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 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.
       Boolean expressions that evaluate to null are treated as false in  con-
       ditionals.

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 than 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.

       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.

       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: LOGGING
       Logging statements may be used to send information to the standard log-
       ging 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  con-
       cat 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  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-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 Inter-
       net Systems Consortium can be found at http://www.isc.org.



                                                                  dhcp-eval(5)

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