When configuring a Linux server's firewall using iptables, one common requirement is to allow all outbound traffic originating from the local machine. This includes DNS queries, HTTP/HTTPS requests, SSH connections, and any other services initiated by the server itself.

Many administrators use the simple approach of setting the default OUTPUT policy to ACCEPT:

iptables -P OUTPUT ACCEPT

While this works, it's not considered the most secure approach as it creates a blanket permission for all outbound traffic.

For better control while still allowing all locally originated traffic, you can use these rules:

# Allow established connections
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow all new outbound connections from local processes
iptables -A OUTPUT -m state --state NEW -j ACCEPT

This solution provides more granular control than simply setting OUTPUT to ACCEPT because:

• It explicitly allows only new connections originating from the local machine
• It maintains the ability to track connection states
• It works well with other firewall rules you might have

Here's a more complete example that includes common security practices:

# Flush all existing rules
iptables -F

# Set default policies
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Allow loopback traffic
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow all new outbound connections
iptables -A OUTPUT -m state --state NEW -j ACCEPT

# Allow specific inbound services (example: SSH)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

After applying these rules, you can verify them with:

iptables -L -v

This will show you the current rules and packet/byte counters for each rule.

Remember that iptables rules are not persistent across reboots. To save them:

iptables-save > /etc/iptables.rules

And to restore them at boot, add this to your startup scripts or use your distribution's method (like iptables-persistent on Debian/Ubuntu).

The simplest way to allow all outbound traffic from a server is to set the default OUTPUT chain policy to ACCEPT:

iptables -P OUTPUT ACCEPT

This configuration permits any traffic initiated by the server itself while maintaining stricter controls on incoming connections through the INPUT chain.

When combined with a restrictive INPUT policy (DROP or REJECT), this approach provides good security because:

• External hosts cannot establish connections to your server unless explicitly allowed
• Your server can freely communicate with external services it needs
• The stateful nature of TCP ensures responses to your outbound connections are allowed

To check your current iptables rules and policies:

iptables -L -n -v
iptables -L -n -v -t nat  # For NAT rules

For more granular control while still allowing all outbound traffic, you could:

# Allow established/related connections
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow all new outbound connections
iptables -A OUTPUT -m state --state NEW -j ACCEPT

# Set default OUTPUT policy to DROP (optional)
iptables -P OUTPUT DROP

• Blocking DNS (port 53) when you need name resolution
• Forgetting about IPv6 (ip6tables) if your system uses it
• Not considering NTP (port 123) for time synchronization

Here's a sample script that allows all outbound while restricting inbound:

#!/bin/sh

# Flush existing rules
iptables -F
iptables -X

# Set default policies
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Allow localhost
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

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

# Add your specific INPUT rules here (SSH, HTTP, etc.)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Log dropped packets (optional)
iptables -A INPUT -j LOG --log-prefix "IPTABLES DROP: "