Laborpruefung/solutions.md

# Practice Exam Solutions

## Task 1: Company Portal Unreachable

### Diagnosis

```bash
# Check nginx status
sudo systemctl status nginx

# Test nginx configuration
sudo nginx -t
# Output: nginx: [emerg] unexpected "}" in /etc/nginx/sites-enabled/company.local:8

# View the config file
cat /etc/nginx/sites-available/company.local
```

### Problem
The nginx configuration file for company.local has a **missing semicolon** after `index index.html`.

### Solution

```bash
# Edit the configuration file
sudo nano /etc/nginx/sites-available/company.local

# Find this line:
    index index.html

# Change to (add semicolon):
    index index.html;

# Test configuration
sudo nginx -t
# Output: nginx: configuration file /etc/nginx/nginx.conf syntax is ok

# Reload nginx
sudo systemctl reload nginx
```

### Verification
```bash
curl http://company.local
# Should return HTML with "Company Portal"
```

---

## Task 2: Online Shop Shows 403 Forbidden

### Diagnosis

```bash
# Check nginx error log
sudo tail /var/log/nginx/shop_error.log
# Output: open() "/var/www/shop/index.html" failed (13: Permission denied)

# Check file permissions
ls -la /var/www/shop/
# Output: ---------- 1 www-data www-data ... index.html
# (No permissions at all!)
```

### Problem
The index.html file has **wrong permissions** (chmod 000 - no read/write/execute for anyone).

### Solution

```bash
# Fix the file permissions
sudo chmod 644 /var/www/shop/index.html

# Verify permissions (should show -rw-r--r--)
ls -la /var/www/shop/index.html
```

### Verification
```bash
curl http://shop.local
# Should return HTML with "Online Shop"
```

---

## Task 3: API Returns 502 Bad Gateway

### Diagnosis

```bash
# Check nginx error log for API
sudo tail /var/log/nginx/api_error.log
# Output: connect() to unix:/var/run/php/php8.x-fpm.sock failed (2: No such file or directory)

# Check PHP-FPM status
sudo systemctl status php*-fpm
# Output: Active: inactive (dead)
```

### Problem
The **PHP-FPM service is stopped and disabled**. Nginx cannot forward PHP requests.

### Solution

```bash
# Find the PHP-FPM version
php -v
# or: ls /etc/php/

# Start PHP-FPM (replace 8.3 with your version if different)
sudo systemctl start php8.3-fpm

# Enable to start on boot
sudo systemctl enable php8.3-fpm

# Verify it's running
sudo systemctl status php8.3-fpm
# Should show: Active: active (running)
```

### Verification
```bash
curl http://api.local:8080
# Should return JSON with "status": "success"
```

---

## Task 4: Docker Application Unreachable

### Diagnosis

```bash
# Check if Docker container is running
sudo docker ps
# Output: practice-webapp is running (HEALTHY)

# Check if port is listening
sudo ss -tulnp | grep 8888
# Output: Shows docker-proxy listening on 8888

# Check firewall status
sudo ufw status
# Output: 8888/tcp DENY Anywhere
```

### Problem
The **firewall (UFW) is blocking port 8888**. The Docker container is running correctly, but connections are denied.

### Solution

```bash
# Remove the deny rule
sudo ufw delete deny 8888/tcp

# Allow port 8888
sudo ufw allow 8888/tcp

# Verify firewall rules
sudo ufw status
# Should show: 8888/tcp ALLOW Anywhere
```

### Verification
```bash
curl http://localhost:8888
# Should return HTML with "Docker Application"
```

---

## Task 5: Database Connection Refused

### Diagnosis

```bash
# Try to connect with the user
mysql -u webuser -p'WebPass123!' practicedb
# Output: ERROR 1045 (28000): Access denied for user 'webuser'@'localhost'

# Check which users exist (as root)
sudo mysql -e "SELECT user, host FROM mysql.user WHERE user='webuser';"
# Output: webuser | 10.0.0.1
# (User is bound to wrong IP, not localhost!)
```

### Problem
The MySQL user `webuser` is bound to **wrong host IP** (10.0.0.1 instead of localhost).

### Solution

```bash
# Connect as root
sudo mysql

# Create user for localhost (or recreate)
DROP USER IF EXISTS 'webuser'@'10.0.0.1';
CREATE USER 'webuser'@'localhost' IDENTIFIED BY 'WebPass123!';
GRANT ALL PRIVILEGES ON practicedb.* TO 'webuser'@'localhost';
FLUSH PRIVILEGES;
EXIT;
```

Or as a one-liner:
```bash
sudo mysql -e "DROP USER IF EXISTS 'webuser'@'10.0.0.1'; CREATE USER 'webuser'@'localhost' IDENTIFIED BY 'WebPass123!'; GRANT ALL PRIVILEGES ON practicedb.* TO 'webuser'@'localhost'; FLUSH PRIVILEGES;"
```

### Verification
```bash
mysql -u webuser -p'WebPass123!' practicedb -e "SELECT * FROM products;"
# Should show table with products (Laptop, Mouse, Keyboard, Monitor)
```

---

## Quick Reference: Problem Summary

| Task | Service | Problem | Key Fix |
|------|---------|---------|---------|
| 1 | company.local | Missing semicolon in nginx config | Add `;` after `index index.html` |
| 2 | shop.local | Wrong file permissions (000) | `chmod 644 /var/www/shop/index.html` |
| 3 | api.local | PHP-FPM service stopped | `systemctl start php*-fpm` |
| 4 | localhost:8888 | Firewall blocking port | `ufw allow 8888/tcp` |
| 5 | MySQL | User bound to wrong host | Create user for `'webuser'@'localhost'` |

---

## Diagnostic Approach

For each problem, follow this systematic approach:

1. **Check the symptom** - What error do you see?
   - Connection refused → Service not running or firewall
   - 403 Forbidden → Permissions issue
   - 502 Bad Gateway → Backend service (PHP-FPM) down
   - Connection timeout → Firewall blocking

2. **Check service status** - Is the service running?
   ```bash
   systemctl status nginx
   systemctl status php*-fpm
   docker ps
   ```

3. **Check logs** - What do the logs say?
   ```bash
   journalctl -u nginx
   tail /var/log/nginx/error.log
   ```

4. **Check configuration** - Is the config valid?
   ```bash
   nginx -t
   cat /etc/nginx/sites-available/...
   ```

5. **Check permissions** - Can the service read the files?
   ```bash
   ls -la /var/www/...
   ```

6. **Check network** - Is the port accessible?
   ```bash
   ss -tulnp
   ufw status
   ```

---

*Practice Exam Solutions - Betriebssysteme und Netzwerke*