Provided by: isc-dhcp-common_4.1.ESV-R4-0ubuntu5_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.

       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
         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.

       lcase (data-expr)

         The  lcase  function  returns  the  result  of  evaluating  data-expr
         converted 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
         converted 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/
         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.  In any context where the  client  whose  request  is  being
         processed  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
         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:

         +o 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.

         +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
           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.

         +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
           message, 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.

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
         dangerous.  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
       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-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
       Internet Systems Consortium can be found at https://www.isc.org.

                                                                  dhcp-eval(5)