bionic (5) dhcp-eval.5.gz

Provided by: isc-dhcp-common_4.3.5-3ubuntu7.4_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)