mirror of
https://github.com/freebsd/freebsd-src.git
synced 2024-11-29 08:52:46 +00:00
211 lines
8.3 KiB
HTML
211 lines
8.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
|
|
<html>
|
|
<head>
|
|
<meta name="generator" content="HTML Tidy, see www.w3.org">
|
|
<title>Access Control Options</title>
|
|
</head>
|
|
<body>
|
|
<h3>Access Control Options</h3>
|
|
|
|
<img align="left" src="pic/pogo6.gif" alt="gif"><a href=
|
|
"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Pogo</i>,
|
|
Walt Kelly</a>
|
|
|
|
<p>The skunk watches for intruders and sprays.<br clear="left">
|
|
</p>
|
|
|
|
<hr>
|
|
<h4>Access Control Support</h4>
|
|
|
|
<tt>ntpd</tt> implements a general purpose address-and-mask based
|
|
restriction list. The list is sorted by address and by mask, and
|
|
the list is searched in this order for matches, with the last match
|
|
found defining the restriction flags associated with the incoming
|
|
packets. The source address of incoming packets is used for the
|
|
match, with the 32- bit address being and'ed with the mask
|
|
associated with the restriction entry and then compared with the
|
|
entry's address (which has also been and'ed with the mask) to look
|
|
for a match. Additional information and examples can be found in
|
|
the <a href="notes.htm">Notes on Configuring NTP and Setting up a
|
|
NTP Subnet</a> page.
|
|
|
|
<p>The restriction facility was implemented in conformance with the
|
|
access policies for the original NSFnet backbone time servers.
|
|
While this facility may be otherwise useful for keeping unwanted or
|
|
broken remote time servers from affecting your own, it should not
|
|
be considered an alternative to the standard NTP authentication
|
|
facility. Source address based restrictions are easily circumvented
|
|
by a determined cracker.</p>
|
|
|
|
<h4>The Kiss-of-Death Packet</h4>
|
|
|
|
<p>Ordinarily, packets denied service are simply dropped with no
|
|
further action except incrementing statistics counters. Sometimes a
|
|
more proactive response is needed, such as a server message that
|
|
explicitly requests the client to stop sending and leave a message
|
|
for the system operator. A special packet format has been created
|
|
for this purpose called the kiss-of-death packet. If the <tt>
|
|
kod</tt> flag is set and either service is denied or the client
|
|
limit is exceeded, the server it returns the packet and sets the
|
|
leap bits unsynchronized, stratum zero and the ASCII string "DENY"
|
|
in the reference source identifier field. If the <tt>kod</tt> flag
|
|
is not set, the server simply drops the packet.</p>
|
|
|
|
<p>A client or peer receiving a kiss-of-death packet performs a set
|
|
of sanity checks to minimize security exposure. If this is the
|
|
first packet received from the server, the client assumes an access
|
|
denied condition at the server. It updates the stratum and
|
|
reference identifier peer variables and sets the access denied
|
|
(test 4) bit in the peer flash variable. If this bit is set, the
|
|
client sends no packets to the server. If this is not the first
|
|
packet, the client assumes a client limit condition at the server,
|
|
but does not update the peer variables. In either case, a message
|
|
is sent to the system log.</p>
|
|
|
|
<h4>Access Control Commands</h4>
|
|
|
|
<dl>
|
|
<dt><tt>restrict <i>numeric_address</i> [mask <i>numeric_mask</i>]
|
|
[<i>flag</i>][...]</tt></dt>
|
|
|
|
<dd>The <i><tt>numeric_address</tt></i> argument, expressed in
|
|
dotted- quad form, is the address of an host or network. The <i>
|
|
<tt>mask</tt></i> argument, also expressed in dotted-quad form,
|
|
defaults to <tt>255.255.255.255</tt>, meaning that the <i><tt>
|
|
numeric_address</tt></i> is treated as the address of an individual
|
|
host. A default entry (address <tt>0.0.0.0</tt>, mask <tt>
|
|
0.0.0.0</tt>) is always included and, given the sort algorithm, is
|
|
always the first entry in the list. Note that, while <i><tt>
|
|
numeric_address</tt></i> is normally given in dotted-quad format,
|
|
the text string <tt>default</tt>, with no mask option, may be used
|
|
to indicate the default entry.</dd>
|
|
|
|
<dd>In the current implementation, <i><tt>flag</tt></i> always
|
|
restricts access, i.e., an entry with no flags indicates that free
|
|
access to the server is to be given. The flags are not orthogonal,
|
|
in that more restrictive flags will often make less restrictive
|
|
ones redundant. The flags can generally be classed into two
|
|
catagories, those which restrict time service and those which
|
|
restrict informational queries and attempts to do run-time
|
|
reconfiguration of the server. One or more of the following flags
|
|
may be specified:</dd>
|
|
|
|
<dd>
|
|
<dl>
|
|
<dt><tt>kod</tt></dt>
|
|
|
|
<dd>If access is denied, send a kiss-of-death packet.</dd>
|
|
|
|
<dt><tt>ignore</tt></dt>
|
|
|
|
<dd>Ignore all packets from hosts which match this entry. If this
|
|
flag is specified neither queries nor time server polls will be
|
|
responded to.</dd>
|
|
|
|
<dt><tt>noquery</tt></dt>
|
|
|
|
<dd>Ignore all NTP mode 6 and 7 packets (i.e. information queries
|
|
and configuration requests) from the source. Time service is not
|
|
affected.</dd>
|
|
|
|
<dt><tt>nomodify</tt></dt>
|
|
|
|
<dd>Ignore all NTP mode 6 and 7 packets which attempt to modify the
|
|
state of the server (i.e. run time reconfiguration). Queries which
|
|
return information are permitted.</dd>
|
|
|
|
<dt><tt>notrap</tt></dt>
|
|
|
|
<dd>Decline to provide mode 6 control message trap service to
|
|
matching hosts. The trap service is a subsystem of the mode 6
|
|
control message protocol which is intended for use by remote event
|
|
logging programs.</dd>
|
|
|
|
<dt><tt>lowpriotrap</tt></dt>
|
|
|
|
<dd>Declare traps set by matching hosts to be low priority. The
|
|
number of traps a server can maintain is limited (the current limit
|
|
is 3). Traps are usually assigned on a first come, first served
|
|
basis, with later trap requestors being denied service. This flag
|
|
modifies the assignment algorithm by allowing low priority traps to
|
|
be overridden by later requests for normal priority traps.</dd>
|
|
|
|
<dt><tt>noserve</tt></dt>
|
|
|
|
<dd>Ignore NTP packets whose mode is other than 6 or 7. In effect,
|
|
time service is denied, though queries may still be permitted.</dd>
|
|
|
|
<dt><tt>nopeer</tt></dt>
|
|
|
|
<dd>Provide stateless time service to polling hosts, but do not
|
|
allocate peer memory resources to these hosts even if they
|
|
otherwise might be considered useful as future synchronization
|
|
partners.</dd>
|
|
|
|
<dt><tt>notrust</tt></dt>
|
|
|
|
<dd>Treat these hosts normally in other respects, but never use
|
|
them as synchronization sources.</dd>
|
|
|
|
<dt><tt>limited</tt></dt>
|
|
|
|
<dd>These hosts are subject to limitation of number of clients from
|
|
the same net. Net in this context refers to the IP notion of net
|
|
(class A, class B, class C, etc.). Only the first <tt>
|
|
client_limit</tt> hosts that have shown up at the server and that
|
|
have been active during the last <tt>client_limit_period</tt>
|
|
seconds are accepted. Requests from other clients from the same net
|
|
are rejected. Only time request packets are taken into account.
|
|
Query packets sent by the <tt>ntpq</tt> and <tt>ntpdc</tt> programs
|
|
are not subject to these limits. A history of clients is kept using
|
|
the monitoring capability of <tt>ntpd</tt>. Thus, monitoring is
|
|
always active as long as there is a restriction entry with the <tt>
|
|
limited</tt> flag.</dd>
|
|
|
|
<dt><tt>ntpport</tt></dt>
|
|
|
|
<dd>This is actually a match algorithm modifier, rather than a
|
|
restriction flag. Its presence causes the restriction entry to be
|
|
matched only if the source port in the packet is the standard NTP
|
|
UDP port (123). Both <tt>ntpport</tt> and <tt>non-ntpport</tt> may
|
|
be specified. The <tt>ntpport</tt> is considered more specific and
|
|
is sorted later in the list.</dd>
|
|
|
|
<dt><tt>version</tt></dt>
|
|
|
|
<dd>Ignore these hosts if not the current NTP version.</dd>
|
|
</dl>
|
|
</dd>
|
|
|
|
<dd>Default restriction list entries, with the flags <tt>ignore,
|
|
interface, ntpport</tt>, for each of the local host's interface
|
|
addresses are inserted into the table at startup to prevent the
|
|
server from attempting to synchronize to its own time. A default
|
|
entry is also always present, though if it is otherwise
|
|
unconfigured; no flags are associated with the default entry (i.e.,
|
|
everything besides your own NTP server is unrestricted).</dd>
|
|
|
|
<dt><tt>clientlimit <i>limit</i></tt></dt>
|
|
|
|
<dd>Set the <tt>client_limit</tt> variable, which limits the number
|
|
of simultaneous access-controlled clients. The default value for
|
|
this variable is 3.</dd>
|
|
|
|
<dt><tt>clientperiod <i>period</i></tt></dt>
|
|
|
|
<dd>Set the <tt>client_limit_period</tt> variable, which specifies
|
|
the number of seconds after which a client is considered inactive
|
|
and thus no longer is counted for client limit restriction. The
|
|
default value for this variable is 3600 seconds.</dd>
|
|
</dl>
|
|
|
|
<hr>
|
|
<a href="index.htm"><img align="left" src="pic/home.gif" alt=
|
|
"gif"></a>
|
|
|
|
<address><a href="mailto:mills@udel.edu">David L. Mills
|
|
<mills@udel.edu></a></address>
|
|
</body>
|
|
</html>
|
|
|