Provided by: dhcp3-common_3.0.3-6ubuntu7_i386 bug

NAME

       dhcp-eval - ISC DHCP conditional evaluation

DESCRIPTION

       The Internet Systems Consortium DHCP client and server both provide the
       ability to perform conditional behavior depending on  the  contents  of
       packets  they  receive.    The  syntax  for specifying this conditional
       behaviour is documented here.

REFERENCE: CONDITIONAL BEHAVIOUR

       Conditional behaviour is specified using the if statement and the  else
       or elsif statements.   A conditional statement can appear anywhere that
       a regular statement (e.g., an option statement)  can  appear,  and  can
       enclose  one or more such statements.   A typical conditional statement
       in a server might be:

       if option dhcp-user-class = "accounting" {
         max-lease-time 17600;
         option domain-name "accounting.example.org";
         option domain-name-servers ns1.accounting.example.org,
                           ns2.accounting.example.org;
       } elsif option dhcp-user-class = "sales" {
         max-lease-time 17600;
         option domain-name "sales.example.org";
         option domain-name-servers ns1.sales.example.org,
                           ns2.sales.example.org;
       } elsif option dhcp-user-class = "engineering" {
         max-lease-time 17600;
         option domain-name "engineering.example.org";
         option domain-name-servers ns1.engineering.example.org,
                           ns2.engineering.example.org;
       } else {
         max-lease-time 600;
         option domain-name "misc.example.org";
         option domain-name-servers ns1.misc.example.org,
                           ns2.misc.example.org;
       }

       On the client side, an example of conditional evaluation might be:

       # example.org filters DNS at its firewall, so we have to use their DNS
       # servers when we connect to their network.   If we are not at
       # example.org, prefer our own DNS server.
       if not option domain-name = "example.org" {
         prepend domain-name-servers 127.0.0.1;
       }

       The if statement and the elsif continuation statement both take boolean
       expressions  as their arguments.   That is, they take expressions that,
       when evaluated, produce a boolean result.   If the expression evaluates
       to  true,  then  the  statements  enclosed  in  braces following the if
       statement are executed, and all subsequent elsif and else  clauses  are
       skipped.    Otherwise,  each  subsequent  elsif  clause’s expression is
       checked, until an elsif clause is encountered whose test  evaluates  to
       true.    If  such a clause is found, the statements in braces following
       it are executed, and then any subsequent elsif  and  else  clauses  are
       skipped.    If  all  the  if  and elsif clauses are checked but none of
       their expressions evaluate true, then if there is an else  clause,  the
       statements  enclosed  in  braces  following  the  else  are  evaluated.
       Boolean expressions that evaluate to  null  are  treated  as  false  in
       conditionals.

BOOLEAN EXPRESSIONS

       The  following  is  the  current  list  of boolean expressions that are
       supported by the DHCP distribution.

       data-expression-1 = data-expression-2

          The  =  operator  compares  the  values  of  two  data  expressions,
          returning  true  if  they  are the same, false if they are not.   If
          either the left-hand side or  the  right-hand  side  are  null,  the
          result is also null.

       boolean-expression-1 and boolean-expression-2

          The  and operator evaluates to true if the boolean expression on the
          left-hand side and the boolean expression  on  the  right-hand  side
          both evaluate to true.  Otherwise, it evaluates to false.  If either
          the expression on the left-hand side or the expression on the right-
          hand side are null, the result is null.

       boolean-expression-1 or boolean-expression-2

          The  or  operator evaluates to true if either the boolean expression
          on the left-hand side or the boolean expression  on  the  right-hand
          side evaluate to true.  Otherwise, it evaluates to false.  If either
          the expression on the left-hand side or the expression on the right-
          hand side are null, the result is null.

       not boolean-expression

          The  not  operator evaluates to true if boolean-expression evaluates
          to false, and returns false if boolean-expression evaluates to true.
          If boolean-expression evaluates to null, the result is also null.

       exists option-name

          The exists expression returns true if the specified option exists in
          the incoming DHCP packet being processed.
       known

          The known expression returns true if the  client  whose  request  is
          currently  being  processed  is  known  - that is, if there’s a host
          declaration for it.
       static

          The static expression returns true if  the  lease  assigned  to  the
          client  whose request is currently being processed is derived from a
          static address assignment.

DATA EXPRESSIONS

       Several of the boolean expressions  above  depend  on  the  results  of
       evaluating  data expressions.   A list of these expressions is provided
       here.

       substring (data-expr, offset, length)

          The substring operator evaluates the data expression and returns the
          substring  of the result of that evaluation that starts offset bytes
          from the beginning, continuing for length bytes.  Offset and  length
          are  both  numeric  expressions.   If  data-expr,  offset  or length
          evaluate to null, then the  result  is  also  null.   If  offset  is
          greater  than  or  equal to the length of the evaluated data, then a
          zero-length data string is returned.  If length is greater then  the
          remaining  length  of  the  evaluated data after offset, then a data
          string containing all data from offset to the end of  the  evaluated
          data is returned.

       suffix (data-expr, length)

          The  suffix operator evaluates data-expr and returns the last length
          bytes of  the  result  of  that  evaluation.  Length  is  a  numeric
          expression.   If  data-expr  or  length  evaluate  to null, then the
          result is also null.  If suffix evaluates to a number  greater  than
          the  length  of  the  evaluated  data,  then  the  evaluated data is
          returned.

       option option-name

          The option operator returns the contents of the specified option  in
          the packet to which the server is responding.

       config-option option-name

          The  config-option  operator  returns  the  value  for the specified
          option that the DHCP client or server has been configured to send.

       hardware

          The hardware operator returns a data string whose first  element  is
          the  type of network interface indicated in packet being considered,
          and whose subsequent elements are client’s link-layer address.    If
          there  is  no  packet, or if the RFC2131 hlen field is invalid, then
          the result is null.   Hardware types include  ethernet  (1),  token-
          ring  (6), and fddi (8).   Hardware types are specified by the IETF,
          and details on how the type numbers are  defined  can  be  found  in
          RFC2131  (in the ISC DHCP distribution, this is included in the doc/
          subdirectory).

       packet (offset, length)

          The packet operator returns the  specified  portion  of  the  packet
          being  considered,  or  null  in  contexts  where no packet is being
          considered.   Offset and length are applied to the  contents  packet
          as in the substring operator.

       string

          A string, enclosed in quotes, may be specified as a data expression,
          and returns the text between the quotes,  encoded  in  ASCII.    The
          backslash (’\’) character is treated specially, as in C programming:
          ’\t’ means TAB, ’\r’ means carriage return, ’\n’ means newline,  and
          ’\b’  means  bell.    Any  octal value can be specified with ’\nnn’,
          where nnn  is  any  positive  octal  number  less  than  0400.   Any
          hexadecimal  value  can  be  specified  with ’\xnn’, where nn is any
          positive hexadecimal number less than or equal to 0xff.

       colon-separated hexadecimal list

          A list of hexadecimal octet values,  separated  by  colons,  may  be
          specified as a data expression.

       concat (data-expr1, ..., data-exprN)
          The  expressions  are  evaluated, and the results of each evaluation
          are concatenated in the sequence that the subexpressions are listed.
          If   any   subexpression  evaluates  to  null,  the  result  of  the
          concatenation is null.

       reverse (numeric-expr1, data-expr2)
          The two expressions are evaluated, and then the result of evaluating
          the  data  expression  is reversed in place, using hunks of the size
          specified in the numeric expression.   For example, if  the  numeric
          expression  evaluates  to four, and the data expression evaluates to
          twelve bytes of data, then the reverse expression will  evaluate  to
          twelve  bytes  of data, consisting of the last four bytes of the the
          input data, followed by the middle four bytes, followed by the first
          four bytes.

       leased-address
          In any context where the client whose request is being processed has
          been assigned an IP address, this data expression  returns  that  IP
          address.

       binary-to-ascii (numeric-expr1, numeric-expr2, data-expr1, data-expr2)
          Converts  the  result  of  evaluating  data-expr2 into a text string
          containing one number for each element of the result  of  evaluating
          data-expr2.    Each number is separated from the other by the result
          of evaluating data-expr1.   The result of  evaluating  numeric-expr1
          specifies  the  base (2 through 16) into which the numbers should be
          converted.   The result of evaluating  numeric-expr2  specifies  the
          width in bits of each number, which may be either 8, 16 or 32.

          As  an  example  of  the  preceding  three  types of expressions, to
          produce the name of a PTR record for the IP address  being  assigned
          to a client, one could write the following expression:

               concat (binary-to-ascii (10, 8, ".",
                                        reverse (1, leased-address)),
                       ".in-addr.arpa.");

       encode-int (numeric-expr, width)
          Numeric-expr  is  evaluated  and  encoded  as  a  data string of the
          specified width,  in  network  byte  order  (most  significant  byte
          first).   If the numeric expression evaluates to the null value, the
          result is also null.

       pick-first-value (data-expr1 [ ... exprn ] )
          The pick-first-value function takes any number of  data  expressions
          as  its arguments.   Each expression is evaluated, starting with the
          first in the list, until  an  expression  is  found  that  does  not
          evaluate to a null value.   That expression is returned, and none of
          the subsequent  expressions  are  evaluated.    If  all  expressions
          evaluate to a null value, the null value is returned.

       host-decl-name
          The host-decl-name function returns the name of the host declaration
          that matched the client whose request is currently being  processed,
          if  any.    If  no  host declaration matched, the result is the null
          value.

NUMERIC EXPRESSIONS

       Numeric expressions are expressions that evaluate to an  integer.    In
       general,  the  maximum size of such an integer should not be assumed to
       be representable in fewer than 32  bits,  but  the  precision  of  such
       integers may be more than 32 bits.

       extract-int (data-expr, width)

          The  extract-int  operator extracts an integer value in network byte
          order from the result of evaluating the specified  data  expression.
          Width  is  the  width in bits of the integer to extract.  Currently,
          the only supported widths are 8, 16 and 32.   If the  evaluation  of
          the  data  expression  doesn’t provide sufficient bits to extract an
          integer of the specified size, the null value is returned.

       lease-time

          The duration of the current lease - that is, the difference  between
          the current time and the time that the lease expires.

       number

          Any  number  between  zero and the maximum representable size may be
          specified as a numeric expression.

       client-state

          The current state of the client instance being processed.   This  is
          only  useful  in  DHCP client configuration files.   Possible values
          are:

           Booting - DHCP client is in the INIT state, and does not yet  have
            an   IP   address.    The  next  message  transmitted  will  be  a
            DHCPDISCOVER, which will be broadcast.

           Reboot - DHCP client is in the INIT-REBOOT state.   It has  an  IP
            address,  but  is  not  yet  using  it.    The  next message to be
            transmitted will be a DHCPREQUEST, which will be  broadcast.    If
            no response is heard, the client will bind to its address and move
            to the BOUND state.

           Select - DHCP client is in the SELECTING state - it  has  received
            at  least  one  DHCPOFFER message, but is waiting to see if it may
            receive other DHCPOFFER messages from other servers.   No messages
            are sent in the SELECTING state.

           Request - DHCP client is in the REQUESTING state - it has received
            at least one DHCPOFFER message, and has chosen which one  it  will
            request.    The  next  message  to  be  sent will be a DHCPREQUEST
            message, which will be broadcast.

           Bound - DHCP client is in the BOUND state - it has an IP  address.
            No messages are transmitted in this state.

           Renew  -  DHCP  client  is  in  the  RENEWING state - it has an IP
            address, and is trying to contact the server to  renew  it.    The
            next  message to be sent will be a DHCPREQUEST message, which will
            be unicast directly to the server.

           Rebind - DHCP client is in the REBINDING state  -  it  has  an  IP
            address,  and  is  trying to contact any server to renew it.   The
            next message to be sent will  be  a  DHCPREQUEST,  which  will  be
            broadcast.

REFERENCE: LOGGING

       Logging  statements  may  be  used  to send information to the standard
       logging channels.  A logging statement includes  an  optional  priority
       (fatal, error, info, or debug), and a data expression.

       log (priority, data-expr)

       Logging  statements  take only a single data expression argument, so if
       you want to output multiple data values,  you  will  need  to  use  the
       concat operator to concatenate them.

REFERENCE: DYNAMIC DNS UPDATES

       The  DHCP  client and server have the ability to dynamically update the
       Domain Name System.  Within the configuration files, you can define how
       you  want  the Domain Name System to be updated.  These updates are RFC
       2136 compliant so any DNS server supporting RFC 2136 should be able  to
       accept updates from the DHCP server.

SECURITY

       Support  for  TSIG  and DNSSEC is not yet available.  When you set your
       DNS server up to allow updates from the DHCP server or client, you  may
       be  exposing  it  to unauthorized updates.  To avoid this, the best you
       can do right now is to use IP address-based packet filtering to prevent
       unauthorized  hosts  from submitting update requests.  Obviously, there
       is currently no way to provide security for client updates - this  will
       require  TSIG  or DNSSEC, neither of which is yet available in the DHCP
       distribution.

       Dynamic DNS (DDNS)  updates  are  performed  by  using  the  dns-update
       expression.   The  dns-update  expression  is a boolean expression that
       takes four parameters.  If the update succeeds, the result is true.  If
       it  fails,  the  result is false.  The four parameters that the are the
       resource record type (RR), the left hand side of the RR, the right hand
       side  of  the RR and the ttl that should be applied to the record.  The
       simplest example of the use  of  the  function  can  be  found  in  the
       reference  section  of the dhcpd.conf file, where events are described.
       In this example several statements are being used to make the arguments
       to the dns-update.

       In  the example, the first argument to the first Bdns-update expression
       is a data expression that evaluates to  the  A  RR  type.   The  second
       argument is constructed by concatenating the DHCP host-name option with
       a  text  string   containing   the   local   domain,   in   this   case
       "ssd.example.net".  The third argument is constructed by converting the
       address the client has been assigned from a 32-bit number into an ascii
       string  with  each  byte  separated by a ".".  The fourth argument, the
       TTL, specifies the amount of time remaining in  the  lease  (note  that
       this  isn’t really correct, since the DNS server will pass this TTL out
       whenever a request comes in, even if that is only a few seconds  before
       the lease expires).

       If  the  first  dns-update statement succeeds, it is followed up with a
       second update to install a PTR RR.  The installation of a PTR record is
       similar  to  installing  an  A RR except that the left hand side of the
       record  is  the  leased   address,   reversed,   with   ".in-addr.arpa"
       concatenated.   The  right hand side is the fully qualified domain name
       of the client to which the address is being leased.

SEE ALSO

       dhcpd.conf(5),   dhcpd.leases(5),    dhclient.conf(5),    dhcp-eval(5),
       dhcpd(8), dhclient(8), RFC2132, RFC2131.

AUTHOR

       The  Internet  Systems  Consortium DHCP Distribution was written by Ted
       Lemon under a contract with Vixie Labs.  Funding for this  project  was
       provided   through  Internet  Systems  Consortium.   Information  about
       Internet Systems Consortium can be found at http://www.isc.org.

                                                                  dhcp-eval(5)