cis-ubuntu1804-v220-2-1-1-1

DevOps & Security

Ensure a single time synchronization daemon is in use

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2.1.1.1 Ensure a single time synchronization daemon is in use (Automated)

Profile

Level 1 - Server
Level 1 - Workstation

Description

System time should be synchronized between all systems in an environment. This is typically done by establishing an authoritative time server or set of servers and having all systems synchronize their clocks to them.

Note:

On virtual systems where host based time synchronization is available consult your virtualization software documentation and verify that host based synchronization is in use and follows local site policy. In this scenario, this section should be skipped
Only one time synchronization method should be in use on the system. Configuring multiple time synchronization methods could lead to unexpected or unreliable results

Rationale

Time synchronization is important to support time sensitive security mechanisms and ensures log files have consistent time records across the enterprise, which aids in forensic investigations.

Audit Procedure

Command Line

On physical systems, and virtual systems where host based time synchronization is not available. One of the three time synchronization daemons should be available; chrony, systemd-timesyncd, or ntp

Run the following script to verify that a single time synchronization daemon is available on the system:

#!/usr/bin/env bash

{
   output="" l_tsd="" l_sdtd="" chrony="" l_ntp=""
   dpkg-query -W chrony > /dev/null 2>&1 && l_chrony="y"
   dpkg-query -W ntp > /dev/null 2>&1 && l_ntp="y" || l_ntp=""
   systemctl list-units --all --type=service | grep -q 'systemd-timesyncd.service' && systemctl is-enabled systemd-timesyncd.service | grep -q 'enabled' && l_sdtd="y"
   if [[ "$l_chrony" = "y" && "$l_ntp" != "y" && "$l_sdtd" != "y" ]]; then
      l_tsd="chrony"
      output="$output\n- chrony is in use on the system"
   elif [[ "$l_chrony" != "y" && "$l_ntp" != "y" && "$l_sdtd" != "y" ]]; then
      l_tsd="ntp"
      output="$output\n- ntp is in use on the system"
   elif [[ "$l_chrony" != "y" && "$l_ntp" != "y" ]]; then
      if systemctl list-units --all --type=service | grep -q 'systemd-timesyncd.service' && systemctl is-enabled systemd-timesyncd.service | grep -Eq '(enabled|disabled|masked)'; then
         l_tsd="sdtd"
         output="$output\n- systemd-timesyncd is in use on the system"
      fi
   else
      [[ "$l_chrony" = "y" && "$l_ntp" = "y" ]] && output="$output\n- both chrony and ntp are in use on the system"
      [[ "$l_chrony" = "y" && "$l_sdtd" = "y" ]] && output="$output\n- both chrony and systemd-timesyncd are in use on the system"
      [[ "$l_ntp" = "y" && "$l_sdtd" = "y" ]] && output="$output\n- both ntp and systemd-timesyncd are in use on the system"
   fi
   if [ -n "$l_tsd" ]; then
      echo -e "\n- PASS:\n$output\n"
   else
      echo -e "\n- FAIL:\n$output\n"
   fi
}

Note: Follow the guidance in the subsection for the time synchronization daemon available on the system and skip the other two time synchronization daemon subsections.

Expected Result

PASS: One of chrony, systemd-timesyncd, or ntp is in use on the system.

Remediation

Command Line

On physical systems, and virtual systems where host based time synchronization is not available. Select one of the three time synchronization daemons; chrony (1), systemd-timesyncd (2), or ntp (3), and follow the remediation procedure for the selected daemon.

Note: enabling more than one synchronization daemon could lead to unexpected or unreliable results.

1. chrony

Run the following command to install chrony:

# apt install chrony

Run the following commands to stop and mask the systemd-timesyncd daemon:

# systemctl stop systemd-timesyncd.service
# systemctl --now mask systemd-timesyncd.service

Run the following command to remove the ntp package:

# apt purge ntp

2. systemd-timesyncd

Run the following command to remove the chrony package:

# apt purge chrony

Run the following command to remove the ntp package:

# apt purge ntp

3. ntp

Run the following command to install ntp:

# apt install ntp

Run the following commands to stop and mask the systemd-timesyncd daemon:

# systemctl stop systemd-timesyncd.service
# systemctl --now mask systemd-timesyncd.service

Run the following command to remove the chrony package:

# apt purge chrony

References

NIST SP 800-53 Rev. 5: AU-3, AU-12

CIS Controls

v8: 8.4 - Standardize Time Synchronization
v7: 6.1 - Utilize Three Synchronized Time Sources