


dhcp-eval(5)                                         dhcp-eval(5)


NNAAMMEE
       dhcp-eval - ISC DHCP conditional evaluation

DDEESSCCRRIIPPTTIIOONN
       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.

RREEFFEERREENNCCEE:: CCOONNDDIITTIIOONNAALL BBEEHHAAVVIIOOUURR
       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 iiff statement and the eellssiiff 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 iiff state-
       ment are executed, and  all  subsequent  eellssiiff  and  eellssee  clauses  are
       skipped.    Otherwise,  each  subsequent  eellssiiff  clause's expression is
       checked, until an elsif clause is encountered whose test  evaluates  to



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dhcp-eval(5)                                         dhcp-eval(5)


       true.    If  such a clause is found, the statements in braces following
       it are executed, and then any subsequent eellssiiff  and  eellssee  clauses  are
       skipped.    If  all  the  iiff  and eellssiiff clauses are checked but none of
       their expressions evaluate true, then if there is an eellssee  clause,  the
       statements  enclosed  in  braces  following  the  eellssee  are  evaluated.
       Boolean expressions that evaluate to null are treated as false in  con-
       ditionals.

BBOOOOLLEEAANN EEXXPPRREESSSSIIOONNSS
       The  following is the current list of boolean expressions that are sup-
       ported by the DHCP distribution.

       _d_a_t_a_-_e_x_p_r_e_s_s_i_o_n_-_1 == _d_a_t_a_-_e_x_p_r_e_s_s_i_o_n_-_2

          The == operator compares the values of two data expressions,  return-
          ing  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.

       _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n_-_1 aanndd _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n_-_2

          The  aanndd 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.

       _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n_-_1 oorr _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n_-_2

          The  oorr  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.

       nnoott _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n

          The  nnoott  operator evaluates to true if _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n evaluates
          to false, and returns false if _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n evaluates to true.
          If _b_o_o_l_e_a_n_-_e_x_p_r_e_s_s_i_o_n evaluates to null, the result is also null.

       eexxiissttss _o_p_t_i_o_n_-_n_a_m_e

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

          The kknnoowwnn expression returns true if the  client  whose  request  is
          currently being processed is known - that is, if there's a host dec-
          laration for it.
       ssttaattiicc

          The ssttaattiicc expression returns true if  the  lease  assigned  to  the
          client  whose request is currently being processed is derived from a



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dhcp-eval(5)                                         dhcp-eval(5)


          static address assignment.

DDAATTAA EEXXPPRREESSSSIIOONNSS
       Several of the boolean expressions above depend on the results of eval-
       uating  data  expressions.    A  list  of these expressions is provided
       here.

       ssuubbssttrriinngg ((_d_a_t_a_-_e_x_p_r,, _o_f_f_s_e_t,, _l_e_n_g_t_h))

          The ssuubbssttrriinngg operator evaluates the data expression and returns the
          substring  of the result of that evaluation that starts _o_f_f_s_e_t bytes
          from the beginning, continuing for _l_e_n_g_t_h bytes.  _O_f_f_s_e_t and  _l_e_n_g_t_h
          are both numeric expressions.  If _d_a_t_a_-_e_x_p_r, _o_f_f_s_e_t or _l_e_n_g_t_h evalu-
          ate to null, then the result is also null.   If  _o_f_f_s_e_t  is  greater
          than  or  equal  to  the  length of the evaluated data, then a zero-
          length data string is returned.   If  _l_e_n_g_t_h  _i_s  _g_r_e_a_t_e_r  _t_h_e_n  _t_h_e
          _r_e_m_a_i_n_i_n_g  _l_e_n_g_t_h  _o_f  _t_h_e  _e_v_a_l_u_a_t_e_d _d_a_t_a _a_f_t_e_r _o_f_f_s_e_t, then a data
          string containing all data from _o_f_f_s_e_t to the end of  the  evaluated
          data is returned.

       ssuuffffiixx ((_d_a_t_a_-_e_x_p_r,, _l_e_n_g_t_h))

          The  ssuuffffiixx operator evaluates _d_a_t_a_-_e_x_p_r and returns the last _l_e_n_g_t_h
          bytes of the result of that evaluation. _L_e_n_g_t_h is a numeric  expres-
          sion.   If  _d_a_t_a_-_e_x_p_r or _l_e_n_g_t_h evaluate to null, then the result is
          also null.  If _s_u_f_f_i_x evaluates to a number greater than the  length
          of the evaluated data, then the evaluated data is returned.

       llccaassee ((_d_a_t_a_-_e_x_p_r))

          The  llccaassee  function returns the result of evaluating _d_a_t_a_-_e_x_p_r con-
          verted to lower case.   If _d_a_t_a_-_e_x_p_r evaluates  to  null,  then  the
          result is also null.

       uuccaassee ((_d_a_t_a_-_e_x_p_r))

          The  uuccaassee  function returns the result of evaluating _d_a_t_a_-_e_x_p_r con-
          verted to upper case.   If _d_a_t_a_-_e_x_p_r evaluates  to  null,  then  the
          result is also null.

       ooppttiioonn _o_p_t_i_o_n_-_n_a_m_e

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

       ccoonnffiigg--ooppttiioonn _o_p_t_i_o_n_-_n_a_m_e

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

       hhaarrddwwaarree

          The  hhaarrddwwaarree  operator returns a data string whose first element is
          the type of network interface indicated in packet being  considered,



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dhcp-eval(5)                                         dhcp-eval(5)


          and  whose subsequent elements are client's link-layer address.   If
          there is no packet, or if the RFC2131 _h_l_e_n 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).

       ppaacckkeett ((_o_f_f_s_e_t,, _l_e_n_g_t_h))

          The  ppaacckkeett  operator  returns  the  specified portion of the packet
          being considered, or null in contexts where no packet is being  con-
          sidered.    _O_f_f_s_e_t  and _l_e_n_g_t_h are applied to the contents packet as
          in the ssuubbssttrriinngg operator.

       _s_t_r_i_n_g

          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 hexadec-
          imal value can be specified with '\xnn', where nn  is  any  positive
          hexadecimal number less than or equal to 0xff.

       _c_o_l_o_n_-_s_e_p_a_r_a_t_e_d _h_e_x_a_d_e_c_i_m_a_l _l_i_s_t

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

       ccoonnccaatt ((_d_a_t_a_-_e_x_p_r_1,, ......,, _d_a_t_a_-_e_x_p_r_N))
          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 concatena-
          tion is null.

       rreevveerrssee ((_n_u_m_e_r_i_c_-_e_x_p_r_1,, _d_a_t_a_-_e_x_p_r_2))
          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.

       lleeaasseedd--aaddddrreessss
          In any context where the client whose request is being processed has
          been  assigned  an  IP address, this data expression returns that IP
          address.  In any context where the client  whose  request  is  being
          processed  has not been assigned an ip address, if this data expres-
          sion is found in executable statements  executed  on  that  client's
          behalf,  a log message indicating "there is no lease associated with



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dhcp-eval(5)                                         dhcp-eval(5)


          this client" is syslogged to the debug  level  (this  is  considered
          dhcpd.conf debugging information).

       bbiinnaarryy--ttoo--aasscciiii ((_n_u_m_e_r_i_c_-_e_x_p_r_1,, _n_u_m_e_r_i_c_-_e_x_p_r_2,, _d_a_t_a_-_e_x_p_r_1,, _d_a_t_a_-_e_x_p_r_2))
          Converts the result of evaluating data-expr2 into a text string con-
          taining 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 pro-
          duce 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.");


       eennccooddee--iinntt ((_n_u_m_e_r_i_c_-_e_x_p_r,, _w_i_d_t_h))
          Numeric-expr is evaluated and encoded as a data string of the speci-
          fied width, in network byte order (most significant byte first).  If
          the  numeric  expression  evaluates to the null value, the result is
          also null.

       ppiicckk--ffiirrsstt--vvaalluuee ((_d_a_t_a_-_e_x_p_r_1 [ ... _e_x_p_rn ] ))
          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 evalu-
          ate  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.

       hhoosstt--ddeeccll--nnaammee
          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.

NNUUMMEERRIICC EEXXPPRREESSSSIIOONNSS
       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 inte-
       gers may be more than 32 bits.

       eexxttrraacctt--iinntt ((_d_a_t_a_-_e_x_p_r,, _w_i_d_t_h))

          The  eexxttrraacctt--iinntt  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



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dhcp-eval(5)                                         dhcp-eval(5)


          integer of the specified size, the null value is returned.

       lleeaassee--ttiimmee

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

       _n_u_m_b_e_r

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

       cclliieenntt--ssttaattee

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

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

          _+_o 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 trans-
            mitted 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.

          _+_o 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.

          _+_o 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 mes-
            sage, which will be broadcast.

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

          _+_o 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.

          _+_o 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.

RREEFFEERREENNCCEE:: AACCTTIIOONN EEXXPPRREESSSSIIOONNSS
       lloogg ((_p_r_i_o_r_i_t_y,, _d_a_t_a_-_e_x_p_r))




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dhcp-eval(5)                                         dhcp-eval(5)


          Logging  statements  may be used to send information to the standard
          logging channels.  A logging statement includes an optional priority
          (ffaattaall, eerrrroorr, iinnffoo, or ddeebbuugg), and a data expression.

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

       eexxeeccuuttee ((_c_o_m_m_a_n_d_-_p_a_t_h [[,, _d_a_t_a_-_e_x_p_r_1,, ...... _d_a_t_a_-_e_x_p_r_N]]));;

          The  eexxeeccuuttee statement runs an external command.  The first argument
          is a string literal containing the name or path of  the  command  to
          run.  The other arguments, if present, are either string literals or
          data- expressions which evaluate to text strings, to  be  passed  as
          command-line arguments to the command.

          eexxeeccuuttee  is  synchronous;  the program will block until the external
          command being run has finished.  Please note  that  lengthy  program
          execution  (for example, in an "on commit" in dhcpd.conf) may result
          in bad performance and timeouts.  Only  external  applications  with
          very short execution times are suitable for use.

          Passing  user-supplied data to an external application might be dan-
          gerous.  Make sure the external application checks input buffers for
          validity.   Non-printable  ASCII  characters  will be converted into
          dhcpd.conf language octal escapes ("777"), make sure  your  external
          command handles them as such.

          It is possible to use the execute statement in any context, not only
          on events. If you put it in a regular  scope  in  the  configuration
          file  you will execute that command every time a scope is evaluated.

RREEFFEERREENNCCEE:: DDYYNNAAMMIICC DDNNSS UUPPDDAATTEESS
       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.

SSEECCUURRIITTYY
       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 ddnnss--uuppddaattee
       expression.  The ddnnss--uuppddaattee 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



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dhcp-eval(5)                                         dhcp-eval(5)


       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 refer-
       ence section of the dhcpd.conf file, where events  are  described.   In
       this example several statements are being used to make the arguments to
       the ddnnss--uuppddaattee.

       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 argu-
       ment is constructed by concatenating the DHCP host-name option  with  a
       text  string  containing  the  local  domain,  in  this case "ssd.exam-
       ple.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, spec-
       ifies  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 ddnnss--uuppddaattee 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" concate-
       nated.  The right hand side is the fully qualified domain name  of  the
       client to which the address is being leased.

SSEEEE AALLSSOO
       dhcpd.conf(5),   dhcpd.leases(5),   dhclient.conf(5),  dhcp-options(5),
       dhcpd(8), dhclient(8), RFC2132, RFC2131.

AAUUTTHHOORR
       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 Inter-
       net Systems Consortium can be found at hhttttppss::////wwwwww..iisscc..oorrgg..























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