1dhcp-eval(5) File Formats Manual dhcp-eval(5)
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6 dhcp-eval - ISC DHCP conditional evaluation
7
9 The Internet Systems Consortium DHCP client and server both provide the
10 ability to perform conditional behavior depending on the contents of
11 packets they receive. The syntax for specifying this conditional be‐
12 haviour is documented here.
13
15 Conditional behaviour is specified using the if statement and the else
16 or elsif statements. A conditional statement can appear anywhere that
17 a regular statement (e.g., an option statement) can appear, and can
18 enclose one or more such statements. A typical conditional statement
19 in a server might be:
20 if option dhcp-user-class = "accounting" {
22 max-lease-time 17600;
23 option domain-name "accounting.example.org";
24 option domain-name-servers ns1.accounting.example.org,
25 ns2.accounting.example.org;
26 } elsif option dhcp-user-class = "sales" {
27 max-lease-time 17600;
28 option domain-name "sales.example.org";
29 option domain-name-servers ns1.sales.example.org,
30 ns2.sales.example.org;
31 } elsif option dhcp-user-class = "engineering" {
32 max-lease-time 17600;
33 option domain-name "engineering.example.org";
34 option domain-name-servers ns1.engineering.example.org,
35 ns2.engineering.example.org;
36 } else {
37 max-lease-time 600;
38 option domain-name "misc.example.org";
39 option domain-name-servers ns1.misc.example.org,
40 ns2.misc.example.org;
41 }
42 On the client side, an example of conditional evaluation might be:
44 # example.org filters DNS at its firewall, so we have to use their DNS
46 # servers when we connect to their network. If we are not at
47 # example.org, prefer our own DNS server.
48 if not option domain-name = "example.org" {
49 prepend domain-name-servers 127.0.0.1;
50 }
51 The if statement and the elsif continuation statement both take boolean
53 expressions as their arguments. That is, they take expressions that,
54 when evaluated, produce a boolean result. If the expression evaluates
55 to true, then the statements enclosed in braces following the if state‐
56 ment are executed, and all subsequent elsif and else clauses are
57 skipped. Otherwise, each subsequent elsif clause's expression is
58 checked, until an elsif clause is encountered whose test evaluates to
59 true. If such a clause is found, the statements in braces following
60 it are executed, and then any subsequent elsif and else clauses are
61 skipped. If all the if and elsif clauses are checked but none of
62 their expressions evaluate true, then if there is an else clause, the
63 statements enclosed in braces following the else are evaluated. Bool‐
64 ean expressions that evaluate to null are treated as false in condi‐
65 tionals.
66
68 The following is the current list of boolean expressions that are sup‐
69 ported by the DHCP distribution.
70 data-expression-1 = data-expression-2
72 The = operator compares the values of two data expressions, returning
74 true if they are the same, false if they are not. If either the
75 left-hand side or the right-hand side are null, the result is also
76 null.
77 data-expression-1 ~= data-expression-2 data-expression-1 ~~ data-
79 expression-2
80 The ~= and ~~ operators (not available on all systems) perform
82 extended regex(7) matching of the values of two data expressions,
83 returning true if data-expression-1 matches against the regular
84 expression evaluated by data-expression-2, or false if it does not
85 match or encounters some error. If either the left-hand side or the
86 right-hand side are null or empty strings, the result is also false.
87 The ~~ operator differs from the ~= operator in that it is case-
88 insensitive.
89 boolean-expression-1 and boolean-expression-2
91 The and operator evaluates to true if the boolean expression on the
93 left-hand side and the boolean expression on the right-hand side both
94 evaluate to true. Otherwise, it evaluates to false. If either the
95 expression on the left-hand side or the expression on the right-hand
96 side are null, the result is null.
97 boolean-expression-1 or boolean-expression-2
99 The or operator evaluates to true if either the boolean expression on
101 the left-hand side or the boolean expression on the right-hand side
102 evaluate to true. Otherwise, it evaluates to false. If either the
103 expression on the left-hand side or the expression on the right-hand
104 side are null, the result is null.
105 not boolean-expression
107 The not operator evaluates to true if boolean-expression evaluates to
109 false, and returns false if boolean-expression evaluates to true.
110 If boolean-expression evaluates to null, the result is also null.
111 exists option-name
113 The exists expression returns true if the specified option exists in
115 the incoming DHCP packet being processed.
116 known
117 The known expression returns true if the client whose request is cur‐
119 rently being processed is known - that is, if there's a host declara‐
120 tion for it.
121 static
122 The static expression returns true if the lease assigned to the
124 client whose request is currently being processed is derived from a
125 static address assignment.
126
128 Several of the boolean expressions above depend on the results of eval‐
129 uating data expressions. A list of these expressions is provided
130 here.
131 substring (data-expr, offset, length)
133 The substring operator evaluates the data expression and returns the
135 substring of the result of that evaluation that starts offset bytes
136 from the beginning, continuing for length bytes. Offset and length
137 are both numeric expressions. If data-expr, offset or length evalu‐
138 ate to null, then the result is also null. If offset is greater than
139 or equal to the length of the evaluated data, then a zero-length data
140 string is returned. If length is greater then the remaining length
141 of the evaluated data after offset, then a data string containing all
142 data from offset to the end of the evaluated data is returned.
143 suffix (data-expr, length)
145 The suffix operator evaluates data-expr and returns the last length
147 bytes of the result of that evaluation. Length is a numeric expres‐
148 sion. If data-expr or length evaluate to null, then the result is
149 also null. If suffix evaluates to a number greater than the length
150 of the evaluated data, then the evaluated data is returned.
151 lcase (data-expr)
153 The lcase function returns the result of evaluating data-expr con‐
155 verted to lower case. If data-expr evaluates to null, then the
156 result is also null.
157 ucase (data-expr)
159 The ucase function returns the result of evaluating data-expr con‐
161 verted to upper case. If data-expr evaluates to null, then the
162 result is also null.
163 option option-name
165 The option operator returns the contents of the specified option in
167 the packet to which the server is responding.
168 config-option option-name
170 The config-option operator returns the value for the specified option
172 that the DHCP client or server has been configured to send.
173 hardware
175 The hardware operator returns a data string whose first element is
177 the type of network interface indicated in packet being considered,
178 and whose subsequent elements are client's link-layer address. If
179 there is no packet, or if the RFC2131 hlen field is invalid, then the
180 result is null. Hardware types include ethernet (1), token-ring
181 (6), and fddi (8). Hardware types are specified by the IETF, and
182 details on how the type numbers are defined can be found in RFC2131
183 (in the ISC DHCP distribution, this is included in the doc/ subdirec‐
184 tory).
185 packet (offset, length)
187 The packet operator returns the specified portion of the packet being
189 considered, or null in contexts where no packet is being considered.
190 Offset and length are applied to the contents packet as in the sub‐
191 string operator.
192 string
194 A string, enclosed in quotes, may be specified as a data expression,
196 and returns the text between the quotes, encoded in ASCII. The
197 backslash ('\') character is treated specially, as in C programming:
198 '\t' means TAB, '\r' means carriage return, '\n' means newline, and
199 '\b' means bell. Any octal value can be specified with '\nnn',
200 where nnn is any positive octal number less than 0400. Any hexadeci‐
201 mal value can be specified with '\xnn', where nn is any positive
202 hexadecimal number less than or equal to 0xff.
203 colon-separated hexadecimal list
205 A list of hexadecimal octet values, separated by colons, may be spec‐
207 ified as a data expression.
208 concat (data-expr1, ..., data-exprN)
210 The expressions are evaluated, and the results of each evaluation are
211 concatenated in the sequence that the subexpressions are listed. If
212 any subexpression evaluates to null, the result of the concatenation
213 is null.
214 reverse (numeric-expr1, data-expr2)
216 The two expressions are evaluated, and then the result of evaluating
217 the data expression is reversed in place, using hunks of the size
218 specified in the numeric expression. For example, if the numeric
219 expression evaluates to four, and the data expression evaluates to
220 twelve bytes of data, then the reverse expression will evaluate to
221 twelve bytes of data, consisting of the last four bytes of the the
222 input data, followed by the middle four bytes, followed by the first
223 four bytes.
224 leased-address
226 In any context where the client whose request is being processed has
227 been assigned an IP address, this data expression returns that IP
228 address. In any context where the client whose request is being pro‐
229 cessed has not been assigned an ip address, if this data expression
230 is found in executable statements executed on that client's behalf, a
231 log message indicating "there is no lease associated with this
232 client" is syslogged to the debug level (this is considered
233 dhcpd.conf debugging information).
234 binary-to-ascii (numeric-expr1, numeric-expr2, data-expr1, data-expr2)
236 Converts the result of evaluating data-expr2 into a text string con‐
237 taining one number for each element of the result of evaluating data-
238 expr2. Each number is separated from the other by the result of
239 evaluating data-expr1. The result of evaluating numeric-expr1 spec‐
240 ifies the base (2 through 16) into which the numbers should be con‐
241 verted. The result of evaluating numeric-expr2 specifies the width
242 in bits of each number, which may be either 8, 16 or 32.
243 As an example of the preceding three types of expressions, to produce
245 the name of a PTR record for the IP address being assigned to a
246 client, one could write the following expression:
247 concat (binary-to-ascii (10, 8, ".",
249 reverse (1, leased-address)),
250 ".in-addr.arpa.");
251 encode-int (numeric-expr, width)
254 Numeric-expr is evaluated and encoded as a data string of the speci‐
255 fied width, in network byte order (most significant byte first). If
256 the numeric expression evaluates to the null value, the result is
257 also null.
258 pick-first-value (data-expr1 [ ... exprn ] )
260 The pick-first-value function takes any number of data expressions as
261 its arguments. Each expression is evaluated, starting with the
262 first in the list, until an expression is found that does not evalu‐
263 ate to a null value. That expression is returned, and none of the
264 subsequent expressions are evaluated. If all expressions evaluate
265 to a null value, the null value is returned.
266 host-decl-name
268 The host-decl-name function returns the name of the host declaration
269 that matched the client whose request is currently being processed,
270 if any. If no host declaration matched, the result is the null
271 value.
272
274 Numeric expressions are expressions that evaluate to an integer. In
275 general, the maximum size of such an integer should not be assumed to
276 be representable in fewer than 32 bits, but the precision of such inte‐
277 gers may be more than 32 bits.
278 extract-int (data-expr, width)
280 The extract-int operator extracts an integer value in network byte
282 order from the result of evaluating the specified data expression.
283 Width is the width in bits of the integer to extract. Currently, the
284 only supported widths are 8, 16 and 32. If the evaluation of the
285 data expression doesn't provide sufficient bits to extract an integer
286 of the specified size, the null value is returned.
287 lease-time
289 The duration of the current lease - that is, the difference between
291 the current time and the time that the lease expires.
292 number
294 Any number between zero and the maximum representable size may be
296 specified as a numeric expression.
297 client-state
299 The current state of the client instance being processed. This is
301 only useful in DHCP client configuration files. Possible values
302 are:
303 · Booting - DHCP client is in the INIT state, and does not yet have
305 an IP address. The next message transmitted will be a DHCPDIS‐
306 COVER, which will be broadcast.
307 · Reboot - DHCP client is in the INIT-REBOOT state. It has an IP
309 address, but is not yet using it. The next message to be trans‐
310 mitted will be a DHCPREQUEST, which will be broadcast. If no
311 response is heard, the client will bind to its address and move to
312 the BOUND state.
313 · Select - DHCP client is in the SELECTING state - it has received at
315 least one DHCPOFFER message, but is waiting to see if it may
316 receive other DHCPOFFER messages from other servers. No messages
317 are sent in the SELECTING state.
318 · Request - DHCP client is in the REQUESTING state - it has received
320 at least one DHCPOFFER message, and has chosen which one it will
321 request. The next message to be sent will be a DHCPREQUEST mes‐
322 sage, which will be broadcast.
323 · Bound - DHCP client is in the BOUND state - it has an IP address.
325 No messages are transmitted in this state.
326 · Renew - DHCP client is in the RENEWING state - it has an IP
328 address, and is trying to contact the server to renew it. The
329 next message to be sent will be a DHCPREQUEST message, which will
330 be unicast directly to the server.
331 · Rebind - DHCP client is in the REBINDING state - it has an IP
333 address, and is trying to contact any server to renew it. The
334 next message to be sent will be a DHCPREQUEST, which will be broad‐
335 cast.
336
338 log (priority, data-expr)
339 Logging statements may be used to send information to the standard
341 logging channels. A logging statement includes an optional priority
342 (fatal, error, info, or debug), and a data expression.
343 Logging statements take only a single data expression argument, so if
345 you want to output multiple data values, you will need to use the
346 concat operator to concatenate them.
347 execute (command-path [, data-expr1, ... data-exprN]);
349 The execute statement runs an external command. The first argument
351 is a string literal containing the name or path of the command to
352 run. The other arguments, if present, are either string literals or
353 data- expressions which evaluate to text strings, to be passed as
354 command-line arguments to the command.
355 execute is synchronous; the program will block until the external
357 command being run has finished. Please note that lengthy program
358 execution (for example, in an "on commit" in dhcpd.conf) may result
359 in bad performance and timeouts. Only external applications with
360 very short execution times are suitable for use.
361 Passing user-supplied data to an external application might be dan‐
363 gerous. Make sure the external application checks input buffers for
364 validity. Non-printable ASCII characters will be converted into
365 dhcpd.conf language octal escapes ("777"), make sure your external
366 command handles them as such.
367 It is possible to use the execute statement in any context, not only
369 on events. If you put it in a regular scope in the configuration file
370 you will execute that command every time a scope is evaluated.
371
373 The DHCP client and server have the ability to dynamically update the
374 Domain Name System. Within the configuration files, you can define how
375 you want the Domain Name System to be updated. These updates are RFC
376 2136 compliant so any DNS server supporting RFC 2136 should be able to
377 accept updates from the DHCP server.
378
380 Support for TSIG and DNSSEC is not yet available. When you set your
381 DNS server up to allow updates from the DHCP server or client, you may
382 be exposing it to unauthorized updates. To avoid this, the best you
383 can do right now is to use IP address-based packet filtering to prevent
384 unauthorized hosts from submitting update requests. Obviously, there
385 is currently no way to provide security for client updates - this will
386 require TSIG or DNSSEC, neither of which is yet available in the DHCP
387 distribution.
388 Dynamic DNS (DDNS) updates are performed by using the dns-update
390 expression. The dns-update expression is a boolean expression that
391 takes four parameters. If the update succeeds, the result is true. If
392 it fails, the result is false. The four parameters that the are the
393 resource record type (RR), the left hand side of the RR, the right hand
394 side of the RR and the ttl that should be applied to the record. The
395 simplest example of the use of the function can be found in the refer‐
396 ence section of the dhcpd.conf file, where events are described. In
397 this example several statements are being used to make the arguments to
398 the dns-update.
399 In the example, the first argument to the first Bdns-update expression
401 is a data expression that evaluates to the A RR type. The second argu‐
402 ment is constructed by concatenating the DHCP host-name option with a
403 text string containing the local domain, in this case "ssd.exam‐
404 ple.net". The third argument is constructed by converting the address
405 the client has been assigned from a 32-bit number into an ascii string
406 with each byte separated by a ".". The fourth argument, the TTL, spec‐
407 ifies the amount of time remaining in the lease (note that this isn't
408 really correct, since the DNS server will pass this TTL out whenever a
409 request comes in, even if that is only a few seconds before the lease
410 expires).
411 If the first dns-update statement succeeds, it is followed up with a
413 second update to install a PTR RR. The installation of a PTR record is
414 similar to installing an A RR except that the left hand side of the
415 record is the leased address, reversed, with ".in-addr.arpa" concate‐
416 nated. The right hand side is the fully qualified domain name of the
417 client to which the address is being leased.
418
420 dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-options(5),
421 dhcpd(8), dhclient(8), RFC2132, RFC2131.
422
424 The Internet Systems Consortium DHCP Distribution was written by Ted
425 Lemon under a contract with Vixie Labs. Funding for this project was
426 provided through Internet Systems Consortium. Information about Inter‐
427 net Systems Consortium can be found at https://www.isc.org.
428 dhcp-eval(5)