Day 7 – NFS, CIFS, Chrony, Cron, Performance Monitoring & Troubleshooting in Linux

 Agenda – Day 7

• NFS (Linux to Linux File Sharing)

• CIFS (Linux to Windows File Sharing)

• Chrony (Time Server Configuration)

• Cron (Job Scheduling)

• Performance Monitoring

• Troubleshooting

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File Sharing Using NFS (Network File System) 

What is NFS?

NFS allows Linux systems to share directories over a network.

• 🗄 NFS Server → Central storage

• 💻 NFS Client → Access shared storage

• 📂 Shared Folder → Exported directory

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NFS Server Configuration

# yum install nfs-utils -y
# mkdir /shared_folder
# vi /etc/exports

Add:

/shared_folder 192.168.1.0/24(rw,sync,no_subtree_check)

Restart:

# systemctl enable nfs-server
# systemctl restart nfs-server

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NFS Client Configuration

# yum install nfs-utils -y
# showmount -e <server_ip>
# mount -t nfs server_ip:/shared_folder /mnt

Permanent:

# echo "server_ip:/shared_folder /mnt nfs defaults 0 0" >> /etc/fstab

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Mounting Windows Shares Using CIFS

CIFS allows Linux to mount Windows shared folders.

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CIFS Configuration

# yum install cifs-utils -y
# vi /root/.smbcredentials

Add:

username=Administrator
password=Pass@1234
domain=dorole.org

Secure:

# chmod 600 /root/.smbcredentials

Mount:

# mount -t cifs -o credentials=/root/.smbcredentials //win01/win-share /mnt

Permanent entry in /etc/fstab:

# //win01/win-share /mnt cifs credentials=/root/.smbcredentials 0 0

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Chrony – Time Server Configuration

Time synchronization is critical for:

✔ Logs
✔ Database consistency
✔ Clusters
✔ Authentication

Modern Linux uses chronyd service.

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Chrony Master (NTP Server)

# yum install chrony -y
# vi /etc/chrony.conf

Add:

allow 192.168.2.0/24
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst

Start:

# systemctl enable chronyd
# systemctl restart chronyd

Allow firewall:

# firewall-cmd --permanent --add-service=ntp
# firewall-cmd --reload

Verify:

# chronyc sources
# chronyc tracking

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Chrony Client

Edit:

# vi /etc/chrony.conf

Add:

server 192.168.1.100 iburst

Restart:

# systemctl restart chronyd

Check:

# chronyc tracking


Integrating a Linux Server with Active Directory 
Prerequisites:
A Linux server with root access (e.g., lamp01).
An existing Active Directory domain (e.g., darole.org).
AD administrative credentials (e.g., vallabh@darole.org).
1. Configure DNS for AD Domain Resolution
Ensure the Linux server can resolve the AD domain by editing /etc/resolv.conf:
# cat /etc/resolv.conf
search darole.org 
nameserver 172.16.1.200 
nameserver 192.168.2.1 
options timeout:2

Verify the DNS SRV records for LDAP and Kerberos:

# dig +short SRV _ldap._tcp.darole.org
# dig +short SRV _kerberos._tcp.darole.org
# dig +short SRV _kerberos._udp.darole.org
2. Install Required Packages
Install the necessary tools for AD integration:
# yum install adcli realmd sssd oddjob oddjob-mkhomedir samba-common-tools krb5-workstation authselect-compat openldap-clients
3. Configure Kerberos
Edit /etc/krb5.conf for Kerberos authentication:
cat /etc/krb5.conf
includedir /etc/krb5.conf.d/
[logging]
    default = FILE:/var/log/krb5libs.log
    kdc = FILE:/var/log/krb5kdc.log
    admin_server = FILE:/var/log/kadmind.log
[libdefaults]
    dns_lookup_realm = true
    dns_lookup_kdc = true
    ticket_lifetime = 24h
    renew_lifetime = 7d
    forwardable = true
    rdns = false
    pkinit_anchors = FILE:/etc/pki/tls/certs/ca-bundle.crt
    spake_preauth_groups = edwards25519
    default_realm = DAROLE.ORG
    default_ccache_name = KEYRING:persistent:%{uid}
    udp_preference_limit = 0
[realms]
    DAROLE.ORG = {
        kdc = 172.16.1.200
        admin_server = 172.16.1.200
    }

Test Kerberos authentication:
# kinit vallabh@DAROLE.ORG
# klist

4. Verify LDAP Connectivity
Use ldapsearch to query the AD domain:
# ldapsearch -H ldap://172.16.1.200 -D "vallabh@darole.org" -W -b "dc=darole,dc=org"
5. Join the Linux Server to AD Domain
Join the AD domain using the realm command:
# realm join -v darole.org -U vallabh

Verify the AD integration:

# id -a vallabh@darole.org
6. Test SSH Login Using AD Credentials
From a remote machine:
ssh lamp01 -l vallabh@darole.org
Conclusion
The Linux server is now successfully integrated into the AD domain. Users can log in with their AD credentials and access resources as authorized.

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Cron/AT – Job Scheduling in Linux

Linux provides different ways to schedule commands and tasks.
AT: Used to execute a command ONE TIME at a specified time.
CRON: Used to execute commands REPEATEDLY according to a schedule.

1. DIFFERENCE BETWEEN AT AND CRON
AT:
- Used for one-time tasks.
- Command runs only once.
- Uses the atd service.
- Check jobs using: atq
- Delete a job using: atrm
CRON:
- Used for recurring tasks.
- Command runs repeatedly.
- Uses the crond service.
- Check jobs using: crontab -l
- Edit jobs using: crontab -e

2. AT COMMAND
The "at" command is used when you want to execute a command
ONE TIME in the future.

Example 1:
Run a command at 10:00 PM.
[root@rocky ~]# at 10:00 PM
Enter the command:
[root@rocky ~]# systemctl restart httpd
Press:
Ctrl+D
The job will be scheduled and executed once at 10:00 PM.

3. AT COMMAND - RUN AFTER 10 MINUTES
To run a command after 10 minutes:
[root@rocky ~]# at now + 10 minutes
Then enter:
[root@rocky ~]# touch /tmp/testfile
Press:
Ctrl+D
The command will execute once after 10 minutes.

4. CHECK AT JOBS
Use "atq" to display scheduled at jobs.
[root@rocky ~]# atq
Example output:
5       Wed Sep 23 22:00:00 2026 a root
Here:
5 = Job ID
Wed Sep 23 22:00:00 2026 = Scheduled time
a = Queue
root = User

5. DELETE AN AT JOB
If the job ID is 5:
[root@rocky ~]# atrm 5
The scheduled job will be removed.

6. AT SERVICE
The "at" command uses the atd service.
Check the service:
[root@rocky ~]# systemctl status atd
Start the service:
[root@rocky ~]# systemctl start atd
Enable the service at boot:
[root@rocky ~]# systemctl enable atd

7. CRON
Cron is used to execute commands or scripts repeatedly
according to a predefined schedule.
Edit the current user's cron jobs:
[root@rocky ~]# crontab -e
View cron jobs:
[root@rocky ~]# crontab -l

8. CRON FORMAT
Cron uses five fields:

* * * * * command
| | | | |
| | | | └── Day of Week (0-7)
| | | └──── Month
| | └────── Day of Month
| └──────── Hour
└────────── Minute

9. CRON EXAMPLE - EVERY DAY
Run backup every day at 10:00 PM:
[root@rocky ~]# crontab -e
Add:
0 22 * * * /root/backup.sh
Meaning:
0  = Minute
22 = Hour
* = Every day of month
* = Every month
* = Every day of week
Therefore:
/root/backup.sh
will run every day at 10:00 PM.

10. CRON EXAMPLE - EVERY 5 MINUTES
Run a command every 5 minutes:
*/5 * * * * df -h >> /var/log/disk_usage.log
Meaning:
*/5 = Every 5 minutes

11. CRON EXAMPLE - EVERY HOUR
Run a script every hour:
0 * * * * /root/test.sh

12. CRON EXAMPLE - EVERY SUNDAY
Run a script every Sunday at 2:00 AM:
0 2 * * 0 /root/cleanup.sh

13. CRON EXAMPLE - MONDAY TO FRIDAY
Run a script at 6:30 AM from Monday to Friday:
30 6 * * 1-5 /root/report.sh

14. CRON SERVICE
Cron uses the crond service.
Check the service:
[root@rocky ~]# systemctl status crond
Start the service:
[root@rocky ~]# systemctl start crond
Enable the service at boot:
[root@rocky ~]# systemctl enable crond

15. QUICK COMPARISON
AT
Purpose       : One-time task
Command       : at
Service       : atd
View jobs     : atq
Delete job    : atrm JOB_ID
CRON
Purpose       : Repeated task
Command       : crontab
Service       : crond
View jobs     : crontab -l
Edit jobs     : crontab -e

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Performance Monitoring in Linux

Monitoring performance helps detect system bottlenecks.

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📊 Important Commands

top

# top

vmstat

# vmstat 2

iostat

# iostat -x 2

sar

# sar -u 1 5

netstat

# netstat -tulnp

mpstat

# mpstat -P ALL 1

free

# free -h

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Troubleshooting in Linux

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🔥 Common Issues

High CPU

Use:

top
mpstat

Disk Full

df -h
du -sh *

Reset Root Password

• Edit GRUB

• Add rd.break

• Use passwd

Recover Deleted LVM

vgcfgrestore

Clear RAM Cache

sync; echo 3 > /proc/sys/vm/drop_caches

DNS Issues

• Check /etc/resolv.conf

• Use nslookup

• Use dig

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7️⃣ Increasing CPU & Memory in Virtual Environments

In production, sometimes performance issues are resolved by scaling resources.

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🖥 Increase CPU & RAM in VMware vCenter

Steps:

1. Login to vCenter

2. Right-click Virtual Machine

3. Click Edit Settings

4. Increase:

   • Number of CPUs

   • Memory (RAM)

5. Click OK

If VM is running:

• Ensure CPU Hot Add and Memory Hot Add are enabled

• Otherwise power off VM before increasing resources

After increasing:

Inside Linux verify:

nproc
free -h
lscpu

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☁ Increase CPU & RAM in Microsoft Azure

Used in cloud environments.

Steps:

1. Login to Azure Portal

2. Go to Virtual Machines

3. Select your VM

4. Click Size

5. Choose larger VM size

6. Click Resize

⚠ Sometimes VM must be stopped before resizing.

After resizing:

Verify inside Linux:

lscpu
free -h

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🎯 Updated Key Takeaways – Day 7

✔ NFS – Linux-to-Linux sharing
✔ CIFS – Linux-to-Windows integration
✔ Chrony – Time synchronization
✔ Cron – Automation
✔ Performance monitoring tools
✔ Root password recovery
✔ LVM troubleshooting
✔ Resource scaling in VMware & Azure

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