draft-ietf-opsec-blackhole-urpf-03.txt   draft-ietf-opsec-blackhole-urpf-04.txt 
Opsec Working Group W. Kumari Opsec Working Group W. Kumari
Internet Draft Google Internet Draft Google
<draft-ietf-opsec-blackhole-urpf-03> D. McPherson <draft-ietf-opsec-blackhole-urpf-04> D. McPherson
Category: Informational Arbor Networks Category: Informational Arbor Networks
Expires: September 30, 2009 Expires: December 4, 2009
March 30, 2009 June 4, 2009
Remote Triggered Black Hole filtering with uRPF Remote Triggered Black Hole filtering with uRPF
<draft-ietf-opsec-blackhole-urpf-03.txt> <draft-ietf-opsec-blackhole-urpf-04.txt>
Status of this Memo Status of this Memo
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Abstract Abstract
Remote Triggered Black Hole (RTBH) filtering is a popular and Remote Triggered Black Hole (RTBH) filtering is a popular and
effective technique for the mitigation of denial-of-service attacks. effective technique for the mitigation of denial-of-service attacks.
This document expands upon destination-based RTBH filtering by This document expands upon destination-based RTBH filtering by
outlining a method to enable filtering by source address as well. outlining a method to enable filtering by source address as well.
Table of Contents Table of Contents
1. Introduction ....................................................3 1. Introduction ....................................................3
2. Terminology .....................................................3 2. Terminology .....................................................4
3. Background ......................................................3 3. Destination address RTBH filtering ..............................4
4. Destination address RTBH filtering ..............................4 3.1. Overview ...................................................4
4.1. Overview ...................................................4 3.2. Detail .....................................................5
4.2. Detail .....................................................5 4. Source address RTBH filtering ...................................8
5. Source address RTBH filtering ...................................7 4.1. Steps to deploy RTBH filtering with uRPF ...................9
5.1. Steps to deploy RTBH filtering with uRPF ...................8 5. Security Considerations .........................................9
6. Security Considerations .........................................9 6. IANA Considerations ............................................10
7. IANA Considerations .............................................9 7. Acknowledgments ................................................10
8. Acknowledgments .................................................9 8. References .....................................................10
9. References ......................................................9 8.1. Normative References ......................................10
9.1. Normative References .......................................9 8.2. Informative References ....................................10
9.2. Informative References ....................................10
A. Cisco Router Configuration Sample...............................11 A. Cisco Router Configuration Sample...............................11
B. Juniper Configuration Sample....................................13 B. Juniper Configuration Sample....................................13
C. A Brief History of RTBH.........................................15 C. A Brief History of RTBH.........................................15
1. Introduction 1. Introduction
This document expands upon the technique outlined in "Configuring BGP This document expands upon the technique outlined in "Configuring BGP
to Block Denial-of-Service Attacks" [RFC3882] to demonstrate a method to Block Denial-of-Service Attacks" [RFC3882] to demonstrate a method
that allows for filtering by source address(es). that allows for filtering by source address(es).
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. [RFC2119].
3. Background
Network operators have developed a variety of techniques for Network operators have developed a variety of techniques for
mitigating denial of service attacks. While different techniques have mitigating denial of service attacks. While different techniques have
varying strengths and weaknesses, from an implementation perspective varying strengths and weaknesses, from an implementation perspective
the selection of which method to use for each type of attack involves the selection of which method to use for each type of attack involves
evaluating the tradeoffs associated with each method. evaluating the tradeoffs associated with each method.
A common DoS attack directed against a customer of a service provider A common DoS attack directed against a customer of a service provider
involves generating a greater volume of attack traffic destined for involves generating a greater volume of attack traffic destined for
the target than will fit down the links from the service provider(s) the target than will fit down the links from the service provider(s)
to the victim (customer). This traffic "starves out" legitimate to the victim (customer). This traffic "starves out" legitimate
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target prefix. All packets towards that destination, attack traffic target prefix. All packets towards that destination, attack traffic
AND legitimate traffic, are then dropped by the participating AND legitimate traffic, are then dropped by the participating
routers,thereby taking the target completely offline. The benefit is routers,thereby taking the target completely offline. The benefit is
that collateral damage to other systems or network availability at that collateral damage to other systems or network availability at
the customer location or in the ISP network is limited, but the the customer location or in the ISP network is limited, but the
negative impact to the target itself is arguably increased. negative impact to the target itself is arguably increased.
By coupling unicast reverse path forwarding (RPF) [RFC3704] By coupling unicast reverse path forwarding (RPF) [RFC3704]
techniques with RTBH filtering, BGP can be used to distribute discard techniques with RTBH filtering, BGP can be used to distribute discard
routes that are based not on destination or target addresses, but routes that are based not on destination or target addresses, but
based on source addresses of unwanted traffic. based on source addresses of unwanted traffic. Note that this will
drop all traffic to / from the address, and not just the traffic to
the victim.
4. Destination address RTBH filtering This document is broken up into three logical parts, the first
outlines how to configure destination based RTBH, the second covers
source based RTBH and the third part has examples and a history of
the technique.
4.1. Overview 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. [RFC2119].
3. Destination address RTBH filtering
3.1. Overview
A "discard" route is installed on each edge router in the network A "discard" route is installed on each edge router in the network
with the destination set to the discard (or null) interface. In order with the destination set to the discard (or null) interface. In order
to use RTBH filtering for a single IP address (or prefix), a BGP to use RTBH filtering for a single IP address (or prefix), a BGP
route for the address to be filtered is announced, with the next-hop route for the address to be filtered is announced, with the next-hop
set as the "discard" route. This causes traffic to the announced set as the "discard" route. This causes traffic to the announced
network prefix to be forwarded to the discard interface so that it network prefix to be forwarded to the discard interface so that it
does not transit the network wasting resources or triggering does not transit the network wasting resources or triggering
collateral damage to other resources along the path towards the collateral damage to other resources along the path towards the
target. target.
While this does "complete" the attack in that the target address(es) While this does "complete" the attack in that the target address(es)
are made unreachable, collateral damage is minimized. It may also be are made unreachable, collateral damage is minimized. It may also be
possible to move the host or service on the target IP address(es) to possible to move the host or service on the target IP address(es) to
another address and keep the service up, for example by updating another address and keep the service up, for example by updating
associated DNS resource records. associated DNS resource records.
4.2. Detail 3.2. Detail
Before deploying RTBH filtering, there is some preparation and
planning that needs to occur and decisions that need to be made.
These include:
- what are the discard addresses?
- what are the discard BGP communities?
- what is the largest prefix that can be black-holed?
- what is the smallest advertisement that your provider will
accept?
Steps to configure destination based RTBH filtering: Steps to configure destination based RTBH filtering:
1. Select your Discard Address schema. An address is chosen to become Step 1. Select your Discard Address schema. An address is chosen to
the "discard address". This is often chosen from 192.0.2.0/24 (TEST- become the "discard address". This is often chosen from 192.0.2.0/24
NET [RFC3330]), or from RFC 1918 [RFC1918] space. Multiple addresses (TEST-NET [RFC3330]), or from RFC 1918 [RFC1918] space. Multiple
allow an operator to configure multiple static routes, one for each addresses allow an operator to configure multiple static routes, one
incident: for each incident:
192.0.2.1 = Incident #1 192.0.2.1 = Incident #1
192.0.2.2 = Incident #2 192.0.2.2 = Incident #2
192.0.2.3 = Incident #3 192.0.2.3 = Incident #3
192.0.2.4 = Incident #4 192.0.2.4 = Incident #4
192.0.2.5 = Incident #5 192.0.2.5 = Incident #5
Customer #1 who has a DDoS attack can be pointed to discard route Customer #1 who has a DDoS attack can be pointed to discard route
192.0.2.1. Customer #2 can be pointed to discard route 192.0.2.2. If 192.0.2.1. Customer #2 can be pointed to discard route 192.0.2.2. If
capable, the router can then count the drops for each, providing some capable, the router can then count the drops for each, providing some
level of telemetry on the volume of drops as well as status of an level of telemetry on the volume of drops as well as status of an
ongoing attack. A consistent address schema facilitates operations. ongoing attack. A consistent address schema facilitates operations.
2. Set the Discard Route(s) on each router. A route for the "discard Step 2. Configure the Discard Route(s) on each router, A route for
address" is installed on the routers that form the edge/perimeter of the "discard address" is installed on the routers that form the
the network, in all routers in the network, or some subset (e.g., edge/perimeter of the network, in all routers in the network, or some
peering, but not customer, etc.). The destination of the route is set subset (e.g., peering, but not customer, etc.). The destination of
to the "discard" or "null" interface. This route is called the the route is set to the "discard" or "null" interface. This route is
"discard route". Implementation experience demonstrates the value of called the "discard route". Implementation experience demonstrates
configuring each ingress router with a capability for dropping the value of configuring each ingress router with a capability for
traffic via RTBH filtering. dropping traffic via RTBH filtering.
3. Configure the RTBH BGP Policy on each router. A BGP policy is Step 3. Configure the RTBH BGP Policy on each router. A BGP policy
configured on all routers that have the discard route so that routes is configured on all routers that have the discard route so that
announced with a chosen community will have their next hop set to the routes announced with a chosen community will have their next hop set
discard address. The BGP policy should be made restrictive so that to the discard address. The BGP policy should be made restrictive so
only BGP routes covering a defined number of hosts addresses will be that only BGP routes covering a defined number of hosts addresses
accepted. That is, typically, only specific /32s are necessary. will be accepted. That is, typically, only specific /32s are
Shorter prefix blocks may also be required or desirable, for example necessary. Shorter prefix blocks may also be required or desirable,
if larger numbers of attack targets are located within a single for example if larger numbers of attack targets are located within a
prefix, or the employment of this mechanism is to drop traffic bound single prefix, or the employment of this mechanism is to drop
for specific networks. When filtering based on shorter prefixes, traffic bound for specific networks. When filtering based on shorter
extreme caution should be used as to avoid collateral damage to other prefixes, extreme caution should be used as to avoid collateral
hosts that reside within those address blocks. Full implementations damage to other hosts that reside within those address blocks. Full
will have multiple communities, with each community used for implementations will have multiple communities, with each community
different parts of a provider's network and for different security used for different parts of a provider's network and for different
incidents. security incidents.
4. Configure the Safety Egress Prefix Filters (Murphy Prefix Step 4. Configure the Safety Egress Prefix Filters. There is always
Filters). There is always a chance that the triggering BGP Update a chance that the triggering BGP Update could leak from the network
could leak from the network. This has happened [ Youtube/Pakistan and so egress prefix filters are strongly recommended. These egress
incident ]. Egress prefix filters are recommended. These egress prefix filter details may vary, but experience has demonstrated that
prefix filter details may varying, but experience has demonstrated the following works:
that the following works:
4.1 Deny all prefixes /30 through /32 from leaving your network. If - Deny all prefixes longer than the longest prefix that you expect
to
announce. For example, if the longest prefix that you expect to
announce is /24, deny all prefixes of length /25 though /32. If
your triggering BGP update is only /32s, then this egress prefix your triggering BGP update is only /32s, then this egress prefix
filter will add a safe measure in case the NO_EXPORT community does filter will add a safe measure in case the NO_EXPORT community
does not work.
- Deny all communities used for triggering RTBH filtering. This is
also a "safety" measure in case the NO_EXPORT community does
not work. not work.
4.2 Deny all communities used for triggering RTBH filtering. This is Step 5: Configure the Trigger Router. Configure the trigger router,
also a "safety" measure in case the NO_EXPORT community does not workstation, or other device so that adding and removing the triggers
work. can be done easily and quickly. The BGP Update should have the
NO_EXPORT community as a mandatory attribute. An egress prefix filter
or policy which prevents RTBH filtering prefixes in the /8 to /24
range is also recommended as a safety tool. The trigger router can be
set up as a iBGP route reflector client which does not receive any
prefixes from its BGP peer. This allows a low cost router /
workstation to be used as the trigger router.
5: Configure the Trigger Router. Set the trigger router, workstation, Using the RTBH filtering:
or other device so that adding and removing the triggers can be done
easily and quickly. The BGP Update should have the NO_EXPORT
community as a mandatory attribute. An egress prefix filter or policy
which prevents RTBHing prefixes in the /8 to /24 range is also
recommended as a safety tool. The trigger router can be set up as a
iBGP route reflector client which does not receive any prefixes from
its BGP peer. This allows a low cost router/workstation to be used
as the trigger router.
6. When RTBH filtering is desired for a specific address, that 1. When RTBH filtering is desired for a specific address, that
address is announced from a trigger router (or route server), tagged address is announced from a trigger router (or route server), tagged
with the chosen "RTBH" community and with the NO_EXPORT community, with the chosen "RTBH" community and with the NO_EXPORT community,
and passed via iBGP. The receiving routers check the BGP policy, set and passed via iBGP. The receiving routers check the BGP policy, set
the next-hop to be the discard route, resulting in a FIB entry the next-hop to be the discard route, resulting in a FIB entry
pointing to a discard address. pointing to a discard address.
7. Traffic entering the network will now be forwarded to the discard 2. Traffic entering the network will now be forwarded to the discard
interface on all edge routers and will therefore be dropped at the interface on all edge routers and will therefore be dropped at the
edge of the network, saving resources. edge of the network, saving resources.
7.1 Multiple Discard Addresses for Incident Granularity. This 2.1 Multiple Discard Addresses for Incident Granularity. This
technique can be expanded by having multiple discard addresses, technique can be expanded by having multiple discard addresses,
routes and communities to allow for monitoring of the discarded routes and communities to allow for monitoring of the discarded
traffic volume on devices that support multiple discard interfaces. traffic volume on devices that support multiple discard interfaces.
As mentioned earlier, each router can have a discard address schema As mentioned earlier, each router can have a discard address schema
to allow the operator to distinguish multiple incidents from each to allow the operator to distinguish multiple incidents from each
other - making it easier to monitor the life-cycle of the incidents. other - making it easier to monitor the life-cycle of the incidents.
7.2 Multiple "Trigger Communities" for Network Wide Granularity. The 2.2 Multiple "Trigger Communities" for Network Wide Granularity. The
network can be sectioned into multiple communities, providing the network can be sectioned into multiple communities, providing the
operator with an ability to drop in discreete parts of their network. operator with an ability to drop in discrete parts of their network.
For example, the network can be divided into the following For example, the network can be divided into the following
communities: communities:
XXX:600 RTBH filtering on all router XXX:600 RTBH filtering on all router
XXX:601 RTBH filtering on only peering router XXX:601 RTBH filtering on only peering router
XXX:602 RTBH filtering on only customers who peer BGP XXX:602 RTBH filtering on only customers who peer BGP
XXX:603 RTBH filtering on Datacenters (to see if the data center XXX:603 RTBH filtering on Datacenters (to see if the data center
is the is the
source of attack) source of attack)
XXX:604 RTBH filtering on all customers (to see how many XXX:604 RTBH filtering on all customers (to see how many
customers are customers are
being used by the attacker) being used by the attacker)
Some diligent thinking is required to develop a community schema Some diligent thinking is required to develop a community schema
which provides flexibility while reflecting topological which provides flexibility while reflecting topological
considerations. considerations.
7.3 "Customer Triggered" RTBH filtering. The technique can also be 2.3 "Customer Triggered" RTBH filtering. The technique can also be
expanded by relaxing the AS path rule to allow customers of a service expanded by relaxing the AS path rule to allow customers of a service
provider to enable RTBH filtering without interacting with the provider to enable RTBH filtering without interacting with the
service provider's trigger routers. If this is configured, an service provider's trigger routers. If this is configured, an
operator MUST only accept announcements for prefixes from the operator MUST only accept announcements for prefixes from the
customer that the customer is authorized to advertise, in order to customer that the customer is authorized to advertise, in order to
prevent the customer from accidentally (or intentionally) black- prevent the customer from accidentally (or intentionally) black-
holing space that they are not allowed to advertise. holing space that they are not allowed to advertise.
A common policy for this type of setup would first permit any more A common policy for this type of setup would first permit any more
specific of an authorized prefix only if the blackhole communities is specific of an authorized prefix only if the blackhole communities is
attached, append NO_EXPORT, NO_ADVERTISE, or similar communities, and attached, append NO_EXPORT, NO_ADVERTISE, or similar communities, and
then also accept from the customer the original aggregate prefix that then also accept from the customer the original aggregate prefix that
will be advertised as standard policy permits. will be advertised as standard policy permits.
Extreme caution should be used in order to avoid leaking any more Extreme caution should be used in order to avoid leaking any more
specifics beyond the local routing domain, unless policy explicitly specifics beyond the local routing domain, unless policy explicitly
aims at doing just that. aims at doing just that.
5. Source address RTBH filtering. 4. Source address RTBH filtering.
In many instances denial-of-service attacks sourced from botnets are In many instances denial-of-service attacks sourced from botnets are
being configured to "follow DNS" (the attacking machines are being configured to "follow DNS" (the attacking machines are
instructed to attack www.example.com, and re-resolve this instructed to attack www.example.com, and re-resolve this
periodically. Historically the attacks were aimed simply at an IP periodically. Historically the attacks were aimed simply at an IP
address and so renumbering www.example.com to a new address was an address and so renumbering www.example.com to a new address was an
effective mitigation). This makes employing technique that allows effective mitigation). This makes employing technique that allows
black-holing to be based on source address desirable. black-holing to be based on source address desirable.
By combining traditional RTBH filtering with unicast Reverse Path By combining traditional RTBH filtering with unicast Reverse Path
Forwarding (uRPF) a network operator can filter based upon the source Forwarding (uRPF) a network operator can filter based upon the source
address. uRPF performs a route lookup of the source address of the address. uRPF performs a route lookup of the source address of the
packet and checks to see if the ingress interface of the packet is a packet and checks to see if the ingress interface of the packet is a
valid egress interface for the packet source address (strict mode) or valid egress interface for the packet source address (strict mode) or
if any route to the source address of the packet exists (loose mode). if any route to the source address of the packet exists (loose mode).
If the check fails, the packet is typically dropped. In loose mode If the check fails, the packet is typically dropped. In loose mode
some vendors also verify that the destination route does not point to some vendors also verify that the destination route does not point to
a discard interface - this allows source based RTBH filtering to be an invalid next-hop - this allows source based RTBH filtering to be
deployed in networks that cannot implement strict (or feasible path) deployed in networks that cannot implement strict (or feasible path)
mode uRPF. mode uRPF. Before enabling uRPF (in any mode), it is vital that you
fully understand the implications of doing so:
- Strict mode will cause the router to drop all ingress traffic
if the best path back to the source address of the traffic is
not the interface from which the traffic was received.
Asymetric routing will cause strict mode uRPF to drop
legitimate traffic.
- Loose mode causes the router to check if a route for the source
address of the traffic exists. This may also cause legitimate
traffic to be discarded.
It is hoped that in the future, vendors will implement a "DoS-
mitigation" mode in addition to the Loose and Strict modes -- in this
mode, the uRPF check will only fail if the next-hop for the source of
the packet is a discard interface.
By enabling the uRPF feature on interfaces at pre-determined points By enabling the uRPF feature on interfaces at pre-determined points
in their network and announcing the source address(es) of attack in their network and announcing the source address(es) of attack
traffic, a network operator can effectively drop the identified traffic, a network operator can effectively drop the identified
attack traffic at specified devices (ideally ingress edge) of their attack traffic at specified devices (ideally ingress edge) of their
network based on source address. network based on source address.
While administrators may choose to drop traffic from any prefix they While administrators may choose to drop traffic from any prefix they
wish, it is recommended when employing source-based RTBH filtering wish, it is recommended when employing source-based RTBH filtering
inter-domain that explicit policy be defined that enables peers to inter-domain that explicit policy be defined that enables peers to
only announce source-based RTBH routes for prefixes which they only announce source-based RTBH routes for prefixes which they
originate. originate.
5.1 Steps to deploy RTBH filtering with uRPF for source filtering. 4.1 Steps to deploy RTBH filtering with uRPF for source filtering.
The same steps that are required to implement destination address The same steps that are required to implement destination address
RTBH filtering are taken with the additional step of enabling unicast RTBH filtering are taken with the additional step of enabling unicast
reverse path forwarding on predetermined interfaces. When a source reverse path forwarding on predetermined interfaces. When a source
address (or network) needs to be blocked, that address (or network) address (or network) needs to be blocked, that address (or network)
is announced using BGP tagged with a community. This will cause the is announced using BGP tagged with a community. This will cause the
route to be installed with a next hop of the discard interface, route to be installed with a next hop of the discard interface,
causing the uRPF check to fail. The destination based RTBH filtering causing the uRPF check to fail and the packets to be discarded. The
community should not be used for source based RTBH filtering, and the destination based RTBH filtering community should not be used for
routes tagged with the selected community should be carefully source based RTBH filtering, and the routes tagged with the selected
filtered. community should be carefully filtered.
The BGP policy will need to be relaxed to accept announcements tagged The BGP policy will need to be relaxed to accept announcements tagged
with this community to be accepted even though they contain addresses with this community to be accepted even though they contain addresses
not controlled by the network announcing them. These announcements not controlled by the network announcing them. These announcements
must NOT be propagated outside the local AS and should carry the must NOT be propagated outside the local AS and should carry the
NO_EXPORT community. NO_EXPORT community.
As a matter of policy, operators SHOULD NOT accept source-based RTBH As a matter of policy, operators SHOULD NOT accept source-based RTBH
announcements from their peers or customers, they should only be announcements from their peers or customers, they should only be
installed by local or attack management systems within their installed by local or attack management systems within their
administrative domain. administrative domain.
6. Security Considerations 5. Security Considerations
The techniques presented here provide enough power to cause The techniques presented here provide enough power to cause
significant traffic forwarding loss if incorrectly deployed. It is significant traffic forwarding loss if incorrectly deployed. It is
imperative that the announcements that trigger the black-holing are imperative that the announcements that trigger the black-holing are
carefully checked and that the BGP policy that accepts these carefully checked and that the BGP policy that accepts these
announcements is implemented in such a manner that the announcements: announcements is implemented in such a manner that the announcements:
- Are not propagated outside the AS (NO_EXPORT). - Are not propagated outside the AS (NO_EXPORT).
- Are not accepted from outside the AS (except from customers). - Are not accepted from outside the AS (except from customers).
- Except where source based filtering is deployed, that the network - Except where source based filtering is deployed, that the network
contained in the announcement falls within the address ranges contained in the announcement falls within the address ranges
controlled by the announcing AS (i.e.: for customers that the controlled by the announcing AS (i.e.: for customers that the
address falls within their space). address falls within their space).
7. IANA Considerations 6. IANA Considerations
No action required. No action required.
8. Acknowledgments 7. Acknowledgments
I would like to thank Joe Abley, Rodney Dunn, Alfred Hoenes, Donald I would like to thank Joe Abley, Ron Bonica, Rodney Dunn, Alfred
Smith, Joel Jaeggli and Steve Williams for their assistance, feedback Hoenes, Donald Smith, Joel Jaeggli, and Steve Williams for their
and not laughing *too* much at the quality of the initial drafts. assistance, feedback and not laughing *too* much at the quality of the
initial drafts.
I would also like to thank all of the regular contributors to the I would also like to thank all of the regular contributors to the
OpSec Working Group and Google for 20% time :-) OpSec Working Group and Google for 20% time :-)
The authors would also like to thank Steven L Johnson and Barry Greene The authors would also like to thank Steven L Johnson and Barry Greene
for getting this implemented and Chris Morrow for publicizing the for getting this implemented and Chris Morrow for publicizing the
technique in multiple talks. technique in multiple talks.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets", and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996. BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September
2002. 2002.
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