Annotation of embedaddon/dhcp/common/dhcp-eval.5, revision 1.1.1.1
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31: .TH dhcp-eval 5
32: .SH NAME
33: dhcp-eval - ISC DHCP conditional evaluation
34: .SH DESCRIPTION
35: The Internet Systems Consortium DHCP client and server both provide
36: the ability to perform conditional behavior depending on the contents
1.1.1.1 ! misho 37: of packets they receive. The syntax for specifying this conditional
1.1 misho 38: behaviour is documented here.
39: .SH REFERENCE: CONDITIONAL BEHAVIOUR
40: Conditional behaviour is specified using the if statement and the else
1.1.1.1 ! misho 41: or elsif statements. A conditional statement can appear anywhere
1.1 misho 42: that a regular statement (e.g., an option statement) can appear, and
1.1.1.1 ! misho 43: can enclose one or more such statements. A typical conditional
1.1 misho 44: statement in a server might be:
45: .PP
46: .nf
47: if option dhcp-user-class = "accounting" {
48: max-lease-time 17600;
49: option domain-name "accounting.example.org";
1.1.1.1 ! misho 50: option domain-name-servers ns1.accounting.example.org,
1.1 misho 51: ns2.accounting.example.org;
52: } elsif option dhcp-user-class = "sales" {
53: max-lease-time 17600;
54: option domain-name "sales.example.org";
1.1.1.1 ! misho 55: option domain-name-servers ns1.sales.example.org,
1.1 misho 56: ns2.sales.example.org;
57: } elsif option dhcp-user-class = "engineering" {
58: max-lease-time 17600;
59: option domain-name "engineering.example.org";
1.1.1.1 ! misho 60: option domain-name-servers ns1.engineering.example.org,
1.1 misho 61: ns2.engineering.example.org;
62: } else {
63: max-lease-time 600;
64: option domain-name "misc.example.org";
1.1.1.1 ! misho 65: option domain-name-servers ns1.misc.example.org,
1.1 misho 66: ns2.misc.example.org;
67: }
68: .fi
69: .PP
70: On the client side, an example of conditional evaluation might be:
71: .PP
72: .nf
73: # example.org filters DNS at its firewall, so we have to use their DNS
1.1.1.1 ! misho 74: # servers when we connect to their network. If we are not at
1.1 misho 75: # example.org, prefer our own DNS server.
76: if not option domain-name = "example.org" {
77: prepend domain-name-servers 127.0.0.1;
78: }
1.1.1.1 ! misho 79: .fi
1.1 misho 80: .PP
81: The
82: .B if
83: statement and the
84: .B elsif
85: continuation statement both take boolean expressions as their
1.1.1.1 ! misho 86: arguments. That is, they take expressions that, when evaluated,
! 87: produce a boolean result. If the expression evaluates to true, then
! 88: the statements enclosed in braces following the
1.1 misho 89: .B if
90: statement are executed, and all subsequent
91: .B elsif
92: and
93: .B else
1.1.1.1 ! misho 94: clauses are skipped. Otherwise, each subsequent
1.1 misho 95: .B elsif
96: clause's expression is checked, until an elsif clause is encountered
1.1.1.1 ! misho 97: whose test evaluates to true. If such a clause is found, the
1.1 misho 98: statements in braces following it are executed, and then any
99: subsequent
100: .B elsif
101: and
102: .B else
1.1.1.1 ! misho 103: clauses are skipped. If all the
1.1 misho 104: .B if
105: and
106: .B elsif
107: clauses are checked but none
108: of their expressions evaluate true, then if there is an
109: .B else
110: clause, the statements enclosed in braces following the
111: .B else
1.1.1.1 ! misho 112: are evaluated. Boolean expressions that evaluate to null are
1.1 misho 113: treated as false in conditionals.
114: .SH BOOLEAN EXPRESSIONS
115: The following is the current list of boolean expressions that are
116: supported by the DHCP distribution.
117: .PP
118: .I data-expression-1 \fB=\fR \fIdata-expression-2\fR
119: .RS 0.25i
120: .PP
121: The \fB=\fR operator compares the values of two data expressions,
1.1.1.1 ! misho 122: returning true if they are the same, false if they are not. If
1.1 misho 123: either the left-hand side or the right-hand side are null, the
124: result is also null.
125: .RE
126: .PP
127: .I data-expression-1 \fB~=\fR \fIdata-expression-2\fR
128: .I data-expression-1 \fB~~\fR \fIdata-expression-2\fR
129: .RS 0.25i
130: .PP
131: The \fB~=\fR and \fB~~\fR operators (not available on all systems) perform
132: extended regex(7) matching of the values of two data expressions, returning
133: true if \fIdata-expression-1\fR matches against the regular expression
134: evaluated by \fIdata-expression-2\fR, or false if it does not match or
1.1.1.1 ! misho 135: encounters some error. If either the left-hand side or the right-hand side
1.1 misho 136: are null or empty strings, the result is also false. The \fB~~\fR operator
137: differs from the \fB~=\fR operator in that it is case-insensitive.
138: .RE
139: .PP
140: .I boolean-expression-1 \fBand\fR \fIboolean-expression-2\fR
141: .PP
142: .RS 0.25i
143: The \fBand\fR operator evaluates to true if the boolean expression on
144: the left-hand side and the boolean expression on the right-hand side
145: both evaluate to true. Otherwise, it evaluates to false. If either
146: the expression on the left-hand side or the expression on the
147: right-hand side are null, the result is null.
148: .RE
149: .PP
150: .I boolean-expression-1 \fBor\fR \fIboolean-expression-2\fR
151: .PP
152: .RS 0.25i
153: The \fBor\fR operator evaluates to true if either the boolean
154: expression on the left-hand side or the boolean expression on the
155: right-hand side evaluate to true. Otherwise, it evaluates to false.
156: If either the expression on the left-hand side or the expression on
157: the right-hand side are null, the result is null.
158: .RE
159: .PP
160: .B not \fIboolean-expression
161: .PP
162: .RS 0.25i
163: The \fBnot\fR operator evaluates to true if \fIboolean-expression\fR
164: evaluates to false, and returns false if \fIboolean-expression\fR evaluates
1.1.1.1 ! misho 165: to true. If \fIboolean-expression\fR evaluates to null, the result
1.1 misho 166: is also null.
167: .RE
168: .PP
169: .B exists \fIoption-name\fR
170: .PP
171: .RS 0.25i
172: The \fBexists\fR expression returns true if the specified option
173: exists in the incoming DHCP packet being processed.
174: .RE
175: .B known
176: .PP
177: .RS 0.25i
178: The \fBknown\fR expression returns true if the client whose request is
179: currently being processed is known - that is, if there's a host
180: declaration for it.
181: .RE
182: .B static
183: .PP
184: .RS 0.25i
185: The \fBstatic\fR expression returns true if the lease assigned to the
186: client whose request is currently being processed is derived from a static
187: address assignment.
188: .RE
189: .SH DATA EXPRESSIONS
190: Several of the boolean expressions above depend on the results of
1.1.1.1 ! misho 191: evaluating data expressions. A list of these expressions is provided
1.1 misho 192: here.
193: .PP
194: .B substring (\fIdata-expr\fB, \fIoffset\fB, \fIlength\fB)\fR
195: .PP
196: .RS 0.25i
197: The \fBsubstring\fR operator evaluates the data expression and returns
198: the substring of the result of that evaluation that starts
199: \fIoffset\fR bytes from the beginning, continuing for \fIlength\fR
200: bytes. \fIOffset\fR and \fIlength\fR are both numeric expressions.
201: If \fIdata-expr\fR, \fIoffset\fR or \fIlength\fR evaluate to null,
202: then the result is also null. If \fIoffset\fR is greater than or
203: equal to the length of the evaluated data, then a zero-length data
204: string is returned. If \fIlength\fI is greater then the remaining
205: length of the evaluated data after \fIoffset\fR, then a data string
206: containing all data from \fIoffset\fR to the end of the evaluated data
207: is returned.
208: .RE
209: .PP
210: .B suffix (\fIdata-expr\fB, \fIlength\fB)\fR
211: .PP
212: .RS 0.25i
213: The \fBsuffix\fR operator evaluates \fIdata-expr\fR and returns the
1.1.1.1 ! misho 214: last \fIlength\fR bytes of the result of that evaluation. \fILength\fR
1.1 misho 215: is a numeric expression. If \fIdata-expr\fR or \fIlength\fR evaluate
216: to null, then the result is also null. If \fIsuffix\fR evaluates to a
217: number greater than the length of the evaluated data, then the
218: evaluated data is returned.
219: .RE
220: .PP
221: .B lcase (\fIdata-expr\fB)\fR
222: .PP
223: .RS 0.25i
224: The \fBlcase\fR function returns the result of evaluating
1.1.1.1 ! misho 225: \fIdata-expr\fR converted to lower case. If \fIdata-expr\fR evaluates
1.1 misho 226: to null, then the result is also null.
227: .RE
228: .PP
229: .B ucase (\fIdata-expr\fB)\fR
230: .PP
231: .RS 0.25i
232: The \fBucase\fR function returns the result of evaluating
1.1.1.1 ! misho 233: \fIdata-expr\fR converted to upper case. If \fIdata-expr\fR evaluates
1.1 misho 234: to null, then the result is also null.
235: .RE
236: .PP
237: .B option \fIoption-name\fR
238: .PP
239: .RS 0.25i
240: The \fBoption\fR operator returns the contents of the specified option in
241: the packet to which the server is responding.
242: .RE
243: .PP
244: .B config-option \fIoption-name\fR
245: .PP
246: .RS 0.25i
247: The \fBconfig-option\fR operator returns the value for the specified option
248: that the DHCP client or server has been configured to send.
249: .RE
250: .PP
251: .B hardware
252: .PP
253: .RS 0.25i
254: The \fBhardware\fR operator returns a data string whose first element
255: is the type of network interface indicated in packet being considered,
1.1.1.1 ! misho 256: and whose subsequent elements are client's link-layer address. If
1.1 misho 257: there is no packet, or if the RFC2131 \fIhlen\fR field is invalid,
1.1.1.1 ! misho 258: then the result is null. Hardware types include ethernet (1),
! 259: token-ring (6), and fddi (8). Hardware types are specified by the
1.1 misho 260: IETF, and details on how the type numbers are defined can be found in
261: RFC2131 (in the ISC DHCP distribution, this is included in the doc/
262: subdirectory).
263: .RE
264: .PP
265: .B packet (\fIoffset\fB, \fIlength\fB)\fR
266: .PP
267: .RS 0.25i
268: The \fBpacket\fR operator returns the specified portion of the packet
269: being considered, or null in contexts where no packet is being
1.1.1.1 ! misho 270: considered. \fIOffset\fR and \fIlength\fR are applied to the
1.1 misho 271: contents packet as in the \fBsubstring\fR operator.
272: .RE
273: .PP
274: .I string
275: .PP
276: .RS 0.25i
277: A string, enclosed in quotes, may be specified as a data expression,
1.1.1.1 ! misho 278: and returns the text between the quotes, encoded in ASCII. The
1.1 misho 279: backslash ('\\') character is treated specially, as in C programming: '\\t'
280: means TAB, '\\r' means carriage return, '\\n' means newline, and '\\b' means
1.1.1.1 ! misho 281: bell. Any octal value can be specified with '\\nnn', where nnn is any
1.1 misho 282: positive octal number less than 0400. Any hexadecimal value can be
283: specified with '\\xnn', where nn is any positive hexadecimal number less
284: than or equal to 0xff.
285: .RE
286: .PP
287: .I colon-separated hexadecimal list
288: .PP
289: .RS 0.25i
290: A list of hexadecimal octet values, separated by colons, may be
291: specified as a data expression.
292: .RE
293: .PP
294: .B concat (\fIdata-expr1\fB, ..., \fIdata-exprN\fB)\fR
295: .RS 0.25i
296: The expressions are evaluated, and the results of each evaluation are
1.1.1.1 ! misho 297: concatenated in the sequence that the subexpressions are listed. If
1.1 misho 298: any subexpression evaluates to null, the result of the concatenation
299: is null.
300: .RE
301: .PP
302: .B reverse (\fInumeric-expr1\fB, \fIdata-expr2\fB)\fR
303: .RS 0.25i
304: The two expressions are evaluated, and then the result of evaluating
305: the data expression is reversed in place, using hunks of the size
1.1.1.1 ! misho 306: specified in the numeric expression. For example, if the numeric
! 307: expression evaluates to four, and the data expression evaluates to
1.1 misho 308: twelve bytes of data, then the reverse expression will evaluate to
309: twelve bytes of data, consisting of the last four bytes of the the
310: input data, followed by the middle four bytes, followed by the first
311: four bytes.
312: .RE
313: .PP
314: .B leased-address
315: .RS 0.25i
316: In any context where the client whose request is being processed has
317: been assigned an IP address, this data expression returns that IP
318: address. In any context where the client whose request is being
319: processed has not been assigned an ip address, if this data expression
320: is found in executable statements executed on that client's behalf,
321: a log message indicating "there is no lease associated with this client"
322: is syslogged to the debug level (this is considered dhcpd.conf debugging
323: information).
324: .RE
325: .PP
326: .B binary-to-ascii (\fInumeric-expr1\fB, \fInumeric-expr2\fB,
327: .B \fIdata-expr1\fB,\fR \fIdata-expr2\fB)\fR
328: .RS 0.25i
329: Converts the result of evaluating data-expr2 into a text string
330: containing one number for each element of the result of evaluating
1.1.1.1 ! misho 331: data-expr2. Each number is separated from the other by the result of
! 332: evaluating data-expr1. The result of evaluating numeric-expr1
1.1 misho 333: specifies the base (2 through 16) into which the numbers should be
1.1.1.1 ! misho 334: converted. The result of evaluating numeric-expr2 specifies the
1.1 misho 335: width in bits of each number, which may be either 8, 16 or 32.
336: .PP
337: As an example of the preceding three types of expressions, to produce
338: the name of a PTR record for the IP address being assigned to a
339: client, one could write the following expression:
340: .RE
341: .PP
342: .nf
343: concat (binary-to-ascii (10, 8, ".",
344: reverse (1, leased-address)),
345: ".in-addr.arpa.");
346:
347: .fi
348: .RE
349: .PP
350: .B encode-int (\fInumeric-expr\fB, \fIwidth\fB)\fR
351: .RS 0.25i
352: Numeric-expr is evaluated and encoded as a data string of the
353: specified width, in network byte order (most significant byte first).
354: If the numeric expression evaluates to the null value, the result is
355: also null.
356: .RE
357: .PP
358: .B pick-first-value (\fIdata-expr1\fR [ ... \fIexpr\fRn ] \fB)\fR
359: .RS 0.25i
360: The pick-first-value function takes any number of data expressions as
1.1.1.1 ! misho 361: its arguments. Each expression is evaluated, starting with the first
1.1 misho 362: in the list, until an expression is found that does not evaluate to a
1.1.1.1 ! misho 363: null value. That expression is returned, and none of the subsequent
! 364: expressions are evaluated. If all expressions evaluate to a null
1.1 misho 365: value, the null value is returned.
366: .RE
367: .PP
368: .B host-decl-name
369: .RS 0.25i
370: The host-decl-name function returns the name of the host declaration
371: that matched the client whose request is currently being processed, if
1.1.1.1 ! misho 372: any. If no host declaration matched, the result is the null value.
1.1 misho 373: .RE
374: .SH NUMERIC EXPRESSIONS
1.1.1.1 ! misho 375: Numeric expressions are expressions that evaluate to an integer. In
1.1 misho 376: general, the maximum size of such an integer should not be assumed to
377: be representable in fewer than 32 bits, but the precision of such
378: integers may be more than 32 bits.
379: .PP
380: .B extract-int (\fIdata-expr\fB, \fIwidth\fB)\fR
381: .PP
382: .RS 0.25i
383: The \fBextract-int\fR operator extracts an integer value in network
384: byte order from the result of evaluating the specified data
1.1.1.1 ! misho 385: expression. Width is the width in bits of the integer to extract.
! 386: Currently, the only supported widths are 8, 16 and 32. If the
1.1 misho 387: evaluation of the data expression doesn't provide sufficient bits to
388: extract an integer of the specified size, the null value is returned.
389: .RE
390: .PP
391: .B lease-time
392: .PP
393: .RS 0.25i
394: The duration of the current lease - that is, the difference between
395: the current time and the time that the lease expires.
396: .RE
397: .PP
398: .I number
399: .PP
400: .RS 0.25i
401: Any number between zero and the maximum representable size may be
402: specified as a numeric expression.
403: .RE
404: .PP
405: .B client-state
406: .PP
407: .RS 0.25i
1.1.1.1 ! misho 408: The current state of the client instance being processed. This is
! 409: only useful in DHCP client configuration files. Possible values are:
1.1 misho 410: .TP 2
411: .I \(bu
412: Booting - DHCP client is in the INIT state, and does not yet have an
1.1.1.1 ! misho 413: IP address. The next message transmitted will be a DHCPDISCOVER,
1.1 misho 414: which will be broadcast.
415: .TP
416: .I \(bu
1.1.1.1 ! misho 417: Reboot - DHCP client is in the INIT-REBOOT state. It has an IP
! 418: address, but is not yet using it. The next message to be transmitted
! 419: will be a DHCPREQUEST, which will be broadcast. If no response is
1.1 misho 420: heard, the client will bind to its address and move to the BOUND state.
421: .TP
422: .I \(bu
423: Select - DHCP client is in the SELECTING state - it has received at
424: least one DHCPOFFER message, but is waiting to see if it may receive
1.1.1.1 ! misho 425: other DHCPOFFER messages from other servers. No messages are sent in
1.1 misho 426: the SELECTING state.
427: .TP
428: .I \(bu
429: Request - DHCP client is in the REQUESTING state - it has received at
430: least one DHCPOFFER message, and has chosen which one it will
1.1.1.1 ! misho 431: request. The next message to be sent will be a DHCPREQUEST message,
1.1 misho 432: which will be broadcast.
433: .TP
434: .I \(bu
1.1.1.1 ! misho 435: Bound - DHCP client is in the BOUND state - it has an IP address. No
1.1 misho 436: messages are transmitted in this state.
437: .TP
438: .I \(bu
439: Renew - DHCP client is in the RENEWING state - it has an IP address,
1.1.1.1 ! misho 440: and is trying to contact the server to renew it. The next message to
1.1 misho 441: be sent will be a DHCPREQUEST message, which will be unicast directly
442: to the server.
443: .TP
444: .I \(bu
445: Rebind - DHCP client is in the REBINDING state - it has an IP address,
1.1.1.1 ! misho 446: and is trying to contact any server to renew it. The next message to
1.1 misho 447: be sent will be a DHCPREQUEST, which will be broadcast.
448: .RE
449: .SH REFERENCE: ACTION EXPRESSIONS
450: .PP
451: .B log (\fIpriority\fB, \fIdata-expr\fB)\fR
452: .RS 0.25i
453: .PP
454: Logging statements may be used to send information to the standard logging
455: channels. A logging statement includes an optional priority (\fBfatal\fR,
456: \fBerror\fR, \fBinfo\fR, or \fBdebug\fR), and a data expression.
457: .PP
458: Logging statements take only a single data expression argument, so if you
459: want to output multiple data values, you will need to use the \fBconcat\fR
460: operator to concatenate them.
461: .RE
462: .PP
463: .B execute (\fIcommand-path\fB [, \fIdata-expr1\fB, ... \fIdata-exprN\fB]);\fR
464: .RS 0.25i
465: .PP
466: The \fBexecute\fR statement runs an external command. The first argument
467: is a string literal containing the name or path of the command to run.
468: The other arguments, if present, are either string literals or data-
469: expressions which evaluate to text strings, to be passed as command-line
470: arguments to the command.
471: .PP
472: \fBexecute\fR is synchronous; the program will block until the external
473: command being run has finished. Please note that lengthy program
474: execution (for example, in an "on commit" in dhcpd.conf) may result in
475: bad performance and timeouts. Only external applications with very short
476: execution times are suitable for use.
477: .PP
478: Passing user-supplied data to an external application might be dangerous.
479: Make sure the external application checks input buffers for validity.
480: Non-printable ASCII characters will be converted into dhcpd.conf language
1.1.1.1 ! misho 481: octal escapes ("\\nnn"), make sure your external command handles them as
1.1 misho 482: such.
483: .PP
484: It is possible to use the execute statement in any context, not only
1.1.1.1 ! misho 485: on events. If you put it in a regular scope in the configuration file
1.1 misho 486: you will execute that command every time a scope is evaluated.
487: .RE
488: .SH REFERENCE: DYNAMIC DNS UPDATES
489: .PP
490: The DHCP client and server have the ability to dynamically update the
491: Domain Name System. Within the configuration files, you can define
492: how you want the Domain Name System to be updated. These updates are
493: RFC 2136 compliant so any DNS server supporting RFC 2136 should be
494: able to accept updates from the DHCP server.
495: .SH SECURITY
496: Support for TSIG and DNSSEC is not yet available. When you set your
497: DNS server up to allow updates from the DHCP server or client, you may
498: be exposing it to unauthorized updates. To avoid this, the best you
499: can do right now is to use IP address-based packet filtering to
500: prevent unauthorized hosts from submitting update requests.
501: Obviously, there is currently no way to provide security for client
502: updates - this will require TSIG or DNSSEC, neither of which is yet
503: available in the DHCP distribution.
504: .PP
505: Dynamic DNS (DDNS) updates are performed by using the \fBdns-update\fR
506: expression. The \fBdns-update\fR expression is a boolean expression
507: that takes four parameters. If the update succeeds, the result is
508: true. If it fails, the result is false. The four parameters that the
509: are the resource record type (RR), the left hand side of the RR, the
510: right hand side of the RR and the ttl that should be applied to the
511: record. The simplest example of the use of the function can be found
512: in the reference section of the dhcpd.conf file, where events are
513: described. In this example several statements are being used to make
514: the arguments to the \fBdns-update\fR.
515: .PP
516: In the example, the first argument to the first \f\Bdns-update\fR
517: expression is a data expression that evaluates to the A RR type. The
518: second argument is constructed by concatenating the DHCP host-name
519: option with a text string containing the local domain, in this case
520: "ssd.example.net". The third argument is constructed by converting
521: the address the client has been assigned from a 32-bit number into an
522: ascii string with each byte separated by a ".". The fourth argument,
523: the TTL, specifies the amount of time remaining in the lease (note
524: that this isn't really correct, since the DNS server will pass this
525: TTL out whenever a request comes in, even if that is only a few
526: seconds before the lease expires).
527: .PP
528: If the first \fBdns-update\fR statement succeeds, it is followed up
529: with a second update to install a PTR RR. The installation of a PTR
530: record is similar to installing an A RR except that the left hand side
531: of the record is the leased address, reversed, with ".in-addr.arpa"
532: concatenated. The right hand side is the fully qualified domain name
533: of the client to which the address is being leased.
534: .SH SEE ALSO
535: dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-options(5), dhcpd(8),
536: dhclient(8), RFC2132, RFC2131.
537: .SH AUTHOR
538: The Internet Systems Consortium DHCP Distribution was written by Ted
539: Lemon under a contract with Vixie Labs. Funding for
540: this project was provided through Internet Systems Consortium.
541: Information about Internet Systems Consortium can be found at
542: .B https://www.isc.org.
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