ntp_auth(5) ntp_auth(5)
NAME
ntp_auth - Authentication Options
AUTHENTICATION SUPPORT
Authentication support allows the NTP client to verify that the server
is in fact known and trusted and not an intruder intending accidentally
or on purpose to masquerade as that server. The NTPv3 specification
RFC-1305 defines a scheme which provides cryptographic authentication
of received NTP packets. Originally, this was done using the Data
Encryption Standard (DES) algorithm operating in Cipher Block Chaining
(CBC) mode, commonly called DES-CBC. Subsequently, this was 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 pri-
vate 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 consid-
ered more secure than symmetric key cryptography, since the security is
based on a private value which is generated by each host and never
revealed. With the exception of the group key described later, all key
distribution and management functions involve only public values, which
considerably simplifies key distribution and storage. Public key man-
agement is based on X.509 certificates, which can be provided by com-
mercial services or produced by utility programs in the OpenSSL soft-
ware library or the NTPv4 distribution.
While the algorithms for symmetric key cryptography are included in the
NTPv4 distribution, public key cryptography requires the OpenSSL soft-
ware library to be installed before building the NTP distribution. This
library is available from http://www.openssl.org and can be installed
using the procedures outlined in the Building and Installing the Dis-
tribution page. Once installed, the configure and build process auto-
matically detects the library and links the library routines required.
Authentication is configured separately for each association using the
key or autokey subcommand on the peer, server, broadcast and manycast-
client configuration commands as described in the 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 config-
ured as described below. If a NTP packet arrives including a message
authentication code (MAC), it is accepted only if it passes all crypto-
graphic checks. The checks require correct key ID, key value and mes-
sage 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 configura-
tion commands sent by a ntpdc program running on another machine. If
this flag is enabled, which is the default case, new broadcast/manycast
client and symmetric passive associations and remote configuration com-
mands must be cryptographically authenticated using either symmetric
key or public key cryptography. If this flag is disabled, these opera-
tions are effective even if not cryptographic authenticated. It should
be understood that operating with the auth flag disabled invites a sig-
nificant vulnerability where a rogue hacker can masquerade as a
truechimer 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 mes-
sages and remote configuration commands and, in particular, the flag
has no effect on the authentication process itself.
The security model and protocol schemes for both symmetric key and pub-
lic key cryptography are summarized below; further details are in the
briefings, papers and reports at the NTP project page linked from
www.ntp.org.
SYMMETRIC KEY CRYPTOGRAPHY
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 informa-
tion 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 associa-
tions, additional keys can be used as passwords for the ntpq and ntpdc
utility programs. Ordinarily, the ntp.keys file is generated by the
ntp-keygen program. When ntpd 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
trustedkey command before use. This allows, for instance, the installa-
tion of possibly several batches of keys and then activating or deacti-
vating each batch remotely using ntpdc. 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
utility, while the controlkey command selects the key used as the pass-
word for the ntpq utility.
PUBLIC KEY CRYPTOGRAPHY
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 sev-
eral digest/signature schemes. Optional identity schemes described on
the Identity Schemes page and based on cryptographic challenge/response
algorithms are also available. Using these schemes provides strong
security against replay with or without modification, spoofing, mas-
querade 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 sum-
mary, 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 program. This includes a required host key file, required
host 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 engi-
neered so that, even under anticipated failure conditions, the NTP sub-
net will form such that every group host can find a trail to at least
one trusted host.
NAMING AND ADDRESSING
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() 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.
CONFIGURATION
Autokey has an intimidating number of options, most of which are not
necessary in typical scenarios. The simplest configuration consists of
a subnet with one or more servers at the same low stratum acting as
trusted hosts and with dependent clients at higher strata and sharing a
single secure group and identity scheme. Each trusted host generates a
host key, trusted certificate and group key. Each client generates a
host key, normal certificate and installs the group key of each trusted
host using secure means and renames it as the name of the trusted host.
For example, trusted host Alice generates keys using
ntp-keygen -H -T -I -p xyz
where H specifies a new host key, T the trusted certificate, I the IFF
identity scheme and p the password used to encrypt the private key
files. The group key file is ntpkey_IFFpar_alice.filestamp, where
filestamp represents the NTP time in seconds when the file was gener-
ated.
Host Bob generate keys using
ntp-keygen -H -p abc
where abc is different for each group host. The trusted host generates
a password-protected group key using
ntp-keygen -q xyz -p abc -e >temp
where xyz is the trusted host password, abc is the password supplied by
the client and temp is a temporary file. This file is transmitted to
Bob using secure means and renamed to the fully qualified host name for
Alice preceded by the string ntpkey_iff_.
OPERATION
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 cryp-
totypes. 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 inter-
operate with each other, but may not represent good security practice.
The cryptotype of an association is determined at the time of mobiliza-
tion, either at configuration time or some time later when a message of
appropriate cryptotype arrives. When mobilized by a server or peer con-
figuration 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.
KEY MANAGEMENT
The cryptographic values used by the Autokey protocol are incorporated
as a set of files generated by the ntp-keygen utility program, includ-
ing 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 certifi-
cate authorities. Note that symmetric keys are necessary for the ntpq
and ntpdc 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 iden-
tifier 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 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]
[ident scheme] [iffpar file] [gqpar 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 unspeci-
fied, 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 /etc/ntp. Following are the subcom-
mands:
cert file
Specifies the location of the required host public cer-
tificate file. This overrides the link ntp-
key_cert_hostname in the keys directory.
gqpar file
Specifies the location of the client 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.
ident scheme
Requests the server identity scheme, which can be IFF,
GQ or MV. This is used when the host will not be a
server for a dependent client.
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 client leapsecond file.
This overrides the link ntpkey_leap in the keys direc-
tory.
mv Requests the MV server identity scheme.
mvpar file
Specifies the location of the client MV parameters
file. This overrides the link ntpkey_mv_hostname in the
keys directory.
pw password
Specifies the password to decrypt files containing pri-
vate 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, ntpq and
ntpdc 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 crypto-
graphic keys, parameters and certificates. The default is
/etc/ntp.
requestkey key
Specifies the key identifier to use with the ntpdc utility pro-
gram, which uses a proprietary protocol specific to this imple-
mentation of ntpd. 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 [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 pur-
poses of authenticating peers with symmetric key cryptography,
as well as keys used by the ntpq and ntpdc 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.
ERROR CODES
Errors can occur due to mismatched configurations, unexpected restarts,
expired certificates and unfriendly people. In most cases the protocol
state machine recovers automatically by retransmission, timeout and
restart, where necessary. Some errors are due to mismatched keys,
digest schemes or identity schemes and must be corrected by installing
the correct media and/or correcting the configuration file. One of the
most common errrors is expired certificates, which must be regenerated
and signed at least once per year using the ntp-keygen program.
The following error codes are reported via the NTP control and monitor-
ing protocol trap mechanism.
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 genera-
tion error.
104 (bad or missing public key)
The public key is missing, has incorrect format or is an unsup-
ported type.
105 (unsupported digest type)
The server requires an unsupported digest/signature scheme.
106 (unsupported identity type)
The client or server has requested an identity scheme the other
does not support.
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 (host certificate expired)
The old server certificate has expired.
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 group key)
The identity key is missing, corrupt or bogus.
115 (protocol error)
The protocol state machine has wedged due to unexpected restart
116 (server certificate expired)
The old server certificate has expired.
FILES
See the ntp-keygen page.
LEAPSECONDS TABLE
The NIST provides a file documenting the epoch for all historic occa-
sions of leap second insertion since 1972. The leapsecond table shows
each epoch of insertion along with the offset of International Atomic
Time (TAI) with respect to Coordinated Universal Time (UTC), as dissem-
inated by NTP. The table can be obtained directly from NIST national
time servers using ftp as the ASCII file pub/leap-seconds.
While not strictly a security function, the Autokey protocol provides
means to securely retrieve the leapsecond table from a server or peer.
Servers load the leapsecond table directly from the file specified in
the crypto command, with default ntpkey_leap, while clients can obtain
the table indirectly from the servers using the Autokey protocol. Once
loaded, the table can be provided on request to other clients and
servers.
SEE ALSO
ntp.conf(5), ntpd(8)
Primary source of documentation: /usr/share/doc/ntp-*
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