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)