Network Working Group D. Turk
Request for Comments: 3882 Bell Canada
Category: Informational September 2004
Configuring BGP to Block Denial-of-Service Attacks
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes an operational technique that uses BGP
communities to remotely trigger black-holing of a particular
destination network to block denial-of-service attacks. Black-holing
can be applied on a selection of routers rather than all BGP-speaking
routers in the network. The document also describes a sinkhole
tunnel technique using BGP communities and tunnels to pull traffic
into a sinkhole router for analysis.
Table of Contents
1. Existing BGP-Triggered Black holing Techniques . . . . . . . . 2
2. Enhanced BGP-Triggered Black holing Technique. . . . . . . . . 3
3. Sinkhole Tunnels . . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations. . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 7
6. Informative References . . . . . . . . . . . . . . . . . . . . 7
7. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . 7
8. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 8
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1. Existing BGP-Triggered Black-holing Techniques
Current BGP-triggered black-holing techniques rely on altering the
BGP next hop address of a network targeted by an attack throughout
the iBGP network. A customized iBGP advertisement is generated from
a router participating in the destination/attacked AS where the next
hop address for the targeted network or host is modified to point to
an RFC 1918 [RFC1918] (private internet) address. Most routers on
the Internet, especially edge routers, have static routes pointing
RFC 1918 addresses to the null interface. Those static routes drive
all traffic destined to the network under attack to the null
interface.
When an iBGP-speaking router inside the destination AS receives the
iBGP update, the advertised prefix will be added to the routing table
with a next hop of one of the networks listed in RFC 1918. The
router will then attempt to resolve the RFC 1918 next-hop in order to
qualify the route and derive a forwarding interface. This process
will return a valid next hop as the null interface. Assuming the
router is properly configured to direct RFC 1918 destined traffic to
a null interface, traffic destined to the attacked network gets
dropped, making the attacked network unreachable to the attacker and
everyone else.
While this technique shields the internal infrastructure from the
attack, protecting a large number of devices, it has the undesirable
side effect of rendering the targeted/attacked network unreachable
throughout the entire destination AS. Even if a static route
pointing an RFC 1918 address to a null interface is not configured on
all routers within the destination AS, the modified next hop makes
the traffic un-routable to its legitimate destination.
Network operators usually use the BGP-triggered black holes for a
short period of time. The technique causes traffic drops on all
ingress points of the AS for traffic destined to the attacked
network. By default, routers dropping traffic into a null interface
should send an "ICMP unreachable" message to the source address
belonging to the origin/attacking AS.
Once the procedure reaches this point, one of the source addresses of
the attack traffic is hijacked by introducing a device with the same
source IP address into the BGP domain of the destination/attacked AS.
The device hijacking the source address collects the ICMP unreachable
packets. The source addresses of these ICMP unreachable packets
reveal which edge routers within the destination/attacked AS the
attack is coming from. The network operator may then opt to manually
stop the traffic on the routers from which attack traffic is
entering.
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RFC 3882 Configuring BGP to Block DoS Attacks September 2004
2. Enhanced BGP-Triggered Black-holing Technique
This paper describes a technique developed to instruct a selected set
of routers to alter the next hop address of a particular prefix by
use of the BGP protocol. The next hop can either be a null interface
or, as discussed later on in this paper, a sinkhole tunnel interface.
This technique does not invoke an access list or rate limiting
statement to treat attack traffic, nor does it involve a network wide
change of the attacked prefix next hop address. The next hop will
only be changed on a selection of routers with the aid of BGP
communities within the destination/attacked AS.
To prepare the network for this technique, the network operator needs
to define a unique community value for each destination AS border
router that could potentially drive attack traffic to the victim.
For example, a network with a BGP autonomous system number 65001 has
two border routers (R1 and R2). Community value 65001:1 is assigned
to identify R1, community value 65001:2 is assigned to identify R2,
and community value 65001:666 is assigned to identify both R1 and R2.
After the BGP community assignment, R1 and R2 must be configured with
the following:
1. Static route pointing an RFC 1918 network to a null interface.
2. AS-Path access list that matches local BGP prefix advertisement.
3. BGP community access list to match the community value assigned by
the network operator for the particular router (i.e., 65001:1 for
R1).
4. BGP community access list to match the community value assigned by
the network operator for all routers (i.e., 65001:666 for R1 and
R2)
5. Under the BGP process, an iBGP import route policy should be
applied on received iBGP advertisements to do the following logic.
(Statements are in a logical AND order)
a. A policy statement to permit routes that match the following
criteria and apply the following changes.
i. Match for a community specific to that router (i.e.,
65001:1, for R1).
ii. Match the AS-Path to locally generated BGP advertisements.
iii. Set the BGP next hop to an RFC 1918 network.
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iv. Overwrite the BGP community with the well-known community
(no-advertise).
b. A policy statement to permit routes that match the following
criteria and apply the following changes.
i. Match for a community that covers all routers (i.e.,
65001:666).
ii. Match the AS-Path to locally generated BGP advertisements.
iii. Set the BGP next hop to an RFC 1918 network.
iv. Overwrite the BGP community with the well-known community
(no-advertise).
After the policies have been configured on R1 and R2, the network
operator can, in the case of an attack, advertise the targeted
network that could be one or more /32 "host" routes into iBGP of the
destination/attacked AS. The advertisement must contain the
community value associated with the router(s) where the attack is
arriving in addition to the well-known community (no-export). Using
BGP communities preserves the original next hop address of the
targeted network on all routers where the special route policy
configuration is not present. iBGP will then carry the prefix
advertisement to all routers in the destination/attacked AS. All
routers within the destination AS, except the ones that match the
community stamped on the prefix, will be oblivious to the community
value and will install the network route with the legitimate next hop
address. Routers that match the community will also install the
network route into their routing table but will alter the next hop
address to an RFC 1918 network and then to a null interface as per
the route policies configuration and recursive route lookup. The
reason for matching locally announced networks is to make sure that
no eBGP peer can misuse this community to drive any network to a null
interface. Blackholing the targeted/attacked hosts is recommended,
but not the entire address block they belong to so that the blackhole
effect has the minimum impact on the attacked network.
This technique stops traffic from getting forwarded to the legitimate
destination on routers identified as transit routers for attack
traffic and that have route map matches for the community value
associated with the network advertisement. All other traffic on the
network will still get forwarded to the legitimate destination thus
minimizing the impact on the targeted network.
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3. Sinkhole Tunnels
Following the "Enhanced BGP-Triggered Black-holing Technique", it may
become a requirement to take a look at the attack traffic for further
analysis. This requirement adds to the complexity of the exercise.
Usually with broadcast interfaces, network operators install network
sniffers on a spanned port of a switch for analysis of traffic.
Another method would be to announce a network prefix that covers the
attack host address into iBGP, altering the next hop into a sinkhole
device that can log traffic for analysis. The latter technique
results in taking down the services offered on the targeted/attacked
IP addresses. Inter-AS traffic will be sucked into the sinkhole,
along with Intra-AS traffic. Packet level analysis involves
redirecting traffic away from the destination host to a sniffer or a
router. As a result, if the traffic being examined includes
legitimate traffic, that legitimate traffic will never make it to the
destination host. This will result in denial of service for the
legitimate traffic.
A better alternative would be to use a sinkhole tunnel. A sinkhole
tunnel is implemented at all possible entry points from which attacks
can pass into the destination/attacked AS. Using the BGP community
technique, traffic destined to the attacked/targeted host could be
re-routed to a special path (tunnel) where a sniffer could capture
the traffic for analysis. After being analyzed, traffic will exit
the tunnel and be routed normally to the destination host. In other
words, the traffic will pass through the network to a sniffer without
altering the next hop information of the destination network. All
routers within the destination/attacked AS iBGP domain will have the
proper next hop address. Only the entry point router will have the
altered next hop information.
To detail the procedure, a sinkhole router with an optional sniffer
attached to its interface is installed and configured to participate
in the IGP and iBGP of the attacked AS. Next, a tunnel is created,
using MPLS Traffic Engineering as an example, from all border routers
attacks can potentially enter from (Inter-AS traffic) to the sinkhole
router. When a host or network is under attack, a customized iBGP
advertisement is sent to announce the network address of the attacked
host(s) with the proper next hop that insures traffic will reach
those hosts or networks. The customized advertisement will also have
a community string value that matches the set of border routers the
attack is entering from, as described in section 2. The new next hop
address configured within the route policy section of all border
routers should be the sinkhole IP address. This IP address belongs
to the /30 subnet assigned to the tunnel connecting the border router
to the sinkhole router.
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RFC 3882 Configuring BGP to Block DoS Attacks September 2004
Routers that do not have a match for the community string will do
regular routing. Lack of a community string match on these routers
will insure that the special route policy does not change the next
hop address. Traffic entering from border routers that do not have a
match to the special community will pass through regular router
interfaces to the legitimate destination. It might also be required
to allow the traffic to reach its destination after being captured.
In this case, a default network route is configured to point to any
interface attached and configured on the iBGP network. This would
also include the same physical interface the tunnel is built on.
Since the next hop address is not changed on the sinkhole device,
traffic entering this device from the tunnel will be sent back to the
network due to the presence of the default route. Routing protocols
will then take care of properly routing the traffic to its original
destination (attacked network).
It becomes apparent that this technique can also be used for purposes
other than analyzing attack traffic. Legitimate traffic could also
be pulled out of normal routing into a tunnel and then reinserted
into the backbone without altering the next hop addressing scheme
throughout the iBGP network.
MPLS Traffic Engineering with its many features, is a good method of
sliding traffic to the sinkhole device. Features like QoS policies
can be applied on the attack traffic, thus preventing it from
competing on resources with legitimate traffic.
To be able to alter the next hop on the border router, a subnet of an
RFC 1918 network is statically routed to the tunnel interface. An
example of the static route is:
ip route 192.168.0.12 255.255.255.255 Tunnel0
Setting the next hop of the target IP address to 192.168.0.12/32 will
force the traffic to go through the tunnel.
Traffic is received at the sinkhole interface via the TE tunnel.
Subsequently, three methods could be installed, namely rate-limiting
policies, QoS policies, and access lists. These policies could rate
limit or drop traffic classified as attack traffic. This process
would be completed on the interface of the sinkhole device. Another
useful application for a sinkhole router is to pull in traffic via
tunnels to an inbound interface and have a default route statement
forwarding the traffic out to an Ethernet interface. The Ethernet
interface is connected to the iBGP network and guarantees proper
delivery of traffic however, it still allows the use of a packet
sniffer to further analyze the attack traffic.
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This becomes very useful when it is not feasible to apply an Access
list or a rate limiting statement on the BGP border router or last
hop router before the attacked host or network because of hardware or
software limitations. Hence, instead of upgrading interfaces at the
point of entry of attack traffic, the latter could be pulled into the
sinkhole and treated on that device. Operational costs can be
rendered minimal if the sinkhole router is a powerful device.
4. Security Considerations
It is very important to practice tight control over eBGP peering
points before implementing the techniques described in this paper.
eBGP customers might be able to blackhole a particular subnet using
the Blackhole communities. To eliminate the risk, the match for
locally generated BGP advertisements in the special route policy
should not be neglected.
5. Acknowledgments
The author of this document would like to acknowledge the developers
of the remotely triggered black-holing technique and the developers
of the backscatter technique for collecting backscatter traffic. The
author would also like to thank all members of the IP Engineering
department for their help in verifying the functionality of this
technique.
6. Informative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
7. Author's Addresses
Doughan Turk
Bell Canada
100 Wynford Drive
EMail: doughan.turk@bell.ca
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8. Full Copyright Statement
Copyright (C) The Internet Society (2004).
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