dhcp-eval <root
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 behaviour 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 conditional statement can appear anywhere
       that  a  regular  statement (e.g., an option statement) can appear, and can enclose one or
       more such statements.

       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  fol‐
       lowing  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 state‐
       ments 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  follow‐
       ing  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  expressions  or  numeric
       expressions.  Within a switch statement they all must be the same type.  The server evalu‐
       ates the expression from the switch statement and then it evaluates the  expressions  from
       the case statements 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 pro‐
       ceeds 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 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)  match‐
         ing  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 evalu‐
         ates 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  pro‐
         cessed 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  expres‐
       sions.  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.

       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 terminator).  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 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 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  net‐
         work  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.

       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 DHCPDISCOVER, 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 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.

         · Select - DHCP client is in the SELECTING state - it has received at least one  DHCPOF‐
           FER  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 message, 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 dangerous.  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 exter‐
         nal 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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