Provided by: isc-dhcp-common_4.3.3-5ubuntu12_amd64 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  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
       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.

       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
       evaluates 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
       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 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.

       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
         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.

       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
           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 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
         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)