Firewall and iptables¶

★★★★★ Intermediate

iptables configures the Linux kernel's netfilter packet filtering framework. Rules are organized into tables and chains. This entry covers rule management, common patterns, persistence, and modern alternatives.

Key Facts¶

Default table: filter (for most firewall rules)
Rules are evaluated top-to-bottom; first match wins
Rules are NOT persistent by default - lost on reboot
Modern replacement: nftables; simplified frontend: ufw

Chains¶

Chain	Traffic Direction
`INPUT`	Incoming to local host
`OUTPUT`	Outgoing from local host
`FORWARD`	Passing through (routing)
`PREROUTING`	Before routing decision (NAT)
`POSTROUTING`	After routing decision (NAT)

Targets (Actions)¶

Target	Effect
`ACCEPT`	Allow packet
`DROP`	Silently discard
`REJECT`	Discard + send error to sender
`LOG`	Log to syslog (does not stop processing)

Viewing Rules¶

sudo iptables -L                      # list filter table rules
sudo iptables -L -nv                  # verbose, numeric (no DNS)
sudo iptables -L -nv --line-numbers   # include rule numbers
sudo iptables -t nat -L               # list NAT table rules

Adding Rules¶

# Append to chain
sudo iptables -A INPUT -s 192.168.0.104 -j DROP     # drop from IP
sudo iptables -A INPUT -p tcp --dport 22 -j REJECT   # block SSH
sudo iptables -A OUTPUT -p tcp --dport 80 -j REJECT  # block outgoing HTTP

# Insert at top (highest priority)
sudo iptables -I INPUT -p tcp --dport 22 -j ACCEPT   # allow SSH first

# Log all traffic
sudo iptables -A INPUT -j LOG

Deleting Rules¶

sudo iptables -F INPUT            # flush all rules in INPUT chain
sudo iptables -F                  # flush all chains in filter table
sudo iptables -D INPUT 4          # delete rule number 4

Default Policies¶

sudo iptables -P INPUT DROP       # deny all incoming by default
sudo iptables -P OUTPUT DROP      # deny all outgoing
sudo iptables -P FORWARD DROP     # deny all forwarded
sudo iptables -P INPUT ACCEPT     # allow all incoming

Safe DROP Pattern¶

# ALWAYS allow SSH before setting DROP policy
sudo iptables -I INPUT -p tcp --dport 22 -j ACCEPT
sudo iptables -P INPUT DROP

Common Rule Patterns¶

# Allow established/related connections (stateful)
sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow loopback
sudo iptables -A INPUT -i lo -j ACCEPT

# Allow ping (ICMP)
sudo iptables -A INPUT -p icmp -j ACCEPT

# Allow port range
sudo iptables -A INPUT -p tcp --dport 8000:8080 -j ACCEPT

# Block subnet
sudo iptables -A INPUT -s 10.0.0.0/8 -j DROP

# NAT masquerade (internet sharing)
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Saving and Restoring¶

sudo iptables-save > ~/iptables_config        # save to file
sudo iptables-restore < ~/iptables_config     # restore from file

# Persistent across reboots (Debian/Ubuntu)
sudo apt install iptables-persistent
sudo netfilter-persistent save

nftables (Modern Alternative)¶

sudo nft list ruleset       # view all rules

ufw (Uncomplicated Firewall)¶

sudo ufw allow 22/tcp       # allow SSH
sudo ufw allow ssh           # same, by service name
sudo ufw deny 80/tcp         # block HTTP
sudo ufw enable              # activate firewall
sudo ufw status              # show rules
sudo ufw disable             # deactivate

Network Utilities¶

hostname -I         # local IP addresses
ip a                # network interfaces and IPs

Gotchas¶

Setting INPUT DROP without first allowing SSH locks you out of remote servers
Rules are ordered - -I (insert) adds at top, -A (append) adds at bottom
DROP gives no response (attacker doesn't know port exists); REJECT sends error back
iptables -F flushes rules but does NOT reset default policies
IPv6 requires separate ip6tables rules
nftables is the future - iptables is legacy on newer kernels