linux ip table firewall

04-27-2004

#1 (permalink)

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linux ip table firewall

Hi
I wrote a firewall for my router (see below)... But because I'm a linux noob I'm not fully convinced of the firewall itself. Please take some time to read it and if you know something I need to add or remove... please reply

Thx in advance

(ps: nat support)

Quote:

#variables

EXT=eth1
INT=eth0

#forwarding kernel level
echo 1 > /proc/sys/net/ipv4/ip_forward

#loading nat module (just to be shure

)
modprobe iptable_nat

#telling iptables to use nat masquerading
iptables -t nat -A POSTROUTING -o $EXT -j MASQUERADE

#drop everything from outside
iptables -P INPUT DROP

#accept output
iptables -P OUTPUT ACCEPT

#accept forwarding (nat, remember)
iptables -P FORWARD ACCEPT

#block some ports in & out
#

#netbus
iptables -A OUTPUT -o $EXT -p tcp --dport 31337 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 31337 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 31337 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 31337 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 31335 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 31335 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 31335 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 31335 -j REJECT

iptables -A OUTPUT -o $EXT -p tcp --dport 27444 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 27444 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 27444 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 27444 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 27665 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 27665 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 27665 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 27665 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 20034 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 20034 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 113 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 113 -j DROPT

#winnie
iptables -A OUTPUT -o $EXT -p tcp --dport 9704 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 9704 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 9704 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 9704 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 137 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 137 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 137 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 137 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 138 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 138 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 138 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 138 -j DROP

iptables -A OUTPUT -o $EXT -p tcp --dport 139 -j DROP
iptables -A OUTPUT -o $EXT -p tcp --sport 139 -j DROP

iptables -A FORWARD -i $EXT -p tcp --dport 139 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 139 -j DROP

#
#end

#no ident request
iptables -A FORWARD -i $EXT -p tcp --dport 113 -j REJECT
iptables -A INPUT -i $EXT -p tcp --dport 113 -j REJECT

#icmp rules
iptables -A INPUT -i $EXT -p icmp --icmp-type 8 -j DROP
iptables -A FORWARD -i $EXT -p icmp --icmp-type 8 -j DROP
iptables -A OUTPUT -o $EXT -p icmp --icmp-type 8 -j ACCEPT

#Forward all packets from eth1 (internal network) to eth0 (the internet).
iptables -A FORWARD -i $INT -o $EXT -j ACCEPT

#Forward packets that are part of existing and related connections from eth0 to eth1.
iptables -A FORWARD -i $EXT -o $INT -m state --state ESTABLISHED,RELATED -j ACCEPT

#Permit packets in to firewall itself that are part of existing and related connections.
iptables -A INPUT -i $EXT -m state --state ESTABLISHED,RELATED -j ACCEPT

#Allow all inputs to firewall from the internal network and local interfaces
iptables -A INPUT -i $INT -s 0/0 -d 0/0 -j ACCEPT
iptables -A INPUT -i lo -s 0/0 -d 0/0 -j ACCEPT

#Accept all tcp SYN packets for protocol SSH
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 22 --syn -j ACCEPT

#For DHCP server
iptables -A INPUT -i $INT -p tcp --sport 68 --dport 67 -j ACCEPT
iptables -A INPUT -i $INT -p udp --sport 68 --dport 67 -j ACCEPT

iptables -A INPUT -i $EXT -p tcp --sport 68 --dport 67 -j DROP
iptables -A INPUT -i $EXT -p udp --sport 68 --dport 67 -j DROP

#For DNS server
iptables -A INPUT -i $INT -p tcp --sport 53 --dport 53 -j ACCEPT
iptables -A INPUT -i $INT -p udp --sport 53 --dport 53 -j ACCEPT

iptables -A INPUT -i $EXT -p tcp --sport 53 --dport 53 -j DROP
iptables -A INPUT -i $EXT -p udp --sport 53 --dport 53 -j DROP

#extra
iptables -A INPUT -s 0/0 -d 0/0 -p udp -j DROP
iptables -A INPUT -s 0/0 -d 0/0 -p tcp --syn -j DROP

04-27-2004	#2 (permalink)
jasonlambert Linux Guru Join Date: Apr 2003 Location: London, UK Posts: 3,284	Generally it is better to DROP everything, then only allow what you need, rather than allowing everything, and trying to figure out what bits to drop. Take a look at my firewall script here for some more inspiration: http://www.jasonlambert.com/download.php Jason

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