# RH342 - Red Hat Enterprise Linux Diagnostics and Troubleshooting

Cheat sheets extracted from the RH342 (RHEL 8.4) student guide.

## Contents

1. [[01 Collecting Information and Logs]] - journalctl, rsyslog, log locations
2. [[02 Boot Loader Troubleshooting]] - GRUB2, grubby, efibootmgr, rescue mode
3. [[03 Hardware Identification]] - dmesg, lscpu, dmidecode, rasdaemon
4. [[04 Storage Stack and Troubleshooting]] - LVM, device mapper, XFS repair, Stratis
5. [[05 Network Troubleshooting]] - nmcli, ping, nmap, firewalld, tcpdump
6. [[06 SELinux Troubleshooting]] - contexts, booleans, ports, ausearch, sealert
7. [[07 Kernel Crash Dumps (kdump)]] - kdump configuration, vmcore analysis

## Quick Reference

### Logs
```bash
journalctl -b                    # Current boot logs
journalctl -u httpd.service      # Service logs
ausearch -m avc -ts recent       # SELinux denials
```

### Boot Issues
```bash
grubby --default-kernel          # Show default kernel
grubby --set-default /boot/vmlinuz-xxx
systemd.unit=rescue.target       # Boot to rescue mode
```

### Hardware
```bash
lscpu                            # CPU info
dmidecode -t memory              # Memory info
dmesg -T                         # Kernel messages
```

### Storage
```bash
lsblk -f                         # Block devices
xfs_repair /dev/xxx              # Repair XFS
lvs; vgs; pvs                    # LVM info
```

### Network
```bash
nmcli device status              # Network devices
ss -tuln                         # Listening ports
firewall-cmd --list-all          # Firewall rules
```

### SELinux
```bash
getenforce                       # SELinux mode
ausearch -m avc -ts recent       # Recent denials
restorecon -Rv /path             # Fix contexts
setsebool -P boolean on          # Set boolean
```

### Kdump
```bash
kdumpctl status                  # Kdump status
echo c > /proc/sysrq-trigger     # Test crash (dangerous!)
crash vmlinux vmcore             # Analyze dump
```