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 may be  specified using the if
          statement and the else or elsif statements or the switch and
          case 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.

          CONDITIONAL BEHAVIOUR: IF

          A typical conditional if 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.

     Page 1                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

          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 expres-
          sion 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 expres-
          sions evaluate true, then if there is an else clause, the
          statements enclosed in braces following the else are evalu-
          ated.  Boolean expressions that evaluate to null are treated
          as false in conditionals.

          CONDITIONAL BEHAVIOUR: SWITCH

          The above example can be rewritten using a switch construct
          as well.

          switch (option dhcp-user-class) {
            case "accounting":
              max-lease-time 17600;
              option domain-name "accounting.example.org";
              option domain-name-servers ns1.accounting.example.org,
                                ns2.accounting.example.org;
            case "sales":
              max-lease-time 17600;
              option domain-name "sales.example.org";
              option domain-name-servers ns1.sales.example.org,
                                ns2.sales.example.org;
              break;
            case "engineering":
              max-lease-time 17600;
              option domain-name "engineering.example.org";
              option domain-name-servers ns1.engineering.example.org,
                                ns2.engineering.example.org;
              break;
            default:
              max-lease-time 600;
              option domain-name "misc.example.org";
              option domain-name-servers ns1.misc.example.org,
                                ns2.misc.example.org;
              break;
          }

     Page 2                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

          The switch statement and the case statements can both be
          data expressions or numeric expressions.  Within a switch
          statement they all must be the same type.  The server evalu-
          ates the expression from the switch statement and then it
          evaluates the expressions from the case statements until it
          finds a match.

          If it finds a match it starts executing statements from that
          case until the next break statement.  If it doesn't find a
          match it starts from the default statement and again pro-
          ceeds to the next break statement.  If there is no match and
          no default it does nothing.

     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 expres-
            sions, 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 expres-
            sion 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

     Page 3                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

            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 eval-
            uates 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 off-
            set, 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 eval-
            uated data, then the evaluated data is returned.

     Page 4                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

          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 spec-
            ified option that the DHCP client or server has been con-
            figured 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 terminator).  This can be used
            for example to configure dhclient to send the local host-
            name 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/ 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.

     Page 5                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

          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 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.

     Page 6                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

            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 signifi-
            cant 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 evalu-
            ated, 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 sub-
            sequent 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 cur-
            rently 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.

          In addition to the following operators several standard math
          functions are available.  They are:
          operation    symbol
          add            +
          subtract       -
          divide         /
          multiply       *
          modulus        %
          bitwise and    &

     Page 7                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

          bitwise or     |
          bitwise xor    ^

          extract-int (data-expr, width)

            The extract-int operator extracts an integer value in net-
            work byte order from the result of evaluating the speci-
            fied 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 expres-
            sion 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 differ-
            ence 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 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 message, which will be
              broadcast.

     Page 8                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

            +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 ("\nnn"), make sure your external command
            handles them as such.

            It is possible to use the execute statement in any con-
            text, not only on events.  If you put it in a regular
            scope in the configuration file you will execute that

     Page 9                       Plan 9             (printed 5/26/22)

     dhcp-eval(5)                                         dhcp-eval(5)

            command every time a scope is evaluated.

          parse-vendor-option;

            The parse-vendor-option statement attempts to parse a ven-
            dor option (code 43).  It is only useful while processing
            a packet on the server and requires that the administrator
            has already used the vendor-option-space statement to
            select a valid vendor space.

            This functionality may be used if the server needs to take
            different actions depending on the values the client
            placed in the vendor option and the sub-options are not at
            fixed locations.  It is handled as an action to allow an
            administrator to examine the incoming options and choose
            the correct vendor space.

     REFERENCE: DYNAMIC DNS UPDATES
          See the dhcpd.conf and dhclient.conf man pages for more
          information about DDNS.

     SEE ALSO
          dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-
          options(5), dhcpd(8), dhclient(8), RFC2132, RFC2131.

     AUTHOR
          Information about Internet Systems Consortium can be found
          at https://www.isc.org.

     Page 10                      Plan 9             (printed 5/26/22)