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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 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 en-
       close 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	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
       statements enclosed in braces following the else	are evaluated.	 Bool-
       ean expressions that evaluate to	null are treated as  false  in	condi-
       tionals.

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.

       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.

       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/ subdirec-
	 tory).

       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
	 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 hexadeci-
	 mal  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
	 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 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 spec-
	 ifies 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	evalu-
	 ate 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.

       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
	   address, but	is not yet using it.   The next	message	to  be	trans-
	   mitted  will	be a DHCPREQUEST, which	will be	broadcast.   If	no re-
	   sponse 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 ("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  ex-
       pression.  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 re-
       source 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	refer-
       ence  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 argu-
       ment  is	 constructed by	concatenating the DHCP host-name option	with a
       text string containing  the  local  domain,  in	this  case  "ssd.exam-
       ple.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, spec-
       ifies 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"  concate-
       nated.	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	Inter-
       net 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 | SECURITY | SEE ALSO | AUTHOR

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