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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.

       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/ 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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