NTP.CONF(5)

NAME
ntp.conf - Network Time Protocol (NTP) daemon configuration file

SYNOPSIS
/etc/ntp.conf

DESCRIPTION
The ntp.conf configuration file is read at initial startup by the ntpd(8) daemon in order to specify the synchronization sources, modes and other related information. Usually, it is installed in the /etc directory, but could be installed elsewhere (see the daemon's -c command line option).

The file format is similar to other UNIX configuration files. Comments begin with a ‘#’ character and extend to the end of the line; blank lines are ignored. Configuration commands consist of an initial keyword followed by a list of arguments, some of which may be optional, separated by whitespace. Commands may not be continued over multiple lines. Arguments may be host names, host addresses written in numeric, dotted-quad form, integers, floating point numbers (when specifying times in seconds) and text strings.

The rest of this page describes the configuration and control options. The "Notes on Configuring NTP and Setting up a NTP Subnet" page (available as part of the HTML documentation provided in /usr/share/doc/ntp) contains an extended discussion of these options. In addition to the discussion of general Configuration Options, there are sections describing the following supported functionality and the options used to control it:

•   Authentication Support

•   Monitoring Support

•   Access Control Support

•   Reference Clock Support

Following these is a section describing Miscellaneous Options. While there is a rich set of options available, the only required option is one or more server, peer, broadcast or manycastclient commands.

Configuration Support
Following is a description of the configuration commands in NTPv4. These commands have the same basic functions as in NTPv3 and in some cases new functions and new arguments. There are two classes of commands, configuration commands that configure a persistent association with a remote server or peer or reference clock, and auxiliary commands that specify environmental variables that control various related operations.

  Configuration Commands
The various modes are determined by the command keyword and the type of the required IP address. Addresses are classed by type as (s) a remote server or peer (IP class A, B and C), (b) the broadcast address of a local interface, (m) a multicast address (IP class D), or (r) a reference clock address (127.127.x.x). Note that only those options applicable to each command are listed below. Use of options not listed may not be caught as an error, but may result in some weird and even destructive behavior.

server address
    [key key | autokey] [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]

peer address
    [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]

broadcast address
    [key key | autokey] [version version] [prefer] [minpoll minpoll] [ttl ttl]

manycastclient address
    [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll] [ttl ttl]

These four commands specify the time server name or address to be used and the mode in which to operate. The address can be either a DNS name or an IP address in dotted-quad notation. Additional information on association behavior can be found in the "Association Management" page.

server
    For type s and r addresses, this command mobilizes a persistent client mode association with the specified remote server or local radio clock. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock. This command should not be used for type b or m addresses.

peer
    For type s addresses (only), this command mobilizes a persistent symmetric-active mode association with the specified remote peer. In this mode the local clock can be synchronized to the remote peer or the remote peer can be synchronized to the local clock. This is useful in a network of servers where, depending on various failure scenarios, either the local or remote peer may be the better source of time. This command should NOT be used for type b, m or r addresses.

broadcast
    For type b and m addresses (only), this command mobilizes a persistent broadcast mode association. Multiple commands can be used to specify multiple local broadcast interfaces (subnets) and/or multiple multicast groups. Note that local broadcast messages go only to the interface associated with the subnet specified, but multicast messages go to all interfaces. In broadcast mode the local server sends periodic broadcast messages to a client population at the address specified, which is usually the broadcast address on (one of) the local network(s) or a multicast address assigned to NTP. The IANA has assigned the multicast group address 224.0.1.1 exclusively to NTP, but other nonconflicting addresses can be used to contain the messages within administrative boundaries. Ordinarily, this specification applies only to the local server operating as a sender; for operation as a broadcast client, see the broadcastclient or multicastclient commands below.

manycastclient
    For type m addresses (only), this command mobilizes a manycast client mode association for the multicast address specified. In this case a specific address must be supplied which matches the address used on the manycastserver command for the designated manycast servers. The NTP multicast address 224.0.1.1 assigned by the IANA should NOT be used, unless specific means are taken to avoid spraying large areas of the Internet with these messages and causing a possibly massive implosion of replies at the sender. The manycastserver command specifies that the local server is to operate in client mode with the remote servers that are discovered as the result of broadcast/multicast messages. The client broadcasts a request message to the group address associated with the specified address and specifically enabled servers respond to these messages. The client selects the servers providing the best time and continues as with the server command. The remaining servers are discarded as if never heard.

Options:

autokey
    All packets sent to and received from the server or peer are to include authentication fields encrypted using the autokey scheme described in Authentication Options.

burst
    when the server is reachable and at each poll interval, send a burst of eight packets instead of the usual one packet. The spacing between the first and the second packets is about 16s to allow a modem call to complete, while the spacing between the remaining packets is about 2s. This is designed to improve timekeeping quality with the server command and s addresses.

iburst
    When the server is unreachable and at each poll interval, send a burst of eight packets instead of the usual one. As long as the server is unreachable, the spacing between packets is about 16s to allow a modem call to complete. Once the server is reachable, the spacing between packets is about 2s. This is designed to speed the initial synchronization acquisition with the server command and s addresses and when ntpd(8) is started with the -q option.

key key
    All packets sent to and received from the server or peer are to include authentication fields encrypted using the specified key identifier with values from 1 to 65534, inclusive. The default is to include no encryption field.

minpoll minpoll

maxpoll maxpoll
    These options specify the minimum and maximum poll intervals for NTP messages, in seconds to the power of two. The maximum poll interval defaults to 10 (1,024 s), but can be increased by the maxpoll option to an upper limit of 17 (36.4 h). The minimum poll interval defaults to 6 (64 s), but can be decreased by the minpoll option to a lower limit of 4 (16 s).

prefer
    Marks the server as preferred. All other things being equal, this host will be chosen for synchronization among a set of correctly operating hosts. See the "Mitigation Rules and the prefer Keyword" page for further information.

ttl ttl
    This option is used only with broadcast server and manycast client modes. It specifies the time-to-live ttl to use on broadcast server and multicast server and the maximum ttl for the expanding ring search with manycast client packets. Selection of the proper value, which defaults to 127, is something of a black art and should be coordinated with the network administrator.

version version
    Specifies the version number to be used for outgoing NTP packets. Versions 1-4 are the choices, with version 4 the default.

  Auxiliary Commands

broadcastclient
    This command enables reception of broadcast server messages to any local interface (type b) address. Upon receiving a message for the first time, the broadcast client measures the nominal server propagation delay using a brief client/server exchange with the server, then enters the broadcast client mode, in which it synchronizes to succeeding broadcast messages. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or public-key authentication as described in Authentication Options.

manycastserver address ...
    This command enables reception of manycast client messages to the multicast group address(es) (type m) specified. At least one address is required, but the NTP multicast address 224.0.1.1 assigned by the IANA should NOT be used, unless specific means are taken to limit the span of the reply and avoid a possibly massive implosion at the original sender. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or public-key authentication as described in Authentication Options.

multicastclient address ...
    This command enables reception of multicast server messages to the multicast group address(es) (type m) specified. Upon receiving a message for the first time, the multicast client measures the nominal server propagation delay using a brief client/server exchange with the server, then enters the broadcast client mode, in which it synchronizes to succeeding multicast messages. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or public-key authentication as described in Authentication Options.

Authentication Support
Authentication support allows the NTP client to verify that the server is in fact known and trusted and not an intruder intending accidentally or on purpose to masquerade as that server. The NTPv3 specification RFC-1305 defines a scheme which provides cryptographic authentication of received NTP packets. Originally, this was done using the Data Encryption Standard (DES) algorithm operating in Cipher Block Chaining (CBC) mode, commonly called DES-CBC. Subsequently, this was augmented by the RSA Message Digest 5 (MD5) algorithm using a private key, commonly called keyed-MD5. Either algorithm computes a message digest, or one-way hash, which can be used to verify the server has the correct private key and key identifier.

NTPv4 retains the NTPv3 schemes, properly described as symmetric-key cryptography and, in addition, provides a new Autokey scheme based on public-key cryptography. Public-key cryptography is generally considered more secure than symmetric-key cryptography, since the security is based on a private value which is generated by each server and never revealed. With Autokey all key distribution and management functions involve only public values, which considerably simplifies key distribution and storage.

Authentication is configured separately for each association using the key or autokey subcommands on the peer, server, broadcast and manycastclient commands as described in Configuration Options. The authentication options described below specify the suite of keys, select the key for each configured association and manage the configuration operations.

The auth flag controls whether new associations or remote configuration commands require cryptographic authentication. This flag can be set or reset by the enable and disable configuration commands and also by remote configuration commands sent by a ntpdc(8) program running in another machine. If this flag is enabled, which is the default case, new broadcast client and symmetric passive associations and remote configuration commands must be cryptographically authenticated using either symmetric-key or public-key schemes. If this flag is disabled, these operations are effective even if not cryptographic authenticated. It should be understood that operating in the latter mode invites a significant vulnerability where a rogue hacker can seriously disrupt client timekeeping.

In networks with firewalls and large numbers of broadcast clients it may be acceptable to disable authentication, since that avoids key distribution and simplifies network maintenance. However, when the configuration file contains host names, or when a server or client is configured remotely, host names are resolved using the DNS and a separate name resolution process. In order to protect against bogus name server messages, name resolution messages are authenticated using an internally generated key which is normally invisible to the user. However, if cryptographic support is disabled, the name resolution process will fail. This can be avoided either by specifying IP addresses instead of host names, which is generally inadvisable, or by enabling the flag for name resolution and disabled it once the name resolution process is complete.

An attractive alternative where multicast support is available is manycast mode, in which clients periodically troll for servers. Cryptographic authentication in this mode uses public-key schemes as described below. The principle advantage of this manycast mode is that potential servers need not be configured in advance, since the client finds them during regular operation, and the configuration files for all clients can be identical.

In addition to the default symmetric-key cryptographic support, support for public-key cryptography is available if the requisite rsaref20 software distribution has been installed before building the distribution. Public-key cryptography provides secure authentication of servers without compromising accuracy and stability. The security model and protocol schemes for both symmetric-key and public-key cryptography are described below.

  Symmetric-Key Scheme
The original RFC-1305 specification allows any one of possibly 65,534 keys, each distinguished by a 32-bit key identifier, to authenticate an association. The servers and clients involved must agree on the key and key identifier to authenticate their messages. Keys and related information are specified in a key file, usually called ntp.keys, which should be exchanged and stored using secure procedures beyond the scope of the NTP protocol itself. Besides the keys used for ordinary NTP associations, additional keys can be used as passwords for the ntpq(8) and ntpdc(8) utility programs.

When ntpd(8) is first started, it reads the key file specified in the keys command and installs the keys in the key cache. However, the keys must be activated with the trusted command before use. This allows, for instance, the installation of possibly several batches of keys and then activating or deactivating each batch remotely using ntpdc(8). This also provides a revocation capability that can be used if a key becomes compromised. The requestkey command selects the key used as the password for the ntpdc(8) utility, while the controlkey command selects the key used as the password for the ntpq(8) utility.

  Public-Key Scheme
The original NTPv3 authentication scheme described in RFC-1305 continues to be supported; however, in NTPv4 an additional authentication scheme called Autokey is available. It uses MD5 message digest, RSA public-key signature and Diffie-Hellman key agreement algorithms available from several sources, but not included in the NTPv4 software distribution. In order to be effective, the rsaref20 package must be installed as described in the README.rsa file. Once installed, the configure and build process automatically detects it and compiles the routines required. The Autokey scheme has several modes of operation corresponding to the various NTP modes supported. RSA signatures with timestamps are used in all modes to verify the source of cryptographic values. All modes use a special cookie which can be computed independently by the client and server. In symmetric modes the cookie is constructed using the Diffie-Hellman key agreement algorithm. In other modes the cookie is constructed from the IP addresses and a private value known only to the server. All modes use in addition a variant of the S-KEY scheme, in which a pseudo-random key list is generated and used in reverse order. These schemes are described along with an executive summary, current status, briefing slides and reading list, in the "Autonomous Authentication" page.

The cryptographic values used by the Autokey scheme are incorporated as a set of files generated by the ntp-genkeys(8) program, including the symmetric private keys, public/private key pair, and the agreement parameters. See the ntp.keys(5) page for a description of the formats of these files. They contain cryptographic values generated by the algorithms of the rsaref20 package and are in printable ASCII format. All file names include the timestamp, in NTP seconds, following the default names given below. Since the file data are derived from random values seeded by the system clock and the file name includes the timestamp, every generation produces a different file and different file name.

The ntp.keys file contains the DES/MD5 private keys. It must be distributed by secure means to other servers and clients sharing the same security compartment and made visible only to root. While this file is not used with the Autokey scheme, it is needed to authenticate some remote configuration commands used by the ntpdc(8), ntpq(8) utilities. The ntpkey file contains the RSA private key. It is useful only to the machine that generated it and never shared with any other daemon or application program, so must be made visible only to root.

The ntp_dh file contains the agreement parameters, which are used only in symmetric (active and passive) modes. It is necessary that both peers beginning a symmetric-mode association share the same parameters, but it does not matter which ntp_dh file generates them. If one of the peers contains the parameters, the other peer obtains them using the Autokey protocol. If both peers contain the parameters, the most recent copy is used by both peers. If a peer does not have the parameters, they will be requested by all associations, either configured or not; but, none of the associations can proceed until one of them has received the parameters. Once loaded, the parameters can be provided on request to other clients and servers. The ntp_dh file can be also be distributed using insecure means, since the data are public values.

The ntpkey_host file contains the RSA public key, where host is the name of the host. Each host must have its own ntpkey_host file, which is normally provided to other hosts using the Autokey protocol. Each server or peer association requires the public key associated with the particular server or peer to be loaded either directly from a local file or indirectly from the server using the Autokey protocol. These files can be widely distributed and stored using insecure means, since the data are public values.

The optional ntpkey_certif_host file contains the PKI certificate for the host. This provides a binding between the host hame and RSA public key. In the current implementation the certificate is obtained by a client, if present, but the contents are ignored.

Due to the widespread use of interface-specific naming, the host names used in configured and mobilized associations are determined by the UNIX gethostname(3) library routine. Both the ntp-genkeys(8) program and the Autokey protocol derive the name of the public key file using the name returned by this routine. While every server and client is required to load their own public and private keys, the public keys for each client or peer association can be obtained from the server or peer using the Autokey protocol. Note however, that at the current stage of development the authenticity of the server or peer and the cryptographic binding of the server name, address and public key is not yet established by a certificate authority or web of trust.

  Leapseconds Table
The NIST provides a table showing the epoch for all historic occasions of leap second insertion since 1972. The leapsecond table shows each epoch of insertion along with the offset of International Atomic Time (TAI) with respect to Coordinated Universal Time (UTC), as disseminated by NTP. The table can be obtained directly from NIST national time servers using FTP as the ASCII file pub/leap-seconds.

While not strictly a security function, the Autokey scheme provides means to securely retrieve the leapsecond table from a server or peer. Servers load the leapsecond table directly from the file specified in the crypto command, while clients can load the table indirectly from the servers using the Autokey protocol. Once loaded, the table can be provided on request to other clients and servers.

  Key Management
All key files are installed by default in /usr/local/etc, which is normally in a shared file system in NFS-mounted networks and avoids installing them in each machine separately. The default can be overridden by the keysdir configuration command. However, this is not a good place to install the private key file, since each machine needs its own file. A suitable place to install it is in /etc, which is normally not in a shared file system.

The recommended practice is to keep the timestamp extensions when installing a file and to install a link from the default name (without the timestamp extension) to the actual file. This allows new file generations to be activated simply by changing the link. However, ntpd(8) parses the link name when present to extract the extension value and sends it along with the public key and host name when requested. This allows clients to verify that the file and generation time are always current. However, the actual location of each file can be overridden by the crypto configuration command.

All cryptographic keys and related parameters should be regenerated on a periodic and automatic basis, like once per month. The ntp-genkeys(8) program uses the same timestamp extension for all files generated at one time, so each generation is distinct and can be readily recognized in monitoring data. While a public/private key pair must be generated by every server and client, the public keys and agreement parameters do not need to be explicitly copied to all machines in the same security compartment, since they can be obtained automatically using the Autokey protocol. However, it is necessary that all primary servers have the same agreement parameter file. The recommended way to do this is for one of the primary servers to generate that file and then copy it to the other primary servers in the same compartment using the UNIX rdist(1) command. Future versions of the Autokey protocol are to contain provisions for an agreement protocol to do this automatically.

Servers and clients can make a new generation in the following way. All machines have loaded the old generation at startup and are operating normally. At designated intervals, each machine generates a new public/private key pair and makes links from the default file names to the new file names. The ntpd(8) is then restarted and loads the new generation, with result clients no longer can authenticate correctly. The Autokey protocol is designed so that after a few minutes the clients time out and restart the protocol from the beginning, with result the new generation is loaded and operation continues as before. A similar procedure can be used for the agreement parameter file, but in this case precautions must be take to be sure that all machines with this file have the same copy.

  Authentication Commands

autokey [logsec]
    Specifies the interval between regenerations of the session key list used with the Autokey protocol. Note that the size of the key list for each association depends on this interval and the current poll interval. The default value is 12 (4096 s or about 1.1 hours). For poll intervals above the specified interval, a session key list with a single entry will be regenerated for every message sent.

controlkey key
    Specifies the key identifier to use with the ntpq(8) utility, which uses the standard protocol defined in RFC-1305. The key argument is the key identifier for a trusted key, where the value can be in the range 1 to 65534, inclusive.

crypto
    [flags flags] [privatekey file] [publickey file] [dhparms file] [leap file] This command requires the NTP daemon build process be configured with the RSA library. This command activates public-key cryptography and loads the required RSA private and public key files and the optional Diffie-Hellman agreement parameter file, if present. If one or more files are left unspecified, the default names are used as described below. Following are the subcommands:

    privatekey file
        Specifies the location of the RSA private key file, which otherwise defaults to /usr/local/etc/ntpkey.

    publickey file
        Specifies the location of the RSA public key file, which otherwise defaults to /usr/local/etc/ntpkey_host, where host is the name of the generating machine.

    dhparms file
        Specifies the location of the Diffie-Hellman parameters file, which otherwise defaults to /usr/local/etc/ntpkey_dh.

    leap file
        Specifies the location of the leapsecond table file, which otherwise defaults to /usr/local/etc/ntpkey_leap.

keys keyfile
    Specifies the location of the DES/MD5 private key file containing the keys and key identifiers used by ntpd(8), ntpq(8) and ntpdc(8) when operating in symmetric-key mode.

keysdir path
    This command requires the NTP daemon build process be configured with the RSA library. It specifies the default directory path for the private key file, agreement parameters file and one or more public key files. The default when this command does not appear in the configuration file is /usr/local/etc.

requestkey key
    Specifies the key identifier to use with the ntpdc(8) utility program, which uses a proprietary protocol specific to this implementation of ntpd(8). The key argument is a key identifier for the trusted key, where the value can be in the range 1 to 65534, inclusive.

revoke logsec
    Specifies the interval between re-randomization of certain cryptographic values used by the Autokey scheme, as a power of 2 in seconds. These values need to be updated frequently in order to deflect brute-force attacks on the algorithms of the scheme; however, updating some values is a relatively expensive operation. The default interval is 16 (65,536 s or about 18 hours). For poll intervals above the specified interval, the values will be updated for every message sent.

trustedkey key ...
    Specifies the key identifiers which are trusted for the purposes of authenticating peers with symmetric-key cryptography, as well as keys used by the ntpq(8) and ntpdc(8) programs. The authentication procedures require that both the local and remote servers share the same key and key identifier for this purpose, although different keys can be used with different servers. The key arguments are 32-bit unsigned integers with values from 1 to 65,534.

Monitoring Support
ntpd(8) includes a comprehensive monitoring facility suitable for continuous, long term recording of server and client timekeeping performance. See the statistics command below for a listing and example of each type of statistics currently supported. Statistic files are managed using file generation sets and scripts in the ./scripts directory of this distribution. Using these facilities and UNIX cron(8) jobs, the data can be automatically summarized and archived for retrospective analysis.

  Monitoring Commands

statistics name ...
    Enables writing of statistics records. Currently, four kinds of name statistics are supported.

    loopstats
        Enables recording of loop filter statistics information. Each update of the local clock outputs a line of the following form to the file generation set named loopstats:

            50935 75440.031 0.000006019 13.778190 0.000351733 0.013380 6

        The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The next five fields show time offset (seconds), frequency offset (parts per million - PPM), RMS jitter (seconds), Allan deviation (PPM) and clock discipline time constant.

    peerstats
        Enables recording of peer statistics information. This includes statistics records of all peers of a NTP server and of special signals, where present and configured. Each valid update appends a line of the following form to the current element of a file generation set named peerstats:

            48773 10847.650 127.127.4.1 9714 -0.001605 0.00000 0.00142

        The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The next two fields show the peer address in dotted-quad notation and status, respectively. The status field is encoded in hex in the format described in Appendix A of the NTP specification RFC 1305. The final three fields show the offset, delay and RMS jitter, all in seconds.

    clockstats
        Enables recording of clock driver statistics information. Each update received from a clock driver appends a line of the following form to the file generation set named clockstats:

            49213 525.624 127.127.4.1 93 226 00:08:29.606 D

        The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The next field shows the clock address in dotted-quad notation. The final field shows the last timecode received from the clock in decoded ASCII format, where meaningful. In some clock drivers a good deal of additional information can be gathered and displayed as well. See information specific to each clock for further details.

    rawstats
        Enables recording of raw-timestamp statistics information. This includes statistics records of all peers of a NTP server and of special signals, where present and configured. Each NTP message received from a peer or clock driver appends a line of the following form to the file generation set named rawstats:

            50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000

        The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The next two fields show the remote peer or clock address followed by the local address in dotted-quad notation. The final four fields show the originate, receive, transmit and final NTP timestamps in order. The timestamp values are as received and before processing by the various data smoothing and mitigation algorithms.

statsdir directory_path
    Indicates the full path of a directory where statistics files should be created (see below). This keyword allows the (otherwise constant) filegen filename prefix to be modified for file generation sets, which is useful for handling statistics logs.

filegen name
    [file filename] [type typename] [link | nolink] [enable | disable] Configures setting of generation file set name. Generation file sets provide a means for handling files that are continuously growing during the lifetime of a server. Server statistics are a typical example for such files. Generation file sets provide access to a set of files used to store the actual data. At any time at most one element of the set is being written to. The type given specifies when and how data will be directed to a new element of the set. This way, information stored in elements of a file set that are currently unused are available for administrational operations without the risk of disturbing the operation of ntpd(8). (Most important: they can be removed to free space for new data produced.) Note that this command can be sent from the ntpdc(8) program running at a remote location.

    name
        This is the type of the statistics records, as shown in the statistics command.

    file filename
        This is the file name for the statistics records. Filenames of set members are built from three concatenated elements prefix, filename and suffix:

        prefix
            This is a constant filename path. It is not subject to modifications via the filegen option. It is defined by the server, usually specified as a compile-time constant. It may, however, be configurable for individual file generation sets via other commands. For example, the prefix used with loopstats and peerstats generation can be configured using the statsdir option explained above.

        filename
            This string is directly concatenated to the prefix mentioned above (no intervening ‘/’ (slash)). This can be modified using the file argument to the filegen statement. No ‘..’ elements are allowed in this component to prevent filenames referring to parts outside the file system hierarchy denoted by prefix.

        suffix
            This part is reflects individual elements of a file set. It is generated according to the type of a file set.

    type typename
        A file generation set is characterized by its type. The following types are supported:

        none
            The file set is actually a single plain file.

        pid One element of file set is used per incarnation of a ntpd(8) server. This type does not perform any changes to file set members during runtime, however it provides an easy way of separating files belonging to different ntpd(8) server incarnations. The set member filename is built by appending a ‘’. (dot) to concatenated prefix and filename strings, and appending the decimal representation of the process ID of the ntpd(8) server process.

        day One file generation set element is created per day. A day is defined as the period between 00:00 and 24:00 UTC. The file set member suffix consists of a ‘’. (dot) and a day specification in the form YYYYMMdd. YYYY is a 4-digit year number (e.g., 1992). MM is a two digit month number. dd is a two digit day number. Thus, all information written at 10 December 1992 would end up in a file named ~prefix/filename/19921210.

        week
            Any file set member contains data related to a certain week of a year. The term week is defined by computing day-of-year modulo 7. Elements of such a file generation set are distinguished by appending the following suffix to the file set filename base: A dot, a 4-digit year number, the letter Ql W , and a 2-digit week number. For example, information from January, 10th 1992 would end up in a file with suffix .1992W1.

        month
            One generation file set element is generated per month. The file name suffix consists of a dot, a 4-digit year number, and a 2-digit month.

        year
            One generation file element is generated per year. The filename suffix consists of a dot and a 4 digit year number.

        age This type of file generation sets changes to a new element of the file set every 24 hours of server operation. The filename suffix consists of a dot, the letter ‘a’, and an 8-digit number. This number is taken to be the number of seconds the server is running at the start of the corresponding 24-hour period. Information is only written to a file generation by specifying enable; output is prevented by specifying disable.

    link | nolink
        It is convenient to be able to access the current element of a file generation set by a fixed name. This feature is enabled by specifying link and disabled using nolink. If link is specified, a hard link from the current file set element to a file without suffix is created. When there is already a file with this name and the number of links of this file is one, it is renamed appending a dot, the letter ‘C’, and the pid of the ntpd(8) server process. When the number of links is greater than one, the file is unlinked. This allows the current file to be accessed by a constant name.

    enable | disable
        Enables or disables the recording function.

Access Control Support
ntpd(8) 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 "Notes on Configuring NTP and Setting up a NTP Subnet" page.

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.

  The Kiss-of-Death Packet
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 kod flag is set and either service is denied or the client limit is exceeded, the server returns the packet and sets the leap bits unsynchronized, stratum zero and the ASCII string "DENY" in the reference source identifier field. If the kod flag is not set, the server simply drops the packet.

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.

  Access Control Commands

restrict numeric_address
    [mask numeric_mask] [flag ...] The numeric_address argument, expressed in dotted-quad form, is the address of a host or network. The mask, also expressed in dotted-quad form, defaults to 255.255.255.255, meaning that the numeric_address is treated as the address of an individual host. A default entry (address 0.0.0.0, mask 0.0.0.0) is always included and, given the sort algorithm, is always the first entry in the list. Note that, while numeric_address is normally given in dotted-quad format, the text string ‘default’, with no mask option, may be used to indicate the default entry. In the current implementation, flag 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 categories, 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:

    kod If access is denied, send a kiss-of-death packet.

    ignore
        Ignore all packets from hosts which match this entry. If this flag is specified neither queries nor time server polls will be responded to.

    noquery
        Ignore all NTP mode 6 and 7 packets (i.e., information queries and configuration requests) from the source. Time service is not affected.

    nomodify
        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.

    notrap
        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.

    lowpriotrap
        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.

    noserve
        Ignore NTP packets whose mode is other than 6 or 7. In effect, time service is denied, though queries may still be permitted.

    nopeer
        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.

    notrust
        Treat these hosts normally in other respects, but never use them as synchronization sources.

    limited
        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 client_limit hosts that have shown up at the server and that have been active during the last client_limit_period 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 ntpq(8) and ntpdc(8) programs are not subject to these limits. A history of clients is kept using the monitoring capability of ntpd(8). Thus, monitoring is always active as long as there is a restriction entry with the limited flag.

    ntpport
        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 ntpport and non-ntpport may be specified. The ntpport is considered more specific and is sorted later in the list.

    version
        Ignore these hosts if not the current NTP version.

    Default restriction list entries, with the flags ignore, interface, ntpport, 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).

clientlimit limit
    Set the client_limit variable, which limits the number of simultaneous access-controlled clients. The default value for this variable is 3.

clientperiod period
    Set the client_limit_period 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.

Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio, satellite and modem reference clocks plus a special pseudo-clock used for backup or when no other clock source is available. Detailed descriptions of individual device drivers and options can be found in the "Reference Clock Drivers" page (available as part of the HTML documentation provided in /usr/share/doc/ntp). Additional information can be found in the pages linked there, including the "Debugging Hints for Reference Clock Drivers" and "How To Write a Reference Clock Driver" pages. In addition, support for a PPS signal is available as described in the "Pulse-per-second (PPS) Signal Interfacing" page. Many drivers support special line discipline/streams modules which can significantly improve the accuracy using the driver. These are described in the "Line Disciplines and Streams Drivers" page.

A reference clock will generally (though not always) be a radio timecode receiver which is synchronized to a source of standard time such as the services offered by the NRC in Canada and NIST and USNO in the US. The interface between the computer and the timecode receiver is device dependent, but is usually a serial port. A device driver specific to each reference clock must be selected and compiled in the distribution; however, most common radio, satellite and modem clocks are included by default. Note that an attempt to configure a reference clock when the driver has not been compiled or the hardware port has not been appropriately configured results in a scalding remark to the system log file, but is otherwise non hazardous.

For the purposes of configuration, ntpd(8) treats reference clocks in a manner analogous to normal NTP peers as much as possible. Reference clocks are identified by a syntactically correct but invalid IP address, in order to distinguish them from normal NTP peers. Reference clock addresses are of the form 127.127.t.u, where t is an integer denoting the clock type and u indicates the unit number in the range 0-3. While it may seem overkill, it is in fact sometimes useful to configure multiple reference clocks of the same type, in which case the unit numbers must be unique.

The server command is used to configure a reference clock, where the address argument in that command is the clock address. The key, version and ttl options are not used for reference clock support. The mode option is added for reference clock support, as described below. The prefer option can be useful to persuade the server to cherish a reference clock with somewhat more enthusiasm than other reference clocks or peers. Further information on this option can be found in the "Mitigation Rules and the prefer Keyword" page. The minpoll and maxpoll options have meaning only for selected clock drivers. See the individual clock driver document pages for additional information.

The fudge command is used to provide additional information for individual clock drivers and normally follows immediately after the server command. The address argument specifies the clock address. The refid and stratum options can be used to override the defaults for the device. There are two optional device-dependent time offsets and four flags that can be included in the fudge command as well.

The stratum number of a reference clock is by default zero. Since the ntpd(8) daemon adds one to the stratum of each peer, a primary server ordinarily displays an external stratum of one. In order to provide engineered backups, it is often useful to specify the reference clock stratum as greater than zero. The stratum option is used for this purpose. Also, in cases involving both a reference clock and a pulse-per-second (PPS) discipline signal, it is useful to specify the reference clock identifier as other than the default, depending on the driver. The refid option is used for this purpose. Except where noted, these options apply to all clock drivers.

  Reference Clock Commands

server
    127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int] This command can be used to configure reference clocks in special ways. The options are interpreted as follows:

    prefer
        Marks the reference clock as preferred. All other things being equal, this host will be chosen for synchronization among a set of correctly operating hosts. See the "Mitigation Rules and the prefer Keyword" page for further information.

    mode int
        Specifies a mode number which is interpreted in a device-specific fashion. For instance, it selects a dialing protocol in the ACTS driver and a device subtype in the parse drivers.

    minpoll int

    maxpoll int
        These options specify the minimum and maximum polling interval for reference clock messages, in seconds to the power of two. For most directly connected reference clocks, both minpoll and maxpoll default to 6 (64 s). For modem reference clocks, minpoll defaults to 10 (17.1 m) and maxpoll defaults to 14 (4.5 h). The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.

fudge
    127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [mode int] [flag1 0 | 1] [flag2 0 | 1] [flag3 0 | 1] [flag4 0 | 1] This command can be used to configure reference clocks in special ways. It must immediately follow the server command which configures the driver. Note that the same capability is possible at run time using the ntpdc(8) program. The options are interpreted as follows:

    time1 sec
        Specifies a constant to be added to the time offset produced by the driver, a fixed-point decimal number in seconds. This is used as a calibration constant to adjust the nominal time offset of a particular clock to agree with an external standard, such as a precision PPS signal. It also provides a way to correct a systematic error or bias due to serial port or operating system latencies, different cable lengths or receiver internal delay. The specified offset is in addition to the propagation delay provided by other means, such as internal DIPswitches. Where a calibration for an individual system and driver is available, an approximate correction is noted in the driver documentation pages. Note: in order to facilitate calibration when more than one radio clock or PPS signal is supported, a special calibration feature is available. It takes the form of an argument to the enable command described in Miscellaneous Options page and operates as described in the "Reference Clock Drivers" page.

    time2 secs
        Specifies a fixed-point decimal number in seconds, which is interpreted in a driver-dependent way. See the descriptions of specific drivers in the "reference clock drivers" page.

    stratum int
        Specifies the stratum number assigned to the driver, an integer between 0 and 15. This number overrides the default stratum number ordinarily assigned by the driver itself, usually zero.

    refid string
        Specifies an ASCII string of from one to four characters which defines the reference identifier used by the driver. This string overrides the default identifier ordinarily assigned by the driver itself.

    mode int
        Specifies a mode number which is interpreted in a device-specific fashion. For instance, it selects a dialing protocol in the ACTS driver and a device subtype in the parse drivers.

    flag1 0 | 1

    flag2 0 | 1

    flag3 0 | 1

    flag4 0 | 1
        These four flags are used for customizing the clock driver. The interpretation of these values, and whether they are used at all, is a function of the particular clock driver. However, by convention flag4 is used to enable recording monitoring data to the clockstats file configured with the filegen command. Further information on the filegen command can be found in Monitoring Options.

Miscellaneous Options

broadcastdelay seconds
    The broadcast and multicast modes require a special calibration to determine the network delay between the local and remote servers. Ordinarily, this is done automatically by the initial protocol exchanges between the client and server. In some cases, the calibration procedure may fail due to network or server access controls, for example. This command specifies the default delay to be used under these circumstances. Typically (for Ethernet), a number between 0.003 and 0.007 seconds is appropriate. The default when this command is not used is 0.004 seconds.

driftfile driftfile
    This command specifies the name of the file used to record the frequency offset of the local clock oscillator. If the file exists, it is read at startup in order to set the initial frequency offset and then updated once per hour with the current frequency offset computed by the daemon. If the file does not exist or this command is not given, the initial frequency offset is assumed zero. In this case, it may take some hours for the frequency to stabilize and the residual timing errors to subside.

    The file format consists of a single line containing a single floating point number, which records the frequency offset measured in parts-per-million (PPM). The file is updated by first writing the current drift value into a temporary file and then renaming this file to replace the old version. This implies that ntpd(8) must have write permission for the directory the drift file is located in, and that file system links, symbolic or otherwise, should be avoided.

enable
    [ auth | bclient | calibrate | kernel | monitor | ntp | stats]

disable
    [ auth | bclient | calibrate | kernel | monitor | ntp | stats] Provides a way to enable or disable various server options. Flags not mentioned are unaffected. Note that all of these flags can be controlled remotely using the ntpdc(8) utility program.

    bclient
        When enabled, this is identical to the broadcastclient command. The default for this flag is disable.

    calibrate
        Enables the calibration facility, which automatically adjusts the time1 values for each clock driver to display the same offset as the currently selected source or kernel discipline signal. See the "Reference Clock Drivers" page for further information. The default for this flag is disable.

    kernel
        Enables the precision-time kernel support for the adjtime(2) system call, if implemented. Ordinarily, support for this routine is detected automatically when the NTP daemon is compiled, so it is not necessary for the user to worry about this flag. It is provided primarily so that this support can be disabled during kernel development. The default for this flag is enable.

    monitor
        Enables the monitoring facility. See the ntpdc(8) program and the monlist command or further information. The default for this flag is enable.

    ntp Enables the server to adjust its local clock by means of NTP. If disabled, the local clock free-runs at its intrinsic time and frequency offset. This flag is useful in case the local clock is controlled by some other device or protocol and NTP is used only to provide synchronization to other clients. In this case, the local clock driver can be used to provide this function and also certain time variables for error estimates and leap-indicators. See the "Reference Clock Drivers" page for further information. The default for this flag is enable.

    stats
        Enables the statistics facility. See the "Monitoring Options" page for further information. The default for this flag is enable.

logconfig configkeyword
    This command controls the amount and type of output written to the system syslog(3) facility or the alternate logfile log file. By default, all output is turned on. All configkeyword keywords can be prefixed with ‘=’, ‘+’ and ‘-’, where ‘=’ sets the syslog(3) priority mask, ‘+’ adds and ‘-’ removes messages. syslog(3) messages can be controlled in four classes ( clock, peer, sys and sync). Within these classes four types of messages can be controlled. Informational messages (info) control configuration information. Event messages (events) control logging of events (reachability, synchronization, alarm conditions). Statistical output is controlled with the statistics keyword. The final message group is the status messages. This describes mainly the synchronizations status. Configuration keywords are formed by concatenating the message class with the event class. The all prefix can be used instead of a message class. A message class may also be followed by the all keyword to enable/disable all messages of the respective message class. Thus, a minimal log configuration could look like this:

        logconfig =syncstatus +sysevents

    This would just list the synchronizations state of ntpd(8) and the major system events. For a simple reference server, the following minimum message configuration could be useful:

        logconfig =syncall +clockall

    This configuration will list all clock information and synchronization information. All other events and messages about peers, system events and so on is suppressed.

logfile logfile
    This command specifies the location of an alternate log file to be used instead of the default system syslog(3) facility.

setvar variable [default]
    This command adds an additional system variable. These variables can be used to distribute additional information such as the access policy. If the variable of the form name=value is followed by the default keyword, the variable will be listed as part of the default system variables ( ntpq(8) rv command )). These additional variables serve informational purposes only. They are not related to the protocol other that they can be listed. The known protocol variables will always override any variables defined via the setvar mechanism. There are three special variables that contain the names of all variable of the same group. The sys_var_list holds the names of all system variables. The peer_var_list holds the names of all peer variables and the clock_var_list holds the names of the reference clock variables.

tinker
    [ step step | panic panic | dispersion dispersion | stepout stepout | minpoll minpoll | allan allan | huffpuff huffpuff] This command can be used to alter several system variables in very exceptional circumstances. It should occur in the configuration file before any other configuration options. The default values of these variables have been carefully optimized for a wide range of network speeds and reliability expectations. In general, they interact in intricate ways that are hard to predict and some combinations can result in some very nasty behavior. Very rarely is it necessary to change the default values; but, some folks can't resist twisting the knobs anyway and this command is for them. Emphasis added: twisters are on their own and can expect no help from the support group.

    All arguments are in floating point seconds or seconds per second. The minpoll argument is an integer in seconds to the power of two. The variables operate as follows:

    step step
        The argument becomes the new value for the step threshold, normally 0.128 s. If set to zero, step adjustments will never occur. In general, if the intent is only to avoid step adjustments, the step threshold should be left alone and the -x command line option be used instead.

    panic panic
        The argument becomes the new value for the panic threshold, normally 1000 s. If set to zero, the panic sanity check is disabled and a clock offset of any value will be accepted.

    dispersion dispersion
        The argument becomes the new value for the dispersion increase rate, normally .000015.

    stepout stepout
        The argument becomes the new value for the watchdog timeout, normally 900 s.

    minpoll minpoll
        The argument becomes the new value for the minimum poll interval used when configuring multicast client, manycast client and , symmetric passive mode association. The value defaults to 6 (64 s) and has a lower limit of 4 (16 s).

    allan allan
        The argument becomes the new value for the minimum Allan intercept, which is a parameter of the PLL/FLL clock discipline algorithm. The value defaults to 1024 s, which is also the lower limit.

    huffpuff huffpuff
        The argument becomes the new value for the experimental huff-n'-puff filter span, which determines the most recent interval the algorithm will search for a minimum delay. The lower limit is 900 s (15 m), but a more reasonable value is 7200 (2 hours). There is no default, since the filter is not enabled unless this command is given.

trap host_address
    [port port_number] [interface interface_address] This command configures a trap receiver at the given host address and port number for sending messages with the specified local interface address. If the port number is unspecified, a value of 18447 is used. If the interface address is not specified, the message is sent with a source address of the local interface the message is sent through. Note that on a multihomed host the interface used may vary from time to time with routing changes.

    The trap receiver will generally log event messages and other information from the server in a log file. While such monitor programs may also request their own trap dynamically, configuring a trap receiver will ensure that no messages are lost when the server is started.

FILES
/etc/ntp.conf
    the default name of the configuration file
ntp.keys
    private MD5 keys
ntpkey
    RSA private key
ntpkey_host
    RSA public key
ntp_dh
    Diffie-Hellman agreement parameters

SEE ALSO
ntpd(8), ntpdc(8), ntpq(8)

In addition to the manual pages provided, comprehensive documentation is available on the world wide web at http://www.ntp.org/. A snapshot of this documentation is available in HTML format in /usr/share/doc/ntp. David L. Mills, Network Time Protocol (Version 3), RFC1305.

BUGS
The syntax checking is not picky; some combinations of ridiculous and even hilarious options and modes may not be detected.

The ntpkey_host files are really digital certificates. These should be obtained via secure directory services when they become universally available.