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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 be-
       haviour 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	condi-
       tional statement	can appear anywhere that a regular statement (e.g., an
       option  statement)  can appear, and can enclose one or more such	state-
       ments.

       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	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	state-
       ments enclosed in braces	following the else are evaluated.  Boolean ex-
       pressions 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  expres-
       sions  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	state-
       ments 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 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.

       data-expression-1  ~=  data-expression-2	 data-expression-1 ~~ data-ex-
       pression-2

	 The ~=	and ~~ operators (not available	on all	systems)  perform  ex-
	 tended	 regex(7)  matching of the values of two data expressions, re-
	 turning true if data-expression-1 matches against the regular expres-
	 sion 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-insensi-
	 tive.

       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 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  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.

       lcase (data-expr)

	 The  lcase  function  returns the result of evaluating	data-expr con-
	 verted	to lower case.	If data-expr evaluates to null,	then  the  re-
	 sult is also null.

       ucase (data-expr)

	 The  ucase  function  returns the result of evaluating	data-expr con-
	 verted	to upper case.	If data-expr evaluates to null,	then  the  re-
	 sult 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 termina-
	 tor).	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 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 back-
	 slash ('\') 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 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 ex-
	 pression 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 ad-
	 dress.	 In any	context	where the client whose request is  being  pro-
	 cessed	 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 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	speci-
	 fies  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 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 inte-
       gers 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:

	 _o 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.

	 _o Reboot - DHCP client	is in the INIT-REBOOT state.  It has an	IP ad-
	   dress, 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 re-
	   ceive 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 mes-
	   sage, 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 ad-
	   dress, 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  ad-
	   dress,  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 ex-
	 ecution (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  dan-
	 gerous.   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-op-
	 tion-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  op-
	 tion  and  the	sub-options are	not at fixed locations.	 It is handled
	 as an action to allow an administrator	to examine  the	 incoming  op-
	 tions 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)

NAME | DESCRIPTION | REFERENCE: CONDITIONAL BEHAVIOUR | BOOLEAN EXPRESSIONS | DATA EXPRESSIONS | NUMERIC EXPRESSIONS | REFERENCE: ACTION EXPRESSIONS | REFERENCE: DYNAMIC DNS UPDATES | SEE ALSO | AUTHOR

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