Internet DRAFT - draft-jdurand-bgp-security
draft-jdurand-bgp-security
Internet Engineering Task Force J. Durand
Internet-Draft CISCO Systems, Inc.
Intended status: BCP I. Pepelnjak
Expires: March 25, 2013 NIL
G. Doering
SpaceNet
September 21, 2012
BGP operations and security
draft-jdurand-bgp-security-02.txt
Abstract
BGP (Border Gateway Protocol) is the protocol almost exclusively used
in the Internet to exchange routing information between network
domains. Due to this central nature, it's important to understand
the security measures that can and should be deployed to prevent
accidental or intentional routing disturbances.
This document describes measures to protect the BGP sessions itself
(like TTL, MD5, control plane filtering) and to better control the
flow of routing information, using prefix filtering and
automatization of prefix filters, max-prefix filtering, AS path
filtering, route flap dampening and BGP community scrubbing.
Foreword
A placeholder to list general observations about this document.
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 [1].
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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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 March 25, 2013.
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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protection of BGP router . . . . . . . . . . . . . . . . . . . 4
4. Protection of BGP sessions . . . . . . . . . . . . . . . . . . 4
4.1. Protection of TCP sessions used by BGP . . . . . . . . . . 4
4.2. BGP TTL security . . . . . . . . . . . . . . . . . . . . . 5
5. Prefix filtering . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Definition of prefix filters . . . . . . . . . . . . . . . 5
5.1.1. Prefixes that MUST not be routed by definition . . . . 5
5.1.2. Prefixes not allocated . . . . . . . . . . . . . . . . 6
5.1.3. Prefixes too specific . . . . . . . . . . . . . . . . 9
5.1.4. Filtering prefixes belonging to local AS . . . . . . . 9
5.1.5. Internet exchange point (IXP) LAN prefixes . . . . . . 10
5.1.6. Default route . . . . . . . . . . . . . . . . . . . . 11
5.2. Prefix filtering recommendations in full routing
networks . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1. Filters with internet peers . . . . . . . . . . . . . 12
5.2.2. Filters with customers . . . . . . . . . . . . . . . . 13
5.2.3. Filters with upstream providers . . . . . . . . . . . 14
5.3. Prefix filtering recommendations for leaf networks . . . . 14
5.3.1. Inbound filtering . . . . . . . . . . . . . . . . . . 14
5.3.2. Outbound filtering . . . . . . . . . . . . . . . . . . 15
6. BGP route flap dampening . . . . . . . . . . . . . . . . . . . 15
7. Maximum prefixes on a peering . . . . . . . . . . . . . . . . 15
8. AS-path filtering . . . . . . . . . . . . . . . . . . . . . . 16
9. Next-Hop Filtering . . . . . . . . . . . . . . . . . . . . . . 17
10. BGP community scrubbing . . . . . . . . . . . . . . . . . . . 17
11. Change logs . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Diffs between draft-jdurand-bgp-security-01 and
draft-jdurand-bgp-security-00 . . . . . . . . . . . . . . 18
11.2. Diffs between draft-jdurand-bgp-security-02 and
draft-jdurand-bgp-security-01 . . . . . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
14. Security Considerations . . . . . . . . . . . . . . . . . . . 20
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
15.1. Normative References . . . . . . . . . . . . . . . . . . . 20
15.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
BGP [7] is the protocol used in the internet to exchange routing
information between network domains. This protocol does not directly
include mechanisms that control that routes exchanged conform to the
various rules defined by the Internet community. This document
intends to summarize most common existing rules and help network
administrators applying simply coherent BGP policies.
2. Definitions
o BGP peering: any TCP BGP connection on the Internet.
3. Protection of BGP router
The BGP router needs to be protected from stray packets. This
protection should be achieved by an access-list (ACL) which would
discard all packets directed to TCP port 179 on the local device and
sourced from an address not known to be a BGP neighbor. If
supported, an ACL specific to the control-plane of the router should
be used (receive-ACL, control-plane policing, etc.), to avoid
filtering transit traffic if not needed. If the hardware can not do
that, interface ACLs can be used to block packets to the local
router.
Some routers automatically program such an ACL upon BGP
configuration. On other devices this ACL should be configured and
maintained manually or using scripts.
The filtering of packets destined to the local router is a wider
topic than "just for BGP" (if you bring down a router by overloading
one of the other protocols from remote, BGP is harmed as well). For
a more detailed recommendation, see RFC6192 [19].
4. Protection of BGP sessions
4.1. Protection of TCP sessions used by BGP
Attacks on TCP sessions used by BGP (ex: sending spoofed TCP
RST packets) could bring down the TCP session. Following a
successful ARP spoofing attack (or other similar Man-in-the-Middle
attack), the attacker might even be able to inject packets into
the TCP stream (routing attacks).
TCP sessions used by BGP can be secured with a variety of mechanisms.
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MD5 protection of TCP session header [2] is the most common one, but
one could also use IPsec or TCP Authentication Option (TCP-AO, [10]).
The drawback of TCP session protection is additional configuration
and management overhead for authentication information (ex: MD5
password) maintenance. Protection of TCP sessions used by BGP is
thus recommended when peerings are established over shared networks
where spoofing can be done (like internet exchanges, IXPs).
You should block spoofed packets (packets with source IP address
belonging to your IP address space) at all edges of your network,
making the protection of TCP sessions used by BGP unnecessary on iBGP
session or EBGP sessions run over point-to-point links.
4.2. BGP TTL security
BGP sessions can be made harder to spoof with the TTL security [9].
Instead of sending TCP packets with TTL value = 1, the routers send
the TCP packets with TTL value = 255 and the receiver checks that the
TTL value equals 255. Since it's impossible to send an IP packet
with TTL = 255 to a non-directly-connected IP host, BGP TTL security
effectively prevents all spoofing attacks coming from third parties
not directly connected to the same subnet as the BGP-speaking
routers.
Note: Like MD5 protection, TTL security has to be configured on both
ends of a BGP session.
5. Prefix filtering
The main aspect of securing BGP resides in controlling the prefixes
that are received/advertised on the BGP peerings. Prefixes exchanged
between BGP peers are controlled with inbound and outbound filters
that can match on IP prefixes (prefix filters, Section 5), AS paths
(as-path filters, Section 8) or any other attributes of a BGP prefix
(for example, BGP communities, Section 10).
5.1. Definition of prefix filters
This section list the most commonly used prefix filters. Following
sections will clarify where these filters should be applied.
5.1.1. Prefixes that MUST not be routed by definition
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5.1.1.1. IPv4
At the time of the writing of this document, there is no dynamic IPv4
registry listing special prefixes and their status on the internet.
On the other hand static document RFC5735 [17] clarifies "special"
IPv4 prefixes and their status in the Internet. Since publication of
that RFC another prefix has been added on the list of the special use
prefixes. Following prefixes MUST NOT cross network boundaries (ie.
ASN) and therefore MUST be filtered:
o Prefixes defined in RFC5735 [17] and more specifics
o Shared address space [31] - 100.64.0.0/10 and more specifics
5.1.1.2. IPv6
IPv6 registry [26] maintains the list of IPv6 special purpose
prefixes. With the exception of the 6to4 2002::/16 prefix in that
registry, all other prefixes that are mentioned and more specifics
MUST not cross network boundaries and therefore MUST be filtered.
The 6to4 prefix 2002::/16 is an exception because the prefix itself
can be advertised, but more specifics MUST be filtered according to
[4], section 5.2.3.
At the time of the writing of this document, the list of IPv6
prefixes that MUST not cross network boundaries can be simplified as
IANA allocates at the time being prefixes to RIR's only in 2000::/3
prefix [25]. All other prefixes (ULA's, link-local, multicast... are
outside of that prefix) and therefore the simplified list becomes:
o 2001:DB8::/32 and more specifics - documentation [13]
o Prefixes more specifics than 2002::/16 - 6to4 [4]
o 3FFE::/16 and more specifics - was initially used for the 6Bone
(worldwide IPv6 test network) and returned to IANA
o All prefixes that are outside 2000::/3 prefix
5.1.2. Prefixes not allocated
IANA allocates prefixes to RIRs which in turn allocate prefixes to
LIRs. It is wise not to accept in the routing table prefixes that
are not allocated. This could mean allocation made by IANA and/or
allocations done by RIRs. This section details the options for
building list of allocated prefixes at every level. It is important
to understand that filtering prefixes not allocated requires constant
updates as IANA and RIRs keep allocating prefixes. Therefore
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automation of such prefix filters is key for the success of this
approach. One should probably not consider solutions described in
this section if it is not capable of maintaining updated prefix
filters: damage would probably be worse than the intended security
policy.
5.1.2.1. IANA allocated prefixes filters
IANA has allocated all the IPv4 available space. Therefore there is
no reason why one would keep checking prefixes are in the IANA
allocated address space [24]. No specific filter need to be put in
place by administrators who want to make sure that IPv4 prefixes they
receive have been allocated by IANA.
For IPv6, given the size of the address space, it can be seen as wise
accepting only prefixes derived from those allocated by IANA.
Administrators can dynamically build this list from the IANA
allocated IPv6 space [27]. As IANA keeps allocating prefixes to
RIRs, the aforementioned list should be checked regularly against
changes and if they occur, prefix filter should be computed and
pushed on network devices. As there is delay between the time a RIR
receives a new prefix and the moment it starts allocating portions of
it to its LIRs, there is no need doing this step quickly and
frequently. At least process in place should make sure there is no
more than one month between the time the IANA IPv6 allocated prefix
list changes and the moment all IPv6 prefix filters have been
updated.
If process in place (manual or automatic) cannot guarantee that the
list is updated regularly then it's better not to configure any
filter based on allocated networks. The IPv4 experience has shown
that many network operators implemented filters for prefixes not
allocated by IANA but did not update them on a regular basis. This
created problems for latest allocations and required a extra work for
RIR's that had to "de-boggonize" the newly allocated prefixes.
5.1.2.2. RIR allocated prefixes filters
A more precise check can be performed as one would like to make sure
that prefixes they receive are being originated by the autonomous
system which actually own the prefix. It has been observed in the
past that one could easily advertise someone else's prefix (or more
specific prefixes) and create black holes or security threats. To
overcome that risk, administrators would need to make sure BGP
advertisements correspond to information located in the existing
registries. At this stage 2 options can be considered (short and
long term options). They are described in the following subsections.
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5.1.2.3. Prefix filters creation from Internet Routing Registries (IRR)
An Internet Routing Registry (IRR) is a database containing internet
routing information, described using Routing Policy Specification
Language objects [14]. Network engineers are given privileges to
describe routing policies of their own networks in the IRR and
information is published, usually publicly. Most of Regional
Internet Registries do also operate an IRR and can control that
registered routes conform to allocations made.
It is possible to use IRR information in order to build for a given
BGP neighbor a list of prefixes, with corresponding originating
autonomous system. This can be done relatively easily using scripts
and existing tools capable of retrieving this information in the
registries. This approach is exactly the same for both IPv4 and
IPv6.
The macro-algorithm for the script is described as follows. For the
peer that is considered, the distant network administrator has
provided the autonomous system and may be able to provide an AS-SET
object (aka AS-MACRO). An AS-SET is an object which contains AS
numbers or other AS-SET's. An operator may create an AS-SET defining
all the AS numbers of its customers. A tier 1 transit provider might
create an AS-SET describing the AS-SET of connected operators, which
in turn describe the AS numbers of their customers. Using recursion,
it is possible to retrieve from an AS-SET the complete list of AS
numbers that the peer is susceptible to announce. For each of these
AS numbers, it is also easy to check in the corresponding IRR all
associated prefixes. With these 2 mechanisms a script can build for
a given peer the list of allowed prefixes and the AS number from
which they should be originated.
As prefixes, AS numbers and AS-SET's may not all be under the same
RIR authority, a difficulty resides choosing for each object the
appropriate IRR to poll. Some IRR have been created and are not
restricted to a given region or authoritative RIR. They allow RIRs
to publish information contained in their IRR in a common place.
They also make it possible for any subscriber (probably under
contract) to publish information too. When doing requests inside
such an IRR, it is possible to specify the source of information in
order to have the most reliable data. One could check the central
registry and only check that the source is one of the 5 RIRs. The
probably most famous registry of that kind is the RADB [28] (Routing
Assets Database).
As objects in IRR's may quickly vary over time, it is important that
prefix filters computed using this mechanism are refreshed regularly.
A daily basis could even been considered as some routing changes must
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be done sometimes in a certain emergency and registries may be
updated at the very last moment. It has to be noted that this
approach significantly increases the complexity of the router
configurations as it can quickly add more than ten thousands
configuration lines for some important peers.
5.1.2.4. SIDR - Secure Inter Domain Routing
IETF has created a working group called SIDR (Secure Inter-Domain
Routing) in order to create an architecture to secure internet
advertisements. At the time this document is written, many document
has been published and a framework is proposed so that advertisements
can be checked against signed routing objects in RIR routing
registries. Implementing mechanisms proposed by this working group
is the solution that will solve at a longer term the BGP routing
security. But as it may take time objects are signed and deployments
are done such a solution will need to be combined at the time being
with other mechanisms proposed in this document. The rest of this
section assumes the reader understands all technologies associated
with SIDR.
Each received route on a router should be checked against the RPKI
data set: if a corresponding ROA is found and is valid then the
prefix should be accepted. It the ROA is found and is INVALID then
the prefix should be discarded. If an ROA is not found then the
prefix should be accepted but corresponding route should be given a
low preference.
5.1.3. Prefixes too specific
Most ISPs will not accept advertisements beyond a certain level of
specificity (and in return do not announce prefixes they consider as
too specific). That acceptable specificity is decided for each
peering between the 2 BGP peers. Some ISP communities have tried to
document acceptable specificity. This document does not make any
judgement on what the best approach is, it just recalls that there
are existing practices on the internet and recommends the reader to
refer to what those are. As an example RIPE community has documented
that IPv4 prefixes longer than /24 and IPv6 prefixes longer than /48
are generally not announced/accepted in the internet [21] [22].
5.1.4. Filtering prefixes belonging to local AS
A network SHOULD filter its own prefixes on peerings with all its
peers (inbound direction). This prevents local traffic (from a local
source to a local destination) to leak over an external peering in
case someone else is announcing the prefix over the Internet. This
also protects the infrastructure which may directly suffer in case
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backbone's prefix is suddenly preferred over the Internet. To an
extent, such filters can also be configured on a network for the
prefixes of its downstreams in order to protect them too. Such
filters must be defined with caution as they can break existing
redundancy mechanisms. For example in case an operator has a
multihomed customer, it should keep accepting the customer prefix
from its peers and upstreams. This will make it possible for the
customer to keep accessing its operator network (and other customers)
via the internet in case the BGP peering between the customer and the
operator is down.
5.1.5. Internet exchange point (IXP) LAN prefixes
5.1.5.1. Network security
When a network is present on an exchange point (IXP) and peers with
other IXP members over a common subnet (IXP LAN prefix), it MUST NOT
accept more specific prefixes for the IXP LAN prefix from any of all
its external BGP peers. Accepting these routes would create a black
hole for connectivity to the IXP LAN.
If the IXP LAN prefix is accepted as an "exact match", care needs to
be taken to avoid other routers in the network sending IXP traffic
towards the externally-learned IXP LAN prefix (recursive route lookup
pointing into the wrong direction). This can be achieved by
preferring IGP routes before eBGP, or by using "BGP next-hop-self" on
all routes learned on that IXP.
If the IXP LAN prefix is accepted at all, it MUST only be accepted
from the ASes that the IXP authorizes to announce it - which will
usually be automatically achieved by filtering announcements by IRR
DB.
5.1.5.2. pMTUd and loose uRPF problem
In order to have pMTUd working in the presence of loose uRPF, it is
necessary that all the networks that may source traffic that could
flow through the IXP (ie. IXP members and their downstreams) have a
route for the IXP LAN prefix. This is necessary as "packet too big"
ICMP messages sent by IXP members' routers may be sourced using an
address of the IXP LAN prefix. In the presence of loose uRPF, this
ICMP packet is dropped if there is no route for the IXP LAN prefix or
a less specific route covering IXP LAN prefix.
In that case, any IXP member SHOULD make sure it has a route for the
IXP LAN prefix or a less specific prefix on all its routers and that
it announces the IXP LAN prefix or less specific (up to a default
route) to its downstreams. The announcements done for this purpose
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SHOULD pass IRR-generated filters described in Section 5.1.2.3 as
well as "prefixes too specific" filters described in Section 5.1.3.
The easiest way to implement this is that the IXP itself takes care
of the origination of its prefix and advertises it to all IXP members
through a BGP peering. Most likely the BGP route servers would be
used for this. The IXP would most likely send its entire prefix
which would be equal or less specific than the IXP LAN prefix.
5.1.5.3. Example
Let's take as an example an IXP in RIPE region for IPv4. It would be
allocated a /22 by RIPE NCC (X.Y.0.0/22 in our example) and use a /23
of this /22 for the IXP LAN (let say X.Y.0.0/23). This IXP LAN
prefix is the one used by IXP members to configure eBGP peerings.
The IXP could also be allocated an AS number (AS64496 in our
example).
Any IXP member MUST make sure it filters prefixes more specific than
X.Y.0.0/23 from all its eBGP peers. If it received X.Y.0.0/24 or
X.Y.1.0/24 this could seriously impact its routing.
The IXP SHOULD originate X.Y.0.0/22 and advertise it to its members
through its BGP route servers (configured with AS64496).
The IXP members SHOULD accept the IXP prefix only if it passes the
IRR generated filters (see Section 5.1.2.3)
IXP members SHOULD then advertise X.Y.0.0/22 prefix to their
downstreams. This announce would pass IRR based filters as it is
originated by the IXP.
5.1.6. Default route
5.1.6.1. IPv4
0.0.0.0/0 prefix MUST NOT be announced on the Internet but it is
usually exchanged on upstream/customer peerings.
5.1.6.2. IPv6
::/0 prefix MUST NOT be announced on the Internet but it is usually
exchanged on upstream/customer peerings.
5.2. Prefix filtering recommendations in full routing networks
For networks that have the full internet BGP table, some policies
should be applied on each BGP peer for received and advertised
routes. It is recommended that each autonomous system configures
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rules for advertised and received routes at all its borders as this
will protect the network and its peer even in case of
misconfiguration. The most commonly used filtering policy is
proposed in this section.
5.2.1. Filters with internet peers
5.2.1.1. Inbound filtering
There are basically 2 options, the loose one where no check will be
done against RIR allocations and the strict one where it will be
verified that announcements strictly conform to what is declared in
routing registries.
5.2.1.1.1. Inbound filtering loose option
In that case, the following prefixes received from a BGP peer will be
filtered:
o Prefixes not routable (Section 5.1.1)
o Prefixes not allocated by IANA (IPv6 only) (Section 5.1.2.1)
o Routes too specific (Section 5.1.3)
o Prefixes belonging to local AS (Section 5.1.4)
o Exchange points LAN prefixes (Section 5.1.5)
o Default route (Section 5.1.6)
5.2.1.1.2. Inbound filtering strict option
In that case, filters are applied to make sure advertisements
strictly conform to what is declared in routing registries
Section 5.1.2.2. It must be checked that in case of script failure
all routes are rejected.
In addition to this, one could apply following filters beforehand in
case routing registry used as source of information by the script is
not fully trusted:
o Prefixes not routable (Section 5.1.1)
o Routes too specific (Section 5.1.3)
o Prefixes belonging to local AS (Section 5.1.4)
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o Exchange points LAN prefixes (Section 5.1.5)
o Default route (Section 5.1.6)
5.2.1.2. Outbound filtering
Configuration in place will make sure that only appropriate prefixes
are sent. These can be for example prefixes belonging to the
considered networks and those of its customers. This can be done
using BGP communities or many other solution. Whatever scenario
considered, it can be desirable that following filters are positioned
before to avoid unwanted route announcement due to bad configuration:
o Prefixes not routable (Section 5.1.1)
o Routes too specific (Section 5.1.3)
o Exchange points LAN prefixes (Section 5.1.5)
o Default route (Section 5.1.6)
In case it is possible to list the prefixes to be advertised, then
just configuring the list of allowed prefixes and denying the rest is
sufficient.
5.2.2. Filters with customers
5.2.2.1. Inbound filtering
Inbound policy with end customers is pretty straightforward: only
customers prefixes must be accepted, all others MUST be discarded.
The list of accepted prefixes can be manually specified, after having
verified that they are valid. This validation can be done with the
appropriate IP address management authorities.
Same rules apply in case the customer is also a network connecting
other customers (for example a tier 1 transit provider connecting
service providers). An exception can be envisaged in case it is
known that the customer network applies strict inbound/outbound
prefix filtering, and the number of prefixes announced by that
network is too large to list them in the router configuration. In
that case filters as in Section 5.2.1.1 can be applied.
5.2.2.2. Outbound filtering
Outbound policy with customers may vary according to the routes
customer wants to receive. In the simplest possible scenario,
customer wants to receive only the default route, which can be done
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easily by applying a filter with the default route only.
In case the customer wants to receive the full routing (in case it is
multihomed or if wants to have a view on the internet table), the
following filters can be simply applied on the BGP peering:
o Prefixes not routable (Section 5.1.1)
o Routes too specific (Section 5.1.3)
o Default route (Section 5.1.6)
There can be a difference for the default route that can be announced
to the customer in addition to the full BGP table. This can be done
simply by removing the filter for the default route. As the default
route may not be present in the routing table, one may decide to
originate it only for peerings where it has to be advertised.
5.2.3. Filters with upstream providers
5.2.3.1. Inbound filtering
In case the full routing table is desired from the upstream, the
prefix filtering to apply is more or less the same than the one for
peers Section 5.2.1.1. There can be a difference for the default
route that can be desired from an upstream provider even if it
advertises the full BGP table. In case the upstream provider is
supposed to announce only the default route, a simple filter will be
applied to accept only the default prefix and nothing else.
5.2.3.2. Outbound filtering
The filters to be applied should not differ from the ones applied for
internet peers (Section 5.2.1.2).
5.3. Prefix filtering recommendations for leaf networks
5.3.1. Inbound filtering
The leaf network will position the filters corresponding to the
routes it is requesting from its upstream. In case a default route
is requested, simple inbound filter will be applied to accept only
that default route (Section 5.1.6). In case the leaf network is not
capable of listing the prefix because the amount is too large (for
example if it requires the full internet routing table) then it
should configure filters to avoid receiving bad announcements from
its upstream:
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o Prefixes not routable (Section 5.1.1)
o Routes too specific (Section 5.1.3)
o Prefixes belonging to local AS (Section 5.1.4)
o Default route (Section 5.1.6) depending if the route is requested
or not
5.3.2. Outbound filtering
A leaf network will most likely have a very straightforward policy:
it will only announce its local routes. It can also configure the
following prefixes filters described in Section 5.2.1.2 to avoid
announcing invalid routes to its upstream provider.
6. BGP route flap dampening
BGP route flap dampening mechanism makes it possible to give
penalties to routes each time they change in the BGP routing table.
Initially this mechanism was created to protect the entire internet
from multiple events impacting a single network. RIPE community now
recommends not using BGP route flap dampening [20]. Author of this
document proposes to follow the proposal of the RIPE community.
7. Maximum prefixes on a peering
It is recommended to configure a limit on the number of routes to be
accepted from a peer. Following rules are generally recommended:
o From peers, it is recommended to have a limit lower than the
number of routes in the internet. This will shut down the BGP
peering if the peer suddenly advertises the full table. One can
also configure different limits for each peer, according to the
number of routes they are supposed to advertise plus some headroom
to permit growth.
o From upstreams which provide full routing, it is recommended to
have a limit much higher than the number of routes in the
internet. A limit is still useful in order to protect the network
(and in particular the routers' memory) if too many routes are
sent by the upstream. The limit should be chosen according to the
number of routes that can actually be handled by routers.
It is important to regularly review the limits that are configured as
the internet can quickly change over time. Some vendors propose
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mechanisms to have 2 thresholds: while the higher number specified
will shutdown the peering, the first threshold will only trigger a
log and can be used to passively adjust limits based on observations
made on the network.
8. AS-path filtering
The following rules should be applied on BGP AS-paths:
o Do not accept anything other than customer's AS number from the
customer. Alternatively, only accept AS-paths with a single AS
number (potentially repeated several times) from your customers.
The latter option is easier to configure than per-customer AS-path
filters: the default BGP logic will make sure in that case that
the first AS number in the AS-path is the one of the peer.
o Do not accept overly long AS path prepending from the customer.
o Do not accept more than two distinct AS path numbers in the AS
path if your customer is an ISP with customers. This rule is not
adding anything extra in case prefix filters are built from
registries as described in Section 5.1.2.3.
o Do not advertise prefixes with non-empty AS-path if you're not
transit.
o Do not advertise prefixes with upstream AS numbers in the AS path
to your peering AS.
o Do not accept private AS numbers except from customers
o Do not advertise private AS numbers. Exception: Customers using
BGP without having their own AS number must use private AS numbers
to advertise their prefixes to their upstream. The private AS
number is usually provided by the upstream.
o Do not accept prefixes when the first AS number in the AS-path is
not the one of the peer. In case the peering is done toward a BGP
route-server [30] (connection on an Internet eXchange Point - IXP)
with transparent AS path handling, this verification needs to be
de-activated as the first AS number will be the one of an IXP
member whereas the peer AS number will be the one of the BGP
route-server.
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9. Next-Hop Filtering
If peering on a shared network, like an Exchange-Point, BGP can
advertise prefixes with a 3rd-party next-hop, thus directing packets
not to the peer announcing the prefix but somewhere else.
This is a desirable property for BGP route-server setups [30], where
the route-server will relay routing information, but has neither
capacity nor desire to receive the actual data packets. So the BGP
route-server will announce prefixes with a next-hop setting pointing
to the router that originally announced the prefix to the route-
server.
In direct peerings between ISPs, this is undesirable, as one of the
peers could trick the other one to send packets into a black hole
(unreachable next-hop) or to an unsuspecting 3rd party who would then
have to carry the traffic. Especially for black-holing, the root
cause of the problem is hard to see without inspecting BGP prefixes
at the receiving router at the IXP.
Therefore, the authors recommend to, by default, apply an inbound
route policy to IXP peerings which sets the next-hop for accepted
prefixes to the BGP peer that sent the prefix (which is what "next-
hop-self" would enforce on the sending side, but you can not rely on
the other party to always send correct information).
This policy MUST NOT be used on route-server peerings, or on peerings
where you intentionally permit the other side to send 3rd-party next-
hops.
10. BGP community scrubbing
Optionally we can consider the following rules on BGP AS-paths:
o Scrub inbound communities with your AS number in the high-order
bits - allow only those communities that customers/peers can use
as a signaling mechanism
o Do not remove other communities: your customers might need them to
communicate with upstream providers. In particular do not
(generally) remove the no-export community as it is usually
announced by your peer for a certain purpose.
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11. Change logs
11.1. Diffs between draft-jdurand-bgp-security-01 and
draft-jdurand-bgp-security-00
Following changes have been made since previous document
draft-jdurand-bgp-security-00:
o "This documents" typo corrected in the former abstract
o Add normative reference for RFC5082 in former section 3.2
o "Non routable" changed in title of former section 4.1.1
o Correction of typo for IPv4 loopback prefix in former section
4.1.1.1
o Added shared transition space 100.64.0.0/10 in former section
4.1.1.1
o Clarification that 2002::/16 6to4 prefix can cross network
boundaries in former section 4.1.1.2
o Rationale of 2000::/3 explained in former section 4.1.1.2
o Added 3FFE::/16 prefix forgotten initially in the simplified list
of prefixes that MUST not be routed by definition in former
section 4.1.1.2
o Warn that filters for prefixes not allocated by IANA must only be
done if regular refresh is guaranteed, with some words about the
IPv4 experience, in former section 4.1.2.1
o Replace RIR database with IRR. A definition of IRR is added in
former section 4.1.2.2
o Remove any reference to anti-spoofing in former section 4.1.4
o Clarification for IXP LAN prefix and pMTUd problem in former
section 4.1.5
o "Autonomous filters" typo (instead of Autonomous systems)
corrected in the former section 4.2
o Removal of an example for manual address validation in former
section 4.2.2.1
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o RFC5735 obsoletes RFC3300
o Ingress/Egress replaced by Inbound/Outbound in all the document
11.2. Diffs between draft-jdurand-bgp-security-02 and
draft-jdurand-bgp-security-01
Following changes have been made since previous document
draft-jdurand-bgp-security-01:
o 2 documentation prefixes were forgotten due to errata in RFC5735.
But all prefixes were removed from that document which now point
to other references for sake of not creating a new "registry" that
would become outdated sooner or later.
o Change MD5 section with global TCP security session and
introducing TCP-AO in former section 3.1. Added reference to
BCP38
o Added new section 3 about BGP router protection with forwarding
plane ACL
o Change text about prefix acceptable specificity in former section
4.1.3 to explain this doc does not try to make recommendations
o Refer as much as possible to existing registries to avoid creating
a new one in former section 4.1.1.1 and 4.1.1.2
o Abstract reworded
o 6to4 exception described (only more specifics must be filtered)
o More specific -> more specifics
o should -> MUST for the prefixes an ISP needs to filter from its
customers in former section 4.2.2.1
o Added "plus some headroom to permit growth" in former section 7
o Added new section on Next-Hop filtering
12. Acknowledgements
Authors would like to thank the following people for their comments
and support: Marc Blanchet, Ron Bonica, Daniel Ginsburg, David
Groves, Tim Kleefass, Hagen Paul Pfeifer, Thomas Pinaud, Carlos
Pignataro, Matjaz Straus, Tony Tauber, Gunter Van de Velde, Sebastian
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Wiesinger.
13. IANA Considerations
This memo includes no request to IANA.
14. Security Considerations
This document is entirely about BGP operational security.
15. References
15.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997,
<http://xml.resource.org/public/rfc/html/rfc2119.html>.
[2] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[3] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[4] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[5] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[6] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[7] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4
(BGP-4)", RFC 4271, January 2006.
[8] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[9] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. Pignataro,
"The Generalized TTL Security Mechanism (GTSM)", RFC 5082,
October 2007.
[10] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication
Option", RFC 5925, June 2010.
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15.2. Informative References
[11] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[12] 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.
[13] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004.
[14] Blunk, L., Damas, J., Parent, F., and A. Robachevsky, "Routing
Policy Specification Language next generation (RPSLng)",
RFC 4012, March 2005.
[15] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[16] Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
April 2008.
[17] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[18] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
[19] Dugal, D., Pignataro, C., and R. Dunn, "Protecting the Router
Control Plane", RFC 6192, March 2011.
[20] Smith, P. and C. Panigl, "RIPE-378 - RIPE Routing Working Group
Recommendations On Route-flap Damping", May 2006.
[21] Smith, P., Evans, R., and M. Hughes, "RIPE-399 - RIPE Routing
Working Group Recommendations on Route Aggregation",
December 2006.
[22] Smith, P. and R. Evans, "RIPE-532 - RIPE Routing Working Group
Recommendations on IPv6 Route Aggregation", November 2011.
[23] Doering, G., "IPv6 BGP Filter Recommendations", November 2009,
<http://www.space.net/~gert/RIPE/ipv6-filters.html>.
[24] "IANA IPv4 Address Space Registry", <http://www.iana.org/
assignments/ipv4-address-space/ipv4-address-space.xml>.
[25] "IANA IPv6 Address Space", <http://www.iana.org/assignments/
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ipv6-address-space/ipv6-address-space.xml>.
[26] "IANA IPv6 Special Purpose Registry", <http://www.iana.org/
assignments/iana-ipv6-special-registry/
iana-ipv6-special-registry.xml>.
[27] "IANA IPv6 Address Space Registry", <http://www.iana.org/
assignments/ipv6-unicast-address-assignments/
ipv6-unicast-address-assignments.xml>.
[28] "Routing Assets Database", <http://www.radb.net>.
[29] "Secure Inter-Domain Routing IETF working group",
<http://datatracker.ietf.org/wg/sidr/>.
[30] "Internet Exchange Route Server", <http://tools.ietf.org/id/
draft-jasinska-ix-bgp-route-server-03.txt>.
[31] "IANA Reserved IPv4 Prefix for Shared Address Space", <http://
tools.ietf.org/id/
draft-weil-shared-transition-space-request-15.txt>.
Authors' Addresses
Jerome Durand
CISCO Systems, Inc.
11 rue Camille Desmoulins
Issy-les-Moulineaux 92782 CEDEX
FR
Email: jerduran@cisco.com
Ivan Pepelnjak
NIL Data Communications
Tivolska 48
Ljubljana 1000
Slovenia
Email: ip@nil.com
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Gert Doering
SpaceNet AG
Joseph-Dollinger-Bogen 14
Muenchen D-80807
Germany
Email: gert@space.net
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