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Opsec WG                                                       P. Savola
Internet-Draft                                                 CSC/FUNET
Intended status: Informational                             June 12, 2006
Expires: December 14, 2006

                   Experiences from Using Unicast RPF

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   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 Failures . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Unused Address Space Ping-Pong . . . . . . . . . . . . . .  4
     2.2.  Private Address Leak . . . . . . . . . . . . . . . . . . .  4
     2.3.  Wrong IP Address . . . . . . . . . . . . . . . . . . . . .  5
   3.  Multihoming uRPF Failures  . . . . . . . . . . . . . . . . . .  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  . . . . . .  6
   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  . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  8
   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 failures and their mitigation techniques.  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 Failures

   We'll describe the most common uRPF failures which apply to both
   single- and multi-homed network, and respective fixes.

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2.1.  Unused Address Space Ping-Pong

   By far, the most common cause for uRPF failures 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].

   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
   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 based on that, figures out what is
   causing such address leaks and fixes the misconfiguration or other

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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 Failures

   We'll describe a few uRPF failure modes which only occur in scenarios
   with a multihomed/multi-connected network or host.

   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

   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

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   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

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.

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

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   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 packets 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.

   Private IP addresses for infrastructure are a bad idea.  But even
   worse than that is 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 and Matsuzaki Yoshinobu provided comments on the
   first revision of this document.

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

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   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

              Huitema, C., "Ingress filtering compatibility for IPv6
              multihomed sites",
              draft-huitema-shim6-ingress-filtering-00 (work in
              progress), September 2005.

              Damas, J. and F. Neves, "Preventing Use of Nameservers in
              Reflector Attacks",
              draft-ietf-dnsop-reflectors-are-evil-00 (work in
              progress), May 2006.

              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
              ISP", May 2006,

   [SPOOFER]  MIT ANA, "Spoofer Project",

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Author's Address

   Pekka Savola

   Email: psavola@funet.fi

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