A simple Firewall

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Firewall 1
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iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

NB:
Allow network connections which have already been established (started by host) and related to your connection. FTP requires this as it may use various ports in support of the file transfer.)
Allow network input/output from self (lo).

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Firewall 2
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Another approach to firewalls is to drop everything and then grant access to each port you may need.

iptables -F : Flush
iptables -A INPUT -i lo -p all -j ACCEPT : Allow loopback interface
iptables -A OUTPUT -o lo -p all -j ACCEPT
iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT : Accept established connections
iptables -A INPUT -p tcp --tcp-option ! 2 -j REJECT --reject-with tcp-reset

# Open ftp port (tcp +/- udp)
iptables -A INPUT -p tcp -i eth0 --dport 21 -j ACCEPT
iptables -A INPUT -p udp -i eth0 --dport 21 -j ACCEPT
# Open secure shell port
iptables -A INPUT -p tcp -i eth0 --dport 22 -j ACCEPT
iptables -A INPUT -p udp -i eth0 --dport 22 -j ACCEPT
# Open HTTP port
iptables -A INPUT -p tcp -i eth0 --dport 80 -j ACCEPT
iptables -A INPUT -p udp -i eth0 --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --syn -s 192.168.10.0/24 --destination-port 139 -j ACCEPT : Accept local Samba connection
iptables -A INPUT -p tcp --syn -s trancas --destination-port 139 -j ACCEPT
iptables -P INPUT DROP : Drop all other connection attempts.Only connections defined above are allowed.