Provided by: isc-dhcp-common_4.1.ESV-R4-0ubuntu5_amd64 bug

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
       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  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)
         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 or empty strings, 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 currently being
         processed 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
       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  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
         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 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:

         · 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.

         · 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.

         · 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.

         · 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.

         · Bound - DHCP client is in the BOUND state - it has an IP address.    No  messages  are
           transmitted in this state.

         · 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.

         · 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 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
         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 reference 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 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
       contract 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)