IETF TCPM WG J. Touch
Internet Draft USC/ISI
Expires: April 2006 October 8, 2005
Defending TCP Against Spoofing Attacks
draft-ietf-tcpm-tcp-antispoof-02.txt
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Abstract
Recent analysis of potential attacks on core Internet infrastructure
indicates an increased vulnerability of TCP connections to spurious
resets (RSTs), sent with forged IP source addresses (spoofing). TCP
has always been susceptible to such RST spoofing attacks, which were
indirectly protected by checking that the RST sequence number was
inside the current receive window, as well as via the obfuscation of
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TCP endpoint and port numbers. For pairs of well-known endpoints
often over predictable port pairs, such as BGP or between web servers
and well-known large-scale caches, increases in the path bandwidth-
delay product of a connection have sufficiently increased the receive
window space that off-path third parties can guess a viable RST
sequence number. The susceptibility to attack increases as the square
of the bandwidth, thus presents a significant vulnerability for
recent high-speed networks. This document addresses this
vulnerability, discussing proposed solutions at the transport level
and their inherent challenges, as well as existing network level
solutions and the feasibility of their deployment. This document
focuses on vulnerabilities due to spoofed TCP segments, and includes
a discussion of related ICMP spoofing attacks on TCP connections.
Table of Contents
1. Introduction...................................................3
2. Background.....................................................4
2.1. Review of TCP Windows.....................................5
2.2. Recent BGP Attacks Using TCP RSTs.........................5
2.3. TCP RST Vulnerability.....................................6
2.4. What Changed - the Ever Opening Advertised Receive Window.7
3. Proposed Solutions and Mitigations.............................9
3.1. Transport Layer Solutions................................10
3.1.1. TCP MD5 Authentication..............................10
3.1.2. TCP RST Window Attenuation..........................10
3.1.3. TCP Timestamp Authentication........................11
3.1.4. Other TCP Cookies...................................12
3.1.5. Other TCP Considerations............................12
3.1.6. Other Transport Protocol Solutions..................13
3.2. Network Layer (IP) Solutions.............................13
3.2.1. Address filtering...................................13
3.2.2. IPsec...............................................14
4. ICMP..........................................................15
5. Issues........................................................16
5.1. Transport Layer (e.g., TCP)..............................16
5.2. Network Layer (IP).......................................17
5.3. Application Layer........................................18
5.4. Shim Transport/Application Layer.........................19
5.5. Link Layer...............................................19
5.6. Issues Discussion........................................19
6. Security Considerations.......................................20
7. IANA Considerations...........................................20
8. Conclusions...................................................20
9. Acknowledgments...............................................21
10. References...................................................21
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10.1. Normative References....................................21
10.2. Informative References..................................21
Author's Addresses...............................................24
Intellectual Property Statement..................................24
Disclaimer of Validity...........................................25
Copyright Statement..............................................25
Acknowledgment...................................................25
1. Introduction
Analysis of the Internet infrastructure has been recently
demonstrated new version of a vulnerability in BGP connections
between core routers using an attack known for nearly six years
[8][9][18][40]. These connections, typically using TCP, can be
susceptible to off-path third-party reset (RST) segments with forged
source addresses (spoofed), which terminate the TCP connection. BGP
routers react to a terminated TCP connection in various ways which
can amplify the impact of an attack, ranging from restarting the
connection to deciding that the other router is unreachable and thus
flushing the BGP routes [32]. This sort of attack affects other
protocols besides BGP, involving any long-lived connection between
well-known endpoints. The impact on Internet infrastructure can be
substantial (esp. for the BGP case), and warrants immediate
attention.
TCP, like many other protocols, can be susceptible to these off-path
third-party spoofing attacks. Such attacks rely on the increase of
commodity platforms supporting public access to previously privileged
resources, such as root-level access. Given such access, it is
trivial for anyone to generate a packet with any header desired.
This, coupled with the lack of sufficient address filtering to drop
such spoofed traffic, can increase the potential for off-path third-
party spoofing attacks. Proposed solutions include the deployment of
existing Internet network and transport security as well as
modifications to transport protocols that reduce its vulnerability to
generated attacks.
One way to defeat spoofing is to validate the segments of a
connection, either at the transport level or the network level. TCP
with MD5 extensions provides this authentication at the transport
level, and IPsec provides authentication at the network level. In
both cases their deployment overhead may be prohibitive, e.g., it may
not feasible for public services, such as web servers, to be
configured with the appropriate certificate authorities of large
numbers of peers (for IPsec using IKE), or shared secrets (for IPsec
in shared-secret mode, or TCP/MD5), because many clients may need to
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be configured rapidly without external assistance. Services from
public web servers connecting to large-scale caches to BGP with
larger numbers of peers can experience this challenge.
The remainder of this document outlines the recent attack scenario in
detail and describes and compares a variety of solutions, including
existing solutions based on TCP/MD5 and IPsec, as well as recently
proposed solutions, including modifications to TCP's RST processing
[10], modifications to TCP's timestamp processing [30], and
modifications to IPsec and TCP/MD5 keying [38]. This document focuses
on spoofing of TCP segments, although a discussion of related
spoofing of ICMP packets based on spoofed TCP contents is also
discussed.
Note that the description of these attacks is not new; attacks using
RSTs on BGP have been known since 1998, and were the reason for the
development of TCP/MD5 [18]. The recent attack scenario was first
documented by Convery at a NANOG meeting in 2003, but that analysis
assumed the entire sequence space (2^32 packets) needed to be covered
for an attack to succeed [9]. Watson's more detailed analysis
discovered that a single packet anywhere in the current window could
succeed at an attack [40]. This document adds the observation that
susceptibility to attack goes as the square of bandwidth, due to the
coupling between the linear increase in receive window size and
linear increase in rate an attacker, as well as comparing the variety
of more recent proposals, including modifications to TCP, use of
IPsec, and use of TCP/MD5 to resist such attacks.
2. Background
The recent analysis of potential attacks on BGP has again raised the
issue of TCP's vulnerability to off-path third-party spoofing attacks
[8][9][40]. A variety of such attacks have been known for several
years, including sending RSTs, SYNs, and even ACKs in an attempt to
affect an existing connection or to load down servers. Overall, such
attacks are countered by the use of some form of authentication at
the network (e.g., IPsec), transport (e.g., SYN cookies, TCP/MD5), or
other layers. TCP already includes a weak form of such
authentication in its check of segment sequence numbers against the
current receiver window. Increases in the bandwidth-delay product
for certain long connections have sufficiently weakened this type of
weak authentication in recent weeks, rendering it moot.
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2.1. Review of TCP Windows
Before proceeding, it is useful to review the terminology and
components of TCP's windowing algorithm. TCP connections have three
kinds of windows [1][31]:
o Send window (SND.WND): the latest advertised send window size.
o Receive window (RCV.WND): the latest advertised receive window
size.
o Congestion window (CWND): the window determined by congestion
feedback that limits how much of RCV.WND can be in-flight in a
round trip time.
For most modern TCP connections, SND.WND and RCV.WND are the size of
the corresponding send and receive socket buffers, and are
configurable using socket buffer resizing commands.
CWND determines how much data can be in transit in a round trip time,
SND.WND determines how much data the sender is willing to store on
its side for possible retransmission due to loss, and RCV.WND
determines the ability of the receiver to accommodate that loss and
reorder received packets. CWND never grows beyond RCV.WND.
High bandwidth-delay product networks need CWND to be sufficiently
large to accommodate as much data would be in transit in a round trip
time, otherwise their performance will suffer. As a result, it is
recommended that users and various automatic programs increase
RCV.WND to at least the size of bandwidth*delay (the bandwidth-delay
product) [19][33].
As the bandwidth-delay product of the network increases, however,
such increases in the advertised receive window can cause increased
susceptibility to spoofing attacks, as the remainder of this document
shows. This assumes, however, that the receive window size (e.g., via
increased receive socket buffer configuration) is increased with the
increased bandwidth-delay product; if not, then connection
performance will degrade, but susceptibility to spoofing attacks will
increase only linearly (with the rate of the attacker to send spoofed
packets), not as the square of the bandwidth.
2.2. Recent BGP Attacks Using TCP RSTs
BGP represents a particular vulnerability to spoofing attacks because
it uses TCP connectivity to infer routability, so losing a TCP
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connection with a BGP peer can result in the flushing of routes to
that peer [32].
Until six years ago, such connections were assumed difficult to
attack because they were described by a few comparatively obscure
parameters [18]. Most TCP connections are protected by multiple
levels of obfuscation except at the endpoints of the connection:
o Both endpoint addresses are usually not well-known; although server
addresses are advertised, clients are somewhat anonymous.
o Both port numbers are usually not well-known; the server's usually
is advertised (representing the service), but the client's is
typically sufficiently unpredictable to an off-path third-party.
o Valid sequence number space is not well-known.
o Connections are relatively short-lived and valid sequence space
changes, so any guess of the above information is unlikely to be
useful.
BGP represents an exception to the above criteria (though not the
only case). Both endpoints can be well-known, using hints from part
of an AS path. The destination port is typically fixed to indicate
the BGP service. The source port used by a BGP router is sometimes
fixed and advertised to enable firewall configuration; even when not
fixed, there are only 65,384 valid source ports which may be
exhaustively attacked. Connections are long-lived, and as noted
before some BGP implementations interpret successive TCP connection
failures as routing failures, discarding the corresponding routing
information. As importantly and as will be shown below, the valid
sequence number space once thought to provide some protection has
been rendered useless by increasing advertised receive window sizes.
2.3. TCP RST Vulnerability
TCP has a known vulnerability to third-party spoofed segments. SYN
flooding consumes server resources in half-open connections,
affecting the server's ability to open new connections. ACK spoofing
can cause connections to transmit too much data too quickly, creating
network congestion and segment loss, causing connections to slow to a
crawl. In the most recent attacks on BGP, RSTs cause connections to
be dropped. As noted earlier, some BGP implementations interpret TCP
connection termination, or a series of such failures, as a network
failure [32]. This causes routers to drop the BGP routing
information already exchanged, in addition to inhibiting their
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ongoing exchanges, thus amplifying the impact of the attack. The
result can affect routing paths throughout the Internet.
The dangerous effects of RSTs on TCP have been known for many years,
even when used by the legitimate endpoints of a connection. TCP RSTs
cause the receiver to drop all connection state; because the source
is not required to maintain a TIME_WAIT state, such a RST can cause
premature reuse of address/port pairs, potentially allowing segments
from a previous connection to contaminate the data of a new
connection, known as TIME_WAIT assassination [7]. In this case,
assassination occurs inadvertently as the result of duplicate
segments from a legitimate source, and can be avoided by blocking RST
processing while in TIME_WAIT. However, assassination can be useful
to deliberately reduce the state held at servers; this requires that
the source of the RSTs go into TIME_WAIT state to avoid such hazards,
and that RSTs are not blocked in the TIME_WAIT state [11].
Firewalls and load balancers, so-called 'middleboxes', sometimes emit
RSTs on behalf of transited connections to optimize server
performance [13]. This is effectively an on-path RST attack in which
the RSTs are sent for benign or beneficial intent. There are
numerous hazards with such use of RSTs, outlined in that RFC.
2.4. What Changed - the Ever Opening Advertised Receive Window
RSTs represent a hazard to TCP, especially when completely unchecked.
Fortunately, there are a number of obfuscation mechanisms that make
it difficult for off-path third parties to forge (spoof) valid RSTs,
as noted earlier. We have already shown it is easy to learn both
endpoint addresses and ports for some protocols, notably BGP. The
final obfuscation is the segment sequence number.
TCP segments include a sequence number which enables out-of-order
receiver processing as well as duplicate detection. The sequence
number space is also used to manage congestion, and indicates the
index of the next byte to be transmitted or received. For RSTs, this
is relevant because legitimate RSTs use the next sequence number in
the transmitter window, and the receiver checks that incoming RSTs
have a sequence number in the expected receive window. Such
processing is intended to eliminate duplicate segments (somewhat moot
for RSTs, though), and to drop RSTs which were part of previous
connections.
TCP uses two window mechanisms, a primary mechanism which uses a
space of 32 bits, and a secondary mechanism which scales this window
[19][31]. The valid advertised receive window is a fraction, not to
exceed approximately half, of this space, or ~2 billion (2E9, i.e.,
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U.S. billion). Under typical configurations, the majority of TCP
connections open to a very small fraction of this space, e.g.,
10,000-60,000(approximately 5-100 segments). This is because the
advertised receive window typically matches the receive socket buffer
size. It is recommended that this buffer be tuned to match the needs
of the connection, either manually or by automatic external means
[33].
On a low-loss path, the advertised receive window should be
configured to match the path bandwidth-delay product, including
buffering delays (assume 1 packet/hop) [33]. Many paths in the
Internet have end-to-end bandwidths of under 1 Mbps, latencies under
100ms, and are under 15 hops, resulting in fairly small advertised
receive windows as above (under 35,000 bytes). Under these
conditions, and further assuming that the initial sequence number is
suitably (pseudo-randomly) chosen, a valid guessed sequence number
would have odds of 1 in 57,000 of falling within the advertised
receive window. Put differently, a blind (i.e., off-path) attacker
would need to send 57,000 RSTs with suitably spaced sequence number
guesses to successfully reset a connection. At 1 Mbps, 57,000 (40
byte) RSTs would take over 50 minutes to transmit, and, as noted
earlier, most current connections are fairly brief by comparison.
Recent use of high bandwidth paths of 10 Gbps and higher result in
bandwidth-delay products over 125 MB - approximately 1/10 of TCP's
overall maximum advertised receive window size (i.e., assuming the
receive socket buffers are increased as much as possible) excluding
scale, assuming the receiver allocates sufficient buffering (as
discussed in Sec. 2. ). Even under networks that are ten times
slower (1 Gbps), the active advertised receive window covers 1/100th
of the overall window size. At these speeds, it takes only 10-100
packets, or less than 32 microseconds, to correctly guess a valid
sequence number and kill a connection. A table of corresponding
exposure to various amounts of RSTs is shown below, for various line
rates, assuming the more conventional 100ms latencies (though even
100ms is large for BGP cases):
BW BW*delay RSTs needed Time needed
------------------------------------------------------------
10 Gbps 125 MB 35 1 us (microsecond)
1 Gbps 12.5 MB 344 110 us
100 Mbps 1.25 MB 3,436 10 ms (millisecond)
10 Mbps 0.125 MB 34,360 1 second
1 Mbps 0.0125 MB 343,598 2 minutes
100 Kbps 0.00125 MB 3,435,974 3 hours
Figure 1 Time needed to kill a connection
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This table demonstrates that the effect of bandwidth on the
vulnerability is squared; for every increase in bandwidth, there is a
linear decrease in the number of sequence number guesses needed, as
well as a linear decrease in the time needed to send a set of
guesses. Notably, as inter-router link bandwidths approach 1 Mbps,
an 'exhaustive' attack becomes practical. Checking that the RST
sequence number is somewhere in the advertised receive window out of
the overall maximum receive window (2^32) is an insufficient
obfuscation.
Note that this table makes a number of assumptions:
1. the overall bandwidth-delay product is relatively fixed
2. traffic losses are negligible (insufficient to affect the
congestion window over the duration of most of the connection)
3. the advertised receive window is a large fraction of the overall
maximum receive window size, e.g., because the receive socket
buffers are set to match a large bandwidth-delay product
4. the attack bandwidth is similar to the end-to-end path bandwidth
Of these assumptions, the last two are more notable. The issue of
receive socket buffers was discussed in Sec. 2. The issue of the
attack bandwidth is considered reasonable as follows:
1. RSTs are substantially easier to send than data; they can be
precomputed and they are smaller than data packets (40 bytes).
2. although susceptible connections use somewhat less ubiquitous
high-bandwidth paths, the attack may be distributed, at which
point only the ingress link of the attack is the primary
limitation
3. for the purposes of the above table, we assume that the ingress at
the attack has the same bandwidth as the path, as an approximation
The previous sections discussed the nature of the recent attacks on
BGP due to the vulnerability of TCP to RST spoofing attacks, due
largely to recent increases in the fraction of the TCP advertised
receive window space in use for a single, long-lived connection.
3. Proposed Solutions and Mitigations
TCP currently authenticates received RSTs using the address and port
pair numbers, and checks that the sequence number is inside the valid
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receiver window. The previous section demonstrated how TCP has
become more vulnerable to RST spoofing attacks due to the increases
in the receive window size. There are a number of current and
proposed solutions to this vulnerability, all attempting to provide
evidence that a received RST is legitimate.
3.1. Transport Layer Solutions
The transport layer represents the last place that segments can be
authenticated before they affect connection management. TCP has a
variety of current and proposed mechanisms to increase the
authentication of segments, protecting against both off-path and on-
path third-party spoofing attacks. SCTP also has mechanisms to
authenticate segments.
3.1.1. TCP MD5 Authentication
An extension to TCP supporting MD5 authentication was developed in
1998 specifically to authenticate BGP connections (although it can be
used for any TCP connection) [18]. The extension relies on a pre-
shared secret key to authenticate the entire TCP segment, including
the data, TCP header, and TCP pseudo-header (certain fields of the IP
header). All segments are protected, including RSTs, to be accepted
only when their signature matches. This option, although widely
deployed in Internet routers, is considered undeployable for
widespread use because the need for pre-shared keys [3][27]. It
further is considered computationally expensive for either hosts or
routers due to the overhead of MD5 [36][37].
3.1.2. TCP RST Window Attenuation
A recent proposal extends TCP to further constrain received RST to
match the expected next sequence number [10]. This restores TCP's
resistance to spurious RSTs, effectively limiting the receive window
for RSTs to a single number. As a result, an attacker would need to
send 2^32 different packets to correctly guess the sequence number.
The extension further modifies the RST receiver to react to
incorrectly-numbered RSTs, by sending a zero-length ACK. If the RST
source is legitimate, upon receipt of an ACK the closed source would
presumably emit a RST with the sequence number matching the ACK,
correctly resetting the intended recipient. This modification
changes TCP's control processing, adding to its complexity and thus
potentially affecting its correctness (in contrast to adding MD5
signatures, which is orthogonal to TCP control processing
altogether). For example, there may be complications between RSTs of
different connections between the same pair of endpoints because RSTs
flush the TIME-WAIT (as mentioned earlier). Further, this proposal
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modifies TCP so that under some circumstances a RST causes a reply
(an ACK), in violation of generally accepted practice, if not gentle
recommendation - although this can be omitted, allowing timeouts to
suffice. The advantage to this proposal is that it can be deployed
incrementally and has benefit to the endpoint on which it is
deployed.
A variant of this proposal uses a different value to attenuate the
window of viable RSTs. It requires RSTs to carry the initial
sequence number rather than the next expected sequence number, i.e.,
the value negotiated on connection establishment [35][41]. This
proposal has the advantage of using an explicitly negotiated value,
but at the cost of changing the behavior of an unmodified endpoint to
a currently valid RST. It would thus be more difficult, without
additional mechanism, to deploy incrementally.
Another variant of this proposal involves increasing TCP's window
space, rather than decreasing the valid range for RSTs, i.e.,
increasing the sequence space from 32 bits to 64 bits. This has the
equivalent effect - the ratio of the valid sequence numbers for any
segment to the overall sequence number space is significantly
reduced. The use of the larger space, as with current schemes to
establish weak authentication using initial sequence numbers (ISNs),
is contingent on using suitably random values for the ISN. Such
randomness adds additional complexity to TCP both in specification
and implementation, and provides only very weak authentication. Such
a modification is not obviously backward compatible, and would be
thus difficult to deploy.
A converse variant of increasing TCP's window space is to decrease
the receive window, which would further reduce the effectiveness of
spoofed RSTs with random sequence numbers. This alternative may
reduce the throughput of the connection, if the advertised receive
window is smaller than the bandwidth-delay product of the connection.
3.1.3. TCP Timestamp Authentication
Another way to authenticate TCP segments is to utilize its timestamp
option, using the value as a sort of authentication [30]. This
requires that the receiver TCP discard values whose timestamp is
outside the accepted window, which is derived from the timestamps of
other packets from the same connection. This technique uses an
existing TCP option, but also requires modified TCP control
processing (with the same caveats) and may be difficult to deploy
incrementally without further modifications. Additionally, the
timestamp value may be easier to guess because it is derived from a
predictable value.
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3.1.4. Other TCP Cookies
All of the above techniques are variants of cookies, otherwise
meaningless data whose value is used to validate the packet. In the
case of MD5 checksums, the cookie is computed based on a shared
secret. Note that even a signature can be guessed, and presents a 1
in 2^(signature length) probability of attack. The primary
difference is that MD5 signatures are effectively one-time cookies,
not predictable based on on-path snooping, because they are dependent
on packet data and thus do not repeat. Window attenuation sequence
numbers can be guessed by snooping the sequence number of current
packets, and timestamps can be guessed even more remotely. These
variants of cookies are similar in spirit to TCP SYN cookies, again
patching a vulnerability to off-path third-party spoofing attacks
based on a (fairly weak, excepting MD5) form of authentication.
Another form of cookie is the source port itself, which can be
randomized but provides only 16 bits of protection (65,000
combinations), which may be exhaustively attacked. This can be
combined with destination port randomization as well, but that would
require a separate coordination mechanism (so both parties know which
ports to use), which is equivalent to (and as infeasible for large-
scale deployments as) exchanging a shared secret.
3.1.5. Other TCP Considerations
The analysis of the potential for RST spoofing above assumes that the
advertised receive window is opened to the maximum extent suggested
by the bandwidth-delay product of the end-to-end path, and that the
window is opened to an appreciable fraction of the overall sequence
number space. As noted earlier, for most common cases, connections
are too brief or over bandwidths too low for such a large window to
be useful. Expanding TCP's sequence number space is a direct way to
further avoid such vulnerability, even for long connections over
emerging bandwidths. If either manual tuning or automatic tuning of
the advertised receive window (via receive buffer tuning) is not
provided, this is not an issue (although connection performance will
suffer) [33].
It is may be sufficient for the endpoint to limit the advertised
receive window by deliberately leaving it small. If the receive
socket buffer is limited, e.g., to the ubiquitous default of 64KB,
the advertised receive window will not be as vulnerable even for very
long connections over very high bandwidths. The vulnerability will
grow linearly with the increased network speed, but not as the
square. The consequence is lower sustained throughput, where only
one window's worth of data per round trip time (RTT) is exchanged.
This will keep the connection open longer; for long-lived connections
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with continuous sourced data, this may continue to present an attack
opportunity, albeit a sparse and slow-moving target. For the most
recent case where BGP data is being exchanged between Internet
routers, the data is bursty and the aggregate traffic may be small
(i.e., unlikely to cover a substantial portion of the sequence space,
even if long-lived), so is smaller advertised receive windows (via
small receiver buffers) may, in some cases, sufficiently address the
immediate problem. This assumes that the routing tables can be
exchanged quickly enough with bandwidth reduced due to the smaller
buffers, or perhaps that the advertised receive window is opened only
during a large burst exchange (e.g., via some other signal between
the two routers).
3.1.6. Other Transport Protocol Solutions
Segment authentication has been addressed at the transport layer in
other protocols. Both SCTP and DCCP include cookies for connection
establishment and use them to authenticate a variety of other control
messages [26][34]. The inclusion of such mechanism at the transport
protocol, although emerging as standard practice, unnecessarily
complicates the design and implementation of new protocols [28] As
new attacks are discovered (SYN floods, RSTs, etc.), each protocol
must be modified individually to compensate. A network solution may
be more appropriate and efficient.
3.2. Network Layer (IP) Solutions
There are two primary variants of network layer solutions to
spoofing: address filtering and IPsec. Address filtering is an
indirect system which relies on other parties to filter packets sent
upstream of an attack, but does not necessarily require participation
of the packet source. IPsec requires cooperation between the
endpoints wanting to avoid attack on their connection, which
currently involves pre-existing shared knowledge of either a shared
key or shared certificate authority.
3.2.1. Address filtering
Address filtering is often proposed as an alternative to protocol
mechanisms to defeat IP source address spoofing [2][12]. Address
filtering restricts traffic from downstream sources across transit
networks based on the IP source address. It cannot restrict traffic
from the core to edges, i.e., from upstream sources. As a result,
each border router must perform the appropriate filtering for overall
protection to result; failure of any border router to filter defeats
the protection of all participants inside the border, ultimately.
Address filtering at the border can protect those inside the border
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from some kinds of spoofing, because only interior addresses should
originate inside the border. It cannot, however, protect connections
originating outside the border except to restrict where the traffic
enters from, e.g., if it expected from one AS and not another.
As a result, address filtering is not a local solution that can be
deployed to protect communicating pairs, but rather relies on a
distributed infrastructure of trusted gateways filtering forged
traffic where it enters the network. It is not feasible for local,
incremental deployment, and relies too heavily on distributed
cooperation. Although useful to reduce the load of spoofed traffic,
it is insufficient to protect particular connections from attack.
A more recent variant of address filtering checks the IP TTL field,
relying on the TTL set by the other end of the connection [14]. This
technique has been used to provide filtering for BGP. It assumes the
connection source TTL is set to 255; packets at the receiver are
checked for TTL=255, and others are dropped. This restricts traffic
to one hop upstream of the receiver (i.e., a BGP router), but those
hops could include other user programs at those nodes (e.g., the BGP
router's peer) or any traffic those nodes accept via tunnels -
because tunnels need not decrement TTLs [29] (see Sec. 5.1 of [14]).
This method of filtering works best where traffic originates one hop
away, so that the address filtering is based on the trust of only
directly-connected (tunneled or otherwise) nodes. Like conventional
address filtering, this reduces spoofing traffic in general, but is
not considered a reliable security mechanism because it relies on
distributed filtering (e.g., the fact that upstream nodes do not
terminate tunnels arbitrarily).
3.2.2. IPsec
TCP is susceptible to RSTs, but also to other off-path and on-path
spoofing attacks, including SYN attacks. Other transport protocols,
such as UDP and RTP are equally susceptible. Although emerging
transport protocols attempt to defeat such attacks at the transport
layer, such attacks take advantage of network layer identity
spoofing. The packet is coming from an endpoint who is spoofing
another endpoint, either upstream or somewhere else in the Internet.
IPsec was designed specifically to establish and enforce
authentication of a packet's source and contents, to most directly
and explicitly addresses this security vulnerability.
The larger problem with IPsec is that of key distribution and use.
IPsec is often cumbersome, and has only recently been supported in
many end-system operating systems. More importantly, it relies on
preshared keys, signed X.509 certificates, or a third-party (e.g.,
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Kerberos) public key infrastructure to establish and exchange keying
information (e.g., via IKE). These present challenges when using
IPsec to secure traffic to a well-known server, whose clients may not
support IPsec or may not have registered with a previously-known
certificate authority (CA).
These keying challenges are being addressed in the IETF in ways that
will enable servers secure associations with other parties without
advance coordination [38][39]. This can be especially useful for
publicly-available servers, or for protecting connections to servers
that - for whatever reason - have not, or will not deploy
conventional IPsec certificates (i.e., core Internet BGP routers).
4. ICMP
Just as spoofed TCP packets can kill a connection, so too can spoofed
ICMP packets. TCP headers can occur inside certain ICMP messages [6].
There have been recent suggestions to validate the sequence number of
TCP headers when they occur inside ICMP messages [16]. This sequence
checking is similar to checks that would occur for conventional data
packets in TCP, but is being proposed in the spirit of the RST window
attenuation described in Section 3.1.2.
Some such checks may be reasonable, especially where they parallel
the validations already performed by TCP processing, notably where
they emulate the semantics of such processing. For example, the TCP
checksum should be validated (if the entire TCP segment is contained
in the ICMP message) before any fields of the TCP header are
examined, to avoid reacting to corrupted packets. Similarly, if the
TCP MD5 option is present, its signature should probably be validated
before considering the contents of the message.
Such validation can ensure that the packet was not corrupted prior to
the ICMP generation (checksum), that the packet was one sent by the
source (IPsec or TCP/MD5 authenticated), or that the packet was not
in the network for an excess of 2*MSL (valid sequence number).
ICMP presents a particular challenge because some messages can reset
a connection more easily - with less validation - than even some
spoofed TCP segments. However, fixing such messages to be 'in window'
is insufficient protection, as this document shows for spoofed data.
In addition, many networks filter all ICMP packets because validation
may not be possible, especially because they can be injected from any
point on a path, and so cannot be selectively address filtered. As a
result, they are not addressed separately in the issues or security
considerations of this document further.
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5. Issues
There are a number of existing and proposed solutions addressing the
vulnerability of transport protocols in general, and TCP in specific,
to off-path third-party spoofing attacks. As shown, these operate at
the transport or network layer. Transport solutions require separate
modification of each transport protocol, addressing network identity
spoofing separately in the context of each transport association.
Network solutions are computationally intensive and require pervasive
registration of certificate authorities with every possible endpoint.
This section explains these observations further.
5.1. Transport Layer (e.g., TCP)
Transport solutions rely on shared cookies to authenticate segments,
including data, transport header, and even pseudo-header (e.g., fixed
portions of the outer IP header in TCP). Because the Internet relies
on stateless network protocols, it makes sense to rely on state
establishment and maintenance available in some transport layers not
only for the connection but for authentication state. Three-way
handshakes and heartbeats can be used to negotiate authentication
state in conjunction with connection parameters, which can be stored
with connection state easily.
As noted earlier, transport layer solutions require separate
modification of all transport protocols to include authentication.
Not all transport layers support negotiated endpoint state (e.g.,
UDP), and legacy protocols have been notoriously difficult to safely
augment. Not all authentication solutions are created equal either,
and relying on a variety of transport solutions exposes end-systems
to increased potential for incorrectly specified or implemented
solutions. Transport authentication has often been developed piece-
wise, in response to specific attacks, e.g., SYN cookies and RST
window attenuation [4][10].
Transport layer solutions are not only per-protocol, but often per-
connection. Each connection needs to negotiate and maintain
authentication state separately. Some overhead is not amortized over
multiple connections, e.g., overheads in packet exchanges, whereas
other overheads are not amortized over different transport protocols,
e.g., design and implementation complexity - both as would be the
case in a network layer solution. Finally, because the
authentication happens later in packet processing than is required,
additional endpoint resources may be needlessly consumed, e.g., in
demultiplexing received packets, indexing connection identifiers,
etc., only to be dropped later at the transport layer.
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5.2. Network Layer (IP)
A network layer solution avoids the hazards of multiple transport
variants, using a single shared endpoint authentication mechanism
early in receiver packet processing to discard unauthenticated
packets at the network layer instead. This defeats spoofing entirely
because spoofing involves masquerading as another endpoint, and
network layer security validates the endpoint as the source of the
packets it emits. Such a network level solution protects all
transport protocols as a result, including both legacy and emerging
protocols, and reduces the complexity of these protocols as well. A
shared solution also reduces protocol overhead, and decouples the
management (and refreshing) of authentication state from that of
individual transport connections. Finally, a network layer solution
protects not only the transport layer but the network layer as well,
e.g., from ICMP, IGMP, etc., spoofing attacks.
The ubiquitous protocol for network layer authentication is IPsec
[22][25]. IPsec specifies the overall architecture, including header
authentication (AH) [21][23] and encapsulation (ESP) modes [24]. AH
authenticates both the IP header and IP data, whereas ESP
authenticates only the IP data (e.g., transport header and payload).
AH is deprecated since ESP is more efficient and the SPI includes
sufficient information to verify the IP header anyway. These two
modes describe the security applied to individual packets within the
IPsec system; key exchange and management is performed either out-of-
band (via pre-shared keys) or by an automated key exchange protocol
IKE [17][20].
IPsec already provides authentication of an IP header and its data
contents sufficient to defeat both on-path and off-path third-party
spoofing attacks. IKE can configure authentication between two
endpoints on a per-endpoint, per-protocol, or per-connection basis,
as desired. IKE also can perform automatic periodic re-keying,
further defeating crypto-analysis based on snooping (clandestine data
collection). The use of IPsec is already commonly strongly
recommended for protected infrastructure.
Existing IPsec is not appropriate for many deployments. It is
computationally intensive both in key management and individual
packet authentication [36]. As importantly, IKE is not anonymous;
keys can be exchanged between parties only if they trust each others'
X.509 certificates, trust some other third-party to help with key
generation (e.g., Kerberos), or pre-share a key. These certificates
provide identification (the other party knows who you are) only where
the certificates themselves are signed by certificate authorities
(CAs) that both parties already trust. To a large extent, the CAs
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themselves are the pre-shared keys which help IKE establish security
association keys, which are then used in the authentication
algorithms.
Alternative mechanisms are under development to address this
limitation, to allow publicly-accessible servers to secure
connections to clients not known in advance, or to allow unilateral
relaxation of identity validation so that the remaining protections
of IPsec to be available [38][39]. In particular, these mechanisms
can prevent a client (but without knowing who that client is) from
being affected by spoofing from other clients, even when the
attackers are on the communications path.
IPsec, although widely available both in commercial routers and
commodity end-systems, is not often utilized except between parties
that already have a preexisting relationship (employee/employer,
between two ISPs, etc.) Servers to anonymous clients (e.g., customer/
business) or more open services (e.g., BGP, where routers may have
large numbers of peers) are unmanageable, due to the breadth and flux
of CAs. New endpoints cannot establish IPsec associations with such
servers unless their own certificate is signed by a CA already
trusted by the server. Different servers - even within the same
overall system (e.g., BGP) - often cannot or will not trust
overlapping subsets of CAs in general.
5.3. Application Layer
There are a number of application layer authentication mechanisms,
often implicit within end-to-end encryption. Application-layer
security (e.g., TLS, SSH, or MD5 checksums within a BGP stream)
provides the ultimate protection of application data from all
intermediaries, including network routers as well as exposure at
other layers in the end-systems. This is the only way to ultimately
protect the application data.
Application authentication cannot protect either the network or
transport protocols from spoofing attacks, however. Spoofed packets
interfere with network processing or reset transport connections
before the application checks the data. Authentication needs to
winnow these packets and drop them before they interfere at these
lower layers.
An alternate application layer solution would involve resilience to
reset connections. If the application can recover from such
connection interruptions, then such attacks have less impact.
Unfortunately, attackers still affect the application, e.g., in the
cost of restarting connections, delays until connections are
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restarted, or increased connection establishment messages on the
network. Some applications - notably BGP - even interpret TCP
connection reliability as an indicator of route path stability, which
is why attacks on BGP have such substantial consequences.
5.4. Shim Transport/Application Layer
Security can also be provided over the transport layer but below the
application layer, in a kind of 'shim' protocol, such as SSL or TLS.
These protocols provide data protection for a variety of applications
over a single, legacy transport protocol, such as SSL/TCP for HTTPS.
Unfortunately, as with application authentication, they do not
protect the transport layer against spoofing attacks.
5.5. Link Layer
Link layer security operates separately on each hop of an Internet.
Such security can be critical in protecting link resources, such as
bandwidth and link management protocols. Protection at this layer
cannot suffice for network or transport layers, because it cannot
authenticate the endpoint source of a packet. Link authentication
ensures only the source of the current link hop where it is examined.
5.6. Issues Discussion
The issues raised in this section suggest that there are challenges
with all solutions to transport protection from spoofing attacks.
This raises the potential need for alternate security levels. While
it is already widely recognized that security needs to occur
simultaneously at many protocol layers, there also may be utility in
supporting a variety of strengths at a single layer. For example,
IPsec already supports a variety of algorithms (MD5, SHA1, etc. for
authentication), but always assumes that:
4. the entire body of the packet is secured
5. security associations are established only where identity is
authenticated by a know certificate authority or other pre-shared
key
6. both on-path and off-path third-party spoofing attacks must be
defeated
These assumptions are prohibitive, especially in many cases of
spoofing attacks. For spoofing, the primary issue is whether packets
are coming from the same party the server can reach. Only the IP
header is fundamentally in question, so securing the entire packet
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(1) is computational overkill. It is sufficient to authenticate the
other party as "a party you have exchanged packets with", rather than
establishing their trusted identity ("Bill" vs. "Bob") as in (2).
Finally, many cookie systems use clear-text (unencrypted), fixed
cookie values, providing reasonable (1 in 2^{cookie-size}) protection
against off-path third-party spoof attacks, but not addressing on-
path attacks at all. Such potential solutions are discussed in the
ANONsec document, in the BTNS (Better Than Nothing Security) BOF
[5][38][39]. Note also that NULL Encryption in IPsec applies a
variant of this cookie, where the SPI is the cookie, and no further
encryption is applied [15].
6. Security Considerations
This entire document focuses on increasing the security of transport
protocols and their resistance to spoofing attacks. Security is
addressed throughout.
This document describes a number of techniques for defeating spoofing
attacks. Those relying on clear-text cookies, either explicit or
implicit (e.g., window sequence attenuation) do not protect from on-
path spoofing attacks, since valid values can be learned from prior
traffic. Those relying on true authentication algorithms are
stronger, protecting even from on-path attacks, because the
authentication hash in a single packet approaches the behavior of
"one time" cookies.
The security of various levels of the protocol stack is addressed.
Spoofing attacks are fundamentally identity masquerading, so we
believe the most appropriate solutions defeat these at the network
layer, where end-to-end identity lies. Some transport protocols
subsume endpoint identity information from the network layer (e.g.,
TCP pseudo-headers), whereas others establish per-connection identity
based on exchanged nonces (e.g., SCTP). It is reasonable, if not
recommended, to address security at all layers of the protocol stack.
7. IANA Considerations
There are no IANA considerations in this document.
This section should be removed by the RFC-Editor upon publication as
an RFC.
8. Conclusions
This document describes the details of the recent BGP spoofing
attacks involving spurious RSTs which could be used to shutdown TCP
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connections. It summarizes and discusses a variety of current and
proposed solutions at various protocol layers.
9. Acknowledgments
This document was inspired by discussions on the
<http://www.ietf.org/html.charters/tcpm-charter.html> about the
recent spoofed RST attacks on BGP routers, including R. Stewart's
draft (which is now edited by M. Dalal) [10][35]. The analysis of
the attack issues, alternate solutions, and the anonymous security
proposed solutions were the result of discussions on that list as
well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang. Ran
Atkinson suggested the UDP variant of TCP/MD5, Paul Goyette suggested
using the ISN to seed TCP/MD5, and Lloyd Wood suggested using the ISN
to validate RSTs. Other improvements are due to the input of various
members of the IETF's TCPM WG, notably detailed feedback from Pekka
Savola.
10. References
10.1. Normative References
None.
10.2. Informative References
[1] Allman, M., V. Paxson, W. Stephens, "TCP Congestion Control,"
RFC 2581, Apr. 1999.
[2] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks," RFC 2827 / BCP 84, March 2004.
[3] Bellovin, S. and A. Zinin, "Standards Maturity Variance
Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4
Specification," (work in progress),
draft-iesg-tcpmd5app-01.txt, Sept. 2004.
[4] Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html",
1997.
[5] Better Than Nothing Security [BTNS] BOF, IETF-61, Wash. DC.,
http://www.ietf.org/ietf/04nov/btns.txt
[6] Braden, R., "Requirements for Internet Hosts -- Communication
Layers," RFC 1122, Oct. 1989.
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[7] Braden, R., "TIME-WAIT Assassination Hazards in TCP", RFC 1337,
May 1992.
[8] CERT alert: "Technical Cyber Security Alert TA04-111A:
Vulnerabilities in TCP --
http://www.us-cert.gov/cas/techalerts/TA04-111A.html", April 20
2004.
[9] Convery, S. and M. Franz, "BGP Vulnerability Testing:
Separating Fact from FUD", 2003,
http://www.nanog.org/mtg-0306/pdf/franz.pdf
[10] Dalal, M., (ed.), "Transmission Control Protocol security
considerations", draft-ietf-tcpm-tcpsecure-02 (work in
progress), Nov. 2004.
[11] Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP
and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573-
1583, March 1999.
[12] Ferguson, P. and D. Senie, Network Ingress Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Address
Spoofing," RFC 2267 / BCP 38, May 2000.
[13] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP
60, RFC 3360, August 2002.
[14] Gill, V., J. Heasley, and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)," RFC 3682 (Experimental), Feb. 2004.
[15] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
Use With IPsec", RFC 2410 (Standards Track), November 1998.
[16] Gont, F., "ICMP attacks against TCP," (work in progress), Sept.
2005.
[17] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409 (Standards Track), November 1998.
[18] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385 (Standards Track), August 1998.
[19] Jacobson, V., B. Braden, and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[20] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-14 (work in progress), June 2004.
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[21] Kent, S., "IP Authentication Header",
draft-ietf-ipsec-rfc2402bis-07 (work in progress), March 2004.
[22] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401 (Standards Track), November 1998.
[23] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402
(Standards Track), November 1998.
[24] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406 (Standards Track), November 1998.
[25] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress),
April 2005.
[26] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", draft-ietf-dccp-spec-11 (work in
progress), March 2005.
[27] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option," RFC 3562 (Informational), July 2003.
[28] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, October 1991.
[29] Perkins, C., "IP Encapsulation within IP," RFC 2003 (Standards
Track), Oct. 1996.
[30] Poon, K., "Use of TCP timestamp option to defend against blind
spoofing attack," draft-poon-tcp-tstamp-mod-01 (work in
progress), Oct. 2004.
[31] Postel, J., "Transmission Control Protocol," RFC 793 / STD 7,
September 1981.
[32] Rekhter, Y. and T. Li, (eds.), "A Border Gateway Protocol 4
(BGP-4)," RFC 1771 (Standards Track), March 1995.
[33] Semke, J., J. Mahdavi, M. Mathis, "Automatic TCP Buffer
Tuning", ACM SIGCOMM '98/ Computer Communication Review, volume
28, number 4, October 1998
[34] Stewart, R., Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson,
"Stream Control Transmission Protocol," RFC 2960 (Standards
Track), October 2000.
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[35] TCPM: IETF TCPM Working Group and mailing list-
http://www.ietf.org/html.charters/tcpm-charter.html.
[36] Touch, J., "Report on MD5 Performance," RFC 1810
(Informational), June 1995.
[37] Touch, J., "Performance Analysis of MD5," Proc. Sigcomm 1995
77-86., March 1999.
[38] Touch, J., "ANONsec: Anonymous Security to Defend Against
Spoofing Attacks," draft-touch-anonsec-00 (work in progress),
May 2004.
[39] Touch, J., D. Black, and Y. Wang, "Problem and Applicability
Statement for Better Than Nothing Security (BTNS)," draft-ietf-
btns-prob-and-applic-01 (work in progress), Sept. 2005.
[40] Watson, P., "Slipping in the Window: TCP Reset attacks,"
Presentation at 2004 CanSecWest.
http://www.cansecwest.com/archives.html
[41] Wood, L., Post to TCPM mailing list regarding use of ISN in
RSTs, ID=Pine.GSO.4.50.0404232249570.5889-
100000@argos.ee.surrey.ac.uk, Apr. 23, 2004.
http://www1.ietf.org/mail-
archive/web/tcpm/current/msg00213.html
Author's Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Fax: +1 (310) 448-9300
Email: touch@isi.edu
URI: http://www.isi.edu/touch
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