Automated Kerberos Ticket Renewal for SMB Printing on macOS in Enterprise AD Environments


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When implementing ksmbprint (http://deploystudio.com/) for Windows print server integration in macOS environments, the 10-hour Kerberos ticket expiration creates operational hurdles. While the initial authentication works perfectly, expired tickets cause print jobs to silently fail - appearing successful but never reaching the printer.

Manual renewal through Keychain Access -> Ticket Viewer isn't scalable for 300+ machines. The launch agent approach that prompts users at 30-minute thresholds fails because:


# Sample launch agent plist that doesn't work for automated renewal
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.company.kerberos_renew</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/bin/kinit</string>
        <string>-R</string>
    </array>
    <key>StartInterval</key>
    <integer>1800</integer> <!-- 30 minutes -->
</dict>
</plist>

The core issue lies in macOS's security model and AD integration:

  • Keychain Services requires user interaction for credential access
  • kinit -R can only renew existing valid tickets
  • Enterprise policies often prohibit storing plaintext passwords

Here's a three-pronged approach that respects security while minimizing user disruption:

1. Credential Caching with Secure Enclave


// SecItemAdd example for storing credential reference
let query: [String: Any] = [
    kSecClass as String: kSecClassInternetPassword,
    kSecAttrAccount as String: "AD_User",
    kSecAttrServer as String: "domain.controller",
    kSecUseAuthenticationUI as String: kSecUseAuthenticationUIAllow,
    kSecAttrSynchronizable as String: kCFBooleanTrue,
    kSecReturnPersistentRef as String: kCFBooleanTrue
]

2. Background Renewal Daemon


#!/bin/bash

# Check ticket status
TICKET_TIME_REMAINING=$(klist -l | grep -Eo '[0-9]+:[0-9]+:[0-9]+')

if [[ "$TICKET_TIME_REMAINING" < "01:00:00" ]]; then
    # Use secure enclave reference for silent renewal
    security execute-with-privileges kinit -R -t /path/to/ccache
fi

3. Fallback Notification System


// Swift example for user notification
import UserNotifications

func showKerberosAlert() {
    let content = UNMutableNotificationContent()
    content.title = "Printing Credentials Expired"
    content.body = "Please authenticate to maintain printing access"
    
    let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 1, repeats: false)
    let request = UNNotificationRequest(identifier: "KerberosRenewal", 
                                      content: content, 
                                      trigger: trigger)
    UNUserNotificationCenter.current().add(request)
}

When rolling this solution:

  • Test extensively with your domain controllers' ticket policies
  • Consider implementing JAMF or Munki for mass deployment
  • Monitor /var/log/krb5.log for renewal attempts

For environments where possible, certificate authentication completely bypasses the ticket renewal problem:


# In krb5.conf
[realms]
    EXAMPLE.COM = {
        pkinit_anchors = FILE:/etc/ssl/certs/ca-certificates.crt
        pkinit_identity = FILE:/path/to/user-cert.pem,/path/to/user-key.pem
    }

Watch for these gotchas:

  • Clock skew exceeding 5 minutes will break authentication
  • DNS resolution must perfectly match AD SRV records
  • MAC address randomization can interfere with persistent tickets

When implementing ksmbprint (DeployStudio's solution) for enterprise macOS printing through Windows print servers, we hit a critical limitation: the 10-hour Kerberos ticket expiration. After this period, print jobs silently fail despite appearing successful in the queue. Manual renewal through Keychain Access's Ticket Viewer isn't scalable for 300+ machines.

Our initial approach used a launch agent to check ticket expiration and prompt for reauthentication:

#!/bin/bash
# Check ticket lifetime remaining
remaining=$(klist -l | grep 'Default principal' -A2 | grep 'renew until' | awk '{print $3}')
expiry_time=$(date -j -f "%H:%M:%S" "$remaining" +%s)
current_time=$(date +%s)
difference=$(( (expiry_time - current_time) / 60 ))

if [ $difference -lt 30 ]; then
    # This works when run manually but fails as launch agent
    osascript -e 'display dialog "Your Kerberos ticket expires soon. Please authenticate." with title "Security Renewal"'
    kinit --renew
fi

The core issue is that launch agents run in a different security context than user sessions. When attempting GUI prompts or password interactions:

  • No access to user's keychain
  • Cannot display graphical prompts
  • Password prompts fail silently

Option 1: Keychain Integration

Store credentials securely in the keychain and automate renewal:

#!/bin/bash
# Retrieve password from keychain
password=$(security find-generic-password -a "${USER}" -s "kerberos_renew" -w)

if [ -z "$password" ]; then
    # First-run initialization
    password=$(osascript -e 'text returned of (display dialog "Enter domain password:" with hidden answer)')
    security add-generic-password -a "${USER}" -s "kerberos_renew" -w "${password}"
fi

# Renew ticket
echo "$password" | kinit --password-file=STDIN user@DOMAIN

Option 2: PKINIT with Smartcard

For higher-security environments, configure PKINIT authentication:

# /etc/krb5.conf
[libdefaults]
default_realm = DOMAIN.COM
pkinit_anchors = FILE:/etc/ssl/certs/ca-certificates.crt
pkinit_identities = PKCS11:/usr/lib/libusb-pkcs11.so

Option 3: Delegated Renewal Service

Create a privileged helper tool for system-wide renewal:

// RenewalService.m (SMJobBless template)
#import <Foundation/Foundation.h>
#import <GSS/GSS.h>

@implementation RenewalService
- (BOOL)renewKerberosTicketForUser:(NSString *)user {
    OM_uint32 maj_stat, min_stat;
    gss_cred_id_t cred = GSS_C_NO_CREDENTIAL;
    gss_name_t name;
    
    CFErrorRef error = NULL;
    AuthorizationRef authRef;
    AuthorizationCreate(NULL, NULL, 0, &authRef);
    
    // Implement GSSAPI renewal logic here
    // This runs with elevated privileges
    
    return YES;
}
@end

When implementing automated solutions:

  • Never store plaintext passwords
  • Use macOS keychain with proper ACLs
  • Consider implementing TouchID authentication for renewals
  • Log all renewal attempts for auditing

Implement a watchdog system to ensure continuity:

# /Library/LaunchDaemons/com.company.printwatchdog.plist
<?xml version="1.0" encoding="UTF-8"?>
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.company.printwatchdog</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/local/bin/print_watchdog.sh</string>
    </array>
    <key>StartInterval</key>
    <integer>300</integer>
</dict>
</plist>