Day 12: Linux Network Services | 100 Days of DevOps

Content:

Today I worked on troubleshooting a real-world Linux network issue where an Apache service was not accessible on port 8088. The issue required debugging at multiple levels including network connectivity, service conflicts, and firewall rules.

What I Learned

Through this task, I understood:

• How to approach troubleshooting step by step
• How port conflicts can break services
• How to identify which process is using a port
• How to adjust configurations safely
• How firewall rules impact service accessibility

Steps I Followed :

1. Checked Connectivity from Jump Host

telnet stapp01 8088
telnet stapp02 8088
telnet stapp03 8088

👉 I started by testing connectivity from the jump host to all app servers.

• stapp02 and stapp03 were reachable
• stapp01 failed

➡️ This confirmed the issue was specific to stapp01, not a global problem.

2. Logged into the Problem Server

ssh tony@stapp01

👉 After identifying the problematic server, I logged in to investigate further.

3. Checked Apache Service Status

sudo systemctl status httpd

👉 The Apache service was not running. 👉 The error message showed: "Address already in use"

➡️ This indicated that another service was already using port 8088.

4. Installed netstat Tool

sudo yum install -y net-tools

👉 The netstat command was not available, so I installed net-tools to inspect open ports.

5. Identified Port Conflict

sudo netstat -tlnup

👉 I checked which service was using port 8088. 👉 Found that sendmail was running on that port.

➡️ This was the root cause — Apache couldn't start because the port was already occupied.

6. Modified Sendmail Configuration

cd /etc/mail
sudo vi sendmail.mc

👉 I edited the sendmail configuration file and changed its port from 8088 to another unused port.

➡️ This ensured there would be no conflict with Apache.

7. Restarted Sendmail

sudo systemctl restart sendmail

👉 Restarted the service to apply the configuration changes.

8. Verified Port Availability

sudo netstat -tlnup

👉 Checked again to confirm that port 8088 was now free.

9. Tested Apache Before Fix

curl http://localhost:8088

👉 Apache was still not accessible .

10. Checked Firewall Rules

sudo iptables -L -n

👉 I inspected firewall rules and noticed that port 8088 was not explicitly allowed.

➡️ Even if Apache runs, firewall could block access.

11. Allowed Port 8088 in Firewall

sudo iptables -I INPUT 4 -p tcp --dport 8088 -j ACCEPT

👉 Added a rule to allow incoming traffic on port 8088.

12. Restarted Apache Service

sudo systemctl restart httpd

👉 Now that:

• Port conflict was resolved ✅
• Firewall rule added ✅

I restarted Apache.

13. Final Verification

curl http://localhost:8088

👉 Successfully received a response

➡️ This confirmed Apache is running and accessible locally.

My Understanding

This task taught me that service failures are often caused by multiple layered issues — not just one. Here, both port conflict and firewall rules had to be resolved to fully fix the problem.

What I Found Interesting

It was interesting to see how a background service like sendmail can unintentionally block a web server. Also, the importance of checking firewall rules after fixing the service was a great real-world lesson.

📌 Full notes: GitHub link