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ntp-keygen - key generation program for ntpd
All files are in PEM-encoded printable ASCII format, so they can be embedded as MIME attachments in mail to other sites and certificate authorities. By default, files are not encrypted. The -p password option specifies the write password and -q password option the read password for previously encrypted files. The program prompts for the password if it reads an encrypted file and the password is missing or incorrect. If an encrypted file is read successfully and no write password is specified, the read password is used as the write password by default.
The ntpd(8) configuration command crypto pw password specifies the read password for previously encrypted files. The daemon expires on the spot if the password is missing or incorrect. For convenience, if a file has been previously encrypted, the default read password is the name of the host running the program. If the previous write password is specified as the host name, these files can be read by that host with no explicit password.
File names begin with the prefix ntpkey_ and end with the postfix _hostname.filestamp where hostname is the owner name, usually the string returned by the Unix gethostname() routine, and filestamp is the NTP seconds when the file was generated, in decimal digits. This both guarantees uniqueness and simplifies maintenance procedures, since all files can be quickly removed by a rm ntpkey* command or all files generated at a specific time can be removed by a rm *filestamp command. To further reduce the risk of misconfiguration, the first two lines of a file contain the file name and generation date and time as comments.
All files are installed by default in the keys directory /usr/local/etc which is normally in a shared filesystem in NFS-mounted networks. The actual location of the keys directory and each file can be overridden by configuration commands, but this is not recommended. Normally, the files for each host are generated by that host and used only by that host, although exceptions exist as noted later on this page.
Normally, files containing private values, including the host key, sign key and identification parameters, are permitted root read/write-only; while others containing public values are permitted world readable. Alternatively, files containing private values can be encrypted and these files permitted world readable, which simplifies maintenance in shared file systems. Since uniqueness is insured by the hostname and file name extensions, the files for a NFS server and dependent clients can all be installed in the same shared directory.
The recommended practice is to keep the file name extensions when installing a file and to install a soft link from the generic names specified elsewhere on this page to the generated files. This allows new file generations to be activated simply by changing the link. If a link is present, ntpd follows it to the file name to extract the filestamp. If a link is not present, ntpd(8) extracts the filestamp from the file itself. This allows clients to verify that the file and generation times are always current. The program uses the same timestamp extension for all files generated at one time, so each generation is distinct and can be readily recognized in monitoring data.
The host key is used to encrypt the cookie when required and so must be RSA type. By default, the host key is also the sign key used to encrypt signatures. When necessary, a different sign key can be specified and this can be either RSA or DSA type. By default, the message digest type is MD5, but any combination of sign key type and message digest type supported by the OpenSSL library can be specified, including those using the MD2, MD5, SHA, SHA1, MDC2 and RIPE160 message digest algorithms. However, the scheme specified in the certificate must be compatible with the sign key. Certificates using any digest algorithm are compatible with RSA sign keys; however, only SHA and SHA1 certificates are compatible with DSA sign keys.
Private/public key files and certificates are compatible with other OpenSSL applications and very likely other libraries as well. Certificates or certificate requests derived from them should be compatible with extant industry practice, although some users might find the interpretation of X509v3 extension fields somewhat liberal. However, the identification parameter files, although encoded as the other files, are probably not compatible with anything other than Autokey.
Running the program as other than root and using the Unix su command to assume root may not work properly, since by default the OpenSSL library looks for the random seed file .rnd in the user home directory. However, there should be only one .rnd most conveniently in the root directory, so it is convenient to define the $RANDFILE environment variable used by the OpenSSL library as the path to /.rnd
Installing the keys as root might not work in NFS-mounted shared file systems, as NFS clients may not be able to write to the shared keys directory, even as root. In this case, NFS clients can specify the files in another directory such as /etc using the keysdir command. There is no need for one client to read the keys and certificates of other clients or servers, as these data are obtained automatically by the Autokey protocol.
Ordinarily, cryptographic files are generated by the host that uses them, but it is possible for a trusted agent (TA) to generate these files for other hosts; however, in such cases files should always be encrypted. The subject name and trusted name default to the hostname of the host generating the files, but can be changed by command line options. It is convenient to designate the owner name and trusted name as the subject and issuer fields, respectively, of the certificate. The owner name is also used for the host and sign key files, while the trusted name is used for the identity files.
On each trusted host as root, change to the keys directory. To insure a fresh fileset, remove all ntpkey files. Then run -T to generate keys and a trusted certificate. On all other hosts do the same, but leave off the -T flag to generate keys and nontrusted certificates. When complete, start the NTP daemons beginning at the lowest stratum and working up the tree. It may take some time for Autokey to instantiate the certificate trails throughout the subnet, but setting up the environment is completely automatic.
If it is necessary to use a different sign key or different digest/signature scheme than the default, run with the -S type option, where type is either RSA or DSA The most often need to do this is when a DSA-signed certificate is used. If it is necessary to use a different certificate scheme than the default, run with the -c scheme option and selected scheme as needed. If is run again without these options, it generates a new certificate using the same scheme and sign key.
After setting up the environment it is advisable to update certificates from time to time, if only to extend the validity interval. Simply run with the same flags as before to generate new certificates using existing keys. However, if the host or sign key is changed, ntpd(8) should be restarted. When ntpd(8) is restarted, it loads any new files and restarts the protocol. Other dependent hosts will continue as usual until signatures are refreshed, at which time the protocol is restarted.
In some schemes there are separate keys for servers and clients. A server can also be a client of another server, but a client can never be a server for another client. In general, trusted hosts and nontrusted hosts that operate as both server and client have parameter files that contain both server and client keys. Hosts that operate only as clients have key files that contain only client keys.
The PC scheme supports only one trusted host in the group. On trusted host alice run -P -p password to generate the host key file ntpkey_RSAkey_ alice.filestamp and trusted private certificate file ntpkey_RSA-MD5_cert_ alice.filestamp Copy both files to all group hosts; they replace the files which would be generated in other schemes. On each host bob install a soft link from the generic name ntpkey_host_ bob to the host key file and soft link ntpkey_cert_ bob to the private certificate file. Note the generic links are on bob, but point to files generated by trusted host alice. In this scheme it is not possible to refresh either the keys or certificates without copying them to all other hosts in the group.
For the IFF scheme proceed as in the TC scheme to generate keys and certificates for all group hosts, then for every trusted host in the group, generate the IFF parameter file. On trusted host alice run -T -I -p password to produce her parameter file ntpkey_IFFpar_ alice.filestamp which includes both server and client keys. Copy this file to all group hosts that operate as both servers and clients and install a soft link from the generic ntpkey_iff_ alice to this file. If there are no hosts restricted to operate only as clients, there is nothing further to do. As the IFF scheme is independent of keys and certificates, these files can be refreshed as needed.
If a rogue client has the parameter file, it could masquerade as a legitimate server and present a middleman threat. To eliminate this threat, the client keys can be extracted from the parameter file and distributed to all restricted clients. After generating the parameter file, on alice run -e and pipe the output to a file or mail program. Copy or mail this file to all restricted clients. On these clients install a soft link from the generic ntpkey_iff_ alice to this file. To further protect the integrity of the keys, each file can be encrypted with a secret password.
For the GQ scheme proceed as in the TC scheme to generate keys and certificates for all group hosts, then for every trusted host in the group, generate the IFF parameter file. On trusted host alice run -T -G -p password to produce her parameter file ntpkey_GQpar_ alice.filestamp which includes both server and client keys. Copy this file to all group hosts and install a soft link from the generic ntpkey_gq_ alice to this file. In addition, on each host bob install a soft link from generic ntpkey_gq_ bob to this file. As the GQ scheme updates the GQ parameters file and certificate at the same time, keys and certificates can be regenerated as needed.
For the MV scheme, proceed as in the TC scheme to generate keys and certificates for all group hosts. For illustration assume trish is the TA, alice one of several trusted hosts and bob one of her clients. On TA trish run -V n -p password where n is the number of revokable keys (typically 5) to produce the parameter file ntpkeys_MVpar_ trish.filestamp and client key files ntpkeys_MVkeyd_ trish.filestamp where d is the key number (0 < d < n ) Copy the parameter file to alice and install a soft link from the generic ntpkey_mv_ alice to this file. Copy one of the client key files to alice for later distribution to her clients. It doesn't matter which client key file goes to alice, since they all work the same way. Alice copies the client key file to all of her cliens. On client bob install a soft link from generic ntpkey_mvkey_ bob to the client key file. As the MV scheme is independent of keys and certificates, these files can be refreshed as needed.
It is important to understand that entropy must be evolved for each generation, for otherwise the random number sequence would be predictable. Various means dependent on external events, such as keystroke intervals, can be used to do this and some systems have built-in entropy sources. Suitable means are described in the OpenSSL software documentation, but are outside the scope of this page.
The entropy seed used by the OpenSSL library is contained in a file, usually called .rnd which must be available when starting the NTP daemon or the program. The NTP daemon will first look for the file using the path specified by the randfile subcommand of the crypto configuration command. If not specified in this way, or when starting the program, the OpenSSL library will look for the file using the path specified by the RANDFILE environment variable in the user home directory, whether root or some other user. If the RANDFILE environment variable is not present, the library will look for the .rnd file in the user home directory. If the file is not available or cannot be written, the daemon exits with a message to the system log and the program exits with a suitable error message.
The format of the symmetric keys file is somewhat different than the other files in the interest of backward compatibility. Since DES-CBC is deprecated in NTPv4, the only key format of interest is MD5 alphanumeric strings. Following hte heard the keys are entered one per line in the format where keyno is a positive integer in the range 1-65,535, type is the string MD5 defining the key format and key is the key itself, which is a printable ASCII string 16 characters or less in length. Each character is chosen from the 93 printable characters in the range 0x21 through 0x7f excluding space and the `#' character.
Note that the keys used by the ntpq(8) and ntpdc(8) programs are checked against passwords requested by the programs and entered by hand, so it is generally appropriate to specify these keys in human readable ASCII format.
The program generates a MD5 symmetric keys file ntpkey_MD5key_ hostname.filestamp Since the file contains private shared keys, it should be visible only to root and distributed by secure means to other subnet hosts. The NTP daemon loads the file ntp.keys so installs a soft link from this name to the generated file. Subsequently, similar soft links must be installed by manual or automated means on the other subnet hosts. While this file is not used with the Autokey Version 2 protocol, it is needed to authenticate some remote configuration commands used by the ntpq(8) and ntpdc(8) utilities.
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