131 lines
5.9 KiB
HTML
131 lines
5.9 KiB
HTML
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<!-- $NetBSD: authopt.html,v 1.1 1998/12/30 20:20:33 mcr Exp $ -->
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Strict//EN">
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<html><head><title>
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Authentication Options
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</title></head><body><h3>
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Authentication Options
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</h3><hr>
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<p><h4>Authentication Support</h4>
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<p>The NTP standard specifies an extension which provides cryptographic
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authentication of received NTP packets. This is implemented in
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<code>xntpd</code> using the DES or MD5 algorithms to compute a digital
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signature, or message digest. The specification allows any one of
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possibly 4 billion keys, numbered with 32-bit key identifiers, to be
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used to authenticate an association. The servers involved in an
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association must agree on the key and key identifier used to
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authenticate their messages.
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<p>Keys and related information are specified in a key file, which
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should be exchanged and stored using secure procedures beyond the scope
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of the protocol. There are three classes of keys involved in the current
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implementation. One class is used for ordinary NTP associations, another
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for the <a href = "ntpq.html"> <code>ntpq</code></a> utility program and
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the third for the <a href = "xntpdc.html"> <code>xntpdc</code></a>
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utility program.
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<p><h4>Authentication Commands</h4>
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<dl>
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<dt><code>keys <i>keyfile</i></code>
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<dd>Specifies the file name containing the encryption keys and key
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identifiers used by <code>xntpd</code>, <code>ntpq</code> and
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<code>xntpdc</code> when operating in authenticated mode. The format of
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this file is described later in this document.
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<p><dt><code>trustedkey <i>key</i> [ ... ]</code>
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<dd>Specifies the encryption key identifiers which are trusted for the
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purposes of authenticating peers suitable for synchronization. The
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authentication procedures require that both the local and remote servers
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share the same key and key identifier for this purpose, although
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different keys can be used with different servers. The
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<code><i>key</i></code> arguments are 32-bit unsigned integers. Note
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that NTP key 0 is fixed and globally known. If meaningful authentication
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is to be performed the 0 key should not be trusted.
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<p><dt><code>requestkey <i>key</i></code>
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<dd>Specifies the key identifier to use with the <code>xntpdc</code>
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program, which uses a proprietary protocol specific to this
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implementation of <code>xntpd</code>. This program is useful to diagnose
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and repair problems that affect <code>xntpd</code> operation. The
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<code><i>key</i></code> argument to this command is a 32-bit unsigned
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integer. If no <code>requestkey</code> command is included in the
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configuration file, or if the keys don't match, such requests will be
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ignored.
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<p><dt><code>controlkey <i>key</i></code>
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<dd>Specifies the key identifier to use with the <code>ntpq</code>
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program, which uses the standard protocol defined in RFC-1305. This
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program is useful to diagnose and repair problems that affect
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<code>xntpd</code> operation. The <code><i>key</i></code> argument to
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this command is a 32-bit unsigned integer. If no <code>requestkey</code>
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command is included in the configuration file, or if the keys don't
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match, such requests will be ignored.
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</dl>
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<p><h4>Authentication Key File Format</h4>
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<p>In the case of DES, the keys are 56 bits long with, depending on
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type, a parity check on each byte. In the case of MD5, the keys are 64
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bits (8 bytes). <code>xntpd</code> reads its keys from a file specified
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using the <code>-k</code> command line option or the <code>keys</code>
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statement in the configuration file. While key number 0 is fixed by the
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NTP standard (as 56 zero bits) and may not be changed, one or more of
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the keys numbered 1 through 15 may be arbitrarily set in the keys file.
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<p>The key file uses the same comment conventions as the configuration
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file. Key entries use a fixed format of the form
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<p><code><i>keyno type key</i></code>
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<p>where <code><i>keyno</i></code> is a positive integer,
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<code><i>type</i></code> is a single character which defines the key
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format, and <code><i>key</i></code> is the key itself.
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<p>The key may be given in one of three different formats, controlled by
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the <code><i>type</i></code> character. The three key types, and
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corresponding formats, are listed following.
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<dl>
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<dt><code>S</code>
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<dd>The key is a 64-bit hexadecimal number in the format specified in
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the DES specification; that is, the high order seven bits of each octet
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are used to form the 56-bit key while the low order bit of each octet is
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given a value such that odd parity is maintained for the octet. Leading
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zeroes must be specified (i.e., the key must be exactly 16 hex digits
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long) and odd parity must be maintained. Hence a zero key, in standard
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format, would be given as <code>0101010101010101</code>.
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<p><dt><code>N</code>
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<dd>The key is a 64-bit hexadecimal number in the format specified in
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the NTP standard. This is the same as the DES format, except the bits in
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each octet have been rotated one bit right so that the parity bit is now
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the high order bit of the octet. Leading zeroes must be specified and
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odd parity must be maintained. A zero key in NTP format would be
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specified as <code>8080808080808080</code>.
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<p><dt><code>A</code>
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<dd>The key is a 1-to-8 character ASCII string. A key is formed from
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this by using the low order 7 bits of each ASCII character in the
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string, with zeroes added on the right when necessary to form a full
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width 56-bit key, in the same way that encryption keys are formed from
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Unix passwords.
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<p><dt><code>M</code>
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<dd>The key is a 1-to-8 character ASCII string, using the MD5
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authentication scheme. Note that both the keys and the authentication
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schemes (DES or MD5) must be identical between a set of peers sharing
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the same key number.
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</dl>
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<p>Note that the keys used by the <code>ntpq</code> and
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<code>xntpdc</code> programs are checked against passwords requested by
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the programs and entered by hand, so it is generally appropriate to
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specify these keys in ASCII format.
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<hr><address>David L. Mills (mills@udel.edu)</address></body></html>
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