How I Crashed My Entire Network by Misconfiguring Firewall Rules (And What I Learned)

When you think, “Let me just block this small port, it won’t break anything…” 💥

Result:

• No internet access
• No ping responses
• No SSH connectivity
• Even the admin interface disappeared

Total digital silence. 😶

🔍 Key Lessons Learned:

1. Test Rules Temporarily – Always test firewall rules with a temporary policy before applying them permanently.
2. Avoid Blocking All Traffic – Never block both inbound and outbound traffic without exceptions (yes, even DNS and DHCP!).
3. Local Console Access is Critical – Ensure you have physical or out-of-band (OOB) access to the firewall/router.
4. Backup Configurations – Always back up firewall configs before making changes.
5. The Golden Rule – `Deny All` at the end, not the beginning!

You Should Know: Essential Firewall Commands & Practices

Linux (`iptables` / `nftables`)

• List current rules:

  sudo iptables -L -n -v  For iptables
  sudo nft list ruleset  For nftables
  

• Create a temporary rule (expires after reboot):

  sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT  Allow SSH
  

• Save rules permanently (Debian/Ubuntu):

  sudo iptables-save > /etc/iptables/rules.v4
  

• Block all traffic (with exceptions):

  sudo iptables -P INPUT DROP
  sudo iptables -P FORWARD DROP
  sudo iptables -P OUTPUT DROP
  sudo iptables -A OUTPUT -p udp --dport 53 -j ACCEPT  Allow DNS
  

Windows (PowerShell & netsh)

• View firewall rules:

  Get-NetFirewallRule | Format-Table Name,Enabled,Action
  

• Allow a specific port:

  New-NetFirewallRule -DisplayName "Allow SSH" -Direction Inbound -Protocol TCP -LocalPort 22 -Action Allow
  

• Backup firewall config:

  netsh advfirewall export "C:\firewall_backup.wfw"
  

Cisco ASA Firewall

• Test a rule before applying:

  access-list TEST_ACL extended permit tcp any host 192.168.1.1 eq 22
  

• Revert changes if locked out:

  configure factory-default
  reload
  

What Undercode Say

Misconfiguring firewalls is a rite of passage for many network admins. The key takeaway? Always have a rollback plan.
– Use `tcpdump` to monitor traffic before applying rules:

sudo tcpdump -i eth0 port 80

– Log dropped packets for debugging:

sudo iptables -A INPUT -j LOG --log-prefix "DROPPED: "

– Automate backups with cron:

0     /sbin/iptables-save > /backups/iptables_$(date +\%Y\%m\%d).rules

– Use `ufw` for simpler management:

sudo ufw allow 22/tcp
sudo ufw enable

Final Pro Tip: If you get locked out, physical console access is your last resort.

Expected Output:

A hardened, well-monitored firewall with tested rules, backups, and clear logging to avoid total network blackouts.

🔗 Further Reading:

• iptables Documentation
• Cisco ASA Best Practices
• Windows Firewall Deep Dive

References:

Reported By: C Marceau – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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