Note: Descriptions are shown in the official language in which they were submitted.
CA 02473326 2004-07-08
SPECIFICATION
Method to Block Unauthorized Access to
TFTP Server Configuration Files
Inventors: Andrew Danforth and Kenneth Gould
Field of the Invention
(01] The present invention relates to methods reducing or eliminating
unauthorized use of broadband data services by addressing inherent weaknesses
in
the interactions between trivial file transfer protocol servers and cable
modems.
Background of the Invention
[02] Internet use involves accessing one or more remote Internet servers
for
purposes of downloading information or digital files as well as uploading
files and
messages. Access is accomplished by connecting a terminal or terminal means to
a
carrier network. Terminal means include traditional terminals, personal
computers
(PC) and game console devices equipped with network connectivity. Additional
devices are used between the terminal means and the carrier network. Such
devices
include local networking electronic devices as well as electronic devices that
connect
a local network or terminal means to an external network. Examples of local
networking devices include network hubs, network switches, network bridges,
network interface cards, and the like. Examples of devices to connect a local
network to an external network include routers, cable modems, DSL modems, dial-
up
modems, and the like.
[03] As used herein, Customer Premises Equipment (OPE) includes terminal
means (such as terminals, personal computer or game consoles), local
networking
devices and electronic devices to connect a local network to an external
network
such as a carrier network.
[04] As used herein, a "Carrier Network" generally refers to a computer
network
through which users communicate with various service providers (e.g. Internet
web
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servers). The Carrier Network may be an external network extending from the
local
network to other external networks, for example, the Internet or "world wide
web".
The Carrier Network is maintained by a "Carrier," which also may serve as a
service
provider for certain services. For example, a Carrier or a related entity may
serve as
an Internet service provider (ISP).
[05] Carrier Networks include "Shared Access Carrier Networks," in which
data of
multiple users are conveyed together over a shared communications medium
between the users and the Intermediate Network, and "Dedicated Connection
Carrier
Networks," in which data of each user is conveyed alone between the user and
the
Intermediate Network and are not combined with data of other users. One of the
most
prevalent Shared Access Carrier Networks today is found in the Data-Over-Cable
(DOC) Network, which includes the traditional network constructed from coaxial
cable
and the hybrid fiber coaxial (HFC) network constructed with both fiber optical
cabling
and coaxial cable. Other Shared Access Carrier Networks include wireless and
digital
subscriber line (xDSL) networks (the xDS1._ lines typically being aggregated
onto an
oversubscribed backhaul trunk into the Intermediate Network, with the trunk
defining
the shared communications medium).
[06] Network carriers and their equipment providers have adopted industry
standards in order to increase interchangeability and reduce manufacturing
costs for
network hardware. For example, DOC Carriers have adopted industry standards
such as the Data Over Cable Service Interface Specification (DOCSIS). DOCSIS
version 1.0 was issued in 1997 with hardware devices being certified starting
in 1999.
DOCSIS version 1.1 replaced version 1.0 in 1999-2001 and now accounts for the
bulk of installed DOC network equipment. Although released, DOCSIS version 2.0
is
not yet widely available. As a result, networks conforming to DOCSIS (i.e.
DOCSIS-compliant) use DOCSIS version 1.1 hardware in most cases.
[07] Figure 1 illustrates an example of such a typical DOCSIS-compliant
network.
Data packets are transmitted in a downstream direction from a cable modem
termination system (CMTS) 21, which is located in headend 31 (or distribution
hub) of
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a Carrier, over a coaxial cable or combination coaxial cable and fiber optic
cable 22
to respective cable modems (CMs) 14 of user local networks. CMs may attach a
single terminal means to the DOCSIS-compliant network or may further comprise
electronics that function as a network hub (e.g. Ethernet hub) or router
function.
Many times, the CMs are procured with "firewall" software that is used to
block
undesirable accesses to the attached local network.
[08] All of the CMs 14 are attached by the coaxial cable 22 to the CMTS 21
in an
inverted tree configuration, and each CM 14 connected to the coaxial cable 22
listens
to all broadcasts from the CMTS 21 transmitted through the coaxial cable 22
for data
packets addressed to it, and ignores all other data packets addressed to other
CMs
14.
[09] Theoretically, a CM 14 is capable of receiving data in the downstream
direction over a 6 MHz channel with a maximum connection speed of 30-40 Mbps.
Data packets also are transmitted in the upstream direction over a 2 MHz
channel by
the CMs 14 to the CMTS 21 typically using time division multiplexing (TDM) and
at a
maximum connection speed of 1.5-10 Mbps (up to 30 Mbps when DOCSIS version
2.0 is available)
[10] The headend 31 in the DOCSIS Network includes a plurality of CMTSs,
with
each CMTS supporting multiple groups of CMs each connected together by a
respective coaxial cable. Each such group of CMs connected to a CMTS defines a
Shared Access Carrier Network, with the coaxial cable in each representing the
shared communications medium. This arrangement of a group of CMs connected to
a CMTS by a coaxial cable is referred to herein as a "Cable Network."
Accordingly,
the DOCSIS network includes a plurality of Cable Networks 20 originating from
CMTSs at the headend 31 of the Carrier, with a particular Cable Network 21
being
illustrated in an expanded view in Figure 1. The DOCSIS network may also
include
multiple headends, for example, 31, 32 and 33.
[11] Data transmission over a DOCSIS network can be thought of as a
downstream data path and an upstream data path. Downstream paths normally f
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to transmission from a web server to a terminal means, for example a terminal
11 or
personal computer 12. Upstream data transmission is the opposite with data
originating in terminal 11 or personal computer 12.
[12] For purposes of this invention, customer premises equipment 20
includes the
cable modems 14, terminals 11, personal computers 12 and related
interconnections,
power sources, etc.
[13] Figure 2 illustrates a special case of a DOCSIS compatible network
(also
referred to as a "coaxial based broadband access network"). Cable modem and
local
area network hub have been combined into a single cable modem hub 19. Such
configurations have become particularly popular recently and include both
wired and
wireless (short distance FM) connections to terminal means. Characteristics of
a
DOCSIS compatible network include two-way transmission, a maximum 100-mile
distance between the farthest cable modem and the cable modem termination
system, and the coexistence with other services on the cable network.
[14] Each cable modem is manufactured with a media access control (MAC)
address. This 48-bit address is utilized as a "serial" number for purposes of
identifying a unique cable modern.
[15] Before a cable modem is permitted to provide connectivity between
other
CPE devices and the CMTS, it must be initialized. Figure 3 illustrates typical
steps
that occur in CM initialization. Of particular interest to this invention are
step 308
Establish IP Connectivity and step 312 Transfer Operational Parameters. Step
308 uses a dynamic host configuration protocol (DHCP) server to initialize the
cable
modem with an Internet protocol address. Also provided is the address of a
TFTP
server and name of the file stored on the TFTP server containing appropriate
operational parameters.
[16] Step 312 transfers a configuration file from a TFTP server to the
cable
modem. Trivial file transfer protocol (TFTP) servers are required to respond
to
requests for files with very little security checking. This inherent security
weakness is
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often targeted by "hackers" or other individuals intent upon obtaining
unauthorized
use of broadband data services.
[17] For example, some customers will attempt to abuse a broadband cable
modem service by retrieving a cable modem configuration file from a TFTP
server,
placing that file on their personal computer and "dissecting" the file to
determine how
the configuration file instructs the cable modem to perform. The customer will
then
attempt to share the contents of this file with other "hackers" and/or will
attempt to
modify the file and trick their cable modem into using their modified file to
steal
service or upgraded class of service. As a result, broadband data service
providers
would like to prevent rogue customers from obtaining the configuration files.
[18] There are many methods for securing the TFTP server to try to limit
access
so that only legitimate cable modems may request files from the TFTP server.
These
methods typically involve implementing filters on the cable modems or by
placing
network firewalls in front of the TFTP servers. While these methods are often
effective, many times they are not, due to human error and misconfiguration of
the
filters or firewalls.
[19] Thus what would be useful is a system and method that prevents
unauthorized retrieval of cable modem configuration files from an available
file
server. As is demonstrated below, applicants have developed such a method that
is
secure yet fully compatible with DOCSIS specifications.
Brief Summary of the Invention
[20] The invention is an application designed to reduce or eliminate
unauthorized
access to cable modem configuration files. The filename of cable modem
configuration files are transmitted from the DHCP server in a disguised or
encrypted
fashion that rely upon authorization keys unique to a single cable modem and a
coordi
nation pass phrase that may be unknown to the cable modem. Cable modem
configuration
files are stored on a TFTP server and transmitted only upon receipt of a
request for a
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valid disguised name with proper authentication key from a cable modem.
[21] Various embodiments of the invention incorporate differing methods to
generate and respond to the modified cable modem configuration filenames.
Preferred methods and embodiments are compatible with DOCSIS specifications
versions 1.0, 1.1 and 2Ø
Brief Description of the Drawings
[22] Figure 1 illustrates a typical network as known in the art and using
cable
= network connectivity;
[23] Figure 2 is a simplified schematic illustrating a combined cable
modem/ hub;
[24] Figure 3 illustrates the steps for initialization of a cable modem in
a DOCSIS
compatible network;
[25] Figure 4 illustrates a typical network as known in the art identifying
potential
unauthorized users;
[26] Figure 5 illustrates a typical cable modem request and response to
establish
internet protocol connectivity;
[27] Figure 6 illustrates a typical cable modem request and response to
transfer
operational parameters, for example from a trivial file transfer protocol
(TFTP) server;
[28] Figure 7 illustrates a flowchart of steps during a typical cable modem
request
and response to establish internet protocol connectivity in accordance with
some
embodiments of the present invention;
[29] Figure 8 illustrates a flowchart of steps during a TFTP server
response to a
typical cable modem request for operational parameters for some embodiments of
the present invention;
[30] Figure 9 illustrates a flowchart of steps during a TFTP server
response to a
typical cable modem request for operational parameters for some embodiments of
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the present invention incorporating additional steps.
Detailed Description of the Invention
[31] The invention is an application designed to reduce or eliminate
unauthorized
access to cable modem (CM) configuration files. The CM configuration file is
retrieved by an authorized user from a trivial file transfer protocol (TFTP)
server in
response to a user TFTP getfile request.
[32] When a cable modem boots, it sends a DHCP request to a DHCP server as
illustrated as step 308 of Figure 3. As used herein "cable modem boots" refers
to
the startup sequence of steps performed by a cable modem during power up or
initialization. This may occur upon initial powering of the modem, subsequent
to a
loss of synchronization signal, or after a forced reset from the DOC network
carrier.
[33] Figure 5 illustrates step 308 in acquiring an Internet protocol
address in
greater detail. The request for IF address is in the form of a DHCP packet.
Table 1
indicates the general form of a DHCP packet (size of data in octets is
indicated in
parenthesis). Table 1 is organized by bit and octet.
Table 1 - DHCP Packet
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 opcode(1) 1 htype (1) 1 hlen (1) 1 hops (1)
-+ -----------
xid (4)
secs (2) flags (2)
ciaddr (4)
yiaddr (4)
1 siaddr (4) 1
giaddr (4)
chaddr (16)
1
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sname (64)
file (128)
option (64)
[34] For DOCS1S, the field values used in the DHCP Request are
indicated in
Table 2:
Table 2 - DHCP Server Parameters Transmitted in DHCP
Request from Cable Modem (Step 308)
=
Parameters F Value / Use
opcode Operation Code ¨ 1 for DHCP Request, 2
for DHCP
= Reply
htype ' Hardware Type ¨ 1 for Ethernet
hlen Hardware Length ¨ 6 for DOCS1S
=
[ hops CM sets to 0, optionally used by a relay-
agent
xid Transaction ID ¨ random number associated
with
transaction that is generated by the cable modem
secs Seconds elapsed since cable modem started
initialization
= flags , Flags including a broadcast bit
: ciaddr Client Identifier set by cable modem to
48 bit MAC
, address of modem
yiaddr , used for the IP address to be
reserved/used by the
cable modem
siaddr used for TFTP server IP address
giaddr IF address of relay agent, if any
chaddr Client Hardware address ¨ set to 48 bit
MAC address
of cable modem
sname optional server address, or TOD server
address
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tLYVIFIAM
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file ; filename or null prior to DHCP Response
i
options'1 option codes, also identification of cable modem
1 vendor
,
_________ [351 The DHCP server responds to the request with, among other
things, an IP
address to be assigned to the cable modem, a TFTP server IF address, and the
name of the DOCSIS configuration file that the modem should request from the
TFTP
server. These parameters along with other parameters transmitted from the DHCP
server to a cable modem are identified in Table 3.
Table 3 - DHCP Server Parameters Transmitted in DHCP
Response to Cable Modem (Step 308)
DHCP Server ! Description
i
Parameters
,
_______________________________________________________________________________
___ ,
!
IP address for the cable ; This IF address typically is assigned dynamically
but ;
modem's cable the DOC Carrier can also statically assign IP
I
interface
addresses on the basis of each modem's MAC
address.
;
IP subnet mask for the ; This subnet mask typically is used for all cable
cable modem's cable ' modems using the same downstream, but this
=
interface
depends on the setup of the CMTS network as well as !
L i subscribers' needs.
;
,
IP address for the TFTP i This TFTP server provides the DOCSIS configuration
server ' file to the cable modem and is typically a
dedicated 1
I
.1
. server located at the DOC Carriers' headend.
11
= 1
IP address for the
A DHCP relay agent is required if the DHCP server is =
DHCP relay agent located on a different network than the IP
address
assigned to the cable modem's cable interface. The
,
DHCP relay agent is also used if the DHCP server i=:..;
,
providing IF addresses to the CPE devices connec=t,-,4
.
, to the cable modem and the CPE devices are on a
.
=' different subnet than the cable modem.
=
Complete filename for ; This is the filename for the DOCSIS configuration fii,
[ the DOCSIS that the cable modem should download from thf-
,
configuration file =
server.
.
¨ _______________________________________________________________________ ...
.1 IP address for one or ' The cable modem uses the ToD server to aet
the
.._;1
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T
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more time of day (ToD) I current date and time so that it can accurately
servers 1 timestamp its SNMP messages and error log
entries. 1
_________________________________________ ¨ ____________
One or more IP .1 Typically, the CMTS acts as the default
gateway for ;
addresses for the the cable modem.
routers that will forward
IP traffic from the cable
; modem 1
One or more IP The cable modem can send its error log
messages to I
addresses for System 1 the SYSLOG servers, which are optional and typically !
Log (SYSLOG) servers located at the DOC Carriers' headend.
[36] The DOCS1S configuration filename ("file" of Table 2) is typically
limited to
128 octets of data. The naming convention of the file is also required to be
compatible with filename conventions for the TFTP server. TFTP normally uses
filenames in netascii format. Netascii is an eight-bit ASCII protocol with the
first bit
always set high, for error checking. In addition to the TFTP requirement, the
filename needs to conform to any filename convention required by the TFTP
server
operating system. This will normally prevent naming the configuration file
with
non-printing or reserved characters.
[37] As illustrated in Figure 3, once the cable modem has established
Internet
protocol 309, it proceeds with establishing time of day 310 and 311 (from ToD
server
identified in DHCP download). The cable modem then requests a download
transfer
320 of a configuration file containing operational parameters.
[38] Figure 6 illustrates step 320, acquiring a configuration file in more
detail.
Using user datagram protocol (UDP), a CM requests a configuration file from
the
TFTP server. The UDP protocol request is limited to the UDP header and the
configuration file name. UDP headers consist of 8 bytes of data, 2 each for
source =
port address, destination port address, total message length and checksum. The
UDP is transmitted within the data field of an Internet protocol datagram
packet. The
IP datagram packet includes a header identifying the IP address currently in
use by
the cable modem.
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[39] After the request is made to the TFTP server, the cable modern begins
waiting for either a configuration file to arrive and starts a timeout clock
323. Upon
the earlier of timeout 323 or receipt of a configuration file 322, this step
of the
initialization continues. In the case of timeout 323, the retry counter is
incremented
324 and if retries are not exceeded 325, the cable modem transmits an
additional
request for a configuration file 320.
[40] When a configuration file is received 322, the file is verified as
having all of
the mandatory items 327, the message integrity checks (MIC) are valid 328 and
that
there are no TLV type 11 errors 329. There are two separate MIC checks,
designated for the cable modem and cable modem termination system
respectfully:
Use of MIC checks ensures that data in a file has not been altered during
transmission and receipt. Performing a "MD5 digest" of the originating data
creates
them.
[41] TLV type 11 errors 329 occur during the TLV-11 element to PDU
translation
when a configuration file has a requested option that is unsupported by the
cable
modem hardware and firmware.
[42] Providing the received configuration file is properly received and no
errors
are found, the cable modem will then initialize the operational functions and
options
present in the configuration file 330. At this point, configuration file
transfer is
complete 340 and the cable modem initialization is ready to perform
registration (step
341 of Figure 3).
[43] As noted above, the cable modem acquires the parameter configuration
file
from a Trivial File Transfer Protocol (TFTP) server. The contents of a DOCSIS
1.0
compliant configuration file are indicated in Table 3. DOCSIS 1.1 and DOCSIS
2.0
compliant configuration files differ somewhat in their contents, but the
exchange of
configuration files via TFTP is the same in all cases.
Table 3 Cable Modem Configuration File Parameters
Configuration File Description
Parameters
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I
! Downstream
1 Specifies the center frequency (in multiples of 62500 Hz) 1
1 Frequency for the downstream channel to be used by the
router.
! (This parameter does not need to be specified in the
.
1 configuration file because the router will scan the
.
=
, :
! downstream for available frequencies, but typically it is
!
,
1 specified to ensure that the router conforms to the
I,
provider's channel plan.)
1
,
_______________________________ 1
___________________________________________________ 1
1
!. Upstream Channel
1 Specifies channel ID for the upstream channel to be used !
1 ID I by the router. (This parameter does not
need to be I
, specified in the configuration file because it can be set
i ______________________________ I dynamically by the CMTS during
provisioning.) !
1
!
:
! Network Access 1 Determines whether CPE devices attached to
the cable 1
,
1 Configuration i modem are allowed access to the cable
network. The 1
I default is to allow access for C.;IDE devices (which is
i
.
: required for normal operations).
1
. Class of Service ID ! Specifies the ID for this class of service
(1-16). i
1
_______________________________________________________________________________
_______ .i
I
' Maximum
! Specifies the maximum downstream data rate (in bits/sec) i
: Downstream Rate : allowed for traffic associated with this
class of service.
' (This is a limit, not a guarantee of service.)
1
¨
_______________________________________________________________________________
_____ ,
.
,
Maximum Upstream , Specifies the maximum upstream data rate (in bits/sec)
!
, Rate . allowed for traffic associated with this
class of service. .
,
, (This is a limit, not a guarantee of service.)
:
___________________________________________________________ ¨
_______________________
Upstream Channel
Specifies the priority for upstream traffic (0-7, where 7 is .
. Priority highest priority).
_______________________________ L_
.
Minimum Upstream = Specifies the minimum upstream data rate (in bits/sec)
Rate
1 that is guaranteed for traffic associated with this class of !
,
i
service.
,
_______________________________________________________________________________
_______ ,
,. ____________________________
:
1
Maximum Upstream ' Specifies the maximum size of burst traffic to be allowed !
Channel Burst on this upstream channel. The size is
specified in bytes, '
0-65535, where 0 is no limit. If this field is set to a non-
:=zero value, it should be set to at least 1800 so that it is
,
, 1 greater than the maximum Ethernet frame
size of 1518 !
i plus the associated packet overhead).
,
,
_______________________________________________________________________________
_____
Class of Service Specifies whether BPI encryption should be
enabled on
Privacy Enable traffic associated with this class of
service (1 enables BPI
, encryption, 0 disables BPI encryption).
,
_______________________________________________________________________________
______
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! Vendor ID
The three-byte Organization Unique Identifier for the
! vendor, which is also usually the first three bytes of the !
cable modem's MAC address. This value is usually
= I expressed as a hexadecimal number (e.g. 000000)
Vendor-Specific Contains any arbitrary values that are defined by the
Options ! manufacturer of the cable modem.
SNMP Write-Access Allows the service provider to set arbitrary SNMP
; Control and SNMP attributes on the cable modem.
M1B Objects
; Authorize Wait ! Specifies the retransmission interval, in seconds, of
Timeout Authorization Request messages from the Authorize Wait
state. Valid values are 2-30 seconds.
Reauthorize Wait Specifies the retransmission interval, in seconds, of
Timeout Reauthorization Request messages from the Authorize
Wait state. Valid values are 2-30 seconds.
' Authorization Grace ; Specifies the grace period for re-authorization, in
Timeout ; seconds. Valid values are 1-1800 seconds.
Operational Wait i Specifies the retransmission interval, in seconds, of
Key !
Timeout ; Requests from the Operational Wait state. Valid values
. are 1-10 seconds.
¨ _________________________________________________________________________ -
Rekey Wait Timeout Specifies the retransmission interval, in seconds, of
Key
Requests from the Rekey Wait state. Valid values are 1- '.
seconds.
TEK Grace Time ! Specifies the grace period for re-keying, in seconds.
Valid
values are 1-1800 seconds.
Authorize Reject ! Specifies how long, in seconds, a cable modem waits in
Wait Timeout ! the Authorize Reject Wait state after receiving an
; Authorization Reject. Valid values are 60-1800 seconds.
Maximum Number of Determines the maximum number of CPE devices that
CPEs can use the cable modem to connect to the cable
; network.
CPE Ethernet MAC ! Configures the cable modem with the MAC addresses for
; Address one or more CPE devices that are allowed to connect to
the cable network. Cable modems give priority to the CPE
___________________ ; devices whose MAC addresses are in the configuration
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file.
TFTP Software Specifies the IP address for the TFTP server
that will
. Server IP Address i provide software images. This server does not
necessarily
have to be the same TFTP server that provided the
DOCSIS configuration file.
Software Image I Specifies the fully qualified path name for
the software
Filename , image that the cable modem should be running.
If
necessary, the cable modem uses TFTP to download this
image from the software server.
Concatenation Specifies whether the cable modem supports
000S1S 1.1
= Support concatenation of upstream packet
requests.
Use RFC2104 Specifies the algorithm used to compute the
CMTS
HMAC-MD5 Message Integrity Check (MIC). If yes, the
HMAC-MD5
algorithm specified in RFC 2104 is used; otherwise, the
algorithm specified by RFC 1321 is used. (The algorithm
used must match the one used on the CMTS.)
CMTS Specifies an authentication string to be used
between the ;
Authentication provisioning server and the CMTS. It allows
the CMTS to
authenticate the CM provisioning with a central
authentication service, such as a RADIUS server.
L
[44] After the TFTP transfer of the CM configuration file is complete (step
340 of
Figure 3), the CM does a registration with the CMTS 342, establishes baseline
privacy interface (steps 342-345, if enabled) and then is operational 350.
Registration consists of registration request from the CM to the CMTS followed
by
registration response from the CMTS to the CM.
[45] One feature known in the art, is that TFTP protocol allows file
downloads with
very little security. Often the only pre-requisite to downloading from a TFTP
server is
network access, TFTP server address, destination address and filename. One
traditional approach to protecting access to CM configuration files is with a
firewall
that prevents unauthorized users from accessing the server.
[46] Two different types of unauthorized users attempting to obtain a
configuration file are illustrated in Figure 4. User 15c is a valid customer
of the DOC
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_ ___________________________________________________
¨
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network provider but is using services or bandwidth not authorized. User 15d
is not
an unauthorized user who is also not a customer of the DOC network provider.
Commonly such users will imitate a valid customer (i.e. spoof the DOC network
connections). Users such as user 15d may be prevented from acquiring a cable
modem configuration file by use of firewalls, as is known in the art.
Firewalls are
used to prevent unauthorized access to network assets. As user 15d is an
unauthorized user without any authorization to use the DOC network, a firewall
may
be used to successfully thwart attempts to acquire a configuration file.
. [47] One form of firewall is to have CMTS filter out network
messages originating
from cable modems that fail DOCSIS message integrity checks (MIC). Similarly,
cable modems may be prevented from registering with a CMTS (steps 341, 342 of
Figure 3) unless the cable modem is using a configuration file that has been
downloaded from the DOC carriers' TFTP server.
[48] In contrast to user 15d of Figure 4, user 15c is a more difficult to
protect
against. These users are valid customers so they have authorization to connect
to
the DOC network as well as to have their cable modem 19c register with CMTS
21.
These users are invoiced amounts for a particular DOC service level limited as
to
bandwidth, class of service, quality of services, optional features, etc. but
are using
DOC network services or bandwidth in excess of their service agreements. One
means users 15c accomplish this is by capturing a configuration file for a
valid
authorized customer having higher service rates and then downloading this
captured
configuration file into their cable modem 19c.
[49] An alternate method users 15c employ involves retrieving the
configuration
file of their cable modem, editing the file, then re-inserting the edited file
into the
cable modem. When the editing removes bandwidth limits the result may be that
users 15c enjoy the maximum bandwidth available on the network segment
attached
to their cable modem 19c. Using unlimited bandwidth is termed called
"uncappinri"
bandwidth.
[50] As users 15c are also customers, any scheme that prevents 15c from
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unauthorized (and in most cases, unpaid for) network services must not
interrupt the
service such users are authorized to enjoy. Unfortunately, most techniques
that add
methods to restrict 15c unauthorized network usage also make the DOG network
less
robust by being more sensitive to outside events. For example, outside events
include power failures, loss of signal, as well as lowered signal to noise
ratios,
electrostatic interference, an the like.
[51] One approach to 15c users is the strict enforcement of the MIC
checking.
The MIC is often based on a Message Digest 5 (MD5) hash of the contents of the
cable modem configuration file. MD5 is a one-way (non-invertible) hash¨meaning
that the input cannot be recovered from the output¨and the output is
considered
unique for a specific input. If the MIC is not correct, the cable modem
registration
process fails and the cable modem is not allowed to become operational.
[52] Publicly available tools exist to create a DOCSIS-compliant
configuration file,
including a valid MIC. However, a "shared-secret" can be included in the MD5
hash
value. Without the shared secret, it is extremely difficult to produce the
correct
matching MIC, and the cable modem is prevented from registering with the DOC
provider's network. This approach dramatically reduces the ease by which user
15c
can modify the user's configuration file by using simple editing tools.
[53] However, if the shared secret is configured identically on all of the
systems
within a service provider's network and TFTP spoofing is possible, then other
valid
configurations containing different parameters for the same service provider
network
can be interchanged and downloaded to a cable modem. The modem will be allowed
to come on line because the shared secret is the same. In addition, while the
MD5
hash is non-invertible, the shared secret to compute it can be recovered from
the
CMTS router configuration. Presently a cable modem shared secret may be
encrypted, but normally such encryption is not cryptographically secure (For
example, Cisco provides the command "service password-encryption" which
invokes
"mode 7" encryption.)
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[54] The present invention avoids many of the pitfalls of these approaches
by
reducing or eliminating unauthorized downloads of configuration files from the
TFTP
server. Figure 7 and Figure 8 illustrate how the present invention differs
from the
traditional DHCP and TFTP server functions. As illustrated in Figure 7, the
present
invention modifies the configuration filename supplied by the DHCP server
during
establishing of IP connectivity (steps 308, 309 of Figure 3). A modified
filename is
downloaded from the DHCP to the cable modem. The modified filename comprises
the actual filename combined with an authentication key that is generated by
the
DHCP server from the filename, assigned IP address and coordinated pass
phrase.
The authentication key may further incorporate additional data or parameters.
Optionally, the modified filename can be further disguised through the use of
a
cloaking function, as described below.
[55] Typical names of cable modem configuration files include a TFTP server
pathname, filename, and filename extension such as "bin", "cm" or" md5". As
noted
earlier, the filename field used by DHCP servers and cable modems may contain
up
to 128 octets, grouped into netascii characters.
[56] The present invention uses the DHCP server to create the modified
configuration filename and pass it along with the assigned IP address to the
cable
modem. The cable modem, in turn, transmits a request for a file with a name
matching the modified filename to the TFTP server.
[57] In preferred embodiments of the invention, the cable modem uses the
modified filename "as is". In this fashion, existing installed cable modems
(e.g.
DOCS1S 1.0, DOCSIS 1.1 and DOCS1S 2.0 compliant) may be utilized without
modification. As the number of installed cable modems in a typical DOC network
carrier may exceed 3 million modems, the advantages of not requiring the
change or
modification of the cable modems are very significant.
[58] Some of the other embodiments of the invention require that the cable
=
modem create the modified configuration filename by incorporating data not
transmitted in the DHCPDISCOVERY or DHCPREQUEST commands. Although this
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approach is useful where very high security DOC networks are needed, in most
instances the cost of special cable modem hardware and interfaces will be
unjustified.
[59] As used herein "modified CM configuration filename" refers to
filenames
modified in accordance with the present invention, for example as illustrated
by
Figure 7. Similarly, "modified CM configuration filename file" refers to a
cable
modem configuration file associated or otherwise identified by the modified CM
configuration filename.
[60] In Figure 7, the DHCP server receives the IP address request from the
cable
modem 521. As earlier described, prior to DHCP REQUEST 521, the cable modem
transmits one or more DHCP DISCOVER 501 packets and has received one or more
DHCP OFFER 511 packets from DHCP servers. The IP address request 521
contains information about the cable modem including the cable modem MAC
address, and requested IP address (i.e. same IP address as in DHCP OFFER 511
packet).
[61] The DHCP server compares the received cable modem MAC address to
those associated with authorized customers and the service plan authorized for
those
customers 522. Requests using MAC addresses not associated with authorized
customers are discarded and ignored 523. MAC addresses of authorized customers
are assigned the requested IF address along with a configuration filename
corresponding to the authorized or agreed to service plan 531. Instead of
ignoring
requests from unauthorized customers, the DHCP server may optionally respond
with
the name of a "disable" configuration file 524 containing instructions to deny
data
services to the cable modem.
[62] The DHCP server next creates an authentication key and combines the
customer authorized configuration filename with the authentication key to form
a
modified configuration filename 532. Optionally, the DHCP server applies a
cloaking
function to further secure the modified filename 533. This modified filename
is the
modified CM configuration filename and is inserted into the "file" parameter
field of
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the DHCP Response packet and the DHCP server forwards the packet to the cable
modem 550.
[63] Various ways of combining the authentication key with a configuration
filename are known. For example, the authentication key may be appended to the
original filename using traditional text concatenation. In order to facilitate
recognition
by the TFTP server, it may be desirable to separate the original filename from
the
authentication key with one or more delimiter characters.
[64] Taking the example of an original configuration filename platinum.cm,
an
authentication key of 1234567890abcdef and a delimiter could result in a
modified
CM configuration file name of platinum.cm@1234567890abcdef.
[65] Needed by the present invention is an authentication key that depends
upon
various parameters and concurrently protects from discovery the values of
those
parameters. Preferably the authentication key depends upon the assigned cable
modem IP address and the original configuration filename. More preferably the
authentication key will also depend upon a "coordinated pass phrase", known
only by
the DHCP server and the TFTP server. Other parameter values may also be
included, provided they are available to both the TFTP server as well as the
DHCP
server.
[66] Creation of the authentication key may use such methods as block
cipher,
iterated block cipher, stream cipher, hash function, message authentication
codes,
factoring, discrete logarithms, elliptic curves, lattice cryptosystems, or
other one-way
encryption functions. Some of the common functions include, but are not
limited to,
Data Encryption Standard (DES), Data Encryption Algorithm (DEA), extended Data
Encryption Standard (DESX), Advanced Encryption Standard (AES, including MARS,
=
RC6), Digital Signature Algorithm (DSA), Rivests Cipher (RC2), RC4, RC5,
Secure
Hash Algorithm (SHA), Message Digest Algorithms (MD2, MD4, MD5), International
Data Encryption Algorithm (IDEA), Secure And Fast Encryption Routine (SAFER),
Fast Data Encipherment Algorithm (FEAL), Skipjack, Blowfish, Carlisle Adams
and
Stafford Tavares (CAST) and EIGamal.
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[67] Although all of the named cryptography methods are suitable,
particularly
preferred are those that are fast and yet form authentication keys that do not
reveal
the "seed" parameter values. One of the advantages of some preferred
embodiments of the invention is that secure one-way hash totals can be used
and
decryption of the authentication key is unnecessary. Examples of particularly
preferred encryption functions are message digest 5 (MD5), and Rivest's Cipher
RC4, RC5 and RC6.
[68] MD5 creates a 128 bit hash total of the fields it digests. The hash
total is
often represented by a printable 32-character string of hexadecimal digits
(base 16)
and is easily transmitted between a cable modem, CMTS, DHCP server and TFTP
server. As an example, applying MD5 to This is a message yields the hash total
0BD0E17C22869EBD31906E27648E77D4. The hash total may also be represented
by a base 64 22-character string (e.g. LOOF8loaevTGQbidkjnfU).
[69] Most of the more secure authentication keys are affected by not only
the
seed values but also by the order in which they are presented to the
encryption
subroutine. As the result, the order in which parameters are digested by MD5
must
be consistent between the DHCP server and later the TFTP server.
[70] The optional cloaking function 533 may be used to present another
layer of
security to the modified filename. Various methods of cloaking are known and
used
in the cryptography arts. One example, is to add random characters into a text
string.
Another cloaking method is to delete characters from a text string. Further,
another
method is to intersperse two character strings. Other cloaking methods include
increasing the size of an encrypted block by padding with random characters.
Preferable cloaking for the instant invention is substituting three or more of
the
authentication key characters with random characters.
[71] Regardless of whether a cloaking function has been used, the resultant
modified CM configuration filename has embedded within the filename the
original
configuration filename as well as the resultant authentication key.
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[72] Figure 8 and Figure 9 illustrate examples of how a TFTP server in
accordance with the present invention may validate and respond to a TFTP
request
for a modified CM configuration filename. These examples shall not be
considered
limiting, as the various steps may be combined or performed in an alternate
order.
Dashed lines indicate optional steps that may be added to incorporate
additional
desired functions or match DHCP server functions (e.g. as illustrated in
Figure 7).
[73] The compare function 855 of Figure 8 compares the modified CM
configuration filename against a filename generated by the TFTP server. An
alternate approach is to compare the original filename to available filenames
and
also compare the DHCP server authentication key against the TFTP generated
authentication key. In either alternative, the TFTP server generates an
authentication
key 850 using the same method DHCP server utilizes. This is advantageous for
software maintenance.
[74] The TFTP server receives a request for a modified CM configuration
filename 320a and saves the filename in a temporary memory location XF1LENAME
801. Also kept available is the IP address of the requesting cable modem
(retrieved
from the datagram packet header). In the case the modified CM configuration
filename had been cloaked, a de-cloaking function is performed 811. The
modified
CM configuration filename is then parsed to discover the original unmodified
filename
850.
[75] The TFTP server next creates an authentication key using the same
method
and parameters the DHCP server used 850. Once the authentication key is
generated, it is combined with the original un-modified filename discovered by
parsing engine 821. Combination of the un-modified filename and authentication
key
is performed as done by DHCP server. If the DHCP server had used an optional
cloaking function, the TFTP server 533 repeats its use. The key generation
function
at a minimum uses parameters: cable modem IP address, original un-modified
filename and coordination pass phrase.
[76] The resulting modified filename will match the received modified CM
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configuration filename XFILENAME from authorized customers. In this case the
TFTP server will transmit the desired cable modem configuration file 322a.
When the
two filenames do not match, it may be due to unauthorized customer request or
cable
modem malfunction, or other data transmission problems. When the two filenames
do not match, various responses are possible. For example, an error message
can
be logged 856 and/or the TFTP server can transmit a special cable modem
configuration file that disables the unauthorized customer's cable modem 322d.
Alternately, a special "service" configuration file can be transmitted to the
cable
modem 322c. The service configuration file is used by the DOC network carrier
service personnel to aid in diagnosing hardware and network problems. Of
course,
another provision of the TFTP server may be to allow customers to request the
service configuration file directly 830.
[77] Comparing the steps performed in Figure 7 by the DHCP server and those
performed in Figure 8 by the TFTP server highlight the elegance of the present
invention. All that must be maintained for the invention to properly perform
is to keep
the coordination pass phrase and authentication key generation methods
consistent.
[78] Preferably the coordination pass phrase is a random phrase that is
frequently
updated. For highest levels of security, the coordination pass phrase is
updated (e.g.
changed or rotated) at a frequency to preclude use of common network intrusion
software. For example, customer networks comprising cable modems incorporating
wireless networks are susceptible to intrusion attacks by the Airsnort
program. Using
Airsnort, a wireless network encryption is quickly broken once 5 to 10 million
encrypted packets are collected (encrypted per IEEE 802.11). With a connection
speed of 3.5 megabits per second, it is estimated the Airsnort program can be
decrypting messages in approximately 16 minutes. As a result, it is desirable
to
update the coordination pass phrase at intervals less than the intrusion
interval.
[79] As used herein "intrusion interval" refers to the time duration a
commonly
available software program can solve encryption security of a network attached
to the
cable modem. For example, when IEEE 802.11 encrypted wireless networks are
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attached, the intrusion interval is currently 16 minutes.
[80] Figure 9 illustrates some of the other optional steps that may be
present in
other embodiments of the invention. Steps 320a, 801, 811 and 821 are the same
in
both Figure 8 and Figure 9. After the modified CM configuration filename is
parsed
821, Figure 9 illustrates examples of how the TFTP server could respond. As
noted,
parsing engine 821 isolates the original un-modified filename, for example
"platinum.cm". TFTP server compares the un-modified filename against filenames
for particular DOC network service agreements.
[81] When a low service agreement file is requested it may be desirable to
not
require additional authorization key checks. By skipping the authorization
step, the
TFTP server will be able to perform a greater number of transactions in a
given time,
thereby supporting larger numbers of customers. This will also provide a back-
up
means in the event the authentication key process is corrupted or the
coordination
pass phrase is changed or erased in the DHCP server but not in the TFTP
server.
[82] In Figure 9, if the original un-modified filename is "default" 825
then no
authentication is performed and the TFTP server transmits the proper default
configuration file 322b. The default configuration file would typically be
associated
with a base or minimum network service agreement to which all customers are
authorized.
[83] If the original un-mcdified filename is "service" 830 then no
authentication is
performed and the TFTP server transmits the proper service configuration file
322c.
As described earlier, a service configuration file could be used during
troubleshooting
new customers or responding to and diagnosing hardware and network
transmission
problems.
[84] When the original un-modified filename is associated with a high
bandwidth
or premium service, authentication keys optionally include additional
parameter
values. For example, for a "premium" service 835, the cable MAC address can be
retrieved from the TFTP server or other database 836 and included in the
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authentication key generation 850. In contrast to IP address, the MAC address
is not
available in the datagram header of the configuration file request 320a.
[85] The disadvantage of including the MAC address is reducing the
transaction
speed of the TFTP server with additional database look-ups. With thousands of
customers serviced by each TFTP server, this may result in significant
initialization
delays. However, by using the method of Figure 9, only a small delay in TFTP
" processing occurs as the additional MAC address steps are performed only for
premium service customers.
[86] The use of this invention will be limited by the hardware and firmware
incorporate into cable modems and cable modem termination systems. Each
manufacturer of these devices may have differing means of implementing the
DOCSIS standards. As the devices are changed, the invention is easily varied
to
accommodate the new hardware and firmware.
[87] The coordination pass phrase must be equal in both the DHCP server and
the TFTP server in order for the authentication key generation steps to result
in
matching modified filenames. Preferably the pass phrase is changed frequently
in
order to promote security and stifle unauthorized user attempts to siphon
services.
[88] Although the present invention has been illustrated in terms of
specific
embodiments, various ways of accomplishing the enumerated steps are possible
in
accordance with the teachings described herein. For example, the present
invention
may use DHCP servers and TFTP servers on separately networked computers or
integrated into a single provisioning host (as for example a single
provisioning host
located at a headend). Additionally, the claims should not be read as limited
to the
described order of steps unless stated to that effect. The scope of the claims
should
not be limited by the preferred embodiments and the examples, but should be
given
the broadest interpretation consistent with the description as a whole.
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