Opsec WG P. Savola
Internet-Draft CSC/FUNET
Intended status: Informational November 15, 2006
Expires: May 19, 2007
Experiences from Using Unicast RPF
draft-savola-bcp84-urpf-experiences-02.txt
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Copyright (C) The IETF Trust (2006).
Abstract
RFC 3704 (BCP 84) published in March 2004 provided an ingress
filtering technique update to RFC 2827 (BCP 38). This memo tries to
document operational experiences learned practising ingress filtering
techniques, in particular ingress filtering for multihomed networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Common uRPF Drops . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Unused Address Space Ping-Pong . . . . . . . . . . . . . . 4
2.2. Private Address Leak . . . . . . . . . . . . . . . . . . . 4
2.3. Wrong IP Address . . . . . . . . . . . . . . . . . . . . . 5
3. Multihoming uRPF Drops . . . . . . . . . . . . . . . . . . . . 5
3.1. Incorrect Source Address Selection . . . . . . . . . . . . 5
3.2. Point-to-Point Interface Routes . . . . . . . . . . . . . 6
3.3. Multiple Routers on a LAN use LAN for Transit . . . . . . 7
4. Special uRPF Failures Cases . . . . . . . . . . . . . . . . . 7
4.1. PMTUD and Private/Non-routed Addresses . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
RFC 3704 [RFC3704] (BCP 84) published in March 2004 provided an
ingress filtering technique update to RFC 2827 [RFC2827] (BCP 38).
This memo tries to document operational experiences learned
practising ingress filtering techniques, in particular ingress
filtering for multihomed networks.
Specifically, this version describes the lessons learned in author's
network where strict unicast RPF (uRPF) ingress filtering, using
"feasible paths" variant [RFC3704] has been used for all the customer
interfaces (whether single- or multihomed) for over two years. In
feasible paths strict uRPF, only an accepted equal length prefix
(even if not preferred) is considered feasible. While in some cases,
a more specific or even a less specific might be acceptable, such
condition would not necessarily be correct in general.
We use the typical "customer" and "ISP" terms to refer to the subject
of strict uRPF filtering and the party doing filtering. The same
considerations also apply for other business relationships (e.g.,
"internal customers" inside an ISP).
According to a study, there is substantial ingress filtering
deployment, even 75% of addresses were not spoofable [SPOOFER].
We note explicitly that Loose mode RPF is NOT a sufficient solution
in any way to ingress filtering as it creates a false sense of
protection. Even its use as a "contract validation" [RFC3704] is
tenuous at best.
NOTE IN DRAFT: comments should be directed to the author or the OPSEC
mailing list (opsec@ops.ietf.org). However, it is not clear what
should be the next steps wrt. these experiences. Update to the
ingress filtering RFCs? Publish separately? Keep as a standing
document for now? Integrate with OPSEC document work? In any case,
feedback on other experiences is encouraged.
In the second section, we'll first look at the most common types of
uRPF drops and their causes. In the third section, we'll look at a
few special cases observed on multihoming or multi-connecting
scenarios. More special filtering failures are discussed in the
fourth section.
2. Common uRPF Drops
Most uRPF packet drops are in fact due to anomalies which have
nothing to do with spoofing source addresses but are detected (and
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prevented) by the uRPF methodology. In this section, we'll describe
the most common causes for uRPF drops which apply to both single- and
multi-homed networks, and respective ways to eliminate or mitigate
dropping.
2.1. Unused Address Space Ping-Pong
By far, the most common cause for uRPF drops seems to be the case
where a prefix P is routed to the customer (e.g., using a static
route), but the customer doesn't use all of P, and an attacker A is
port-scanning the unused address space.
In that case, typically packets destined to the unused part of "P"
lack a more specific route, and are routed back to the ISP through a
default route. The ISP's router sees these as sourced from attacker
A (an IP address in the Internet), destined to the customer's prefix
P. This fails uRPF check and is dropped.
Note: if uRPF is not employed, the scan may may cause ping-pong
effect up to the remaining hop count/TTL of the packet, consuming
even 250 times the bandwidth and packet processing. This has been
briefly described in [I-D.ietf-ipngwg-p2p-pingpong]. Hence employing
uRPF significantly mitigates the impact of this kind of packet
looping.
The ping-pong effect has also been used in Internet Exchanges to game
peer selection or traffic balance data.
Therefore, the customer should install static discard aggregate
routes (or equivalent) for all of its address space upon assignment,
so that if no better route exists, such probe packets are discarded.
An alternative is applying a similar filtering in egress interface
towards the ISP. There isn't much an ISP can do to prevent this
unless it wants to create customer-specific uRPF access-lists.
2.2. Private Address Leak
Very often, packes from all kinds of private addresses also leak to
the ISP, which are obviously dropped by uRPF. This is probably a
result of misconfigured NATs or inadequate firewall rules. Even
(constant) rates of hundreds of packets per second have been
observed, which makes one wonder which kinds of users' communications
must be failing or otherwise working in a non-optimal fashion due to
this kind of misconfiguration...
This is actually one of the most convincing reasons from the users'
perspective why (they or the ISP) using uRPF could give benefits: it
allows them to notice and fix network misconfiguration and
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malfunction "at the source" and as a result, communication should
work more reliably and new issues would be easier to notice.
The obvious fix is to ensure that the customer is filtering out (and
logging) these packets, and uses that to figure out what is causing
such address leaks and fixes the misconfiguration or other
problem(s).
2.3. Wrong IP Address
It's also not atypical to see other kinds of wrong source addresses.
These can be classified in three main categories: a) nomadic laptops
trying their old IP from a previous network attachment point, b)
spoofed/misconfigured/typoed public, routable IP address, or c) an
unroutable ("bogon") IP address. (It should be noted that Loose uRPF
would only spot the last category.)
Many spoofed attacks are usually a result of a worm or a botnet (DoS)
attack. A recent case was using recursive DNS servers for reflection
[I-D.ietf-dnsop-reflectors-are-evil], but a lot of different usages
have been observed.
The same considerations as for leaking private addresses apply here,
except that these wouldn't typically get this far if the customer had
been using unicast RPF at its LAN interfaces (i.e., uRPF can and
should be applied recursively [RFC3704]).
3. Multihoming uRPF Drops
We'll describe a few multihomed/multi-connected network scenarios
which cause uRPF drops, and how to eliminate these drops. Bearing
these in mind, uRPF can be employed with multihomed networks as well.
We note that a customer can multihome and even perform traffic
engineering with feasible paths uRPF provided that the consistency
requirement is fulfilled. In other words, AS-path prepending,
setting communities to lower local-preference, etc. are all valid
mechanisms to ensure the prefix is advertised to every provider, but
actually may not ever end up being used.
3.1. Incorrect Source Address Selection
Hosts attaching to multiple LANs with different IP address need to be
careful with their source address selection. The same applies to
networks with multiple prefixes as explored in
[I-D.huitema-shim6-ingress-filtering].
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For example, assume the host has a default route through interface 1
with address A1 from prefix P1, and only a more specific route
through interface 2 with address A2 from prefix P2. When a host in
P1 sends a packet to A2, the response may go out through interface 1;
similarly, when a host in P2 sends a packet to A1, the response may
go out through interface 2.
This problem can be fixed by the customers by setting up source-based
routing so packets go through the right route, or by making an
exclusion in the uRPF filter list to allow sourcing from the other
prefix. The latter is typically not a good solution, especially if
the ISP doesn't control both the prefixes, because an ISP originating
these excluded packets would be indistinguishable from IP address
spoofing.
3.2. Point-to-Point Interface Routes
Feasible path strict uRPF works well, but assumes that the routes in
all the directions are consistent (i.e., exist). This principle is
often violated with the interface routes between the ISP and the
customer (ie., point-to-point links).
In some cases, the point-to-point link may be unnumbered but this has
other issues (e.g., eBGP is more complicated). If the links have
addresses, the address blocks usually need to be separate. The
addresses might be more specifics of the customer's aggregate(s) or
from the ISP's address space. In either case, the similar source
address selection issue as described in the previous section applies
for communication (e.g., pinging the CPE's p2p address) to the
customer's point-to-point addresses.
The easiest fix is to add dummy static routes with a higher
preference/distance on all the border routers, so that every router
facing the customer knows all the point-to-point address blocks used
on other routers; using a higher preference implies that the route is
actually never used, but is still valid from uRPF perspective.
Another possibility, if the addresses come from the customer's
aggregate, is to not propagate the point-to-point addresses in iBGP
or IGP at all so that there are no more specifics to mess up the uRPF
feasible path consistency, but this may have manageability concerns
if the aggregate goes down (i.e., can't ping the point-to-point
address except on the router connecting the customer). As already
mentioned, using unnumbered interfaces is also possible in some cases
but may have manageability or configuration concerns.
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3.3. Multiple Routers on a LAN use LAN for Transit
When multiple routers attach to the same network subnet (typically
when e.g., VRRP is used), packets destined to router 2 (R2)'s
interface addresses towards the LAN transiting router 1 use the LAN
interface to reach R2. (In most cases, the primary path between
routers should go via dedicated link(s), not via a LAN.) These
packets fail uRPF check at R2 (and vice versa at R1).
There are two obvious fixes: have R2 advertise such LAN addresses in
iBGP or IGP (or set up static routes), resulting a more specific so
the LAN interface is not used, or make an exception to uRPF
configuration to allow such "transit LAN" usage. However, the latter
allows an attacker in the LAN to spoof an address to the LAN router's
interface address(es) (for example, circumventing remote login access
lists), which usually makes it a suboptimal solution.
4. Special uRPF Failures Cases
4.1. PMTUD and Private/Non-routed Addresses
A disturbing issue is that some large operators seem to think it's
perfectly legitimate to send private-source addressed ICMP messages
(e.g., from PMTUD) across AS boundaries [PRIVIP]. While the
reasoning is different, the result is similar for non-routed, but
uniquely assigned address space. This might prevent applying strict
packet-based source filtering from the direction of that network.
Private IP addresses for infrastructure are a bad idea. But even
worse is doing that and deploying links in such infrastructure which
have lower MTU than the egress link, i.e., are guaranteed to send
ICMP fragmentation needed messages under certain circumstances.
Deploying such networks that require PMTUD to work while happily
originating RFC1918 traffic (and translating it at the edge) seems
like very bad design from network hygiene perspective.
5. IANA Considerations
This memo makes no request to IANA.
6. Acknowledgements
Danny McPherson, Matsuzaki Yoshinobu, and Barry Greene provided
comments on earlier revisions of this document.
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7. Security Considerations
This document describes uRPF experiences. The most important
security impact comes from applying particular fixes to uRPF issues
noted, i.e., what kind of spoofing window or other unintended usage
that would allow.
As already stated, in invalid source address selection scenario,
making an exception to allow prefixes which you don't control is
typically a big mistake, as then you become indistinguishable from
someone spoofing that address. Also as already stated, in the case
of transit LAN, making an exception might allow one to spoof an
address destined to the LAN router's interface address(es) which
usually has a security impact.
8. References
8.1. Normative References
[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.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
8.2. Informative References
[I-D.huitema-shim6-ingress-filtering]
Huitema, C., "Ingress filtering compatibility for IPv6
multihomed sites",
draft-huitema-shim6-ingress-filtering-00 (work in
progress), September 2005.
[I-D.ietf-dnsop-reflectors-are-evil]
Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks",
draft-ietf-dnsop-reflectors-are-evil-02 (work in
progress), September 2006.
[I-D.ietf-ipngwg-p2p-pingpong]
Hagino, J., JINMEI, T., and B. Zill, "Avoiding ping-pong
packets on point-to-point links",
draft-ietf-ipngwg-p2p-pingpong-00 (work in progress),
July 2001.
[PRIVIP] NANOG mailing-list thread, "private IP addresses from
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ISP", May 2006,
<http://www.merit.edu/mail.archives/nanog/msg00279.html>.
[SPOOFER] MIT ANA, "Spoofer Project",
<http://spoofer.csail.mit.edu>.
Author's Address
Pekka Savola
CSC/FUNET
Espoo
Finland
Email: psavola@funet.fi
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