. If there
is no response message after MAX_LEASEQUERY_DELAY, send the
LEASEQUERY message again.
An example of the entry is illustrated in Figure 12.
+---------+-------+---------+-----------------------+-------+
| Anchor |Address| State | Lifetime |TID |
+---------+-------+---------+-----------------------+-------+
| Port_1 | Addr1 |RECOVERY |2*MAX_LEASEQUERY_DELAY |TID |
+---------+-------+---------+-----------------------+-------+
Figure 12: Binding entry in BST on Lease Query
Resulting state: RECOVERY - The SAVI device is querying the
assignment and lease time of the address in the entry through DHCP
Leasequery
7.6.2. Event: EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA)
message received from unexpected system
Remove the entry.
Resulting state: NO_BIND - No binding has been set up
7.6.3. Events not observed in DETECTION
EVE_DATA_UNMATCH: A data packet without matched binding is received
EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received
EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or LEASEQUERY-REPLY is
received
EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received
7.7. Initial State: state RECOVERY - The SAVI device is querying the
assignment and lease time of the address in the entry through DHCP
Leasequery
7.7.1. Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or
LEASEQUERY-REPLY is received
IPv4 address:
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(1) Send an ARP Request with the Target Protocol Address set to the
IP address in the corresponding entry. The ARP Request is only
sent to the attachment which triggers the binding. If there is
no response after DETECTION_TIMEOUT, send another ARP Request.
If there is still no response, remove the entry.
(2) If there is only one identical response, get the sender hardware
address. Check if the 'chaddr' field (hardware address) of the
DHCPLEASEACTIVE message matches the sender hardware address. If
the two addresses do not match, the following actions will not be
performed. If there is more than one response, if any of the
sender hardware addresses matches the 'chaddr' field (hardware
address) of the DHCPLEASEACTIVE message,
* Set life time to the sum of the value encoded in IP Address
Lease Time option of the DHCPLEASEACTIVE message and
MAX_DHCP_RESPONSE_TIME.
* Erase the TID field.
IPv6 address:
(1) Send a Neighbor Solicitation message with the target address set
to the IP address in the corresponding entry. The Neighbor
Solicitation is only sent to the attachment which triggers the
binding. If there is no response after DETECTION_TIMEOUT, send
another Neighbor Solicitation. If there is still no response,
remove the entry.
(2) On receipt of a valid Neighbor Announcement,
* Set the lifetime to the sum of the valid lifetime extracted
from OPTION_CLIENT_DATA option in the LEASEQUERY-REPLY
message and MAX_DHCP_RESPONSE_TIME.
* Erase the TID field.
* After the above checks, if multiple addresses are specified
in the LEASEQUERY-REPLY message and there are no
corresponding binding entries, new entries MUST also be
created correspondingly on the same binding anchor.
In the event that responses are received from multiple DHCP servers,
the conflict resolution mechanisms specified in section 6.8 of
[RFC4388] and section 4.3.4 of [RFC5007] will be used to determine
which message should be used.
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Resulting state: if ARP or ND succeeds (there is a valid response),
BOUND - The binding has been set up. Otherwise, the resulting state
is NO_BIND - No binding has been set up
7.7.2. Event: EVE_ENTRY_EXPIRE
Remove the entry.
Resulting state: NO_BIND - No binding has been set up
7.7.3. Events not observed in RECOVERY
EVE_DATA_UNMATCH: A data packet without matched binding is received
EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message
received from unexpected system
EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received
EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
received
7.8. Initial State: state BOUND - The binding has been set up
Upon entry to the state BOUND, control the system continues as if a
DHCP message assigning the address has been observed, as in
Section 6.4.3. The BST entry has been restored.
Note that the TID field contains no value after the binding state
changes to BOUND. The TID field is recovered from snooping DHCP
Renew/Rebind messages. Because TID is used to associate binding
entries with messages from DHCP servers, it must be recovered; or
else a number of state transits of this mechanism will be not
executed normally.
7.9. Table of State Machine
The main state transits are listed as follows.
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State Event Action Next State
NO_BIND EVE_DATA_UNMATCH Duplication detection DETECTION
DETECTION EVE_ENTRY_EXPIRE Send Leasequery RECOVERY
DETECTION EVE_DATA_CONFLICT Remove entry NO_BIND
RECOVERY EVE_DATA_LEASEQUERY Set lease time BOUND or NO_BIND
RECOVERY EVE_ENTRY_EXPIRE Remove entry NO_BIND
BOUND RENEW/REBIND Record TID BOUND
Figure 13: Table of Transit
RENEW: EVE_DHCP_RENEW
REBIND: EVE_DHCP_REBIND
+-------------+
| |
/---------| NO_BIND |<--------\
| ------>| | | EVE_ENTRY_EXPIRE
| | +-------------+ |(2nd LQ_DELAY)
EVE_DATA_UNMATCH | | |
| | |
1st | | |
DETECTION_TIMEOUT | | | 1st LQ_DELAY
/------\ | | | /---------\
| | | | EVE_DATA_CONFLICT | | |
| v v | | v |
| +-------------+ EVE_ENTRY_EXPIRE +------------+ |
| | |(2nd DETECTION_TIMEOUT) | | |
\----| DETECTION ------------------------>| RECOVERY ----/
| | | |
+-------------+ +------------+
EVE_DATA_LEASEQUERY|
/----------\ (+ 2x DETECTION) |
EVE_DHCP_RENEW| | |
EVE_DHCP_REBIND| +-----v-------+ |
| | | |
\----| BOUND |<----------/
| |
+-------------+
Figure 14: Diagram of Transit
LQ_DELAY: MAX_LEASEQUERY_DELAY
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8. Filtering Specification
This section specifies how to use bindings to filter out packets with
spoofed source addresses.
Filtering policies are different for data packets and control
packets. DHCP, ARP, and NDP (Neighbor Discovery Protocol) [RFC4861]
messages are classified as control packets. All other packets are
classified as data packets.
8.1. Data Packet Filtering
Data packets from attachments with the Validating attribute TRUE MUST
be checked. There is one exception to this rule.
A packet whose source IP address is a link-local address cannot be
checked against DHCP assignments, as it is not assigned using DHCP.
Note: as explained in Section 1, a SAVI solution for link-local
addresses, e.g., the SAVI-FCFS [RFC6620], can be enabled to check
packets with a link-local source address.
If the source IP address of a packet is not a link-local address, but
there is not a matching entry in BST with state BOUND, this packet
MUST be discarded. However, the packet may trigger the Data Snooping
Process Section 7 if the Data-Snooping attribute is set on the
attachment.
Data packets from an attachment with the VALIDATING attribute set
FALSE will be forwarded without being validated.
The SAVI device MAY log packets that fail source address validation.
8.2. Control Packet Filtering
For attachments with the Validating attribute:
DHCPv4 Client-Server messages in which the source IP address is
neither all zeros nor bound with the corresponding binding anchor in
the BST MUST be discarded.
DHCPv6 Client-Server messages in which the source IP address is
neither a link-local address nor bound with the corresponding binding
anchor in the BST MUST be discarded.
NDP messages in which the source IP address is neither a link-local
address nor bound with the corresponding binding anchor MUST be
discarded.
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NA messages in which the target address is neither a link-local
address nor bound with the corresponding binding anchor MUST be
discarded.
ARP messages in which the protocol is IP and sender protocol address
is neither all zeros address nor bound with the corresponding binding
anchor MUST be discarded.
ARP Reply messages in which the target protocol address is not bound
with the corresponding binding anchor MUST be discarded.
For attachments with other attributes:
DHCP Server-to-Client messages not from attachments with the DHCP-
Trust attribute or Trust attribute MUST be discarded.
For attachments with no attribute:
DHCP Server-to-Client messages from such attachments MUST be
discarded.
The SAVI device MAY record any messages that are discarded.
9. State Restoration
If a SAVI device reboots, the information kept in volatile memory
will be lost. This section specifies the restoration of attribute
configuration and BST.
9.1. Attribute Configuration Restoration
The loss of attribute configuration will not break the network: no
action will be performed on traffic from attachments with no
attribute. However, the loss of attribute configuration makes this
SAVI function unable to work.
To avoid the loss of binding anchor attribute configuration, the
configuration MUST be able to be stored in non-volatile storage.
After the reboot of SAVI device, if the configuration of binding
anchor attribute can be found in non-volatile storage, the
configuration MUST be used.
9.2. Binding State Restoration
The loss of binding state will cause the SAVI devices discard
legitimate traffic. Purely using the Data Snooping Process to
recover a large number of bindings is of heavy overhead and
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considerable delay. Thus, to recover bindings from non-volatile
storage, as specified below, is RECOMMENDED.
Binding entries MAY be saved into non-volatile storage whenever a new
binding entry changes to BOUND state. If a binding with BOUND state
is removed, the saved entry MUST be removed correspondingly. The
time when each binding entry is established is also saved.
Immediately after reboot, the SAVI device SHOULD restore binding
states from the non-volatile storage. The system time of save
process MUST be stored. After rebooting, the SAVI device MUST check
whether each entry has been obsolete by comparing the saved lifetime
and the difference between the current time and time when the binding
entry is established.
10. Constants
The following constants are recommended for use in this context:
o MAX_DHCP_RESPONSE_TIME 120s (SOL_MAX_RT from [RFC3315])
o MAX_LEASEQUERY_DELAY 10s (LQ_MAX_RT from [RFC5007])
o DETECTION_TIMEOUT 0.5s (TENT_LT from [RFC6620])
o DATA_SNOOPING_INTERVAL 60s and configurable (recommendation)
o OFFLINK_DELAY 30s (recommendation)
11. Security Considerations
11.1. Security Problems about the Data Snooping Process
There are two security problems about the Data Snooping Process
Section 7:
(1) The Data Snooping Process is costly, but an attacker can trigger
it simply through sending a number of data packets. To avoid
Denial of Services attack against the SAVI device itself, the
Data Snooping Process MUST be rate limited. A constant
DATA_SNOOPING_INTERVAL is used to control the frequency. Two
Data Snooping Processes on one attachment MUST be separated by a
minimum interval time DATA_SNOOPING_INTERVAL. If this value is
changed, the value needs to be large enough to minimize denial of
service attacks.
(2) The Data Snooping Process may set up incorrect bindings if the
clients do not reply to the detection probes Section 7.5.1. An
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attack will pass the duplicate detection if the client assigned
the target address does not reply to the detection probes. The
DHCP Lease query procedure performed by the SAVI device just
tells whether the address is assigned in the network or not.
However, the SAVI device cannot determine whether the address is
just assigned to the triggering attachment from the DHCP
LEASEQUERY Reply.
11.2. Client departure issues
After a binding is set up, the corresponding client may leave its
attachment point. It may depart temporarily due to signal fade, or
permanently by moving to a new attachment point or leaving the
network. In the signal fade case, since the client may return
shortly, the binding should be kept momentarily, lest legitimate
traffic from the client be blocked. However, if the client leaves
permanently, keeping the binding can be a security issue. If the
binding anchor is a property of the attachment point rather than the
client, e.g., the switch port but not incorporating the MAC Address,
an attacker using the same binding anchor can send packets using IP
addresses assigned to the client. Even if the binding anchor is a
property of the client, retaining binding state for a departed client
for a long time is a waste of resources.
Whenever a direct client departs from the network, a link-down event
associated with the binding anchor will be triggered. SAVI-DHCP
monitors such events, and performs the following mechanism.
(1) Whenever a client with the Validating attribute leaves, a timer
of duration OFFLINK_DELAY is set on the corresponding binding
entries.
(2) If a DAD Neighbor Solicitation/Gratuitous ARP request is
received that targets the address during OFFLINK_DELAY, the entry
MAY be removed.
(3) If the client returns on-link during OFFLINK_DELAY, cancel the
timer.
In this way, the bindings of a departing client are kept for
OFFLINK_DELAY. In case of link flapping, the client will not be
blocked. If the client leaves permanently, the bindings will be
removed after OFFLINK_DELAY.
SAVI-DHCP does not handle the departure of indirect clients, because
it will not be notified of such events. Switches supporting indirect
attachment (e.g., through a separate non-SAVI switch) SHOULD use
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information specific to the client such as its MAC address as part of
the binding anchor.
11.3. Duplicate Bindings to the Same Address
The same address may be bound to multiple binding anchors only if the
binding setup processes successfully complete for each binding
anchor. This mechanism is designed to address the case where a
client moves on the local link, and the case where a client has
multiple attachments to a SAVI device.
There are two security issues with such a design:
First, by allowing one address to be bound to multiple binding
anchors, the traceability of the address is weakened. An address can
be traced to multiple attachments.
Second, in the local link movement scenario, the former binding may
not be removed and it can be used by an attacker sharing the same
binding anchor. For example, when a switch port is used as binding
anchor and the port is shared by an attacker and a client with a hub,
the attacker can make use of the address assigned to the client after
the client leaves.
11.4. Compatibility with DNA (Detecting Network Attachment)
DNA [RFC4436][RFC6059] is designed to decrease the handover latency
after re-attachment to the same network. DNA mainly relies on
performing reachability test by sending unicast Neighbor Solicitation
/Router Solicitation/ARP Request message to determine whether a
previously configured address is still valid.
Although DNA provides optimization for clients, there is insufficient
information for this mechanism to migrate the previous binding or
establish a new binding. If a binding is set up only by snooping the
reachability test message, the binding may be invalid. For example,
an attacker can perform reachability test with an address bound to
another client. If binding is migrated to the attacker, the attacker
can successfully obtain the binding from the victim. Because this
mechanism wouldn't set up a binding based on snooping the DNA
procedure, it cannot achieve perfect compatibility with DNA.
However, it only means the re-configuration of the interface is
slowed but not prevented. Details are discussed as follows.
In Simple DNAv6 [RFC6059], the probe is sent with the source address
set to a link-local address, and such messages will not be discarded
by the policy specified in Section 8.2. If a client is re-attached
to a previous network, the detection will be completed, and the
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address will be regarded as valid by the client. However, the
candidate address is not contained in the probe. Thus, the binding
cannot be recovered through snooping the probe. As the client will
perform DHCP exchange at the same time, the binding will be recovered
from the DHCP Snooping Process. The DHCP Request messages will not
be filtered out in this case because they have link-local source
addresses. Before the DHCP procedure is completed, packets will be
filtered out by the SAVI device. In other words, if this SAVI
function is enabled, Simple DNAv6 will not help reduce the handover
latency. If Data-Snooping attribute is configured on the new
attachment of the client, the data triggered procedure may reduce
latency.
In DNAv4 [RFC4436], the ARP probe will be discarded because an
unbound address is used as the sender protocol address. As a result,
the client will regard the address under detection is valid.
However, the data traffic will be filtered. The DHCP Request message
sent by the client will not be discarded, because the source IP
address field should be all zero as required by [RFC2131]. Thus, if
the address is still valid, the binding will be recovered from the
DHCP Snooping Process.
11.5. Binding Number Limitation
A binding entry will consume a certain high-speed memory resources.
In general, a SAVI device can afford only a quite limited number of
binding entries. In order to prevent an attacker from overloading
the resource of the SAVI device, a binding entry limit is set on each
attachment. The binding entry limit is the maximum number of
bindings supported on each attachment with Validating attribute. No
new binding should be set up after the limit has been reached. If a
DHCP Reply assigns more addresses than the remaining binding entry
quota of each client, the message will be discarded and no binding
will be set up.
11.6. Privacy Considerations
A SAVI device MUST delete binding anchor information as soon as
possible (i.e., as soon as the state for a given address is back to
NO_BIND), except where there is an identified reason why that
information is likely to be involved in the detection, prevention, or
tracing of actual source address spoofing. Information about the
majority of hosts that never spoof SHOULD NOT be logged.
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12. IANA Considerations
This memo asks the IANA for no new parameters.
13. Acknowledgment
Special thanks to Jean-Michel Combes, Christian Vogt, Joel M.
Halpern, Eric Levy-Abegnoli, Marcelo Bagnulo Braun, Jari Arkko, Elwyn
Davies, Barry Leiba, Ted Lemon, Leaf Yeh, Ralph Droms and Alberto
Garcia for careful review and valuation comments on the mechanism and
text.
Thanks to Mark Williams, Erik Nordmark, Mikael Abrahamsson, David
Harrington, Pekka Savola, Xing Li, Lixia Zhang, Bingyang Liu, Duanqi
Zhou, Robert Raszuk, Greg Daley, John Kaippallimalil and Tao Lin for
their valuable contributions.
This document was generated using the xml2rfc tool.
14. References
14.1. Normative References
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting
Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
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[RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
"DHCPv6 Leasequery", RFC 5007, September 2007.
[RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
July 2008.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059, November
2010.
[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses", RFC
6620, May 2012.
14.2. Informative References
[I-D.ietf-opsec-dhcpv6-shield]
Gont, F., Will, W., and G. Velde, "DHCPv6-Shield:
Protecting Against Rogue DHCPv6 Servers", draft-ietf-
opsec-dhcpv6-shield-04 (work in progress), July 2014.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, October 2013.
[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport", RFC
7341, August 2014.
Authors' Addresses
Jun Bi
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
EMail: junbi@tsinghua.edu.cn
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Jianping Wu
Tsinghua University
Computer Science, Tsinghua University
Beijing 100084
China
EMail: jianping@cernet.edu.cn
Guang Yao
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
EMail: yaoguang@cernet.edu.cn
Fred Baker
Cisco Systems
Santa Barbara, CA 93117
United States
EMail: fred@cisco.com
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