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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.

       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 than 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.

       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.

       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: LOGGING
       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.

       log (priority, data-expr)

       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.

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-eval(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
       http://www.isc.org.

										     dhcp-eval(5)
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