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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 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 state-
ments. 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 state-
ments 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 evalu-
ates 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,
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 expres-
sions 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 evalu-
ated 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 config-
ured to send.
gethostname()
The gethostname() function returns a data string whose contents are a character string, the results of calling gethost-
name() on the local system with a size limit of 255 bytes (not including NULL terminator). This can be used for exam-
ple 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 car-
riage 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 subexpres-
sions 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:
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.
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 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 cur-
rently no way to provide security for client updates - this will require TSIG or DNSSEC, neither of which is yet avail-
able 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)
