I have a private server on AWS that has no publicly open inbound ports. The only open port is SSH, and it's only accessible from other machines in the same subnet.
This machine reads files from S3 over the network. Recently it stopped working because the server's time had drifted by more than 15 minutes and the requests to S3 were rejected:
<Code>RequestTimeTooSkewed</Code>
<Message>
The difference between the request time and the current time
is too large.
</Message>
<MaxAllowedSkewMilliseconds>900000</MaxAllowedSkewMilliseconds>
The easy fix is to install ntp, but it requires opening UDP port 123. Because UDP is stateless, both inbound and outbound ports must be open.
How can I automatically update the system time using only TCP connections initiated by my server? Is there a standard TCP-based daemon like NTP? I don't care if it's not as precise: even a skew of up to 10 minutes would be acceptable.
Edit 2 Jun 2017
This is how the security groups are set up:
Inbound
Type Protocol Port Range Source SSH TCP 22 172.31.0.0/16Outbound
Type Protocol Port range Destination All traffic All All 0.0.0.0/0
This is the network ACL for the subnet - just the default ACL:
Inbound
Rule # Type Protocol Port Range Source Allow / Deny 100 ALL Traffic ALL ALL 0.0.0.0/0 ALLOW * ALL Traffic ALL ALL 0.0.0.0/0 DENYOutbound
Rule # Type Protocol Port Range Destination Allow / Deny 100 ALL Traffic ALL ALL 0.0.0.0/0 ALLOW * ALL Traffic ALL ALL 0.0.0.0/0 DENY
Edit 2 Jun 2017 #2
Alright now it works without any special security groups, as prediceted by @Tim. I think I was just not testing it properly:
ntpdatedoesn't use the servers in/etc/ntp.conf, so it was reporting an error:no servers can be used, exiting
ntpddoes not attempt to update the clock as soon as it starts; it waits for a minute or so.
Testing instead with ntpdate-debian, and without port 123 open in a security group, works fine; and ntpd updates the time properly if I let it run for a while.
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 able to initiate a connection.
You will of course require NACLs open in both direction for that port.
Update You should read about AWS Security, particularly security groups and NACLs.
NACL is network ACL. This is a firewall that lives outside your instance and only allows traffic to reach your instance if you have ports open. By default all inbound and outbound ports are open, but you can configure any way you like. NACLs are stateless so you need to open ports in each direction. Don't forget ephemeral ports.
Security groups on the other hand enforce network rules on the hypervisor level. They're stateful so if you allow an outgoing port then the response is automatically allowed back into the instance.
NACLs in theory reduce host server load slightly, because they prevent traffic hitting the network interface. Since you only control a small fraction of the server that probably doesn't make much difference.
Update 1st December 2017
AWS have announced the AWS Time Sync Service. In short, they're providing servers inside each data center to ensure the server time is accurate.
The AWS documentation says to use the Chrony software, rather than NTP, but both the NTP service. There's no point me copying the AWS instructions into this post as they may change in future, and AWS documentation is excellent.
Using NTPD
Instead of installing Chrony I've simply added the following to my /etc/ntp.conf to try to get NTP to use the new AWS NTP server
I also made sure none of the other server statements had "prefer" configured.
If you're not opposed to running a cron job on another machine that has accurate time then you can always have one automatically update the date over SSH. If you have SSH permissions on another server that has an accurate date you can do it the other way as well.