dhcp-eval.5

DHCP服务器源码

considered.   \fIOffset\fR and \fIlength\fR are applied to the
contents packet as in the \fBsubstring\fR operator.
.RE
.PP
.I string
.PP
.RS 0.25i
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 0xff.
.RE
.PP
.I colon-separated hexadecimal list
.PP
.RS 0.25i
A list of hexadecimal octet values, separated by colons, may be
specified as a data expression.
.RE
.PP
.B concat (\fIdata-expr1\fB, ..., \fIdata-exprN\fB)\fR
.RS 0.25i
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.
.RE
.PP
.B reverse (\fInumeric-expr1\fB, \fIdata-expr2\fB)\fR
.RS 0.25i
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.
.RE
.PP
.B leased-address
.RS 0.25i
In any context where the client whose request is being processed has
been assigned an IP address, this data expression returns that IP
address.
.RE
.PP
.B binary-to-ascii (\fInumeric-expr1\fB, \fInumeric-expr2\fB,
.B \fIdata-expr1\fB,\fR \fIdata-expr2\fB)\fR
.RS 0.25i
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.
.PP
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:
.RE
.PP
.nf
        concat (binary-to-ascii (10, 8, ".",
                                 reverse (1, leased-address)),
                ".in-addr.arpa.");

.fi
.PP
.B encode-int (\fInumeric-expr\fB, \fIwidth\fB)\fR
.RS 0.25i
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.
.PP
.B pick-first-value (\fIdata-expr1\fR [ ... \fIexpr\fRn ] \fB)\fR
.RS 0.25i
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.
.RE
.PP
.B host-decl-name
.RS 0.25i
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.
.RE
.SH 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.
.PP
.B extract-int (\fIdata-expr\fB, \fIwidth\fB)\fR
.PP
.RS 0.25i
The \fBextract-int\fR 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.
.RE
.PP
.B lease-time
.PP
.RS 0.25i
The duration of the current lease - that is, the difference between
the current time and the time that the lease expires.
.RE
.PP
.I number
.PP
.RS 0.25i
Any number between zero and the maximum representable size may be
specified as a numeric expression.
.RE
.PP
.B client-state
.PP
.RS 0.25i
The current state of the client instance being processed.   This is
only useful in DHCP client configuration files.   Possible values are:
.TP 2
.I \(bu
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.
.TP
.I \(bu
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.
.TP
.I \(bu
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.
.TP
.I \(bu
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.
.TP
.I \(bu
Bound - DHCP client is in the BOUND state - it has an IP address.   No
messages are transmitted in this state.
.TP
.I \(bu
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.
.TP
.I \(bu
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.
.RE
.SH REFERENCE: LOGGING
Logging statements may be used to send information to the standard logging
channels.  A logging statement includes an optional priority (\fBfatal\fR,
\fBerror\fR, \fBinfo\fR, or \fBdebug\fR), and a data expression.
.PP
.B log (\fIpriority\fB, \fIdata-expr\fB)\fR
.PP
Logging statements take only a single data expression argument, so if you
want to output multiple data values, you will need to use the \fBconcat\fR
operator to concatenate them.
.RE
.SH REFERENCE: DYNAMIC DNS UPDATES
.PP
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.
.SH 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.
.PP
Dynamic DNS (DDNS) updates are performed by using the \fBdns-update\fR
expression.  The \fBdns-update\fR 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 \fBdns-update\f\R.
.PP
In the example, the first argument to the first \f\Bdns-update\fR
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).
.PP
If the first \fBdns-update\fR 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.
.SH SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5), dhcpd(8),
dhclient(8), RFC2132, RFC2131.
.SH 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
.B http://www.isc.org.