iptables-essentials

:ballot_box_with_check: TODO

• Add useful Iptables configuration examples
• Add useful Kernel Settings (sysctl) configuration
• Add links to useful external resources
• Add advanced configuration examples, commands, rules

Table of Contents

• Tools to help you configure Iptables
• Manuals/Howtos/Tutorials
• Useful Kernel Settings (sysctl) configuration
  • rp_filter
  • log_martians
  • send_redirects
  • accept_source_route
  • accept_redirects
  • tcp_syncookies
  • icmp_echo_ignore_broadcasts
  • ip_forward
• How it works?
• Iptables Rules
  • Saving Rules
    • Debian Based
    • RedHat Based
  • List out all of the active iptables rules with verbose
  • List out all of the active iptables rules with numeric lines and verbose
  • Print out all of the active iptables rules
  • List Rules as Tables for INPUT chain
  • Print all of the rule specifications in the INPUT chain
  • Show Packet Counts and Aggregate Size
  • To display INPUT or OUTPUT chain rules with numeric lines and verbose
  • Delete Rule by Chain and Number
  • Delete Rule by Specification
  • Flush All Rules, Delete All Chains, and Accept All
  • Flush All Chains
  • Flush a Single Chain
  • Insert Firewall Rules
  • Allow Loopback Connections
  • Allow Established and Related Incoming Connections
  • Allow Established Outgoing Connections
  • Internal to External
  • Drop Invalid Packets
  • Block an IP Address
  • Block and IP Address and Reject
  • Block Connections to a Network Interface
  • Allow All Incoming SSH
  • Allow Incoming SSH from Specific IP address or subnet
  • Allow Outgoing SSH
  • Allow Incoming Rsync from Specific IP Address or Subnet
  • Allow All Incoming HTTP
  • Allow All Incoming HTTPS
  • Allow All Incoming HTTP and HTTPS
  • Allow MySQL from Specific IP Address or Subnet
  • Allow MySQL to Specific Network Interface
  • PostgreSQL from Specific IP Address or Subnet
  • Allow PostgreSQL to Specific Network Interface
  • Block Outgoing SMTP Mail
  • Allow All Incoming SMTP
  • Allow All Incoming IMAP
  • Allow All Incoming IMAPS
  • Allow All Incoming POP3
  • Allow All Incoming POP3S
  • Drop Private Network Address On Public Interface
  • Drop All Outgoing to Facebook Networks
  • Log and Drop Packets
  • Log and Drop Packets with Limited Number of Log Entries
  • Drop or Accept Traffic From Mac Address
  • Block or Allow ICMP Ping Request
  • Specifying Multiple Ports with multiport
  • Load Balancing with random* or nth*
  • Restricting the Number of Connections with limit and iplimit*
  • Maintaining a List of recent Connections to Match Against
  • Matching Against a string* in a Packet's Data Payload
  • Time-based Rules with time*
  • Packet Matching Based on TTL Values
  • Protection against port scanning
  • SSH brute-force protection
  • Syn-flood protection
    • Mitigating SYN Floods With SYNPROXY
  • Block New Packets That Are Not SYN
  • Force Fragments packets check
  • XMAS packets
  • Drop all NULL packets
  • Block Uncommon MSS Values
  • Block Packets With Bogus TCP Flags
  • Block Packets From Private Subnets (Spoofing)
• Advanced configuration examples
  • Packet handling in Python using NFQUEUE target

Tools to help you configure Iptables

Manuals/Howtos/Tutorials

Useful Kernel Settings (sysctl) Configuration

rp_filter

Disable routing triangulation. Respond to queries out the same interface, not another. Also protects against IP spoofing.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/conf/all/rp_filter = 1
EOF

• rp_filter and LPIC-3 Linux Security
• Linux kernel rp_filter settings (Reverse path filtering)
• Reverse Path Filtering

log_martians

Enable logging of packets with malformed IP addresses.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/conf/all/log_martians = 1
EOF

• What is the usefulness of logging of martians packet?

send_redirects

Disables sending of all IPv4 ICMP redirected packets on all interfaces.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/conf/all/send_redirects = 0
EOF

• Disable source routing
• What are ICMP redirects and should they be blocked?

accept_source_route

Disable source routed packets (packets with the Strict Source Route (SSR) or Loose Source Routing (LSR) option set).

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/conf/all/accept_source_route = 0
EOF

• Disable source routing
• The system must not accept IPv4 source-routed packets by default.

accept_redirects

Disable acceptance of ICMP redirects.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/conf/all/accept_redirects = 0
EOF

• What are ICMP redirects and should they be blocked?
• The Red Hat Enterprise Linux operating system must ignore Internet Protocol version 4 (IPv4) Internet Control Message Protocol (ICMP) redirect messages.

tcp_syncookies

Turn on SYN-flood protections (protection from Denial of Service (DOS) attacks).

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/tcp_syncookies = 1
EOF

• Hardening your TCP/IP Stack Against SYN Floods
• Linux: Turn On TCP SYN Cookie Protection
• Better alternative for tcp_syncookies in Linux

icmp_echo_ignore_broadcasts

Disable responding to ping broadcasts.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/icmp_echo_ignore_broadcasts = 1
EOF

• What is ICMP broadcast good for?
• The system must not respond to ICMPv4 sent to a broadcast address.

ip_forward

Enable IP routing. Required if your firewall is protecting a network, NAT included.

cat << EOF >> /etc/sysctl.d/40-custom.conf
net/ipv4/ip_forward = 1
EOF

• Introduction to routers
• How to Enable IP Forwarding in Linux
• What is kernel ip forwarding?

How it works?

Iptables Rules

Saving Rules

Debian Based

netfilter-persistent save

RedHat Based

service iptables save

List out all of the active iptables rules with verbose

iptables -n -L -v

List out all of the active iptables rules with numeric lines and verbose

iptables -n -L -v --line-numbers

Print out all of the active iptables rules

iptables -S

List Rules as Tables for INPUT chain

iptables -L INPUT

Print all of the rule specifications in the INPUT chain

iptables -S INPUT

Show Packet Counts and Aggregate Size

iptables -L INPUT -v

To display INPUT or OUTPUT chain rules with numeric lines and verbose

iptables -L INPUT -n -v
iptables -L OUTPUT -n -v --line-numbers

Delete Rule by Chain and Number

iptables -D INPUT 10

Delete Rule by Specification

iptables -D INPUT -m conntrack --ctstate INVALID -j DROP

Flush All Rules, Delete All Chains, and Accept All

iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

iptables -t nat -F
iptables -t mangle -F
iptables -F
iptables -X

Flush All Chains

iptables -F

Flush a Single Chain

iptables -F INPUT

Insert Firewall Rules

iptables -I INPUT 2 -s 202.54.1.2 -j DROP

Allow Loopback Connections

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

Allow Established and Related Incoming Connections

iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

Allow Established Outgoing Connections

iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED -j ACCEPT

Internal to External

iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

Drop Invalid Packets

iptables -A INPUT -m conntrack --ctstate INVALID -j DROP

Block an IP Address

iptables -A INPUT -s 192.168.252.10 -j DROP

Block and IP Address and Reject

iptables -A INPUT -s 192.168.252.10 -j REJECT

Block Connections to a Network Interface

iptables -A INPUT -i eth0 -s 192.168.252.10 -j DROP

Allow All Incoming SSH

iptables -A INPUT -p tcp --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow Incoming SSH from Specific IP address or subnet

iptables -A INPUT -p tcp -s 192.168.240.0/24 --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow Outgoing SSH

iptables -A OUTPUT -p tcp --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow Incoming Rsync from Specific IP Address or Subnet

iptables -A INPUT -p tcp -s 192.168.240.0/24 --dport 873 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 873 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming HTTP

iptables -A INPUT -p tcp --dport 80 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 80 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming HTTPS

iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 443 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming HTTP and HTTPS

iptables -A INPUT -p tcp -m multiport --dports 80,443 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow MySQL from Specific IP Address or Subnet

iptables -A INPUT -p tcp -s 192.168.240.0/24 --dport 3306 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 3306 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow MySQL to Specific Network Interface

iptables -A INPUT -i eth1 -p tcp --dport 3306 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o eth1 -p tcp --sport 3306 -m conntrack --ctstate ESTABLISHED -j ACCEPT

PostgreSQL from Specific IP Address or Subnet

iptables -A INPUT -p tcp -s 192.168.240.0/24 --dport 5432 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 5432 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow PostgreSQL to Specific Network Interface

iptables -A INPUT -i eth1 -p tcp --dport 5432 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o eth1 -p tcp --sport 5432 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Block Outgoing SMTP Mail

iptables -A OUTPUT -p tcp --dport 25 -j REJECT

Allow All Incoming SMTP

iptables -A INPUT -p tcp --dport 25 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 25 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming IMAP

iptables -A INPUT -p tcp --dport 143 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 143 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming IMAPS

iptables -A INPUT -p tcp --dport 993 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 993 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming POP3

iptables -A INPUT -p tcp --dport 110 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 110 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Allow All Incoming POP3S

iptables -A INPUT -p tcp --dport 995 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 995 -m conntrack --ctstate ESTABLISHED -j ACCEPT

Drop Private Network Address On Public Interface

iptables -A INPUT -i eth1 -s 192.168.0.0/24 -j DROP
iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

Drop All Outgoing to Facebook Networks

Get Facebook AS:

whois -h v4.whois.cymru.com " -v $(host facebook.com, grep "has address", cut -d " " -f4)", tail -n1, awk '{print $1}'

Drop:

for i in $(whois -h whois.radb.net -- '-i origin AS32934', grep "^route:", cut -d ":" -f2, sed -e 's/^[ \t]*//', sort -n -t . -k 1,1 -k 2,2 -k 3,3 -k 4,4, cut -d ":" -f2, sed 's/$/;/') ; do

  iptables -A OUTPUT -s "$i" -j REJECT

done

Log and Drop Packets

iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j LOG --log-prefix "IP_SPOOF A: "
iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

By default everything is logged to /var/log/messages file:

tail -f /var/log/messages
grep --color 'IP SPOOF' /var/log/messages

Log and Drop Packets with Limited Number of Log Entries

iptables -A INPUT -i eth1 -s 10.0.0.0/8 -m limit --limit 5/m --limit-burst 7 -j LOG --log-prefix "IP_SPOOF A: "
iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

Drop or Accept Traffic From Mac Address

iptables -A INPUT -m mac --mac-source 00:0F:EA:91:04:08 -j DROP
iptables -A INPUT -p tcp --destination-port 22 -m mac --mac-source 00:0F:EA:91:04:07 -j ACCEPT

Block or Allow ICMP Ping Request

iptables -A INPUT -p icmp --icmp-type echo-request -j DROP
iptables -A INPUT -i eth1 -p icmp --icmp-type echo-request -j DROP

Specifying Multiple Ports with multiport

iptables -A INPUT -i eth0 -p tcp -m state --state NEW -m multiport --dports ssh,smtp,http,https -j ACCEPT

Load Balancing with random* or nth*

_ips=("172.31.250.10" "172.31.250.11" "172.31.250.12" "172.31.250.13")

for ip in "${_ips[@]}" ; do
  iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 4 --packet 0 \
    -j DNAT --to-destination ${ip}:80
done

or

_ips=("172.31.250.10" "172.31.250.11" "172.31.250.12" "172.31.250.13")

for ip in "${_ips[@]}" ; do
  iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m random --average 25 \
    -j DNAT --to-destination ${ip}:80
done

Restricting the Number of Connections with limit and iplimit*

iptables -A FORWARD -m state --state NEW -p tcp -m multiport --dport http,https -o eth0 -i eth1 \
    -m limit --limit 20/hour --limit-burst 5 -j ACCEPT

or

iptables -A INPUT -p tcp -m state --state NEW --dport http -m iplimit --iplimit-above 5 -j DROP

Maintaining a List of recent Connections to Match Against

iptables -A FORWARD -m recent --name portscan --rcheck --seconds 100 -j DROP
iptables -A FORWARD -p tcp -i eth0 --dport 443 -m recent --name portscan --set -j DROP

Matching Against a string* in a Packet's Data Payload

iptables -A FORWARD -m string --string '.com' -j DROP
iptables -A FORWARD -m string --string '.exe' -j DROP

Time-based Rules with time*

iptables -A FORWARD -p tcp -m multiport --dport http,https -o eth0 -i eth1 \
    -m time --timestart 21:30 --timestop 22:30 --days Mon,Tue,Wed,Thu,Fri -j ACCEPT

Packet Matching Based on TTL Values

iptables -A INPUT -s 1.2.3.4 -m ttl --ttl-lt 40 -j REJECT

Protection against port scanning

iptables -N port-scanning
iptables -A port-scanning -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s --limit-burst 2 -j RETURN
iptables -A port-scanning -j DROP

SSH brute-force protection

iptables -A INPUT -p tcp --dport ssh -m conntrack --ctstate NEW -m recent --set
iptables -A INPUT -p tcp --dport ssh -m conntrack --ctstate NEW -m recent --update --seconds 60 --hitcount 10 -j DROP

Syn-flood protection

iptables -N syn_flood

iptables -A INPUT -p tcp --syn -j syn_flood
iptables -A syn_flood -m limit --limit 1/s --limit-burst 3 -j RETURN
iptables -A syn_flood -j DROP

iptables -A INPUT -p icmp -m limit --limit  1/s --limit-burst 1 -j ACCEPT

iptables -A INPUT -p icmp -m limit --limit 1/s --limit-burst 1 -j LOG --log-prefix PING-DROP:
iptables -A INPUT -p icmp -j DROP

iptables -A OUTPUT -p icmp -j ACCEPT

Mitigating SYN Floods With SYNPROXY

iptables -t raw -A PREROUTING -p tcp -m tcp --syn -j CT --notrack
iptables -A INPUT -p tcp -m tcp -m conntrack --ctstate INVALID,UNTRACKED -j SYNPROXY --sack-perm --timestamp --wscale 7 --mss 1460
iptables -A INPUT -m conntrack --ctstate INVALID -j DROP

Block New Packets That Are Not SYN

iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP

or

iptables -t mangle -A PREROUTING -p tcp ! --syn -m conntrack --ctstate NEW -j DROP

Force Fragments packets check

iptables -A INPUT -f -j DROP

XMAS packets

iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

Drop all NULL packets

iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

Block Uncommon MSS Values

iptables -t mangle -A PREROUTING -p tcp -m conntrack --ctstate NEW -m tcpmss ! --mss 536:65535 -j DROP

Block Packets With Bogus TCP Flags

iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,ACK FIN -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ACK,URG URG -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ACK,FIN FIN -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ACK,PSH PSH -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL ALL -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL NONE -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL SYN,FIN,PSH,URG -j DROP
iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

Block Packets From Private Subnets (Spoofing)

_subnets=("224.0.0.0/3" "169.254.0.0/16" "172.16.0.0/12" "192.0.2.0/24" "192.168.0.0/16" "10.0.0.0/8" "0.0.0.0/8" "240.0.0.0/5")

for _sub in "${_subnets[@]}" ; do
  iptables -t mangle -A PREROUTING -s "$_sub" -j DROP
done
iptables -t mangle -A PREROUTING -s 127.0.0.0/8 ! -i lo -j DROP

Advanced configuration examples

Packet handling in Python using NFQUEUE target

iptables -A INPUT -j NFQUEUE --queue-num 1

#!/usr/bin/python3

from netfilterqueue import NetfilterQueue
from scapy.all import *

def packetanalyzer(pkt):
    ip=IP(pkt.get_payload())
    if(ip.src=="192.168.122.1"):
        print(f"New packet from {ip.src}")
        pkt.accept()
    else:
	pkt.drop()

nfqueue=NetfilterQueue()
nfqueue.bind(1, packetanalyzer)
nfqueue.run()