Ubuntu Manpage: dhcp-eval - ISC DHCP conditional evaluation

Provided by: isc-dhcp-common_4.4.1-2.1ubuntu5.20.04.5_amd64

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.
       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.

       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;
       }

       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
       evaluates 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
       proceeds  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 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.

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 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/ 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 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.

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

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 ("\nnn"), 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.

parse-vendor-option;

The parse-vendor-option statement attempts to parse a vendor 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.

AUTHOR

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

                                                                                     dhcp-eval(5)