Understanding iptables

Feb 11, 2016Download as PPTX, PDF1 like2,676 views

This document provides an overview of iptables and Linux firewall configuration. It discusses Netfilter hooks and stages, stateless and stateful firewall rules using iptables, logging rules, the tables (filter, nat, mangle, raw) and built-in chains, creating custom chains, using ipsets for constant-time lookups, and useful iptables commands. It also briefly mentions using libnetfilter_queue to divert traffic to userspace applications and provides references for further reading on Linux firewalls and Netfilter.

Understanding iptables
Linux firewall basics

Netfilter hooks stages
Socket
App
NIC
INPUT
PRE_ROUTING POST_ROUTING
OUTPUT
FORWARD

Stateless firewall
iptables -A INPUT -p tcp --dport 80 -j ACCEPT

Stateful firewall
iptables -A INPUT -p tcp -m conntrack --ctstate ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW -j ACCEPT

Logging
iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW -j LOG --log-prefix “In
Http:”

Tables overview
Filter is a default table.
So, if you don’t define
you own table, you’ll
be using filter table.
Each table has a number
of predefined chains
inside.
You can create your own
chain.
Filter
Input
Forward
Output
Nat
Output
Prerouting
Postrouting
Mangle
Input
Prerouting
Postrouting
Output
Forward
Raw
Output
Prerouting

Tables in shell
iptables -t mangle -A POSTROUTING -o $NETCARD -p tcp -m connbytes --connbytes
10000000: --connbytes-mode bytes --connbytes-dir both -j CONNMARK --set-mark 999
iptables -t mangle -A INPUT -i eth0 -p tcp --dport 80 -m string --string ”get /admin http/”
--icase --algo bm -m conntrack --ctstate ESTABLISHED -j DROP
iptables -t filter -A input -p tcp --dport 22 -m time --datestart “” --datestop “” --utc --j
DROP

Custom chains
Create a new chain
iptables -N LOGDROP
Add chain rules
iptables -A LOGDROP -j LOG --log-level 4 --log-prefix 'SourceDrop '
iptables -A LOGDROP -j DROP
Add chain rules to iptables rules
iptables -A INPUT -s 10.0.0.0/8 -j LOGDROP

Netfilter in user land
libnetfilter_queue is used to divert traffic to user application
Packets are not duplicated
User application has to inject a packet back
Useful for debugging rules

ip sets
Constant time hash lookup
modprobe ip_set
ipset -N droplist nethash
ipset -add droplist 192.168.1.0/24
iptables -A INPUT -m set --set droplistsrc -j DROP

Useful commands
Drop all rules
iptables -F
Quickly restore rules
iptables-restore <rules list file>

References
Designing and Implementing Linux Firewalls with QoS using netfilter, iproute2, NAT
and L7-filter
Netfilter & Iptables Elements
Linux Firewall Tutorial: IPTables Tables, Chains, Rules Fundamentals
Understanding Linux Network Internals
iptables book
Iptables targets and jumps
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Understanding iptables

1. Understanding iptables Linux firewall basics

2. Netfilter hooks stages Socket App NIC INPUT PRE_ROUTING POST_ROUTING OUTPUT FORWARD

3. Stateless firewall iptables -A INPUT -p tcp --dport 80 -j ACCEPT

4. Stateful firewall iptables -A INPUT -p tcp -m conntrack --ctstate ESTABLISHED -j ACCEPT iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW -j ACCEPT

5. Logging iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW -j LOG --log-prefix “In Http:”

6. Tables overview Filter is a default table. So, if you don’t define you own table, you’ll be using filter table. Each table has a number of predefined chains inside. You can create your own chain. Filter Input Forward Output Nat Output Prerouting Postrouting Mangle Input Prerouting Postrouting Output Forward Raw Output Prerouting

7. Tables in shell iptables -t mangle -A POSTROUTING -o $NETCARD -p tcp -m connbytes --connbytes 10000000: --connbytes-mode bytes --connbytes-dir both -j CONNMARK --set-mark 999 iptables -t mangle -A INPUT -i eth0 -p tcp --dport 80 -m string --string ”get /admin http/” --icase --algo bm -m conntrack --ctstate ESTABLISHED -j DROP iptables -t filter -A input -p tcp --dport 22 -m time --datestart “” --datestop “” --utc --j DROP

8. Custom chains Create a new chain iptables -N LOGDROP Add chain rules iptables -A LOGDROP -j LOG --log-level 4 --log-prefix 'SourceDrop ' iptables -A LOGDROP -j DROP Add chain rules to iptables rules iptables -A INPUT -s 10.0.0.0/8 -j LOGDROP

9. Netfilter in user land libnetfilter_queue is used to divert traffic to user application Packets are not duplicated User application has to inject a packet back Useful for debugging rules

10. ip sets Constant time hash lookup modprobe ip_set ipset -N droplist nethash ipset -add droplist 192.168.1.0/24 iptables -A INPUT -m set --set droplistsrc -j DROP

11. Useful commands Drop all rules iptables -F Quickly restore rules iptables-restore <rules list file>

12. References Designing and Implementing Linux Firewalls with QoS using netfilter, iproute2, NAT and L7-filter Netfilter & Iptables Elements Linux Firewall Tutorial: IPTables Tables, Chains, Rules Fundamentals Understanding Linux Network Internals iptables book Iptables targets and jumps Security in Linux

13. My blog Learning Network Programming

Understanding iptables

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