dhcp-eval(5) File Formats Manual 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 behav‐
ior 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 fol‐
lowing 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 state‐
ments 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.
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 expres‐
sion on the right-hand side both evaluate to true. Otherwise, it evaluates to false. If either the expres‐
sion 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 pro‐
cessed.
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 pro‐
cessed 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 evalua‐
tion 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)
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 call‐
ing 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/ sub‐
directory).
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
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 sup‐
ported 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 trans‐
mitted 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.
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 val‐
ues, 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- expres‐
sions 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 perfor‐
mance 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 applica‐
tion 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.
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
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)
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