Preguntes Freqüents - FAQ

UFW is a friendly front-end, but it's built on top of the Linux kernel's actual packet filtering systems — iptables (legacy) and nftables (modern replacement). This guide covers when and how to work with them directly for advanced scenarios UFW doesn't cover.

UFW vs iptables vs nftables

When to Go Beyond UFW

• Complex NAT/port forwarding rules
• Rate limiting beyond UFW's basic limit command
• Custom chains for specific traffic classification
• Fine-grained rule ordering UFW's abstraction doesn't expose directly

Viewing Current iptables Rules

sudo iptables -L -v -n

Basic iptables Rule Structure

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

-A INPUT appends to the input chain, -p tcp --dport 22 matches TCP traffic on port 22, -j ACCEPT is the action (accept the packet).

Common iptables Examples

Allow established connections (required for most rule sets to function correctly):

sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Rate-limit new SSH connections:

sudo iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
sudo iptables -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 4 -j DROP

Block a specific IP:

sudo iptables -A INPUT -s 203.0.113.50 -j DROP

Saving iptables Rules Permanently

sudo apt install iptables-persistent -y
sudo netfilter-persistent save

Without this, rules are lost on reboot — a common source of confusion when rules "disappear."

Introduction to nftables (Modern Approach)

sudo nft list ruleset

Basic rule example:

sudo nft add rule inet filter input tcp dport 22 accept

nftables Configuration File

sudo nano /etc/nftables.conf

table inet filter {
    chain input {
        type filter hook input priority 0;
        tcp dport 22 accept
        tcp dport 80 accept
        tcp dport 443 accept
    }
}

sudo systemctl enable nftables
sudo systemctl restart nftables

Checking Which System Is Active

sudo iptables -V

Modern iptables commands are often actually translated to nftables under the hood on current distributions — check the output for confirmation of which backend is in use.

Common Errors

Rules disappear after reboot — not saved persistently; use iptables-persistent or ensure the nftables service is enabled.

Locked out after adding a rule — use your provider's console access; always test rule changes carefully, ideally with a way to revert if something goes wrong.

Best Practices

• Use UFW for standard scenarios — only drop to iptables/nftables directly when you need capabilities UFW doesn't expose
• Always allow established/related connections, or you'll break existing traffic flows
• Save rules persistently, and test carefully before relying on them in production

Related Articles

• How to Configure UFW Firewall on a Linux VPS
• How to Install and Configure Fail2Ban on Ubuntu & Debian
• How to Set Up Port Forwarding on a Linux VPS