Iptables

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TL;DR base desktop rules

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


quick 'n dirty guide to iptables as a firewall

list rules:

iptables --list

save rules: (they will not stay when your interface goes down)

iptables-save > /etc/iptalbes.conf

restore rules:

iptables-restore < /etc/iptables.conf

auto appy rules:

echo "iptables-restore < /etc/iptables" > /etc/network/if-up.d/iptables
chmod +x /etc/network/if-up.d/iptables

after you make changes do not forget to save them with

iptables-save > /etc/iptalbes.conf

set default policy:

iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT

accept established connections: (let what goes out come back in)

iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
  • -A adds a rule to the end of a chain
  • replace with a -I to add a rule at the begenning of a chain

allow communication through loopback or localhost:

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
  • the interface will not show in iptables --list so this will look broken.
  • iptables -S will tell the true story

allow ssh in:

iptables -A INPUT -p tcp --dport 22 -j ACCEPT

allow ssh from a specific network:

iptables -A INPUT -p tcp -s 192.168.100.0/24 --dport 22 -j ACCEPT

allow ping from outside to inside:

iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
iptables -A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT

allow ping from inside to outside:

iptables -A OUTPUT -p icmp --icmp-type echo-request -j ACCEPT
iptables -A INPUT -p icmp --icmp-type echo-reply -j ACCEPT

find more filters:

man iptables

many options starting with a -- require a module to be called ahead of it. Ex:

iptables -I OUTPUT --uid-owner 1000 -j ACCEPT

will fail with an "unknown option --uid-owner". If you load the correct moudle first,

iptables -I OUTPUT -m owner --uid-owner 1000 -j ACCEPT

it will work.

log dropped packets:

iptables -A OUTPUT -m conntrack --ctstate NEW -j LOG --log-prefix "OUTPUT_DROP: " --log-level 7 --log-ip-options
  • the order of rules matters
  • rules are processed from top down
  • when an ACCEPT or DROP is hit processing of a packet will stop

watch logs:

tail -f /var/log/debug | grep DROP

misc blocks for common attacks:

iptables -I INPUT -p tcp ! --syn -m state --state NEW -j DROP
iptables -I INPUT -f -j DROP
iptables -I INPUT -p tcp --tcp-flags ALL ALL -j DROP
iptables -I INPUT -p tcp --tcp-flags ALL NONE -j DROP

block DoS attacks:

iptables -A INPUT -p tcp --dport 80 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT

other cool stuff:

load balance web traffic:

iptables -A PREROUTING -i eth0 -p tcp --dport 443 -m state --state NEW -m nth --counter 0 --every 3 --packet 0 -j DNAT --to-destination 192.168.1.101:443
iptables -A PREROUTING -i eth0 -p tcp --dport 443 -m state --state NEW -m nth --counter 0 --every 3 --packet 1 -j DNAT --to-destination 192.168.1.102:443
iptables -A PREROUTING -i eth0 -p tcp --dport 443 -m state --state NEW -m nth --counter 0 --every 3 --packet 2 -j DNAT --to-destination 192.168.1.103:443

allow a specific user access through a port:

iptables -I OUTPUT -p tcp --dport 80 -m owner --uid-owner 1000 -j ACCEPT

allow a specific process access through a port:

iptables -I OUTPUT -p tcp --dport 23 -m owner --pid-owner 23945 -j ACCEPT

allow a specific command access through a port:

iptables -I OUTPUT -p tcp --dport 110 -m owner --cmd-owner claws-mail -j ACCEPT

sources: