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Internet Engineering Task Force (IETF) A. Kirkham
Request for Comments: 6752 Palo Alto Networks
Category: Informational September 2012
ISSN: 2070-1721
Issues with Private IP Addressing in the Internet
Abstract
The purpose of this document is to provide a discussion of the
potential problems of using private, RFC 1918, or non-globally
routable addressing within the core of a Service Provider (SP)
network. The discussion focuses on link addresses and, to a small
extent, loopback addresses. While many of the issues are well
recognised within the ISP community, there appears to be no document
that collectively describes the issues.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6752.
Copyright Notice
Copyright (c) 2012 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................2
2. Conservation of Address Space ...................................3
3. Effects on Traceroute ...........................................3
4. Effects on Path MTU Discovery ...................................6
5. Unexpected Interactions with Some NAT Implementations ...........7
6. Interactions with Edge Anti-Spoofing Techniques .................9
7. Peering Using Loopbacks .........................................9
8. DNS Interaction .................................................9
9. Operational and Troubleshooting Issues .........................10
10. Security Considerations .......................................10
11. Alternate Approaches to Core Network Security .................12
12. References ....................................................13
12.1. Normative References .....................................13
12.2. Informative References ...................................13
Appendix A. Acknowledgments ......................................14
1. Introduction
In the mid to late 1990s, some Internet Service Providers (ISPs)
adopted the practice of utilising private (or non-globally unique)
[RFC1918] IP addresses for the infrastructure links and in some cases
the loopback interfaces within their networks. The reasons for this
approach centered on conservation of address space (i.e., scarcity of
public IPv4 address space) and security of the core network (also
known as core hiding).
However, a number of technical and operational issues occurred as a
result of using private (or non-globally unique) IP addresses, and
virtually all these ISPs moved away from the practice. Tier 1 ISPs
are considered the benchmark of the industry and as of the time of
writing, there is no known tier 1 ISP that utilises the practice of
private addressing within their core network.
The following sections will discuss the various issues associated
with deploying private [RFC1918] IP addresses within ISP core
networks.
The intent of this document is not to suggest that private IP
addresses can not be used with the core of an SP network, as some
providers use this practice and operate successfully. The intent is
to outline the potential issues or effects of such a practice.
Note: The practice of ISPs using "squat" address space (also known
as "stolen" space) has many of the same, plus some additional, issues
(or effects) as that of using private IP address space within core
networks. The term "squat IP address space" refers to the practice
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of an ISP using address space for its own infrastructure/core network
addressing that has been officially allocated by an RIR (Regional
Internet Registry) to another provider, but that provider is not
currently using or advertising within the Internet. Squat addressing
is not discussed further in this document. It is simply noted as an
associated issue.
2. Conservation of Address Space
One of the original intents for the use of private IP addressing
within an ISP core was the conservation of IP address space. When an
ISP is allocated a block of public IP addresses (from an RIR), this
address block was traditionally split in order to dedicate some
portion for infrastructure use (i.e., for the core network) and the
other portion for customer (subscriber) or other address pool use.
Typically, the number of infrastructure addresses needed is
relatively small in comparison to the total address count. So unless
the ISP was only granted a small public block, dedicating some
portion to infrastructure links and loopback addresses (/32) is
rarely a large enough issue to outweigh the problems that are
potentially caused when private address space is used.
Additionally, specifications and equipment capability improvements
now allow for the use of /31 subnets [RFC3021] for link addresses in
place of the original /30 subnets -- further minimising the impact of
dedicating public addresses to infrastructure links by only using two
(2) IP addresses per point-to-point link versus four (4),
respectively.
The use of private addressing as a conservation technique within an
Internet Service Provider (ISP) core can cause a number of technical
and operational issues or effects. The main effects are described
below.
3. Effects on Traceroute
The single biggest effect caused by the use of private addressing
[RFC1918] within an Internet core is the fact that it can disrupt the
operation of traceroute in some situations. This section provides
some examples of the issues that can occur.
A first example illustrates the situation where the traceroute
crosses an Autonomous System (AS) boundary, and one of the networks
has utilised private addressing. The following simple network is
used to show the effects.
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AS64496 EBGP AS64497
IBGP Mesh <---------------> IBGP Mesh
R1 Pool - R6 Pool -
203.0.113.0/26 203.0.113.64/26
198.51.100.8/30
198.51.100.4/30
10.1.1.0/30 10.1.1.4/30 198.51.100.0/30
.9 .10
.1 .2 .5 .6 ------------ .6 .5 .2 .1
R1-----------R2-----------R3--| |--R4----------R5----------R6
R1 Loopback: 10.1.1.101 R4 Loopback: 198.51.100.103
R2 Loopback: 10.1.1.102 R5 Loopback: 198.51.100.102
R3 Loopback: 10.1.1.103 R6 Loopback: 198.51.100.101
Using this example, performing the traceroute from AS64497 to
AS64496, we can see the private addresses of the infrastructure links
in AS64496 are returned.
R6#traceroute 203.0.113.1
Type escape sequence to abort.
Tracing the route to 203.0.113.1
1 198.51.100.2 40 msec 20 msec 32 msec
2 198.51.100.6 16 msec 20 msec 20 msec
3 198.51.100.9 20 msec 20 msec 32 msec
4 10.1.1.5 20 msec 20 msec 20 msec
5 10.1.1.1 20 msec 20 msec 20 msec
R6#
This effect in itself is often not a problem. However, if anti-
spoofing controls are applied at network perimeters, then responses
returned from hops with private IP addresses will be dropped. Anti-
spoofing refers to a security control where traffic with an invalid
source address is discarded. Anti-spoofing is further described in
[BCP38] and [BCP84]. Additionally, any [RFC1918] filtering
mechanism, such as those employed in most firewalls and many other
network devices can cause the same effect.
The effects are illustrated in a second example below. The same
network as in example 1 is used, but with the addition of anti-
spoofing deployed at the ingress of R4 on the R3-R4 interface (IP
Address 198.51.100.10).
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R6#traceroute 203.0.113.1
Type escape sequence to abort.
Tracing the route to 203.0.113.1
1 198.51.100.2 24 msec 20 msec 20 msec
2 198.51.100.6 20 msec 52 msec 44 msec
3 198.51.100.9 44 msec 20 msec 32 msec
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
In a third example, a similar effect is caused. If a traceroute is
initiated from a router with a private (source) IP address, located
in AS64496 and the destination is outside of the ISP's AS (AS64497),
then in this situation, the traceroute will fail completely beyond
the AS boundary.
R1# traceroute 203.0.113.65
Type escape sequence to abort.
Tracing the route to 203.0.113.65
1 10.1.1.2 20 msec 20 msec 20 msec
2 10.1.1.6 52 msec 24 msec 40 msec
3 * * *
4 * * *
5 * * *
6 * * *
R1#
While it is completely unreasonable to expect a packet with a private
source address to be successfully returned in a typical SP
environment, the case is included to show the effect as it can have
implications for troubleshooting. This case will be referenced in a
later section.
In a complex topology, with multiple paths and exit points, the
provider will lose its ability to trace paths originating within its
own AS, through its network, to destinations within other ASes. Such
a situation could be a severe troubleshooting impediment.
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For completeness, a fourth example is included to show that a
successful traceroute can be achieved by specifying a public source
address as the source address of the traceroute. Such an approach
can be used in many operational situations if the router initiating
the traceroute has at least one public address configured. However,
the approach is more cumbersome.
R1#traceroute
Protocol [ip]:
Target IP address: 203.0.113.65
Source address: 203.0.113.1
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]: 10
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Type escape sequence to abort.
Tracing the route to 203.0.113.65
1 10.1.1.2 0 msec 4 msec 0 msec
2 10.1.1.6 0 msec 4 msec 0 msec
3 198.51.100.10 [AS 64497] 0 msec 4 msec 0 msec
4 198.51.100.5 [AS 64497] 0 msec 0 msec 4 msec
5 198.51.100.1 [AS 64497] 0 msec 0 msec 4 msec
R1#
It should be noted that some solutions to this problem have been
proposed in [RFC5837], which provides extensions to ICMP to allow the
identification of interfaces and their components by any combination
of the following: ifIndex, IPv4 address, IPv6 address, name, and
MTU. However, at the time of this writing, little or no deployment
was known to be in place.
4. Effects on Path MTU Discovery
The Path MTU Discovery (PMTUD) process was designed to allow hosts to
make an accurate assessment of the maximum packet size that can be
sent across a path without fragmentation. Path MTU Discovery is
utilised by IPv4 [RFC1191], IPv6 [RFC1981], and some tunnelling
protocols such as Generic Routing Encapsulation (GRE) and IPsec.
The PMTUD mechanism requires that an intermediate router can reply to
the source address of an IP packet with an ICMP reply that uses the
router's interface address. If the router's interface address is a
private IP address, then this ICMP reply packet may be discarded due
to unicast reverse path forwarding (uRPF) or ingress filtering,
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thereby causing the PMTUD mechanism to fail. If the PMTUD mechanism
fails, this will cause transmission of data between the two hosts to
fail silently due to the traffic being black-holed. As a result, the
potential for application-level issues may be created.
5. Unexpected Interactions with Some NAT Implementations
Private addressing is legitimately used within many enterprise,
corporate, or government networks for internal network addressing.
When users on the inside of the network require Internet access, they
will typically connect through a perimeter router, firewall, or
network proxy that provides Network Address Translation (NAT) or
Network Address Port Translation (NAPT) services to a public
interface.
Scarcity of public IPv4 addresses is forcing many service providers
to make use of NAT. CGN (Carrier-Grade NAT) will enable service
providers to assign private [RFC1918] IPv4 addresses to their
customers rather than public, globally unique IPv4 addresses. NAT444
will make use of a double NAT process.
Unpredictable or confusing interactions could occur if traffic such
as traceroute, PMTUD, and possibly other applications were launched
from the NAT IPv4 'inside address', and it passed over the same
address range in the public IP core. While such a situation would be
unlikely to occur if the NAT pools and the private infrastructure
addressing were under the same administration, such a situation could
occur in the more typical situation of a NATed corporate network
connecting to an ISP. For example, say 10.1.1.0/24 is used to
internally number the corporate network. A traceroute or PMTUD
request is initiated inside the corporate network from say 10.1.1.1.
The packet passes through a NAT (or NAPT) gateway, then over an ISP
core numbered from the same range. When the responses are delivered
back to the originator, the returned packets from the privately
addressed part of the ISP core could have an identical source and
destination address of 10.1.1.1.
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NAT Pool -
203.0.113.0/24
10.1.1.0/30 10.1.1.0/30 198.51.100.0/30
198.51.100.12/30 198.51.100.4/30
.1 .2 .14 .13 .1 .2 .6 .5 .2 .1
R1-----------R2-----------R3---------------R4----------R5----------R6
NAT
R6 Loopback:
198.51.100.100
R1#traceroute 198.51.100.100
Type escape sequence to abort.
Tracing the route to 198.51.100.100
1 10.1.1.2 0 msec 0 msec 0 msec
2 198.51.100.13 0 msec 4 msec 0 msec
3 10.1.1.2 0 msec 4 msec 0 msec <<<<
4 198.51.100.5 4 msec 0 msec 4 msec
5 198.51.100.1 0 msec 0 msec 0 msec
R1#
This duplicate address space scenario has the potential to cause
confusion among operational staff, thereby making it more difficult
to successfully debug networking problems.
Certainly a scenario where the same [RFC1918] address space becomes
utilised on both the inside and outside interfaces of a NAT/NAPT
device can be problematic. For example, the same private address
range is assigned by both the administrator of a corporate network
and its ISP. Some applications discover the outside address of their
local Customer Premises Equipment (CPE) to determine if that address
is reserved for special use. Application behaviour may then be based
on this determination. "IANA-Reserved IPv4 Prefix for Shared Address
Space" [RFC6598] provides further analysis of this situation.
To address this scenario and others, "IANA-Reserved IPv4 Prefix for
Shared Address Space" [RFC6598] allocated a dedicated /10 address
block for the purpose of Shared CGN (Carrier Grade NAT) Address
Space: 100.64.0.0/10. The purpose of Shared CGN Address Space is to
number CPE (Customer Premise Equipment) interfaces that connect to
CGN devices. As explained in [RFC6598], [RFC1918] addressing has
issues when used in this deployment scenario.
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6. Interactions with Edge Anti-Spoofing Techniques
Denial-of-Service (DOS) attacks and Distributed Denial-of-Service
(DDoS) attacks can make use of spoofed source IP addresses in an
attempt to obfuscate the source of an attack. Network Ingress
Filtering [RFC2827] strongly recommends that providers of Internet
connectivity implement filtering to prevent packets using source
addresses outside of their legitimately assigned and advertised
prefix ranges. Such filtering should also prevent packets with
private source addresses from egressing the AS.
Best security practices for ISPs also strongly recommend that packets
with illegitimate source addresses should be dropped at the AS
perimeter. Illegitimate source addresses includes private [RFC1918]
IP addresses, addresses within the provider's assigned prefix ranges,
and bogons (legitimate but unassigned IP addresses). Additionally,
packets with private IP destination addresses should also be dropped
at the AS perimeter.
If such filtering is properly deployed, then traffic either sourced
from or destined for privately addressed portions of the network
should be dropped, hence the negative consequences on traceroute,
PMTUD, and regular ping-type traffic.
7. Peering Using Loopbacks
Some ISPs use the loopback addresses of Autonomous System Border
Routers (ASBRs) for peering, in particular, where multiple
connections or exchange points exist between the two ISPs. Such a
technique is used by some ISPs as the foundation of fine-grained
traffic engineering and load balancing through the combination of IGP
metrics and multi-hop BGP. When private or non-globally reachable
addresses are used as loopback addresses, this technique is either
not possible or considerably more complex to implement.
8. DNS Interaction
Many ISPs utilise their DNS to perform both forward and reverse
resolution for infrastructure devices and infrastructure addresses.
With a privately numbered core, the ISP itself will still have the
capability to perform name resolution of its own infrastructure.
However, others outside of the autonomous system will not have this
capability. At best, they will get a number of unidentified
[RFC1918] IP addresses returned from a traceroute.
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It is also worth noting that in some cases, the reverse resolution
requests may leak outside of the AS. Such a situation can add load
to public DNS servers. Further information on this problem is
documented in "AS112 Nameserver Operations" [RFC6304].
9. Operational and Troubleshooting Issues
Previous sections of this document have noted issues relating to
network operations and troubleshooting. In particular, when private
IP addressing within an ISP core is used, the ability to easily
troubleshoot across the AS boundary may be limited. In some cases,
this may be a serious troubleshooting impediment. In other cases, it
may be solved through the use of alternative troubleshooting
techniques.
The key point is that the flexibility of initiating an outbound ping
or traceroute from a privately numbered section of the network is
lost. In a complex topology, with multiple paths and exit points
from the AS, the provider may be restricted in its ability to trace
paths through the network to other ASes. Such a situation could be a
severe troubleshooting impediment.
For users outside of the AS, the loss of the ability to use a
traceroute for troubleshooting is very often a serious issue. As
soon as many of these people see a row of "* * *" in a traceroute
they often incorrectly assume that a large part of the network is
down or inaccessible (e.g., behind a firewall). Operational
experience in many large providers has shown that significant
confusion can result.
With respect to [RFC1918] IP addresses applied as loopbacks, in this
world of acquisitions, if an operator needed to merge two networks,
each using the same private IP ranges, then the operator would likely
need to renumber one of the two networks. In addition, assume an
operator needed to compare information such as NetFlow / IP Flow
Information Export (IPFIX) or syslog, between two networks using the
same private IP ranges. There would likely be an issue as the unique
ID in the collector is, in most cases, the source IP address of the
UDP export, i.e., the loopback address.
10. Security Considerations
One of the arguments often put forward for the use of private
addressing within an ISP is an improvement in the network security.
It has been argued that if private addressing is used within the
core, the network infrastructure becomes unreachable from outside the
provider's autonomous system, hence protecting the infrastructure.
There is legitimacy to this argument. Certainly, if the core is
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privately numbered and unreachable, it potentially provides a level
of isolation in addition to what can be achieved with other
techniques, such as infrastructure Access Control Lists (ACLs), on
their own. This is especially true in the event of an ACL
misconfiguration, something that does commonly occur as the result of
human error.
There are three key security gaps that exist in a privately addressed
IP core.
1. The approach does not protect against reflection attacks if edge
anti-spoofing is not deployed. For example, if a packet with a
spoofed source address corresponding to the network's
infrastructure address range is sent to a host (or other device)
attached to the network, that host will send its response
directly to the infrastructure address. If such an attack was
performed across a large number of hosts, then a successful
large-scale DoS attack on the infrastructure could be achieved.
This is not to say that a publicly numbered core will protect
from the same attack; it won't. The key point is that a
reflection attack does get around the apparent security offered
in a privately addressed core.
2. Even if anti-spoofing is deployed at the AS boundary, the border
routers will potentially carry routing information for the
privately addressed network infrastructure. This can mean that
packets with spoofed addresses, corresponding to the private
infrastructure addressing, may be considered legitimate by edge
anti-spoofing techniques (such as Unicast Reverse Path Forwarding
- Loose Mode) and forwarded. To avoid this situation, an edge
anti-spoofing algorithm (such as Unicast Reverse Path Forwarding
- Strict Mode) would be required. Strict approaches can be
problematic in some environments or where asymmetric traffic
paths exist.
3. The approach on its own does not protect the network
infrastructure from directly connected customers (i.e., within
the same AS). Unless other security controls, such as access
control lists (ACLs), are deployed at the ingress point of the
network, customer devices will normally be able to reach, and
potentially attack, both core and edge infrastructure devices.
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11. Alternate Approaches to Core Network Security
Today, hardware-based ACLs, which have minimal to no performance
impact, are now widespread. Applying an ACL at the AS perimeter to
prevent access to the network core may be a far simpler approach and
provide comparable protection to using private addressing; such a
technique is known as an infrastructure ACL (iACL).
In concept, iACLs provide filtering at the edge network, which allows
traffic to cross the network core but not to terminate on
infrastructure addresses within the core. Proper iACL deployment
will normally allow required network management traffic to be passed,
such that traceroutes and PMTUD can still operate successfully. For
an iACL deployment to be practical, the core network needs to have
been addressed with a relatively small number of contiguous address
blocks. For this reason, the technique may or may not be practical.
A second approach to preventing external access to the core is IS-IS
core hiding. This technique makes use of a fundamental property of
the IS-IS protocol, which allows link addresses to be removed from
the routing table while still allowing loopback addresses to be
resolved as next hops for BGP. The technique prevents parties
outside the AS from being able to route to infrastructure addresses,
while still allowing traceroutes to operate successfully. IS-IS core
hiding does not have the same practical requirement for the core to
be addressed from a small number of contiguous address blocks as with
iACLs. From an operational and troubleshooting perspective, care
must be taken to ensure that pings and traceroutes are using source
and destination addresses that exist in the routing tables of all
routers in the path, i.e., not hidden link addresses.
A third approach is the use of either an MPLS-based IP VPN or an
MPLS-based IP Core where the 'P' routers (or Label Switch Routers) do
not carry global routing information. As the core 'P' routers (or
Label Switch Routers) are only switching labeled traffic, they are
effectively not reachable from outside of the MPLS domain. The 'P'
routers can optionally be hidden so that they do not appear in a
traceroute. While this approach isolates the 'P' routers from
directed attacks, it does not protect the edge routers ('PE' routers
or Label Edge Routers (LERs)). Obviously, there are numerous other
engineering considerations in such an approach; we simply note it as
an option.
These techniques may not be suitable for every network. However,
there are many circumstances where they can be used successfully
without the associated effects of privately addressing the core.
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12. References
12.1. Normative References
[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", May 2000.
[BCP84] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", March 2004.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[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.
12.2. Informative References
[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
RFC 3021, December 2000.
[RFC5837] Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.
Rivers, "Extending ICMP for Interface and Next-Hop
Identification", RFC 5837, April 2010.
[RFC6304] Abley, J. and W. Maton, "AS112 Nameserver Operations",
RFC 6304, July 2011.
[RFC6598] Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
Space", BCP 153, RFC 6598, April 2012.
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Appendix A. Acknowledgments
The author would like to thank the following people for their input
and review: Dan Wing (Cisco Systems), Roland Dobbins (Arbor
Networks), Philip Smith (APNIC), Barry Greene (ISC), Anton Ivanov
(kot-begemot.co.uk), Ryan Mcdowell (Cisco Systems), Russ White (Cisco
Systems), Gregg Schudel (Cisco Systems), Michael Behringer (Cisco
Systems), Stephan Millet (Cisco Systems), Tom Petch (BT Connect), Wes
George (Time Warner Cable), and Nick Hilliard (INEX).
The author would also like to acknowledge the use of a variety of
NANOG mail archives as references.
Author's Address
Anthony Kirkham
Palo Alto Networks
Level 32, 101 Miller St
North Sydney, New South Wales 2060
Australia
Phone: +61 7 33530902
EMail: tkirkham@paloaltonetworks.com
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