Provided by: isc-dhcp-common_4.2.4-7ubuntu12.13_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 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 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 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 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:
• 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.
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 currently no way to provide security for client updates
- this will require TSIG or DNSSEC, neither of which is yet available 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)