81 lines
1.9 KiB
Markdown
81 lines
1.9 KiB
Markdown
---
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title: "iptables"
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tags: [ "Documentation", "Networking" ]
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---
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# Intro
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This is a basic Linux firewall program.
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Look at your firewalls:
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> iptables -L
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We see the output of input, output and forwarding rules.
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# Forward
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I don't need any forwarding, so I'm going to drop all forwarding:
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> iptables -P FORWARD DROP
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# Input
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Let's 'A'dd, or 'A'ppend a rule with -A. Let's drop all input from a nearby IP
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> iptables -A INPUT -s 192.168.0.23 -j DROP
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Or we can block all input from a particular port on the full Network.
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> iptables -A INPUT -s 192.168.0.0/24 -p tcp --destination-port 25 -j DROP
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> iptables -A INPUT --dport 80 -j ACCEPT
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This allows http traffic to an Apache web server over port 80.
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However, rules are accepted in order - so a packet cannot be rejected and then accepted.
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To delete rule 2 from the INPUT chain:
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> iptables -D INPUT 3
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Alternatively, you can 'I'nsert a rule at the start, rather than 'A'ppending it.
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> iptables -I INPUT -s 192.168.0.13 DROP
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# Catchalls
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Catchall rules state that anything which is not permitted is forbidden. They must be allowed last.
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# -Jurice-Diction
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The -j flag accepts ACCEPT/REJECT/DROP. The last two are identical except that "REJECT" acknowledges the rejection.
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Flush all existing rules with:
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> iptables -F
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# Examples
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```
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# Allow all loopback (lo0) traffic and drop all traffic to 127/8
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# that doesn't use lo0
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iptables -A INPUT -i lo -j ACCEPT
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iptables -A OUTPUT -o lo -j ACCEPT
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iptables -A INPUT -d 127.0.0.0/8 ! -i lo -j REJECT --reject-with icmp-port-unreachable
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# Allow established sessions to receive traffic
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iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
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# Allow ICMP pings
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iptables -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT
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# Allow SSH remote
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iptables -I INPUT -p tcp --dport 22 -j ACCEPT
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# Reject all other inbound connections
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iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable
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iptables -A FORWARD -j REJECT --reject-with icmp-port-unreachable
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```
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