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Want to link to this manual page? Skip site navigation 1 Skip section navigation 2 Header And Logo. The file format is similar to other UNIX configuration files.
Configuration commands consist of an initial keyword fol- lowed by a list of arguments, some of which may be optional, separated by whitespace. Commands may not be continued over multiple lines. Argu- ments 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. In addition to the discussion of general Configuration Options , there are sections describ- ing the following supported functionality and the options used to control it: While there is a rich set of options available, the only required option is one or more pool , server , peer , broadcast or manycastclient commands. 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, configu- ration 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 IPv4 class A, B and C , b the broadcast address of a local interface, m a multicast address IPv4 class D , or r a refer- ence clock address Note that only those options applica- ble 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. In a few cases, including the reslist billboard generated by ntpq 8 or ntpdc 8 , IPv6 addresses are automatically generated. IPv6 addresses can be used almost everywhere where IPv4 addresses can be used, with the excep- tion of reference clock addresses, which are always IPv4.
Note that in contexts where a host name is expected, a -4 qualifier pre- ceding the host name forces DNS resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to the IPv6 namespace.
See IPv6 ref- erences for the equivalent classes for that address family. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchro- nized to the local clock.
This command should not be used for type b or m addresses. 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 vari- ous 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. Note that local broadcast mes- sages go only to the interface associated with the subnet speci- fied, 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. 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.
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 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. 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.
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.
The default is to include no encryption field. The minimum poll inter- val defaults to 6 64 s , but can be decreased by the minpoll option to a lower limit of 4 16 s. The server is discarded by the selection algroithm. Use this option only for test- ing. All other things being equal, this host will be chosen for synchronization among a set of cor- rectly operating hosts. This option should almost certainly only be used while testing an association.
It specifies the time-to-live ttl to use on broad- cast 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 , is something of a black art and should be coordinated with the network administrator. Versions 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. 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.
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 Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using symmetric-key or pub- lic-key authentication as described in Authentication Options.
This command enables reception of multicast server messages to the multicast group address es type m specified. Note that, in order to avoid accidental or malicious disruption in this mode, both the server and client should operate using sym- metric-key or public-key authentication as described in Authentication Options.
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. 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.
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 scheme, properly described as symmetric key cryp- tography 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. Pub- lic key management is based on X. While the algorithms for symmetric key cryptography are included in the NTPv4 distribution, public key cryptography requires the OpenSSL software library to be installed before building the NTP distribution.
Directions for doing that are on the Building and Installing the Distribution page. Authentication is configured separately for each association using the key or autokey subcommand on the peer , server , broadcast and manycastclient configuration commands as described in Configuration Options page.
The authentication options described below specify the locations of the key files, if other than default, which symmetric keys are trusted and the interval between various operations, if other than default.
Authentication is always enabled, although ineffective if not configured as described below. If a NTP packet arrives including a message authen- tication code MAC , it is accepted only if it passes all cryptographic checks.
The checks require correct key ID, key value and message digest. If the packet has been modified in any way or replayed by an intruder, it will fail one or more of these checks and be discarded. Furthermore, the Autokey scheme requires a preliminary protocol exchange to obtain the server certificate, verify its credentials and initialize the protocol 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 commands and also by remote configuration commands sent by a ntpdc 8 program running on another machine. If this flag is enabled, which is the default case, new broadcast client and sym- metric passive associations and remote configuration commands must be cryptographically authenticated using either symmetric key or public key cryptography.
If this flag is disabled, these operations are effective even if not cryptographic authenticated. It should be understood that operating with the auth flag disabled invites a significant vulnerability where a rogue hacker can masquerade as a falseticker and seriously dis- rupt system timekeeping. It is important to note that this flag has no purpose other than to allow or disallow a new association in response to new broadcast and symmetric active messages and remote configuration com- mands and, in particular, the flag has no effect on the authentication process itself.
An attractive alternative where multicast support is available is many- cast mode, in which clients periodically troll for servers as described in the Automatic NTP Configuration Options page. Either symmetric key or public key cryptographic authentication can be used in this mode.
The principle advantage of 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.
The security model and protocol schemes for both symmetric key and public key cryptography are summarized below; further details are in the brief- ings, papers and reports at the NTP project page linked from http: Symmetric-Key Cryptography The original RFC specification allows any one of possibly 65, keys, each distinguished by a bit key identifier, to authenticate an association.
The servers and clients involved must agree on the key and key identifier to authenticate NTP packets. Keys and related information are specified in a key file, usually called ntp.
Besides the keys used for ordinary NTP associations, addi- tional 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 configuration command and installs the keys in the key cache. How- ever, 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. Using all of these schemes provides strong security against replay with or without modification, spoofing, masquer- ade and most forms of clogging attacks.
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 sta- tus, 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.
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 cer- tificates for all hosts along the trail to one or more trusted hosts.