This document describes the configuration file for the NTP Project's
ntpd
program.
This document applies to version 4.2.7p364 of ntp.conf
.
The behavior of ntpd
can be changed by a configuration file,
by default ntp.conf
.
The
ntp.conf
configuration file is read at initial startup by the
ntpd(1ntpdmdoc)
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 an 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:
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
pool
,
server
,
peer
,
broadcast
or
manycastclient
commands.
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.
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 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.
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 ntpdc, 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 | Cm autokey][burst
][iburst
][version
version][prefer
][minpoll
minpoll][maxpoll
maxpoll]peer
address[key
key | Cm autokey][version
version][prefer
][minpoll
minpoll][maxpoll
maxpoll]broadcast
address[key
key | Cm autokey][version
version][prefer
][minpoll
minpoll][ttl
ttl]manycastclient
address[key
key | Cm 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
server
peer
broadcast
broadcastclient
or
multicastclient
commands
below.
manycastclient
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
burst
server
command and s addresses.
iburst
server
command and s addresses and when
ntpd(1ntpdmdoc)
is started with the
-q
option.
key
keyminpoll
minpollmaxpoll
maxpollmaxpoll
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
prefer
ttl
ttlversion
versionbroadcastclient
manycastserver
address ...multicastclient
address ...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 replaced 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 scheme, 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. Public key management is based on X.509 certificates, which can be provided by commercial services or produced by utility programs in the OpenSSL software library or the NTPv4 distribution.
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 authentication 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(1ntpdcmdoc)
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 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
disrupt 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 commands and, in particular,
the flag has no effect on
the authentication process itself.
An attractive alternative where multicast support is available is manycast 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 briefings, papers
and reports at the NTP project page linked from
http://www.ntp.org/
.
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 NTP packets.
Keys and
related information are specified in a key
file, usually called
ntp.keys,
which must be distributed and stored using
secure means 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(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
utility programs.
When
ntpd(1ntpdmdoc)
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(1ntpdcmdoc)
.
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(1ntpdcmdoc)
utility, while the
controlkey
command selects the key used as the password for the
ntpq(1ntpqmdoc)
utility.
NTPv4 supports the original NTPv3 symmetric key scheme described in RFC-1305 and in addition the Autokey protocol, which is based on public key cryptography. The Autokey Version 2 protocol described on the Autokey Protocol page verifies packet integrity using MD5 message digests and verifies the source with digital signatures and any of several digest/signature schemes. Optional identity schemes described on the Identity Schemes page and based on cryptographic challenge/response algorithms are also available. Using all of these schemes provides strong security against replay with or without modification, spoofing, masquerade 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 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.
It is important to note that Autokey does not use DNS to resolve addresses, since DNS can't be completely trusted until the name servers have synchronized clocks. The cryptographic name used by Autokey to bind the host identity credentials and cryptographic values must be independent of interface, network and any other naming convention. The name appears in the host certificate in either or both the subject and issuer fields, so protection against DNS compromise is essential.
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 destinatino 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.
A specific combination of authentication scheme (none, symmetric key, public key) and identity scheme is called a cryptotype, although not all combinations are compatible. There may be management configurations where the clients, servers and peers may not all support the same cryptotypes. A secure NTPv4 subnet can be configured in many ways while keeping in mind the principles explained above and in this section. Note however that some cryptotype combinations may successfully interoperate with each other, but may not represent good security practice.
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(1ntpqmdoc)
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.
The cryptographic values used by the Autokey protocol are
incorporated as a set of files generated by the
ntp-keygen(1ntpkeygenmdoc)
utility program, including symmetric key, host key and
public certificate files, as well as sign key, identity parameters
and leapseconds files.
Alternatively, host and sign keys and
certificate files can be generated by the OpenSSL utilities
and certificates can be imported from public certificate
authorities.
Note that symmetric keys are necessary for the
ntpq(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
utility programs.
The remaining files are necessary only for the
Autokey protocol.
Certificates imported from OpenSSL or public certificate
authorities have certian 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.
autokey
[logsec]controlkey
keyntpq(1ntpqmdoc)
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 65,534, inclusive.
crypto
[cert
file][leap
file][randfile
file][host
file][sign
file][gq
file][gqpar
file][iffpar
file][mvpar
file][pw
password]keysdir
command or default
/usr/local/etc.
Following are the subcommands:
cert
filegqpar
filehost
fileiffpar
fileleap
filemvpar
filepw
passwordrandfile
filesign
filekeys
keyfilentpd(1ntpdmdoc)
,
ntpq(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
when operating with symmetric key cryptography.
This is the same operation as the
-k
command line option.
keysdir
pathrequestkey
keyntpdc(1ntpdcmdoc)
utility program, which uses a
proprietary protocol specific to this implementation of
ntpd(1ntpdmdoc)
.
The
key
argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,534, inclusive.
revoke
logsectrustedkey
key ...ntpq(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
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.
The following error codes are reported via the NTP control and monitoring protocol trap mechanism.
ntpd(1ntpdmdoc)
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.
statistics
name ...clockstats
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
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
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
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
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
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.
36000
81965
0
9546
56
71793
512
540
10
147
statsdir
directory_pathfilegen
filename prefix to be modified for file generation sets, which
is useful for handling statistics logs.
filegen
namefile
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(1ntpdcmdoc)
program running at a remote location.
statistics
command.
filefilename
This is the file name for the statistics records.
Filenames of set
members are built from three concatenated elements
Cmprefix,
Cmfilename
and
Cmsuffix:
ntpd(1ntpdmdoc)
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(1ntpdmdoc)
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
filenameNs.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
No.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
.
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 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|
Cm
disable
Enables or disables the recording function.
The
ntpd(1ntpdmdoc)
daemon implements a general purpose address/mask based restriction
list.
The list contains address/match entries sorted first
by increasing address values and and then by increasing mask values.
A match occurs when the bitwise AND of the mask and the packet
source address is equal to the bitwise AND of the mask and
address in the list.
The list is searched in order with the
last match found defining the restriction flags associated
with the entry.
Additional information and examples can be found in the
"NotesonConfiguringNTPandSettingupaNTPSubnet"
page
(available as part of the HTML documentation
provided in
/usr/share/doc/ntp).
The restriction facility was implemented in conformance with the access policies for the original NSFnet backbone time servers. Later the facility was expanded to deflect cryptographic and clogging attacks. While this facility may be useful for keeping unwanted or broken or malicious clients from congesting innocent servers, it should not be considered an alternative to the NTP authentication facilities. Source address based restrictions are easily circumvented by a determined cracker.
Clients can be denied service because they are explicitly included in the restrict list created by the restrict command or implicitly as the result of cryptographic or rate limit violations. Cryptographic violations include certificate or identity verification failure; rate limit violations generally result from defective NTP implementations that send packets at abusive rates. Some violations cause denied service only for the offending packet, others cause denied service for a timed period and others cause the denied service for an indefinate period. When a client or network is denied access for an indefinate period, the only way at present to remove the restrictions is by restarting the server.
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" (KoD) packet.
KoD packets have the leap bits set unsynchronized and stratum set
to zero and the reference identifier field set to a four-byte
ASCII code.
If the
noserve
or
notrust
flag of the matching restrict list entry is set,
the code is "DENY"; if the
limited
flag is set and the rate limit
is exceeded, the code is "RATE".
Finally, if a cryptographic violation occurs, the code is "CRYP".
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.
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.
restrictaddress
[mask
mask][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.
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: