NAME
dhcp-eval - ISC DHCP conditional evaluationDESCRIPTION
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.