from Pogo, Walt Kelly

For putting out compiler fires.

2. Table of Contents

3. Introduction

NTS is a method for using TLS/SSL to authenticate NTP traffic on the net. That means that bad guys can’t forge packets that will give your system bogus time.

The RFC hasn’t been published yet (December 2019). Nothing has changed recently, but there may be minor adjustments when it is finalized.

Note: The NTP Pool does not currently support NTS.

It is strongly suggested that you get a "normal", unauthenticated, NTP server working before enabling NTS. This may reduce the time spent debugging. See the Client Quick Start Guide.

4. NTS Client Configuration

Append the keyword nts to the end of your server lines. Do this only for servers that speak NTS. If the server uses a port other than 123 for NTS key exchange, you also need to specify the port number. As of December 2019, the following should work:

Public NTP servers supporting NTS:

server nts	# Global, anycast
server nts		# Sweden

CloudFlare supports only TLS 1.3. To use TLS 1.3, you must have OpenSSL 1.1.1 or higher.

Development machines, so there may be gaps in availability:

server nts	# Singapore
server nts	# San Francisco
server nts	# London

Note that you must use the same host name that was used to create the server’s certificate. IP addresses will not work.

This assumes that the server is using a certificate covered by your OS/distro’s root certificate collection.

NetBSD needs the mozilla-rootcerts-openssl package and ca /etc/openssl/certs/ added to the server line.

Restart ntpd, and skip to Verification, below.

5. NTS Server Configuration

Being an NTS server requires a well-formed SSL certificate. The easiest way to do this is to use Let’s Encrypt. It needs a FQDN. Please see the certbot client site for instructions.

The following worked on Fedora:

$ sudo dnf install certbot

# Install
$ sudo certbot certonly --standalone

# Renew
$ sudo certbot renew
$ sudo killall -HUP ntpd

If you already have an SSL certificate for your server, and you are serving time using the same FQDN, you can reuse that certificate.

Next, add the line: nts enable to your ntp.conf file.

Locate the following two files:

  • Your certificate private key

  • Your certificate chain (i.e. your certificate followed by any intermediate CA certificates)

Then add the lines below to your ntp.conf, replacing with your pathnames.

Example, using Let’s Encrypt:

nts key /etc/letsencrypt/live/
nts cert /etc/letsencrypt/live/

Note that ntpd must be able to read both files and you want to make sure that the bad guys can’t read your private key. It may be simpler to copy those files over to /etc/ntp/ and adjust their owner and mode so ntpd running as user ntp can read them.

You may need to tell your system where to store the keys used to encrypt cookies. The default is /var/lib/ntp/nts-keys. Some distros use /var/db/ rather than /var/lib/.

nts cookie /var/lib/ntp/nts-keys

Again, make sure the bad guys can’t read that file.

Restart your server, and skip to Verification, below.

6. Verification

Check your log file. The current client side NTS implementation is quite chatty. The log lines may change, but what you see should be similar to below.

As a client, you should see lines like this:

2019-03-22T08:06:33 ntpd[12915]: DNS: dns_probe:, cast_flags:1, flags:21801
2019-03-22T08:06:33 ntpd[12915]: NTSc: DNS lookup of took 0.003 sec
2019-03-22T08:06:33 ntpd[12915]: NTSc: nts_probe connecting to =>
2019-03-22T08:06:34 ntpd[12915]: NTSc: Using TLSv1.2, AES256-GCM-SHA384 (256)
2019-03-22T08:06:34 ntpd[12915]: NTSc: certificate subject name: /
2019-03-22T08:06:34 ntpd[12915]: NTSc: certificate issuer name: /C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
2019-03-22T08:06:34 ntpd[12915]: NTSc: certificate is valid.
2019-03-22T08:06:34 ntpd[12915]: NTSc: read 880 bytes
2019-03-22T08:06:34 ntpd[12915]: NTSc: Got 8 cookies, length 104, aead=15.
2019-03-22T08:06:34 ntpd[12915]: NTSc: NTS-KE req to took 0.882 sec, OK

For initializing a server, you should see lines like this:

27 Dec 12:03:47 ntpd[962738]: INIT: OpenSSL 1.1.1d FIPS  10 Sep 2019, 1010104f
27 Dec 12:03:47 ntpd[962738]: NTSs: starting NTS-KE server listening on port 123
27 Dec 12:03:47 ntpd[962738]: NTSs: loaded certificate (chain) from /etc/ntp/fullchain.pem
27 Dec 12:03:47 ntpd[962738]: NTSs: loaded private key from /etc/ntp/privkey.pem
27 Dec 12:03:47 ntpd[962738]: NTSs: Private Key OK
27 Dec 12:03:47 ntpd[962738]: NTSs: OpenSSL security level is 1
27 Dec 12:03:47 ntpd[962738]: NTSs: listen4 worked
27 Dec 12:03:47 ntpd[962738]: NTSs: listen6 worked
27 Dec 12:03:47 ntpd[962738]: NTSc: Using system default root certificates.

On a server, each time a client uses TLS to setup cookies, you should see lines like these. If all goes well, there is a single line for each connection.

 1 Dec 22:42:21 ntpd[237777]: NTSs: NTS-KE from, Using TLSv1.3, TLS_CHACHA20_POLY1305_SHA256 (256), took 0.018 sec
 1 Dec 22:42:56 ntpd[237777]: NTSs: NTS-KE from, Using TLSv1.2, ECDHE-RSA-AES256-GCM-SHA384 (256), took 0.075 sec

Servers on the big bad internet will get a lot of garbage connections. The common cases produce a single line. Less common cases will have additional lines with OpenSSL error data.

26 Dec 18:13:55 ntpd[940892]: NTSs: SSL accept from failed: wrong version number, took 0.000 sec
26 Dec 18:24:20 ntpd[940892]: NTSs: SSL accept from failed, took 0.101 sec
26 Dec 18:24:20 ntpd[940892]: NTS: error:14094418:SSL routines:ssl3_read_bytes:tlsv1 alert unknown ca

alert means the client on the other end encountered troubles and is reporting them back to the server.

The logging prefix NTSs is for the NTS server component. The NTSc component is for the NTS client part, where you are talking to NTS servers.

6.1. Check with ntpq

The output of ntpq will be slightly different when NTS is in use, note the t column. Example:

root@ntpmon:/var/www/html/ntp# ntpq -p
     remote                                   refid      st t when poll reach   delay   offset   jitter
*SHM(1)                                  .PPS.            0 l   20   64  377   0.0000   0.0007   0.0281
xSHM(0)                                  .GPS.            0 l   19   64  377   0.0000 233.3966  19.2212                          .PPS.            1 8   56   64  371 197.4484   0.0932   0.9660                   2 8   17   64  273 210.7230  -1.3924   0.6086                   2 8   50   64  277 178.5749   3.8921   0.9611                  2 8    -   64  177 185.7582  -2.6534   0.0275
 2407:8000:8001:80::8                    .DNS.           16 u    - 1024    0   0.0000   0.0000   0.0005                       .GPS.            1 u  105   64  356 221.5282  -2.4354   0.0293

The t column shows how many cookies your NTS client is holding for the appropriate servers. The number should be 8. Lower numbers indicate dropped packets. (7 could be a packet in flight.)

The RFC calls for the server to rotate the private key used to encrypt cookies every 24 hours. The server also saves the previous key so old cookies will work for at least 24 hours. 24 hours and 8 cookies will work for a polling interval of up to 3 hours. That’s much longer than the default maxpoll of 10 (1024 seconds).

6.2. Check ntp variables

Try ntpq -c nts. This will show various counters related to NTS. This feature is under active development, so the format might change. An example:

root@ntpmon:/var/www/html/ntp# ntpq -c nts
NTS client sends:            7491
NTS client recvs:            6562
NTS client recvs w error:    0
NTS server recvs:            5591
NTS server recvs w error:    38
NTS server sends:            5553
NTS make cookies:            6392
NTS decode cookies:          4734
NTS decode cookies old:      819
NTS decode cookies too old:  0
NTS decode cookies error:    0
NTS KE probes:               8
NTS KE probes_bad:           0
NTS KE serves:               75
NTS KE serves_bad:           56

7. References