50 votes

Force systemd timesyncd to sync time with NTP server immediately

Without installing any more packages... Turn NTP off, manually set the time to be close enough, turn NTP back on: Set NTP Service inactive $ timedatectl set-ntp false Set the time manually Get the ...
John Mee's user avatar
  • 2,698
30 votes
Accepted

What is NTP dispersion and how do I control it?

I see some confusion going on in the answers here. For starters, ntpclient, at least in -s mode, isn't acting as a full NTP client, it's only sending and receiving one packet, so there's no "last 8 ...
hobbs's user avatar
  • 556
27 votes
Accepted

Force systemd timesyncd to sync time with NTP server immediately

To use an actual NTP implementation, you need to install and configure one, chrony or maybe ntpd. Do so if you require any monitoring of time performance. I will assume chrony. Add iburst to your pool ...
John Mahowald's user avatar
25 votes
Accepted

NTP Service Auto-Discovery

You can specify an NTP server via DHCP, or Group Policy, or DNS SRV records. DHCP: GPO: SRV Record: If your client is unable to leverage any of those mechanisms... you can also pretty safely ...
Ryan Ries's user avatar
  • 55.9k
23 votes
Accepted

Things to consider when running public NTP servers

Firstly, good for you; it's a helpful and public-spirited thing to do. That said, and given your clarification that you're planning on creating one or more DMZ VMs which will sync to and make ...
MadHatter's user avatar
  • 80.6k
21 votes

Dealing with NTP reflection attacks in IPTables

Essentially, you're outta luck if the DDoS attack manages to fill whatever pipe you have to the Internet (which is the purpose of any UDP reflection attack -- to fill the pipe). If your upstream link ...
womble's user avatar
  • 97.1k
20 votes
Accepted

How to synchronize time on ESXi Windows virtual machines within one second?

This is 2018. Windows is capable of keeping servers synchronized within 2 ms or so, as required by MIFID II Regulations. So, your problem is a non-problem. Our sysadmins using windows NTP to sync ...
TomTom's user avatar
  • 51.9k
16 votes
Accepted

Does installing NTP mean I'm installing an NTP server?

Short answer: Usually, if you're installing an NTP daemon, you're installing the capability to both sync the local clock, and to provide time to clients over the network. However, there are some ...
Paul Gear's user avatar
  • 4,507
15 votes
Accepted

How to set up local ntp server without internet access on ubuntu?

Don't do this. Seriously. Just don't. People keep coming up with the idea that NTP is designed to allow a bunch of machines all to have the same time. It isn't. It's designed, quite carefully, to ...
MadHatter's user avatar
  • 80.6k
13 votes

What is NTP dispersion and how do I control it?

Just a partial answer for "What is dispersion?": A typical NTP round trip: client | | server t1 |------->| t2 t3 |<-------| t4 This yields two values, offset (the time ...
Sven's user avatar
  • 99.6k
12 votes
Accepted

What are the limits of running NTP servers in virtual machines? (2010)

It is now 2023, and all of the earlier answers to this question are now wrong (and have been since at least late 2016), at least as far as Linux VMs are concerned. [The advice below may not apply to ...
Paul Gear's user avatar
  • 4,507
12 votes

How do I point clients to the NTP server?

It's actually more important, in terms of the domain continuing to work properly, that your clients and member servers use Domain Controllers for their time source than it is that your clients have ...
Rob Moir's user avatar
  • 32k
12 votes

Things to consider when running public NTP servers

Firstly, congrats on an NTP question that is non-facepalm material. :-) I've included some graphs at the bottom of this post to give you a feel for things. The VM in question is set to 100 Mbps in ...
Paul Gear's user avatar
  • 4,507
12 votes

Sync ntp immediately at boot with undiciplined clock

You can use the -g parameter. From the manpage: -g Normally, ntpd exits with a message to the system log if the offset exceeds the panic threshold, which is 1000 s by default. This option allows the ...
basekat's user avatar
  • 456
11 votes
Accepted

Getting NTP to work on AWS without opening inbound port in security group

According to the AWS Documentation you can open UDP:123 in your security group outbound only. Because security groups are stateful replies will get back to you, but no-one outside your VPC will be ...
Tim's user avatar
  • 32.8k
11 votes
Accepted

Ubuntu 20.04 time sync problems and possibly incorrect status information

The timedatectl command reports that NTP is in use when either chronyd or systemd-timesyncd is enabled and running. Both of these are lightweight NTP clients suitable for most servers and workstations....
Michael Hampton's user avatar
10 votes

ntp.conf pool vs server directives

The answer was given by mboehn. To clarify more: See the document he mentioned. Especially the last lines: The pool scheme is configured using one or more pool commands with DNS names indicating ...
Woftor's user avatar
  • 101
10 votes
Accepted

How can I synchronize system time with a NTP server continuously?

No. It is good practice to run ntpd, because that is what it is for - to continuously keep your time as precisely sync'ed to correct time as possible, with the assistance of one or more reference ...
MadHatter's user avatar
  • 80.6k
10 votes

Chrony time synchronization on huge time diff

If your time is way off (days or even months), time synchronization will not work ("it will take a long time") because NTP clients like Chrony adjust the clock gradually by slowing it down or ...
basic6's user avatar
  • 373
10 votes
Accepted

ntpd synchronizes but chronyd fails

The difference is that chronyd by default uses a random source port for each client request. If the firewall doesn't allow client requests from other ports than 123, you will need to specify the port ...
mlichvar's user avatar
  • 116
9 votes
Accepted

NTP server architecture

Depending on how critical time keeping is in your environment, you may not want server1 to be a single point of failure. If you have to take it offline for maintenance or repair for an extended period ...
Aaron Copley's user avatar
  • 12.7k
9 votes

Ntpd on local network - Preventing clock drift causing high offsets

A driftfile records the frequency of the local clock, and is a good thing to have to accurately discipline it. Ubuntu /etc/ntp.conf has a driftfile by default. Appending iburst to the server line ...
John Mahowald's user avatar
9 votes
Accepted

ntpd fails to sync TIME_ERROR: 0x41: Clock Unsynchronized

Your clock is syncing fine. The sync_ntp in your ntpq -c sysinfo proves that. The kernel messages you're referring to are transient during ntpd startup and are not anything to be worried about. The ...
Paul Gear's user avatar
  • 4,507
8 votes

Is there research material on NTP accuracy available?

Vested interest on my part: I'm a Meinberg agent :-) Yes NTP can achieve an end-to-end precision down to approx. 50 us (that's microseconds) of jitter, if you sync a linux "client" on bare ...
frr's user avatar
  • 271
8 votes
Accepted

Chrony doesn't appear to be syncing with the servers I've specified

The servers you've specified: server 0.us.pool.ntp.org server 1.us.pool.ntp.org server 2.us.pool.ntp.org server 3.us.pool.ntp.org are, as the names suggest, part of the NTP Pool Project. When you ...
Michael Hampton's user avatar
8 votes
Accepted

Allow Active Directory users to disable datetime sync

As pointed out by @MDMarra, using a VM is a great way to go because blocking time synchronization can cause significant problems for authentication, TLS certificate validity, etc. However, for true ...
Sam Erde's user avatar
  • 3,469
7 votes

Different offset in "ntpq -p" and "ntpdate -q"

ntpq outputs offset in milliseconds. ntpdate offset is in seconds. Your offset is therefore 209.956 ms vs. 0.207451 sec which is pretty close (209ms vs. 207ms)
LHWizard's user avatar
  • 576
7 votes

What is NTP dispersion and how do I control it?

Your dispersion and skew are enormous, there is a very large offset from the local clock to that peer. You should compare the offsets with the local date and set the clock manually. Get ntpd running ...
John Mahowald's user avatar
7 votes

Allow Active Directory users to disable datetime sync

You really don't want to do this. Time sync is critical to Kerberos functioning properly. Unless you want authentication failures, you'll leave the system clocks where they are. Perhaps a non-domain ...
MDMarra's user avatar
  • 101k
7 votes
Accepted

NTP high-availability behind a Virtual IP

After my initial comment a slightly longer answer. As a rule of thumb when the protocol itself, or the common implementations of said protocol, already natively support resilience, high ...
HBruijn's user avatar
  • 80.3k

Only top scored, non community-wiki answers of a minimum length are eligible