dhcp-eval(5)dhcp-eval(5)NAMEdhcp-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 be‐
haviour 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 state‐
ment 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. Bool‐
ean expressions that evaluate to null are treated as false in condi‐
tionals.
BOOLEAN EXPRESSIONS
The following is the current list of boolean expressions that are sup‐
ported 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 cur‐
rently being processed is known - that is, if there's a host declara‐
tion 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 eval‐
uating 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 evalu‐
ate 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 than 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 expres‐
sion. 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/ subdirec‐
tory).
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 sub‐
string 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 hexadeci‐
mal 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 spec‐
ified 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.
binary-to-ascii (numeric-expr1, numeric-expr2, data-expr1, data-expr2)
Converts the result of evaluating data-expr2 into a text string con‐
taining 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 spec‐
ifies the base (2 through 16) into which the numbers should be con‐
verted. 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 speci‐
fied 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 evalu‐
ate 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 inte‐
gers 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 DHCPDIS‐
COVER, 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 trans‐
mitted 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 mes‐
sage, 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 broad‐
cast.
REFERENCE: LOGGING
Logging statements may be used to send information to the standard log‐
ging 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 con‐
cat 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 refer‐
ence 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 argu‐
ment is constructed by concatenating the DHCP host-name option with a
text string containing the local domain, in this case "ssd.exam‐
ple.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, spec‐
ifies 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" concate‐
nated. The right hand side is the fully qualified domain name of the
client to which the address is being leased.
SEE ALSOdhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(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 Inter‐
net Systems Consortium can be found at http://www.isc.org.
dhcp-eval(5)
