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dhcp-eval(5)                  File Formats Manual                 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 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 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
       statements enclosed in braces following the else are evaluated.   Bool-
       ean expressions that evaluate to null are treated as  false  in  condi-
       tionals.

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.

       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.

       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/ subdirec-
         tory).

       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
         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 hexadeci-
         mal  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 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 spec-
         ifies 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 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.

       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.

       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  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.

         _ 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 broad-
           cast.

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 ("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.

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 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 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 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 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" concate-
       nated.  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-options(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 Inter-
       net Systems Consortium can be found at http://www.isc.org.



                                                                  dhcp-eval(5)