Kerberos Time Skew Detection and Fix Guide

Time synchronization is crucial for Kerberos authentication to function correctly. If the time on the client machine differs from the Key Distribution Center (KDC) by more than the allowed skew (typically 5 minutes), authentication will fail with errors such as KRB5KRB_AP_ERR_SKEW.

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1. Detecting Time Skew Using ntpdate

Use the ntpdate tool to compare the local time with the Domain Controller (KDC). Ensure the Domain Controller is reachable and responds to NTP queries for accurate results:

sudo ntpdate -q <DC-IP-or-hostname>

Example:

sudo ntpdate -q 10.10.10.1

Sample Output:

server 10.10.10.1, stratum 2, offset -7.232454, delay 0.02659

• offset: Time difference in seconds. If this is more than ±300 seconds (5 minutes), Kerberos will fail.

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2. Optional: Using Nmap for Kerberos Detection

nmap can help identify if the target supports Kerberos but does not directly indicate time skew:

nmap -sU -p 88 --script=krb5-enum-users <target>

• Use this to enumerate Kerberos usernames.

• Time skew might cause script failures or lack of responses.

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3. Detecting Skew Errors with Kerberos Tools

Use tools like kinit or Impacket's getTGT to test authentication. These tools require valid credentials and network access to the KDC to function properly.

With kinit:

kinit testuser

With Impacket:

getTGT test.local/testuser:'Passw0rd!'

Common Error Due to Skew:

KRB5KRB_AP_ERR_SKEW: Clock skew too great

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4. Fixing Time Skew Immediately

Synchronize time with the KDC manually (note: administrative privileges are usually required to run ntpdate):

sudo ntpdate <DC-IP>

Example:

sudo ntpdate 10.10.10.1

This will immediately adjust your system clock.

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5. Summary Table

Tool	Purpose	Detects Time Skew?
ntpdate -q <ip>	Check time offset with KDC	✅ Yes
nmap with script	Kerberos enumeration	❌ No
kinit / getTGT	Authenticate & detect errors	⚠️ Shows failure if skew exists
ntpdate <ip>	Fix time immediately	✅ Yes

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Note

• Ensure ntpdate is installed: sudo apt install ntpdate (Ubuntu/Debian)

• For ongoing synchronization, consider using chrony or enabling NTP with timedatectl:

sudo timedatectl set-ntp true

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This guide effectively helps identify and resolve Kerberos authentication issues caused by time mismatches.