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:
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.
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.
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.
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).
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:
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: LOGGING
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.
log (priority, data-expr)
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.
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.
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
http://www.isc.org.
