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



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 be-
       haviour is documented here.

REFERENCE: CONDITIONAL BEHAVIOUR
       Conditional behaviour may be  specified using the if statement and  the
       else  or  elsif statements or the switch and case statements.  A condi-
       tional statement can appear anywhere that a regular statement (e.g., an
       option  statement)  can appear, and can enclose one or more such state-
       ments.

       CONDITIONAL BEHAVIOUR: IF

       A typical conditional if 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 state-
       ment 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  state-
       ments  enclosed  in  braces  following the else are evaluated.  Boolean
       expressions that evaluate to null are treated as false in conditionals.

       CONDITIONAL BEHAVIOUR: SWITCH

       The above example can be rewritten using a switch construct as well.

       switch (option dhcp-user-class) {
         case "accounting":
           max-lease-time 17600;
           option domain-name "accounting.example.org";
           option domain-name-servers ns1.accounting.example.org,
                             ns2.accounting.example.org;
         case "sales":
           max-lease-time 17600;
           option domain-name "sales.example.org";
           option domain-name-servers ns1.sales.example.org,
                             ns2.sales.example.org;
           break;
         case "engineering":
           max-lease-time 17600;
           option domain-name "engineering.example.org";
           option domain-name-servers ns1.engineering.example.org,
                             ns2.engineering.example.org;
           break;
         default:
           max-lease-time 600;
           option domain-name "misc.example.org";
           option domain-name-servers ns1.misc.example.org,
                             ns2.misc.example.org;
           break;
       }

       The  switch  statement and the case statements can both be data expres-
       sions or numeric expressions.  Within a switch statement they all  must
       be  the same type.  The server evaluates the expression from the switch
       statement and then it evaluates the expressions from  the  case  state-
       ments until it finds a match.

       If it finds a match it starts executing statements from that case until
       the next break statement.  If it doesn't find a match  it  starts  from
       the  default  statement and again proceeds to the next break statement.
       If there is no match and no default it does nothing.


BOOLEAN EXPRESSIONS
       The following is the current list of boolean expressions that are  sup-
       ported 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.

       data-expression-1  ~=  data-expression-2  data-expression-1  ~~   data-
       expression-2

         The  ~=  and  ~~  operators  (not  available  on all systems) perform
         extended regex(7) matching of the values  of  two  data  expressions,
         returning  true  if  data-expression-1  matches  against  the regular
         expression evaluated by data-expression-2, or false if  it  does  not
         match  or encounters some error.  If either the left-hand side or the
         right-hand side are null or empty strings, the result is also  false.
         The  ~~  operator  differs  from  the ~= operator in that it is case-
         insensitive.

       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 cur-
         rently being processed is known - that is, if there's a host declara-
         tion 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 eval-
       uating 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  evalu-
         ate 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  expres-
         sion.   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.

       lcase (data-expr)

         The  lcase  function  returns the result of evaluating data-expr con-
         verted to lower case.  If  data-expr  evaluates  to  null,  then  the
         result is also null.

       ucase (data-expr)

         The  ucase  function  returns the result of evaluating data-expr con-
         verted to upper case.  If  data-expr  evaluates  to  null,  then  the
         result is also null.

       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.

       gethostname()

         The gethostname() function returns a data string whose contents are a
         character string, the results of calling gethostname() on  the  local
         system  with  a  size limit of 255 bytes (not including NULL termina-
         tor).  This can be used for example to configure dhclient to send the
         local  hostname  without  knowing  the  local  hostname  at  the time
         dhclient.conf is written.

       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 sub-
         string 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 back-
         slash ('\') 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 spec-
         ified 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  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.  In any context where the client whose request is being pro-
         cessed  has  not been assigned an ip address, if this data expression
         is found in executable statements executed on that client's behalf, a
         log  message  indicating  "there  is  no  lease  associated with this
         client"  is  syslogged  to  the  debug  level  (this  is   considered
         dhcpd.conf debugging information).

       binary-to-ascii (numeric-expr1, numeric-expr2, data-expr1, data-expr2)
         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 speci-
         fies  the  base  (2 through 16) into which the numbers should be con-
         verted.  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 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.

       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 inte-
       gers may be more than 32 bits.

       In  addition to the following operators several standard math functions
       are available.  They are:
       operation    symbol
       add            +
       subtract       -
       divide         /
       multiply       *
       modulus        %
       bitwise and    &
       bitwise or     |
       bitwise xor    ^

       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  DHCPDIS-
           COVER, 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 transmit-
           ted 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  mes-
           sage, 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: ACTION EXPRESSIONS
       log (priority, data-expr)

         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.

         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.

       execute (command-path [, data-expr1, ... data-exprN]);

         The execute 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.

         execute 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 ("\nnn"), 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.

       parse-vendor-option;

         The parse-vendor-option statement attempts to parse a  vendor  option
         (code 43).  It is only useful while processing a packet on the server
         and requires that the administrator  has  already  used  the  vendor-
         option-space statement to select a valid vendor space.

         This  functionality may be used if the server needs to take different
         actions depending on the values  the  client  placed  in  the  vendor
         option and the sub-options are not at fixed locations.  It is handled
         as an action to  allow  an  administrator  to  examine  the  incoming
         options and choose the correct vendor space.

REFERENCE: DYNAMIC DNS UPDATES
       See  the  dhcpd.conf  and  dhclient.conf man pages for more information
       about DDNS.

SEE ALSO
       dhcpd.conf(5),  dhcpd.leases(5),   dhclient.conf(5),   dhcp-options(5),
       dhcpd(8), dhclient(8), RFC2132, RFC2131.

AUTHOR
       Information   about   Internet  Systems  Consortium  can  be  found  at
       https://www.isc.org.



                                                                  dhcp-eval(5)

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