最低限のiptables

iptables

*filter
# 受信/転送を破棄・送信を許可
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]

# ループバックアドレスからの送受信を許可
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT

# ssh
-A INPUT -m state --state NEW -p tcp --dport 22 -s xxx.xxx.xxx.xxx -j ACCEPT
# http, https
-A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
-A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
# smtp
-A INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT

# 接続済みパケットの送受信を許可
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
COMMIT