DNS, Rsync, and Cron Services#

DNS Service (BIND)#

Service Name#

• named (most distributions)

Configuration Location#

• Ubuntu: /etc/bind/
• CentOS: May be in different location

Check Service#

systemctl status named

Basic Concept#

DNS translates domain names to IP addresses (forward lookup) and IP addresses to domain names (reverse lookup).

Forward lookup: example.com → 192.168.1.100 Reverse lookup: 192.168.1.100 → example.com

Key Files (Bind)#

• named.conf - Main configuration
• Zone files - Define DNS records for domains

This is a complex service - requires understanding of:

• Zone files
• DNS record types (A, PTR, CNAME, MX, etc.)
• Forward vs reverse zones
• DNS hierarchy

Rsync - File Synchronization/Backup#

Basic Syntax#

rsync [options] source destination

Common Options#

-a    # Archive mode (preserves permissions, timestamps, etc.)
-v    # Verbose (show what's being copied)
-z    # Compress during transfer
-r    # Recursive (copy directories)
-h    # Human-readable output
--delete  # Delete files in dest that don't exist in source

Local Backup Example#

rsync -av /home/user/stuff/ /home/user/backups/

Note the trailing slash on source - affects behavior:

• /source/ - copy contents of source
• /source - copy source directory itself

Remote Backup via SSH#

rsync -avz /local/path/ user@remote:/remote/path/

Consistency vs. Accumulation#

Consistency (mirror - deletes old files):

rsync -av --delete /source/ /backup/

Accumulation (keeps all files):

rsync -av /source/ /backup/

Check Installed#

rsync --version
# or just run rsync to see options

Cron - Task Automation#

Service Name#

• cron (Ubuntu/Debian)
• crond (CentOS/RHEL)

Check Service#

systemctl status cron
systemctl status crond  # CentOS

Edit Crontab#

crontab -e   # Edit current user's crontab

First time will ask which editor (nano recommended for beginners).

Crontab Syntax#

Five time fields + command:

* * * * * command
│ │ │ │ │
│ │ │ │ └─ Day of week (0-7, 0/7 = Sunday)
│ │ │ └─── Month (1-12)
│ │ └───── Day of month (1-31)
│ └─────── Hour (0-23)
└───────── Minute (0-59)

Asterisk (*) means "every"

Examples#

Every minute:

* * * * * /path/to/command

Every 5 minutes:

*/5 * * * * /path/to/command

Every day at 2:30 AM:

30 2 * * * /path/to/command

Every Monday at 5:00 PM:

0 17 * * 1 /path/to/command

First day of every month at midnight:

0 0 1 * * /path/to/command

Automated Backup Example#

Run rsync backup every night at 2 AM:

0 2 * * * rsync -av --delete /var/www/html/ /backups/website/

Redirect Output#

Send output to file:

* * * * * /path/to/command > /path/to/logfile.txt

Append to file:

* * * * * /path/to/command >> /path/to/logfile.txt

Suppress output:

* * * * * /path/to/command > /dev/null 2>&1

View Crontab#

crontab -l   # List current user's crontab

Remove Crontab#

crontab -r   # Remove current user's crontab

System-Wide Cron#

User-specific: Managed via crontab -e

System-wide cron directories:

• /etc/cron.daily/ - Scripts run daily
• /etc/cron.hourly/ - Scripts run hourly
• /etc/cron.weekly/ - Scripts run weekly
• /etc/cron.monthly/ - Scripts run monthly

Place executable scripts in these directories for automatic execution.

Important Notes#

1. Cron uses absolute paths - always specify full path to commands
2. Cron runs in minimal environment - may need to set PATH, etc.
3. Test commands manually first before adding to cron
4. Cron jobs run as the user who owns the crontab
5. sudo crontab -e edits root's crontab (for privileged tasks)

Combining Rsync + Cron#

Automated nightly backups:

# In crontab -e:
0 2 * * * rsync -avz /var/www/html/ /backups/website/
0 3 * * * rsync -avz /etc/ /backups/configs/

This creates automated, scheduled backups without manual intervention.