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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 behaviour is documented here.

REFERENCE: CONDITIONAL BEHAVIOUR
       Conditional  behaviour  is  specified  using the if statement and the else or elsif state-
       ments.	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 fol-
       lowing  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.

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) match-
	 ing 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 evalu-
	 ates  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 pro-
	 cessed 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 expres-
       sions.	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.

       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 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 net-
	 work 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.

       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 DHCPDISCOVER, 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 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  DHCPOF-
	   FER	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 message, 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  DHCPRE-
	   QUEST, 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 ("777"), make sure your  exter-
	 nal 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  suc-
       ceeds,  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 reference section of the  dhcpd.conf  file,  where
       events are described.  In this example several statements are being used to make the argu-
       ments to the dns-update.

       In the example, the first argument to the first Bdns-update expression is a  data  expres-
       sion that evaluates to the A RR type.  The second argument is constructed by concatenating
       the DHCP host-name option with a text string containing the local  domain,  in  this  case
       "ssd.example.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, specifies 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" concatenated.  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  con-
       tract  with  Vixie  Labs.   Funding for this project was provided through Internet Systems
       Consortium.   Information  about   Internet   Systems   Consortium   can   be   found   at
       https://www.isc.org.

										     dhcp-eval(5)
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