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dhcp-eval(5)							  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
       enclose	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.
       Boolean expressions that	evaluate to null are treated as	false in  con-
       ditionals.

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-
       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, 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 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
	 result	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
	 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.

       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
	 address.  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
	   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
	   receive 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
	   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.

	 _o 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	broad-
	   cast.

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 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
       expression.   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
       resource	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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