Configuration Commands

If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is detected, support for the IPv6 address family is generated in addition to the default support of the IPv4 address family. In a few cases, including the reslist billboard generated by ntpq(8) or ntpdc(8), IPv6 addresses are automatically generated. IPv6 addresses can be identified by the presence of colons “:” in the address field. IPv6 addresses can be used almost everywhere where IPv4 addresses can be used, with the exception of reference clock addresses, which are always IPv4.

Note that in contexts where a host name is expected, a -4 qualifier preceding the host name forces DNS resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to the IPv6 namespace. See IPv6 references for the equivalent classes for that address family.

pool address [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]

 

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

 

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

 

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

 

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

 

These five 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 (available as part of the HTML documentation provided in /usr/share/doc/ntp).

pool

For type s addresses, this command mobilizes a persistent client mode association with a number of remote servers. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock.

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 IPv4 224.0.1.1 and IPv6 ff05::101 (site local) 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, send a burst of eight packets instead of the usual one. The packet spacing is normally 2 s; however, the spacing between the first and second packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete. This is designed to improve timekeeping quality with the server command and s addresses.

iburst

When the server is unreachable, send a burst of eight packets instead of the usual one. The packet spacing is normally 2 s; however, the spacing between the first two packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete. 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 65535, 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, as a power of 2 in seconds 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).

noselect

Marks the server as unused, except for display purposes. The server is discarded by the selection algroithm.

preempt

Says the association can be preempted.

true

Marks the server as a truechimer. Use this option only for testing.

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 (available as part of the HTML documentation provided in /usr/share/doc/ntp) for further information.

true

Forces the association to always survive the selection and clustering algorithms. This option should almost certainly only be used while testing an association.

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.

xleave

Valid in peer and broadcast modes only, this flag enables interleave mode.

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.

mdnstries number

If we are participating in mDNS, after we have synched for the first time we attempt to register with the mDNS system. If that registration attempt fails, we try again at one minute intervals for up to mdnstries times. After all, ntpd may be starting before mDNS. The default value for mdnstries is 5.

Symmetric-Key Cryptography

When ntpd(8) is first started, it reads the key file specified in the keys configuration command and installs the keys in the key cache. However, individual 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 Cryptography

The Autokey protocol has several modes of operation corresponding to the various NTP modes supported. Most modes use a special cookie which can be computed independently by the client and server, but encrypted in transmission. 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 on the Autonomous Authentication page.

The specific cryptographic environment used by Autokey servers and clients is determined by a set of files and soft links generated by the ntp-keygen(1ntpkeygenmdoc) program. This includes a required host key file, required certificate file and optional sign key file, leapsecond file and identity scheme files. The digest/signature scheme is specified in the X.509 certificate along with the matching sign key. There are several schemes available in the OpenSSL software library, each identified by a specific string such as md5WithRSAEncryption, which stands for the MD5 message digest with RSA encryption scheme. The current NTP distribution supports all the schemes in the OpenSSL library, including those based on RSA and DSA digital signatures.

NTP secure groups can be used to define cryptographic compartments and security hierarchies. It is important that every host in the group be able to construct a certificate trail to one or more trusted hosts in the same group. Each group host runs the Autokey protocol to obtain the certificates for all hosts along the trail to one or more trusted hosts. This requires the configuration file in all hosts to be engineered so that, even under anticipated failure conditions, the NTP subnet will form such that every group host can find a trail to at least one trusted host.

Naming and Addressing

By convention, the name of an Autokey host is the name returned by the Unix gethostname(2) system call or equivalent in other systems. By the system design model, there are no provisions to allow alternate names or aliases. However, this is not to say that DNS aliases, different names for each interface, etc., are constrained in any way.

It is also important to note that Autokey verifies authenticity using the host name, network address and public keys, all of which are bound together by the protocol specifically to deflect masquerade attacks. For this reason Autokey includes the source and destination IP addresses in message digest computations and so the same addresses must be available at both the server and client. For this reason operation with network address translation schemes is not possible. This reflects the intended robust security model where government and corporate NTP servers are operated outside firewall perimeters.

Operation

The cryptotype of an association is determined at the time of mobilization, either at configuration time or some time later when a message of appropriate cryptotype arrives. When mobilized by a server or peer configuration command and no key or autokey subcommands are present, the association is not authenticated; if the key subcommand is present, the association is authenticated using the symmetric key ID specified; if the autokey subcommand is present, the association is authenticated using Autokey.

When multiple identity schemes are supported in the Autokey protocol, the first message exchange determines which one is used. The client request message contains bits corresponding to which schemes it has available. The server response message contains bits corresponding to which schemes it has available. Both server and client match the received bits with their own and select a common scheme.

Following the principle that time is a public value, a server responds to any client packet that matches its cryptotype capabilities. Thus, a server receiving an unauthenticated packet will respond with an unauthenticated packet, while the same server receiving a packet of a cryptotype it supports will respond with packets of that cryptotype. However, unconfigured broadcast or manycast client associations or symmetric passive associations will not be mobilized unless the server supports a cryptotype compatible with the first packet received. By default, unauthenticated associations will not be mobilized unless overridden in a decidedly dangerous way.

Some examples may help to reduce confusion. Client Alice has no specific cryptotype selected. Server Bob has both a symmetric key file and minimal Autokey files. Alice's unauthenticated messages arrive at Bob, who replies with unauthenticated messages. Cathy has a copy of Bob's symmetric key file and has selected key ID 4 in messages to Bob. Bob verifies the message with his key ID 4. If it's the same key and the message is verified, Bob sends Cathy a reply authenticated with that key. If verification fails, Bob sends Cathy a thing called a crypto-NAK, which tells her something broke. She can see the evidence using the ntpq(8) program.

Denise has rolled her own host key and certificate. She also uses one of the identity schemes as Bob. She sends the first Autokey message to Bob and they both dance the protocol authentication and identity steps. If all comes out okay, Denise and Bob continue as described above.

It should be clear from the above that Bob can support all the girls at the same time, as long as he has compatible authentication and identity credentials. Now, Bob can act just like the girls in his own choice of servers; he can run multiple configured associations with multiple different servers (or the same server, although that might not be useful). But, wise security policy might preclude some cryptotype combinations; for instance, running an identity scheme with one server and no authentication with another might not be wise.

Key Management

Certificates imported from OpenSSL or public certificate authorities have certain limitations. The certificate should be in ASN.1 syntax, X.509 Version 3 format and encoded in PEM, which is the same format used by OpenSSL. The overall length of the certificate encoded in ASN.1 must not exceed 1024 bytes. The subject distinguished name field (CN) is the fully qualified name of the host on which it is used; the remaining subject fields are ignored. The certificate extension fields must not contain either a subject key identifier or a issuer key identifier field; however, an extended key usage field for a trusted host must contain the value trustRoot;. Other extension fields are ignored.

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 65,535, inclusive.

crypto [cert file] [leap file] [randfile file] [host file] [sign file] [gq file] [gqpar file] [iffpar file] [mvpar file] [pw password]

This command requires the OpenSSL library. It activates public key cryptography, selects the message digest and signature encryption scheme and loads the required private and public values described above. If one or more files are left unspecified, the default names are used as described above. Unless the complete path and name of the file are specified, the location of a file is relative to the keys directory specified in the keysdir command or default /usr/local/etc. Following are the subcommands:

cert file

Specifies the location of the required host public certificate file. This overrides the link ntpkey_cert_hostname in the keys directory.

gqpar file

Specifies the location of the optional GQ parameters file. This overrides the link ntpkey_gq_hostname in the keys directory.

host file

Specifies the location of the required host key file. This overrides the link ntpkey_key_hostname in the keys directory.

iffpar file

Specifies the location of the optional IFF parameters file. This overrides the link ntpkey_iff_hostname in the keys directory.

leap file

Specifies the location of the optional leapsecond file. This overrides the link ntpkey_leap in the keys directory.

mvpar file

Specifies the location of the optional MV parameters file. This overrides the link ntpkey_mv_hostname in the keys directory.

pw password

Specifies the password to decrypt files containing private keys and identity parameters. This is required only if these files have been encrypted.

randfile file

Specifies the location of the random seed file used by the OpenSSL library. The defaults are described in the main text above.

sign file

Specifies the location of the optional sign key file. This overrides the link ntpkey_sign_hostname in the keys directory. If this file is not found, the host key is also the sign key.

keys keyfile

Specifies the complete path and location of the MD5 key file containing the keys and key identifiers used by ntpd(8), ntpq(8) and ntpdc(8) when operating with symmetric key cryptography. This is the same operation as the -k command line option.

keysdir path

This command specifies the default directory path for cryptographic keys, parameters and certificates. The default 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 65,535, 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 65,535.

Error Codes

101

(bad field format or length) The packet has invalid version, length or format.

102

(bad timestamp) The packet timestamp is the same or older than the most recent received. This could be due to a replay or a server clock time step.

103

(bad filestamp) The packet filestamp is the same or older than the most recent received. This could be due to a replay or a key file generation error.

104

(bad or missing public key) The public key is missing, has incorrect format or is an unsupported type.

105

(unsupported digest type) The server requires an unsupported digest/signature scheme.

106

(mismatched digest types) Not used.

107

(bad signature length) The signature length does not match the current public key.

108

(signature not verified) The message fails the signature check. It could be bogus or signed by a different private key.

109

(certificate not verified) The certificate is invalid or signed with the wrong key.

110

(certificate not verified) The certificate is not yet valid or has expired or the signature could not be verified.

111

(bad or missing cookie) The cookie is missing, corrupted or bogus.

112

(bad or missing leapseconds table) The leapseconds table is missing, corrupted or bogus.

113

(bad or missing certificate) The certificate is missing, corrupted or bogus.

114

(bad or missing identity) The identity key is missing, corrupt or bogus.

Monitoring Commands

statistics name ...

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

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.

cryptostats

This option requires the OpenSSL cryptographic software library. It enables recording of cryptographic public key protocol information. Each message received by the protocol module appends a line of the following form to the file generation set named cryptostats:

49213 525.624 127.127.4.1 message
        

The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The next field shows the peer address in dotted-quad notation, The final message field includes the message type and certain ancillary information. See the Authentication Options section for further information.

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 0.0133806
        

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 -0.001605376 0.000000000 0.001424877 0.000958674
        

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 four fields show the offset, delay, dispersion and RMS jitter, all in seconds.

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.

sysstats

Enables recording of ntpd statistics counters on a periodic basis. Each hour a line of the following form is appended to the file generation set named sysstats:

50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147
        

The first two fields show the date (Modified Julian Day) and time (seconds and fraction past UTC midnight). The remaining ten fields show the statistics counter values accumulated since the last generated line.

Time since restart 36000

Time in hours since the system was last rebooted.

Packets received 81965

Total number of packets received.

Packets processed 0

Number of packets received in response to previous packets sent

Current version 9546

Number of packets matching the current NTP version.

Previous version 56

Number of packets matching the previous NTP version.

Bad version 71793

Number of packets matching neither NTP version.

Access denied 512

Number of packets denied access for any reason.

Bad length or format 540

Number of packets with invalid length, format or port number.

Bad authentication 10

Number of packets not verified as authentic.

Rate exceeded 147

Number of packets discarded due to rate limitation.

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. (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 file ... prefix, file ... filename and file ... 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 ‘/’). 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 filesystem 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 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 ‘.’ 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 ‘.’ 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 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.

The Kiss-of-Death Packet

A client receiving a KoD performs a set of sanity checks to minimize security exposure, then updates the stratum and reference identifier peer variables, sets the access denied (TEST4) bit in the peer flash variable and sends a message to the log. As long as the TEST4 bit is set, the client will send no further packets to the server. The only way at present to recover from this condition is to restart the protocol at both the client and server. This happens automatically at the client when the association times out. It will happen at the server only if the server operator cooperates.

Access Control Commands

discard [average avg] [minimum min] [monitor prob]

Set the parameters of the limited facility which protects the server from client abuse. The average subcommand specifies the minimum average packet spacing, while the minimum subcommand specifies the minimum packet spacing. Packets that violate these minima are discarded and a kiss-o'-death packet returned if enabled. The default minimum average and minimum are 5 and 2, respectively. The monitor subcommand specifies the probability of discard for packets that overflow the rate-control window.

restrict address [mask mask] [ippeerlimit int] [flag ...]

The address argument expressed in dotted-quad form is the address of a host or network. Alternatively, the address argument can be a valid host DNS name. The mask argument expressed in dotted-quad form defaults to 255.255.255.255, meaning that the 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 is always the first entry in the list. Note that text string default, with no mask option, may be used to indicate the default entry. The ippeerlimit directive limits the number of peer requests for each IP to int, where a value of -1 means "unlimited", the current default. A value of 0 means "none". There would usually be at most 1 peering request per IP, but if the remote peering requests are behind a proxy there could well be more than 1 per IP. 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:

ignore

Deny packets of all kinds, including ntpq(8) and ntpdc(8) queries.

kod

If this flag is set when an access violation occurs, a kiss-o'-death (KoD) packet is sent. KoD packets are rate limited to no more than one per second. If another KoD packet occurs within one second after the last one, the packet is dropped.

limited

Deny service if the packet spacing violates the lower limits specified in the discard command. 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.

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.

noepeer

Deny ephemeral peer requests, even if they come from an authenticated source. Note that the ability to use a symmetric key for authentication may be restricted to one or more IPs or subnets via the third field of the ntp.keys file. This restriction is not enabled by default, to maintain backward compatibility. Expect noepeer to become the default in ntp-4.4.

nomodify

Deny ntpq(8) and ntpdc(8) queries which attempt to modify the state of the server (i.e., run time reconfiguration). Queries which return information are permitted.

noquery

Deny ntpq(8) and ntpdc(8) queries. Time service is not affected.

nopeer

Deny unauthenticated packets which would result in mobilizing a new association. This includes broadcast and symmetric active packets when a configured association does not exist. It also includes pool associations, so if you want to use servers from a pool directive and also want to use nopeer by default, you'll want a restrict source ... line as well that does not include the nopeer directive.

noserve

Deny all packets except ntpq(8) and ntpdc(8) queries.

notrap

Decline to provide mode 6 control message trap service to matching hosts. The trap service is a subsystem of the ntpq(8) control message protocol which is intended for use by remote event logging programs.

notrust

Deny service unless the packet is cryptographically authenticated.

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

Deny packets that do not match 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).

Manycasting

Note that the manycasting paradigm does not coincide with the anycast paradigm described in RFC-1546, which is designed to find a single server from a clique of servers providing the same service. The manycast paradigm is designed to find a plurality of redundant servers satisfying defined optimality criteria.

Manycasting can be used with either symmetric key or public key cryptography. The public key infrastructure (PKI) offers the best protection against compromised keys and is generally considered stronger, at least with relatively large key sizes. It is implemented using the Autokey protocol and the OpenSSL cryptographic library available from http://www.openssl.org/. The library can also be used with other NTPv4 modes as well and is highly recommended, especially for broadcast modes.

A persistent manycast client association is configured using the manycastclient command, which is similar to the server command but with a multicast (IPv4 class D or IPv6 prefix FF) group address. The IANA has designated IPv4 address 224.1.1.1 and IPv6 address FF05::101 (site local) for NTP. When more servers are needed, it broadcasts manycast client messages to this address at the minimum feasible rate and minimum feasible time-to-live (TTL) hops, depending on how many servers have already been found. There can be as many manycast client associations as different group address, each one serving as a template for a future ephemeral unicast client/server association.

Manycast servers configured with the manycastserver command listen on the specified group address for manycast client messages. Note the distinction between manycast client, which actively broadcasts messages, and manycast server, which passively responds to them. If a manycast server is in scope of the current TTL and is itself synchronized to a valid source and operating at a stratum level equal to or lower than the manycast client, it replies to the manycast client message with an ordinary unicast server message.

The manycast client receiving this message mobilizes an ephemeral client/server association according to the matching manycast client template, but only if cryptographically authenticated and the server stratum is less than or equal to the client stratum. Authentication is explicitly required and either symmetric key or public key (Autokey) can be used. Then, the client polls the server at its unicast address in burst mode in order to reliably set the host clock and validate the source. This normally results in a volley of eight client/server at 2-s intervals during which both the synchronization and cryptographic protocols run concurrently. Following the volley, the client runs the NTP intersection and clustering algorithms, which act to discard all but the "best" associations according to stratum and synchronization distance. The surviving associations then continue in ordinary client/server mode.

The manycast client polling strategy is designed to reduce as much as possible the volume of manycast client messages and the effects of implosion due to near-simultaneous arrival of manycast server messages. The strategy is determined by the manycastclient, tos and ttl configuration commands. The manycast poll interval is normally eight times the system poll interval, which starts out at the minpoll value specified in the manycastclient, command and, under normal circumstances, increments to the maxpolll value specified in this command. Initially, the TTL is set at the minimum hops specified by the ttl command. At each retransmission the TTL is increased until reaching the maximum hops specified by this command or a sufficient number client associations have been found. Further retransmissions use the same TTL.

The quality and reliability of the suite of associations discovered by the manycast client is determined by the NTP mitigation algorithms and the minclock and minsane values specified in the tos configuration command. At least minsane candidate servers must be available and the mitigation algorithms produce at least minclock survivors in order to synchronize the clock. Byzantine agreement principles require at least four candidates in order to correctly discard a single falseticker. For legacy purposes, minsane defaults to 1 and minclock defaults to 3. For manycast service minsane should be explicitly set to 4, assuming at least that number of servers are available.

If at least minclock servers are found, the manycast poll interval is immediately set to eight times maxpoll. If less than minclock servers are found when the TTL has reached the maximum hops, the manycast poll interval is doubled. For each transmission after that, the poll interval is doubled again until reaching the maximum of eight times maxpoll. Further transmissions use the same poll interval and TTL values. Note that while all this is going on, each client/server association found is operating normally it the system poll interval.

Administratively scoped multicast boundaries are normally specified by the network router configuration and, in the case of IPv6, the link/site scope prefix. By default, the increment for TTL hops is 32 starting from 31; however, the ttl configuration command can be used to modify the values to match the scope rules.

It is often useful to narrow the range of acceptable servers which can be found by manycast client associations. Because manycast servers respond only when the client stratum is equal to or greater than the server stratum, primary (stratum 1) servers fill find only primary servers in TTL range, which is probably the most common objective. However, unless configured otherwise, all manycast clients in TTL range will eventually find all primary servers in TTL range, which is probably not the most common objective in large networks. The tos command can be used to modify this behavior. Servers with stratum below floor or above ceiling specified in the tos command are strongly discouraged during the selection process; however, these servers may be temporally accepted if the number of servers within TTL range is less than minclock.

The above actions occur for each manycast client message, which repeats at the designated poll interval. However, once the ephemeral client association is mobilized, subsequent manycast server replies are discarded, since that would result in a duplicate association. If during a poll interval the number of client associations falls below minclock, all manycast client prototype associations are reset to the initial poll interval and TTL hops and operation resumes from the beginning. It is important to avoid frequent manycast client messages, since each one requires all manycast servers in TTL range to respond. The result could well be an implosion, either minor or major, depending on the number of servers in range. The recommended value for maxpoll is 12 (4,096 s).

It is possible and frequently useful to configure a host as both manycast client and manycast server. A number of hosts configured this way and sharing a common group address will automatically organize themselves in an optimum configuration based on stratum and synchronization distance. For example, consider an NTP subnet of two primary servers and a hundred or more dependent clients. With two exceptions, all servers and clients have identical configuration files including both multicastclient and multicastserver commands using, for instance, multicast group address 239.1.1.1. The only exception is that each primary server configuration file must include commands for the primary reference source such as a GPS receiver.

The remaining configuration files for all secondary servers and clients have the same contents, except for the tos command, which is specific for each stratum level. For stratum 1 and stratum 2 servers, that command is not necessary. For stratum 3 and above servers the floor value is set to the intended stratum number. Thus, all stratum 3 configuration files are identical, all stratum 4 files are identical and so forth.

Once operations have stabilized in this scenario, the primary servers will find the primary reference source and each other, since they both operate at the same stratum (1), but not with any secondary server or client, since these operate at a higher stratum. The secondary servers will find the servers at the same stratum level. If one of the primary servers loses its GPS receiver, it will continue to operate as a client and other clients will time out the corresponding association and re-associate accordingly.

Some administrators prefer to avoid running ntpd(8) continuously and run either sntp(8) or ntpd(8) -q as a cron job. In either case the servers must be configured in advance and the program fails if none are available when the cron job runs. A really slick application of manycast is with ntpd(8) -q. The program wakes up, scans the local landscape looking for the usual suspects, selects the best from among the rascals, sets the clock and then departs. Servers do not have to be configured in advance and all clients throughout the network can have the same configuration file.

Manycast Interactions with Autokey

About once an hour or less often if the poll interval exceeds this, the client regenerates the Autokey key list. This is in general transparent in client/server mode. However, about once per day the server private value used to generate cookies is refreshed along with all manycast client associations. In this case all cryptographic values including certificates is refreshed. If a new certificate has been generated since the last refresh epoch, it will automatically revoke all prior certificates that happen to be in the certificate cache. At the same time, the manycast scheme starts all over from the beginning and the expanding ring shrinks to the minimum and increments from there while collecting all servers in scope.

Broadcast Options

tos [bcpollbstep gate]

This command provides a way to delay, by the specified number of broadcast poll intervals, believing backward time steps from a broadcast server. Broadcast time networks are expected to be trusted. In the event a broadcast server's time is stepped backwards, there is clear benefit to having the clients notice this change as soon as possible. Attacks such as replay attacks can happen, however, and even though there are a number of protections built in to broadcast mode, attempts to perform a replay attack are possible. This value defaults to 0, but can be changed to any number of poll intervals between 0 and 4.

Manycast Options

tos [ceiling ceiling | cohort { 0 | 1 } | floor floor | minclock minclock | minsane minsane]

This command affects the clock selection and clustering algorithms. It can be used to select the quality and quantity of peers used to synchronize the system clock and is most useful in manycast mode. The variables operate as follows:

ceiling ceiling

Peers with strata above ceiling will be discarded if there are at least minclock peers remaining. This value defaults to 15, but can be changed to any number from 1 to 15.

cohort {0 | 1}

This is a binary flag which enables (0) or disables (1) manycast server replies to manycast clients with the same stratum level. This is useful to reduce implosions where large numbers of clients with the same stratum level are present. The default is to enable these replies.

floor floor

Peers with strata below floor will be discarded if there are at least minclock peers remaining. This value defaults to 1, but can be changed to any number from 1 to 15.

minclock minclock

The clustering algorithm repeatedly casts out outlier associations until no more than minclock associations remain. This value defaults to 3, but can be changed to any number from 1 to the number of configured sources.

minsane minsane

This is the minimum number of candidates available to the clock selection algorithm in order to produce one or more truechimers for the clustering algorithm. If fewer than this number are available, the clock is undisciplined and allowed to run free. The default is 1 for legacy purposes. However, according to principles of Byzantine agreement, minsane should be at least 4 in order to detect and discard a single falseticker.

ttl hop ...

This command specifies a list of TTL values in increasing order, up to 8 values can be specified. In manycast mode these values are used in turn in an expanding-ring search. The default is eight multiples of 32 starting at 31.

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 (available as part of the HTML documentation provided in /usr/share/doc/ntp) 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, as a power of 2 in seconds 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 (available as part of the HTML documentation provided in /usr/share/doc/ntp).

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 (available as part of the HTML documentation provided in /usr/share/doc/ntp ).

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.