Difference between revisions of "Iptables"

Latest revision as of 21:16, 19 December 2018

iptables is a user-space utility program that allows a system administrator to configure the tables provided by the Linux kernel firewall (implemented as different Netfilter modules) and the chains and rules it stores. Different kernel modules and programs are currently used for different protocols; iptables applies to IPv4, ip6tables to IPv6, arptables to ARP, and ebtables to Ethernet frames.

iptables requires elevated privileges to operate and must be executed by user root, otherwise it fails to function. On most Linux systems, iptables is installed as /usr/sbin/iptables and documented in its man pages, which can be opened using man iptables when installed. It may also be found in /sbin/iptables, but since iptables is more like a service rather than an "essential binary", the preferred location remains /usr/sbin.

The term iptables is also commonly used to inclusively refer to the kernel-level components. x_tables is the name of the kernel module carrying the shared code portion used by all four modules that also provides the API used for extensions; subsequently, Xtables is more or less used to refer to the entire firewall (v4, v6, arp, and eb) architecture.

The successor of iptables is nftables, which was merged into the Linux kernel mainline in kernel version 3.13, which was released on 19 January 2014.

Stateful filtering

• TCP states: NEW, RELATED, ESTABLISHED, INVALID
• Connection states (to match in the conntrack module): INVALID, NEW, ESTABLISHED, RELATED, UNTRACKED, SNAT, DNAT
• Chain types (this option can either be the name of a user defined chain or any of the builtin chains): INPUT, FORWARD, OUTPUT, PREROUTING, POSTROUTING, SECMARK, CONNSECMARK
• Jump types: ACCEPT, DROP, REJECT

• Basic ruleset:

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [241:19144]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A INPUT -p icmp -j ACCEPT 
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 53 -j ACCEPT
-A INPUT -p udp -m state --state NEW -m udp --dport 53 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited 
-A FORWARD -j REJECT --reject-with icmp-host-prohibited 
COMMIT

-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 --sport 63636 -s 10.0.0.2 -j ACCEPT

• Get a list of ICMP type names:

$ iptables -p icmp -h

• The REJECT target rejects the packet. If you do not specify which ICMP message to reject with, by default, the server will send back "ICMP port unreachable" (type 3, code 3).

REJECT

This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD, and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:

--reject-with type

The type given can be:

icmp-net-unreachable

icmp-host-unreachable

icmp-port-unreachable

icmp-proto-unreachable

icmp-net-prohibited

icmp-host-prohibited

icmp-admin-prohibited (*)

which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes, which frequently occur when sending mail to broken mail hosts (which will not accept your mail otherwise).

(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT

Examples

• Sample `iptables` ruleset:

*filter

# Dropping incoming connections that don't have explecit rules bellow
:INPUT DROP [68:4456]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1628:151823]

# Allow established connections for both public and private connections
-A INPUT -i eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT

# Opening ports wide open
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 21 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 3306 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 53 -j ACCEPT
-A INPUT -p udp -m udp --dport 53 -j ACCEPT

# Opening a port to a specific IP
-A INPUT -p tcp -m tcp --dport 10000 -s 192.168.1.1 -j ACCEPT

# Opening a port to a range of IPs
-A INPUT -p tcp -m tcp --dport 20000 -s 192.168.0.0/24 -j ACCEPT

# Commmiting the rules to the firewall
COMMIT

• Simple Bash script:

#!/bin/bash
# Set INPUT chain default policy to DROP
iptables -P INPUT DROP
# Flushes all rule in the filter table
iptables -F
# ACCEPT all packets from loopback interface
iptables -A INPUT -i lo -j ACCEPT
# ACCEPT all ESTABLISHED,RELATED packets
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# ACCEPT all NEW connections to tcp port 22
iptables -A INPUT -m state --state NEW -p tcp --dport 22 -j ACCEPT
# REJECT all packets from 192.168.1.0/24 network
iptables -A INPUT -s 192.168.1.0/24 -j REJECT
# ACCEPT all icmp traffic from 192.168.0.0/24
iptables -A INPUT -p icmp -s 192.168.0.0/24 -j ACCEPT
# save rules
iptables-save > /etc/sysconfig/iptables
# print out saved rules
iptables-save
iptables -nvL

Network Address Translation (NAT)

• The chains available in the filter table are: INPUT, FORWARD, OUTPUT
• The chains available in the nat table are: PREROUTING, POSTROUTING, OUTPUT
• Example:

$ iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
$ iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to-source 192.168.0.1
$ iptables -t nat -A PREROUTING -i eth0 -m tcp -p tcp --dport 80 -j DNAT --to-destination 192.168.0.100:8080

• The DNAT target can only be used in the PREROUTING chain and the OUTPUT chain of the nat table.
• To enable forwarding persistently across reboots, add net.ipv4.ip_forward=1 to /etc/sysctl.conf then run sysctl -p, or:

sysctl -w net.ipv4.ip_forward=1

Basic command options

Chain manipulation (three default chains, INPUT, FORWARD, OUTPUT, are always present):

• Create a new chain (-N, --new-chain chain)
• Delete an empty chain (-X, --delete-chain [chain])
• Change the policy for a built-in chain (-P, --policy chain target)
• List the rules in a chain (-L, --list [chain])
• Flush the rules out of a chain (-F, --flush [chain])
• Zero the packet and byte counters in all chains (-Z, --zero) (note: It is legal to specify the -L, --list (list) option as well, to see the counters immediately before they are cleared.)

Rule manipulation:

• Append a new rule to a chain (-A, --append chain rule-specification)
• Delete a rule at some position in a chain (-D, --delete chain rule-specification)

NEW 

packet starts a new communication, adds a rule to the state tracking table

ESTABLISHED 

any packet that matches a rule in the state tracking table

RELATED 

traffic "related" in some way to ESTABLISHED traffic; protocols like SSH, FTP, etc.

INVALID 

packet cannot be identified; normally these should be rejected or dropped

NOTE: Running the `iptables` command changes the netfilter kernel module rules in memory, but will not persist across a reboot.

Running `service iptables save` will take the current rules in memory and write them to /etc/sysconfig/iptables which is read during system boot. Run instead:

service iptables restart

Help (-h)

Usage

iptables -[AD] chain rule-specification [options]
iptables -[RI] chain rulenum rule-specification [options]
iptables -D chain rulenum [options]
iptables -[LFZ] [chain] [options]
iptables -[NX] chain
iptables -E old-chain-name new-chain-name
iptables -P chain target [options]
iptables -h (print this help information)

Commands

Either long or short options are allowed.

--append -A chain 

append to chain

--delete -D chain 

delete matching rule from chain

--delete -D chain rulenum 

delete rule rulenum (1 = first) from chain

--insert -I chain [rulenum] 

insert in chain as rulenum (default 1=first)

--replace -R chain rulenum 

replace rule rulenum (1 = first) in chain

--list -L [chain] 

list the rules in a chain or all chains

--flush -F [chain] 

delete all rules in chain or all chains

--zero -Z [chain] 

zero counters in chain or all chains

--new -N chain 

create a new user-defined chain

--delete-chain -X [chain] 

delete a user-defined chain

--policy -P chain target 

change policy on chain to target

--rename-chain -E old-chain new-chain 

change chain name, (moving any references)

Options

--proto -p [!] proto 

protocol: by number or name, eg. 'tcp'

--source -s [!] address[/mask] 

source specification

--destination -d [!] address[/mask] 

destination specification

--in-interface -i [!] input name[+] 

network interface name ([+] for wildcard)

--jump -j target 

target for rule (may load target extension)

--goto -g chain 

jump to chain with no return

--match -m match 

extended match (may load extension)

--numeric -n 

numeric output of addresses and ports

--out-interface -o [!] output name[+] 

network interface name ([+] for wildcard)

--table -t table 

table to manipulate (default: 'filter')

--verbose -v 

verbose mode

--line-numbers 

print line numbers when listing

--exact -x 

expand numbers (display exact values)

[!] --fragment -f 

match second or further fragments only

--modprobe=<command> 

try to insert modules using this command

--set-counters PKTS BYTES 

set the counter during insert/append

[!] --version -V 

print package version.

Netmask

ICMP datagram types

see: RFC 1700 (Assigned Numbers)
/usr/include/netinet/ip_icmp.h

Type Of Service (TOS)

Example script

#!/bin/bash

LOOPBACK="127.0.0.0/8"
CLASS_A="10.0.0.0/8"
CLASS_B="172.16.0.0/12"
CLASS_C="192.168.0.0/16"
CLASS_D="224.0.0.0/4"
CLASS_E="240.0.0.0/5"
BROADCAST_SRC="0.0.0.0"
BROADCAST_DEST="255.255.255.255"

########
# flush iptables
iptables -F
iptables -t nat -F
iptables -t mangle -F

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

########
# policies
iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT
iptables -P FORWARD DROP

iptables -t nat -P PREROUTING ACCEPT
iptables -t nat -P OUTPUT ACCEPT
iptables -t nat -P POSTROUTING ACCEPT

iptables -t mangle -P PREROUTING ACCEPT
iptables -t mangle -P OUTPUT ACCEPT

########
# allow related incoming
iptables -I INPUT 1 -m state --state ESTABLISHED,RELATED -j ACCEPT

########
# programs and stuff (add a line for each service you want to allow)

# SSH on local network
iptables -A INPUT -s $CLASS_A -p tcp --destination-port 22 -j ACCEPT

# apache server (on all interfaces/networks)
iptables -A INPUT -p tcp --destination-port 80 -j ACCEPT

# samba + network share
iptables -A INPUT -s $CLASS_A -p tcp --destination-port 137 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p udp --destination-port 137 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p tcp --destination-port 138 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p udp --destination-port 138 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p tcp --destination-port 139 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p udp --destination-port 139 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p tcp --destination-port 445 -j ACCEPT
iptables -A INPUT -s $CLASS_A -p udp --destination-port 445 -j ACCEPT

Map external IP address onto an internal one

Let's say your external IP address is 128.65.225.10 and your internal IP address is 10.0.17.10, then:

-A PREROUTING -d 128.65.225.10 -j DNAT --to-destination 10.0.17.10
-A POSTROUTING -s 10.0.17.10 -j SNAT --to-source 128.65.225.10

You can then view the results with

iptables --list -n -t nat -v

Scripts

• rc.flush-iptables — resets iptables to default values
• fail2ban

Tips

• Find your external interface:

$ ip route ls
10.0.213.56 via 10.0.27.5e dev eth0  metric 10
192.168.27.0/24 dev eth0  proto kernel  scope link  src 192.168.27.63  metric 10
10.0.27.0/24 dev eth0  proto kernel  scope link  src 10.0.27.63  metric 10
169.254.0.0/16 dev eth0  scope link  metric 10
127.0.0.0/8 dev lo  scope link
default via 10.0.27.153 dev eth0  metric 10

The last line shows eth0 to be your external interface.

• Determine the IP address of your external interface:

$ ip addr ls dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc pfifo_fast qlen 100
   link/ether 00:19:d1:4f:22:60 brd ff:ff:ff:ff:ff:ff
   inet 128.95.27.63/24 brd 10.0.27.255 scope global eth0
   inet 192.168.27.63/24 brd 192.168.27.255 scope global eth0:0
   inet6 fe80::219:d1ff:fe4f:2260/64 scope link
      valid_lft forever preferred_lft forever

• So, the IP address of the external interface is 128.95.27.63.

See also

• How to switch from FirewallD to Iptables on CentOS 7
• Ettercap — a suite for man in the middle attacks on LAN. It features sniffing of live connections, content filtering on the fly and many other interesting tricks.
• netfilter2html — a script wrote using GAWK to process netfilter logs and generate a nice HTML output.
• Wflogs — a firewall log analysis tool. It can be used to produce a log summary report in plain text, HTML and XML, or to monitor firewalling logs in real-time.
• Samba — contains extra iptables rules
• dm-crypt: a device-mapper crypto target
• loop-AES
• tripwire
• wikipedia:Port_address_translation (PAT)

External links

• netfilter.org

Free firewall software

• GNU Firewall Free Software Directory
• Shorewall

Tutorials / Howtos / Examples / etc.

• Iptables Tutorial 1.2.2 — by Oskar Andreasson
• Linux Networking-concepts HOWTO
• Iptables - Example Firewall Rulesets — by James Stephens
• Iptables On A Linksys-Cisco WRT54GL Broadband Router HOWTO — by James Stephens
• the DD-WRT Wiki — a third party developed firmware for many 802.11g wireless routers based on a Broadcom chip reference design.
• Firewall for Single Host with Iptables
• Netfilter Log Format
• Linux Network Administrator's Guide, 2nd Edition - Chapter 9: TCP/IP Firewall
• TCP/IP Network Administration, 3rd Edition - Chapter 9: Local Network Services
• Step-By-Step Configuration of NAT with iptables
• Easy Firewall Generator for IPTables
• Firewalling with netfilter/iptables — on linux.ie
• Beating Sandvine with Linux iptables — for legal downloads only.
• pam_recent: an add-on to make iptables' recent match more useful
• An Introduction to TCP/IP
• Speedup DNS requests with a local cache