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RFC 1492 - An Access Control Protocol, Sometimes Called TACACS


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Network Working Group                                         C. Finseth
Request for Comments: 1492                       University of Minnesota
                                                               July 1993

          An Access Control Protocol, Sometimes Called TACACS

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

Background

   There used to be a network called ARPANET.  This network consisted of
   end nodes (hosts), routing nodes (IMPs) and links.  There were (at
   least) two types of IMPs: those that connected dedicated lines only
   and those that could accept dial up lines.  The latter were called
   "TIPs."

   People being what they were, there was a desire to control who could
   use the dial up lines.  Someone invented a protocol, called "TACACS"
   (Terminal Access Controller Access Control System?), which allowed a
   TIP to accept a username and password and send a query to a TACACS
   authentication server, sometimes called a TACACS daemon or simply
   TACACSD.  This server was normally a program running on a host. The
   host would determine whether to accept or deny the request and sent a
   response back.  The TIP would then allow access or not, based upon
   the response.

   While TIPs are -- shall we say? -- no longer a major presence on the
   Internet, terminal servers are.  Cisco Systems terminal servers
   implement an extended version of this TACACS protocol.  Thus, the
   access control decision is delegated to a host.  In this way, the
   process of making the decision is "opened up" and the algorithms and
   data used to make the decision are under the complete control of
   whoever is running the TACACS daemon.  For example, "anyone with a
   first name of Joe can only login after 10:00 PM Mon-Fri, unless his
   last name is Smith or there is a Susan already logged in."

   The extensions to the protocol provide for more types of
   authentication requests and more types of response codes than were in
   the original specification.

   The original TACACS protocol specification does exist.  However, due
   to copyright issues, I was not able to obtain a copy of this document

   and this lack of access is the main reason for the writing of this
   document.  This version of the specification was developed with the
   assistance of Cisco Systems, who has an implementation of the TACACS
   protocol that is believed to be compatible with the original
   specification.  To be precise, the Cisco Systems implementation
   supports both the simple (non-extended) and extended versions.  It is
   the simple version that would be compatible with the original.

   Please keep in mind that this is an informational RFC and does not
   specify a standard, and that more information may be uncovered in the
   future (i.e., the original specification may become available) that
   could cause parts of this document to be known to be incorrect.

   This RFC documents the extended TACACS protocol use by the Cisco
   Systems terminal servers.  This same protocol is used by the
   University of Minnesota's distributed authentication system.

1. Protocol Semantics

   This section will describe the requests and responses.  The following
   two sections will describe two different ways of encoding the
   protocol.

   A request/response pair is the basic unit of interaction.  In this
   pair, the client sends a request and the server replies with a
   response.  All requests must be acknowledged with a response.  This
   requirement implies that all requests can be denied, although it is
   probably futile to attempt to deny a "logout" request.

1.1 Connections

   In some cases, a string of request/response pairs forms a larger
   unit, called a "connection."

   There are three types of connections:

   1) Authenticate only, no connection:

           client:  sends an AUTH packet
           server:  responds with a REPLY

   2) Login connection:

           client:  sends a LOGIN packet
           server:  responds with a REPLY

           repeat zero or more times:
                   client:  sends a CONNECT packet
                   server:  responds with a REPLY

           client:  sends a LOGOUT packet
           server:  responds with a REPLY

   3) SLIP connection:

           client:  sends a LOGIN packet
           server:  responds with a REPLY

           repeat zero or more times:
                   client:  sends a CONNECT packet
                   server:  responds with a REPLY

           client:  sends a SLIPADDR packet
           server:  responds with a REPLY

           repeat zero or more times:
                   client:  sends a CONNECT packet
                   server:  responds with a REPLY

           client:  sends a SLIPON packet
           server:  responds with a REPLY
           client:  sends a LOGOUT packet (immediate)
           server:  responds with a REPLY

           client:  sends a SLIPOFF packet
           server:  responds with a REPLY

1.2 Requests

   This section lists the requests supported by the protocol.  The
   responses are described in the later encodings sections.

   AUTH(username, password, line, style)

      This request asks for an authentication.  The parameters are:

              - the username
              - the password
              - an indication of which line the request is for, and
              - a style of authentication

      The username is a string that identifies the user.  In principle,
      it can be of any length and contain any characters.  In practice,
      it should be no longer than 128 characters and should contain only
      the ASCII characters "!" (33 decimal) through "~" (126 decimal),
      inclusive.

      The password is a string that is used to authenticate the user
      identified by the username.  In principle, it can be of any length
      and contain any characters.  In practice, it should be no longer
      than 128 characters and should contain only the ASCII characters
      "!" (33 decimal) through "~" (126 decimal), inclusive.

      The line is a non-negative decimal integer.  If the client
      supports multiple physical access channels, this value identifies
      the particular channel.  By convention, lines are numbered
      starting from one, although this should be taken with a grain of
      salt.  For example, Cisco Systems' implementation uses zero to
      designate the console port, then continues with one for the "main"
      serial lines. Clients that support only one channel should use
      line zero.

      The authentication style is a possibly empty string.  It
      identifies the particular style of authentication to be performed.
      Its syntax and semantics are local.

      Example:

              AUTH("fin@unet.umn.edu", "fake-password", 0, "staff")

      This specifies a username of "fin@unet.umn.edu" (which happens to
      be my e-mail address), a password, an indication that no line is
      associated with this request, and a style of "staff".  The
      semantics for this style might be that I am required to be a staff
      member (in addition, of course, to supplying a valid username and
      password).  The server would presumably consult an external
      database to verify the staff status.

      As a local option, the implementation may choose to encode the
      style information by using alternate port numbers.  E.g. port 4001
      would mean style 1, 4002 would be style 2, etc.

      Note that the AUTH request type cannot be sent using the UDP
      encoding.

   LOGIN(username, password, line) returns (result1, result2, result3)

      This request asks for an authentication and signals that -- if the
      authentication succeeds -- a login connection is starting.  The
      parameters are:

      - the username
      - the password
      - an indication of which line the request is for

      The meanings of the input fields are the same as the AUTH request.
      If the request is successful, this request returns three result
      values in addition to the success status.  The result values are
      non-negative integers.  Their interpretation is local.  For
      example, Cisco Systems terminal servers interpret result3 to be
      the identifier of a local access list to use for additional
      validation.

   CONNECT(username, password, line, destinationIP, destinationPort)
   returns (result1, result2, result3)

      This request can only be issued when the username and line specify
      an already-existing connection.  As such, no authentication is
      required and the password will in general be the empty string. It
      asks, in the context of that connection, whether a TCP connection
      can be opened to the specified destination IP address and port.

      The return values are as for LOGIN.

   SUPERUSER(username, password, line)

      This request can only be issued when the username and line specify
      an already-existing connection.  As such, no authentication is
      required and the password will in general be the empty string.  It
      asks, in the context of that connection, whether the user can go
      into "super-user" or "enable" mode on the terminal server.

      As an example of the flexibility inherint in this whole scheme,
      the TACACSD supplied by Cisco Systems ignores the username part
      and instead checks wether the password matches that of the special
      user "$enable$".

   LOGOUT(username, password, line, reason)

      This request can only be issued when the username and line specify
      an already-existing connection.  As such, no authentication is
      required and the password will in general be the empty string.  It
      indicates that the connection should be terminated (but see

      SLIPON).  It must be acknowledged, but the success/fail status of
      the acknowledgment is irrelevant.  The reason value indicates why
      the connection is terminating.  A null reason value is supplied
      when the connection is going into SLIP mode.

   SLIPON(username, password, line, SLIPaddress) returns (result1,
   result2, result3)

      This request can only be issued when the username and line specify
      an already-existing connection.  As such, no authentication is
      required and the password will in general be the empty string.  It
      asks, in the context of that connection, whether the specified
      SLIPaddress can be used for the remote end of the connection.

      If the server replies with a success, the client can proceed to a
      SLIPON request.  (It need not do so right away, however.)

      Note that semantics of "username" can get hairy.  For example, the
      Cisco Systems implementation encodes information in this way:

      - If the user just requested the default address be assigned, this
      field holds the username in lower case.

      - If the user requested a specific IP address or host name for the
      SLIP connection, this field contains the requested host name in
      UPPER case.

      If the server replies with a success, the client will immediately
      send a LOGOUT request.  However, the connection will remain
      established until a SLIPOFF request is sent.  No other
      authentication requests will be sent for that connection.

      SLIPaddress specifies the IP address used by the remote host.  If
      a SLIPADDR request has been made, it will be that address.
      Otherwise, it will be the default address assigned by the client
      (e.g., Cisco terminal server).

      The return values are as for LOGIN.

   SLIPOFF(username, password, line, reason)

      This request can only be issued when the username and line specify
      an already-existing connection that is in "SLIP" mode.  As such,
      no authentication is required and the password will in general be
      the empty string.  It indicates that the connection should be
      terminated.  It must be acknowledged, but the success/fail status
      of the acknowledgment is irrelevant.  The reason value indicates
      why the connection is terminating.

2.0 UDP Encoding: TACACS

   This section describes the UDP encoding of the requests that have
   just been described.  It also describes the responses.  This UDP
   encoding forms the basis of the historical TACACS protocol.

   This protocol uses port 49.  This assignment continues to be
   confirmed by the IANA in the Assigned Numbers RFCs.  (I can't say
   that it was assigned by the IANA as the assignment preceded the
   organization.)

   The basic packet format is shown here.  All multi-bytes values are in
   network byte order.  Unless otherwise specified, all values given are
   in decimal.  Unused fields should be set to zero, but the recipient
   should not depend on that setting.

   As was mentioned earlier, there are both simple and extended forms,
   of which the simple form is a proper subset of the extended form.  A
   server should support both.  I will describe both forms in parallel.

   Simple Form

   The fields are:

      offset       length  field

      0    1       version
      1    1       type
      2    2       nonce value
      4    1       username length (to server) / response (to client)
      5    1       password length (to server) / reason (to client)

   in the usual packet layout format:

    0                   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :    Version    :     Type      :             Nonce             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   : User len/Resp : PW len/Reason :            data...            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Extended Form

   The fields are:

           offset  length  field

           0       1       version
           1       1       type
           2       2       nonce value
           4       1       username length
           5       1       password length
           6       1       response
           7       1       reason
           8       4       result1
           12      4       destination host, IP address
           16      2       destination port
           18      2       line
           20      4       result2
           24      2       result3
           26      varies  data: username + password

   in the usual packet layout format:

    0                   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :    Version    :     Type      :             Nonce             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :   User len    : Password len  :   Response    :    Reason     :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                           Result 1                            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                      Destination Address                      :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :           Dest Port           :             Line              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                           Result 2                            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :           Result 3            :            data...            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1 Fields

   The VERSION field specifies the version number.  It must be zero for
   simple or 128 (80 hexadecimal) for extended.

   The TYPE field encodes the request type.  Values are:

           LOGIN           1
           RESPONSE        2       (server to client only)
           CHANGE          3
           FOLLOW          4
           CONNECT         5
           SUPERUSER       6
           LOGOUT          7
           RELOAD          8
           SLIPON          9
           SLIPOFF         10
           SLIPADDR        11

   Other values below 128 are reserved for future use.  Values from 128
   to 255 are reserved for local use.

   Note that the semantics of the CHANGE, FOLLOW, RELOAD requests have
   not been determined.

   The NONCE field is set by the client to an arbitrary value.  Its
   purpose is to allow clients that may have multiple outstanding
   requests to determine which request a response is for. The server
   must copy this value to the reply unaltered.

   The USERNAME LENGTH field is set by the client to the length of the
   username in characters.  Legal values are 0 to 255, inclusive. The
   server must copy this value to the reply unaltered.

   The PASSWORD LENGTH is set by the client to the length of the
   password in characters. Legal values are 0 to 255, inclusive. The
   server must copy this value to the reply unaltered.

   The RESPONSE field should be set by the client to zero.  The server
   sets the value to one of:

           value   meaning

           1       accepted
           2       rejected

   Other values below 128 are reserved for future use.  Values from 128
   to 255 are reserved for local use.

   The REASON field should be set by the client to zero, except for
   LOGOUT and SLIPOFF requests, which may use values of 4, 5, or 6.  The
   server sets the value to one of:

           value   meaning         notes

           0       none            used for ACCEPTED or if the server
                                   is ornery
           1       expiring
           2       password
           3       denied
           4       quit            user quit normally
           5       idle            idle timeout
           6       drop            carrier dropped
           7       bad             too many bad passwords

   The values from 4 to 6 will only be used for reasons for LOGOUT or
   SLIPOFF requests: they will not be returned by the server.  Other
   values below 128 are reserved for future use.  Values from 128 to 255
   are reserved for local use.

   The RESULT1 field should be set by the client to zero.  For LOGIN or
   CONNECT requests, it is set by the server as specified in the request
   description.  For all other requests, it should be set by the server
   to zero.

   The DESTINATION HOST field is set by the client.  On CONNECT, SLIPON,
   and SLIPOFF requests it specifies an IP address.  It should be set to
   zero on all other requests.  For SLIPON and SLIPOFF request, this
   value should be the IP address assigned to the line.  For CONNECT
   requests, this value is the IP address of the host that the user is
   attempting to connect to.  The server copies this value to the reply.

   The DESTINATION PORT field is set by the client.  On CONNECT requests
   it specifies the port number that the user is attempting to connect
   to.  It should be set to zero on all other requests. The server
   copies this value to the reply.

   The LINE field is set by the client to the line number that the
   request is for.  The server copies this value to the reply.

   The RESULT2 field should be set by the client to zero.  For LOGIN or
   CONNECT requests, it is set by the server as specified in the request
   description.  For all other requests, it should be set by the server
   to zero.

   The RESULT3 field should be set by the client to zero.  For LOGIN or
   CONNECT requests, it is set by the server as specified in the request

   description.  For all other requests, it should be set by the server
   to zero.

   The DATA field contains just the text of the username and password,
   with no separator characters (you use username length and password
   length to sort them out).  The server does not copy the values to the
   reply. (However, the server does copy the username length and
   password length fields to the reply.) The username data may be in
   upper case: comparisons should be case-insensitive.

2.2 What a Client Does

   The client must format and send a UDP request to port 49.  It
   constructs the request by following these steps:

           - set the version to 128
           - set the type to that of the request
           - set the nonce to a unique value that is different from all
             outstanding requests
           - set the username length
           - set the password length
           - set the response to zero
           - set the reason to zero (except for LOGOUT and SLIPOFF)
           - set the result1 to zero
           - if CONNECT, SLIPON, or SLIPOFF, set the destination address
             to the IP address, otherwise set it to zero
           - if CONNECT, set the destination port to the port, otherwise
             set it to zero
           - set the line
           - set the result2 to zero
           - set the result3 to zero
           - copy the username to the location just after result3
           - copy the password to the location just after the end of the
             username

   Send the request.  Wait for a reasonable (and hopefully configurable)
   period of time.  If no response has been received, retry a reasonable
   (and hopefully configurable) number of times.  Reasonable default
   wait times are 5 seconds and retries are 2.

   If a response has been received, use the nonce value (and as many
   other fields as you like) to match it to an outstanding request.  If
   there is no matching outstanding request, take appropriate (and
   hopefully configurable) action such as discarding and/or logging the
   packet.

   If the response matches an outstanding request, examine the response
   and reason codes and take whatever action you deem correct.  For

   responses to LOGIN and CONNECT requests, also incorporate the
   result1, result2, and result3 values as you deem correct.

2.3 What a Server Does

   Upon receipt of a UDP format request, the server examines the data in
   the request packet and determines its response.  It constructs the
   reply by following these steps:

           - set the version to 128
           - set the type to RESPONSE (2)
           - copy the nonce value
           - copy the username length value
           - copy the password length value
           - set the response value to the desired response
           - set the reason value to the desired reason
           - if LOGIN or CONNECT, set the result1 else zero the result1
           - copy the destination host value
           - copy the destination port value
           - copy the line value
           - if LOGIN or CONNECT, set the result2 else zero the result2
           - if LOGIN or CONNECT, set the result3 else zero the result3
           - do NOT copy the username or password data

   (As always, be liberal in what you expect and conservative in what
   you send.) Send the response.  Do not attempt to retry, as you have
   no basis for determining whether a retry is required.  Any retries
   are up to the client.  This, of course, implies that requests are
   idempotent.  They aren't, of course, so the retries must be
   considered when trying to assemble requests into connections.

3.0 TCP Encoding

   This section describes the TCP encoding of the requests and
   responses.  This encoding is not compatible with the historical
   TACACS protocol.  However, it is somewhat more "modern" in that it
   has been updated to provide for current feature needs.

   This protocol does not use a reserved port.  Instead, it must be
   possible to configure the ports used by both the the client and
   server.

   The basic request format is shown here.  The request consists of four
   lines of ASCII text.  All numeric values are expressed in ASCII as
   decimal integers.

           <version> <type> <parameters>
           <username>
           <password>
           <line>

   Each line in the example corresponds to one line of text.  That is,
   the lines are separated with <CR>/<LF> (13/10 decimal) pairs.  In no
   event may "bare" <CR> or <LF> characters appear within a field.  In
   addition, <NUL> (0 decimal) characters may not be sent.

   The <version> and <type> fields are separated with one or more
   <SPACE> (32 decimal) or <TAB> (9 decimal) characters.

   The <parameters> field is optional.  If present, it is separated from
   the <type> field and internal parameters separated from each other by
   or more <SPACE> or <TAB> characters.  Any trailing <SPACE> or <TAB>
   characters present on this line should be ignored by the server: they
   should not be taken to imply a trailing empty field.

   In theory there are no line length limits.  In practice, lines should
   not exceed 255 characters (counting the <CR> and <LF>) and probably
   should be 80 characters or less.

3.1 Fields

   The VERSION field specifies the version number.  It must be 1.  Other
   values below 128 are reserved for future use.  Values from 128 to 255
   are reserved for local use.

   The TYPE field encodes the request type.  Values are:

           AUTH
           LOGIN
           CONNECT
           SUPERUSER
           LOGOUT
           SLIPON
           SLIPOFF

   I.e., the keyword simply encodes itself.  It must be in upper case.
   Keywords that begin with the letter "X" are reserved for local use.

   The USERNAME field contains the text of the username.  Leading and
   trailing <SPACE> or <TAB> characters are considered significant.  The
   username data may be in upper case: comparisons should be case-
   insensitive.

   The PASSWORD field contains the text of the password.  Leading and

   trailing <SPACE> or <TAB> characters are considered significant.

   The LINE field is set by the client to the line number that the
   request is for.

3.2 Responses

   Appendix E of STD 10, RFC 821 describes the general theory of reply
   codes.  The this protocol follows the format described in that
   document.  In a nutshell, replies are of the form:

           <number> <text>

   Where <number> is a three-digit decimal value and <text> is an
   arbitrary text string, presumably containing only printing text
   characters (<SP> (32 decimal) through "~" (126 decimal)).  At least
   one <SP> (32 decimal) character separates the number from the text.
   A <CR>/<LF> sequence follows the text.

   The three digit codes completely determine the response.  The text
   should be considered an explanatory comment for human understanding.
   However, even without knowing all values, the first digit can be used
   to determine the overall nature of the response.  The encodings are:

           1       Positive Preliminary: the request is acceptable,
                   but no action will be taken until an additional
                   request is made (not used in this version of the
                   protocol)

           2       Positive Completion

           3       Positive Intermediate: the request is acceptable
                   so far, but has not been completely transferred
                   (not used in this version of the protocol)

           4       Transient Negative: the request is not acceptable
                   for now.  It is acceptable to retry, as another
                   instance may have a different result.

           5       Permanent Negative: the request is not acceptable

   The text portion is optional (i.e., may be the empty string) and it
   describes the meaning of the message in human readable form.

   While different server implementations will result in different
   messages, the following are suggested:

           201 accepted: # # #
           202 accepted, password is expiring: # # #
           401 no response; retry
           501 invalid format
           502 access denied

   The ": # # #" in the first two messages is the suggested way of
   returning the three result codes for LOGIN and CONNECT requests.

3.3 What a Client Does

   The client opens a TCP connection to the locally-configured address
   and port.  It sends the request by sending:

           - the character "1"
           - one or more <SPACE> or <TAB> characters
           - the request type as an ASCII string
           - if an AUTH, send one or more <SPACE> or <TAB> characters
             and the authentication style
           - if a CONNECT, SLIPON, or SLIPOFF, send one or more <SPACE>
             or <TAB> characters and the IP address in dotted decimal
             notation
           - if a CONNECT, send one or more <SPACE> or <TAB> characters
             and the port number in decimal
           - a <CR>/<LF>
           - the username (or hostname for SLIPADDR)
           - a <CR>/<LF>
           - the password
           - a <CR>/<LF>
           - the line
           - a <CR>/<LF>

   Then read one line from the connection and close the connection.
   This encoding lets TCP take care of waiting, retries, and matching up
   requests and responses.

   Examine the response line and take whatever action you deem correct.

3.4 What the Server Does

   The server waits on the locally-specified port for requests.  When
   one is made, it reads four lines of input.

   It examines the first line for a valid version number and request.
   It also records any optional parameters.

   It uses the username, password, and line number along with any other
   information it deems fit to determine its response.

   It then sends exactly one line of response, terminated by a
   <CR>/<LF>, and closes the connection.

4.0 Pros and Cons

   Advantages to using the UDP format:

           - lower overhead
           - compatible with historical standard
           - some existing equipment supports it

   Advantages to using the TCP format:

           - easier to implement, especially on machines with no or
             poor UDP support
           - simpler, cleaner syntax
           - potentially wider range of error codes, and support for
             temporary and negotiated authentication sequences

5.0 Security Notes

   While the protocol itself has been described, there are a number of
   other considerations worth mentioning.

   First, the protocol carries the username and password in clear text
   in either a single UDP packet or a TCP stream.  As such, if an
   attacker is capable of monitoring that data, the attacker could
   capture username/password pairs.  Implementations can take several
   steps to minimize this danger:

   - Use point-to-point links where possible.

   - Physically secure the transmission medium.

   - If packets must traverse multiple network segments, use a secure
   routing subsystem.  This implies:

           - Tight control over router configurations.
           - Tight control over routing protocols.
           - Avoid use of bridges, as they can be silently fooled into
             duplicating packets.

   Second, this protocol potentially opens up a new way of probing
   usernames and passwords.  Thus, implementations may wish to have

   servers:

           - limit responses to a controlled list of clients,
           - throttle the rate of responding to requests,
           - log all failures (and possibly successes, too).

   Third, this protocol essentially allows clients to offload
   accept/reject decisions to servers.  While an obvious implementation
   would simply use the server's native login mechanism to make the
   determination, there is no reason to limit implementations to that
   mechanism. Servers could:

           - use alternate lists of accounts (e.g., password files),
           - use alternate mechanisms for accessing the accounts (e.g.,
             a database, NIS),
           - use alternate algorithms (e.g., SecureID cards),
           - translate the request to another protocol and use that
             protocol to make the determination (e.g., Kerberos).

   Fourth, the use of a "fanout" server (described in the next section)
   allows for:

           - centralized logging of usage for attack analysis
           - centralized policy:

                   - ability to block selected specific users
                   - ability to block selected user names (e.g., don't
                     allow "root" or "guest")
                   - ability to block poor passwords (e.g., none or weak)

6.0 Case Study

   This section presents the basic steps used by the implementation at
   the University of Minnesota.  Two examples will be used.  The first
   is a basic terminal login.  The second is a database access
   verification.

   Usernames are in one of three forms:

           First.M.Last-#@umn.edu
           First.M.Last-#
           user@host

   A name in the first form is converted to one in the second.

   A name in the second form is looked up in the University-wide
   directory system.  If found, the associated electronic mail address
   is treated as if the third form was entered.

   The third form specifics the name of a computer whose manager has
   agreed to perform validations and the name of an account on that
   computer.

   The system that we use allows for many requesting clients (typically
   modem pools).  Further, each client can support multiple distinct
   pools of users.  For example, lines 1-20 could be general access, but
   lines 21-25 could be 800-numbers with a restricted set of valid
   users.  The system supports this distinction by specifying which
   validation computers are legal for each modem pool.

6.1 Terminal Login

   On the Cisco Terminal Server:

   - accept a connection
   - request a username and password
   - pack the request into a UDP TACACS packet and send to the central
     fanout

   Central Fanout:

   - accept a request
   - if the request is not in a valid format, return "nope"
   - log the request
   - if the source IP address is not in a list of valid clients,
     status = "nope"
   - else if the username contains invalid characters, status = "nope"
   - else
           if the username is of the form First.M.Last-#@umn.edu,
                   convert to First.M.Last-#
           if the username is of the form First.M.Last-#,
                   look up the name in the directory
                   if the name is not found, status = "nope"
                   otherwise, use the e-mail address as the username

           if the user is on a special "blocked" list, status = "nope"
                   and send mail warning that access to a blocked
                   account was attempted
           split the username into user and host parts
           if the host is not on a list of known servers,
                   status = "nope"
           else if the host is not allowed to validate this type of
                   request for this pool, status = "nope"

           now format a request for validation of the user and send it
           to the specified host

           if no response, status = "nope"
           otherwise set the status to the returned status

   - log what response is going to be returned
   - return the response

   Validation Host:

   This machine can run a "stripped down" version of the central fanout.
   It need perform no special validation or logging, with one exception.

   - accept a request
   - if the request is not in a valid format, return "nope"
   - if the request is not from the central fanout, return "nope"
   - figure the return status
   - return the response

6.2 Database Access Verification

   In this example, assume that a database is only to be accessed by
   faculty and staff.

   Mainframe:

   - the user is on the mainframe and makes a request
   - the program requests username and password
   - the program packs the request into a UDP TACACS packet and send to
     the central fanout

   Central Fanout:

   - accept a request
   - if the request is not in a valid format, return "nope"
   - log the request
   - if the source IP address is not in a list of valid clients,
     status = "nope"
   - else if the username contains invalid characters, status = "nope"
   - else
           if the username is of the form First.M.Last-#@umn.edu,
                   convert to First.M.Last-#
           if the username is of the form First.M.Last-#,
                   look up the name in the directory
                   if the name is not found, status = "nope"
                   otherwise, use the e-mail address as the username
                      and obtain the staff status from the directory
           if the user is on a special "blocked" list, status = "nope"
                   and send mail warning that access to a blocked
                   account was attempted

           split the username into user and host parts
           if the host is not on a list of known servers,
                   status = "nope"
           else if the host is not allowed to validate this type of
                   request for this pool, status = "nope"

           now format a request for validation of the user and send it
           to the specified host

           if no response or status is "nope", status = "nope"

           else if the user originally gave a user@host mail address,
                   do a directory lookup and obtain the staff status

           set the status to the staff status
   - log what response is going to be returned
   - return the response

   Note that the validation host is unchanged.

References

   [RFC821] Postel, J. "Simple Mail Transfer Protocol", STD 10, RFC 821,
   USC/Information Sciences Institute, August 1982.

   [RFC1340] Reynolds, J. and J. Postel, "Assigned Numbers," STD 2, RFC
   1340, USC/Information Sciences Institute, July 1992.

   Anderson, Brian; Ruth, Greg; Ditmars, Peter; Eisner, Sharon;
   Delsignore, John (1985) TAC Access Control System Protocols, Second
   Edition: August 16 1985. BBN Tech Memo CC-0045.

   Cisco Systems, Inc. (September 1992) Communications Server
   Configuration and Reference.  Menlo Park, California.

Security Considerations

   Security issues are the main topic of this memo.

Author's Address

   Craig A. Finseth
   Networking Services
   Computer and Information Services
   University of Minnesota
   130 Lind Hall
   207 Church St SE
   Minneapolis MN 55455-0134

   Phone: +1 612 624 3375
   Fax:   +1 612 626 1002

   EMail: Craig.A.Finseth-1@umn.edu, or
          fin@unet.umn.edu

 

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