Difference between revisions of "Iptables"

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__TOC__
 
__TOC__
 +
'''NOTE:''' IP tables has been replaced with [[nftables]]. ([[iptables]] command converts into nft rulesets)
 
[[iptables]] is a packet filtering, NAT/PAT and packet mangling tool for Linux. Used by a variety of Linux distributions including Ubuntu, Redhat and CentOS. Iptables is highly configurable and are used in many firewall solutions.
 
[[iptables]] is a packet filtering, NAT/PAT and packet mangling tool for Linux. Used by a variety of Linux distributions including Ubuntu, Redhat and CentOS. Iptables is highly configurable and are used in many firewall solutions.
−
*See also [[NAT Linux]] and [[Ip linux command]]
+
*Se også [[NAT Linux]] og [[Ip linux command]]
 
== Using IPTABLES ==
 
== Using IPTABLES ==
 
Iptables is a commandline command with numerous options, and need some training to master.
 
Iptables is a commandline command with numerous options, and need some training to master.
Line 42: Line 43:
  
 
== IPTABLES firewall solutions ==
 
== IPTABLES firewall solutions ==
−
*Shorewall firewall [http://www.shorewall.net/ Shorewall.net 9
+
*Shorewall firewall [http://www.shorewall.net/  
 
[[Category:Linux]]
 
[[Category:Linux]]
 +
 
== iptables syntax ==
 
== iptables syntax ==
 
<center>'''Format''': iptables ''<nowiki>[</nowiki>''-t'' tabel]'' ''kommando regel handling''</center>
 
<center>'''Format''': iptables ''<nowiki>[</nowiki>''-t'' tabel]'' ''kommando regel handling''</center>
Line 175: Line 177:
 
|}
 
|}
  
−
== very basic Firewall ==
+
== Very basic firewall ==
 
Accepts only ping from inside to the outside. The machine is open to the Internet. Could be started in /etc/rc.local.
 
Accepts only ping from inside to the outside. The machine is open to the Internet. Could be started in /etc/rc.local.
 
<pre>
 
<pre>
Line 188: Line 190:
 
EXT_IF="eth0"
 
EXT_IF="eth0"
  
−
# Fjern alle tidligere regler
+
# Fjern alle tidligere regler og ikke indbyggede kæder
 
$FW –F
 
$FW –F
 
$FW -F -t nat
 
$FW -F -t nat
 +
$FW –X
 +
$FW -X -t nat
  
 
# Sæt default politik til afvisning
 
# Sæt default politik til afvisning
Line 208: Line 212:
 
</pre>
 
</pre>
  
−
== basic firewall ==
+
== Basic firewall ==
 
iptables-save udskriver en liste over regler. Kommandoen
 
iptables-save udskriver en liste over regler. Kommandoen
  
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INT_IF="ens192"                    # Internal interface
 
INT_IF="ens192"                    # Internal interface
  
−
# Fjern alle tidligere regler
+
# Fjern alle tidligere regler og ikke indbyggede kæder
−
$FW -F
+
$FW –F
 
$FW -F -t nat
 
$FW -F -t nat
 +
$FW –X
 +
$FW -X -t nat
  
 
# Sæt default politik til afvisning
 
# Sæt default politik til afvisning
Line 325: Line 331:
 
</source>
 
</source>
  
−
==Complete firewall example==
+
==Complete firewall examples==
 +
===Using SNAT (Source NAT)===
  
 
<source lang=cli>
 
<source lang=cli>
Line 345: Line 352:
 
EXT_IF="ens160"                    # External interface
 
EXT_IF="ens160"                    # External interface
 
INT_IF="ens192"                    # Internal interface
 
INT_IF="ens192"                    # Internal interface
−
 
+
# Fjern alle tidligere regler og ikke indbyggede kæder
−
# Fjern alle tidligere regler
+
$FW –F
−
$FW -F
 
 
$FW -F -t nat
 
$FW -F -t nat
−
 
+
$FW –X
 +
$FW -X -t nat
 
# Sæt default politik til afvisning
 
# Sæt default politik til afvisning
 
$FW -P FORWARD DROP
 
$FW -P FORWARD DROP
  
 
#Accepting established and relatec TCP/UDP and ICMP connections
 
#Accepting established and relatec TCP/UDP and ICMP connections
 +
## Accept return traffic from inside users.
 
$FW -A FORWARD -i $EXT_IF -o $INT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
 
$FW -A FORWARD -i $EXT_IF -o $INT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
 +
## Accept return traffic from insode servers (port forwarding)
 +
$FW -A FORWARD -i $INT_IF -o $EXT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
  
 
# Tillad ping indefra
 
# Tillad ping indefra
Line 365: Line 375:
 
$FW -t nat -A PREROUTING -p tcp -i $EXT_IF  -d $EXT_IP --dport 8080 \
 
$FW -t nat -A PREROUTING -p tcp -i $EXT_IF  -d $EXT_IP --dport 8080 \
 
         -j DNAT --to 192.168.1.2:80
 
         -j DNAT --to 192.168.1.2:80
 +
#Allow forwarded traffic through firewall
 
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT
 
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT
−
$FW -A FORWARD -p tcp -i $INT_IF -d 0.0.0.0/0 -s 192.168.1.2 --sport 80 -j ACCEPT
 
  
 
#Tillad DNS (udp port 53)
 
#Tillad DNS (udp port 53)
Line 379: Line 389:
 
echo "1" > /proc/sys/net/ipv4/ip_forward
 
echo "1" > /proc/sys/net/ipv4/ip_forward
  
 +
 +
</source>
 +
 +
===Using Masqerading===
 +
Use masqerading when the external IP address is unknown. For example when using [[DHCP]]. 
 +
<source lang=cli>
 +
#!/bin/bash
 +
#
 +
# Start firewall
 +
#
 +
# Tillader ping indefra og ud.
 +
# Lukker for al anden trafik.
 +
#
 +
# OBS! Denne firewall rører udelukkende ved FORWARD
 +
#
 +
 +
# Diverse erklæringer
 +
FW=iptables
 +
INT_NET="192.168.1.0/24"          # Internal network behind firewall
 +
EXT_IF="ens160"                    # External interface
 +
INT_IF="ens192"                    # Internal interface
 +
 +
# Fjern alle tidligere regler og ikke indbyggede kæder
 +
$FW –F
 +
$FW -F -t nat
 +
$FW –X
 +
$FW -X -t nat
 +
 +
# Sæt default politik til afvisning
 +
$FW -P FORWARD DROP
 +
 +
#Accepting established and relatec TCP/UDP and ICMP connections
 +
## Accept return traffic from inside users.
 +
$FW -A FORWARD -i $EXT_IF -o $INT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
 +
## Accept return traffic from insode servers (port forwarding)
 +
$FW -A FORWARD -i $INT_IF -o $EXT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
 +
 +
# Tillad ping indefra
 +
$FW -A FORWARD -s $INT_NET -p icmp --icmp-type echo-request -j ACCEPT
 +
 +
iptables -t nat -A POSTROUTING -s $INT_NET -d 0.0.0.0/0 -o $EXT_IF -j MASQUERADE
 +
#Forward port 8080 on $EXT_IP to internal webserver at 192.168.1.2 port 80
 +
iptables -t nat -A PREROUTING -p tcp -i $EXT_IF --dport 8080 -j DNAT --to-destination 192.168.1.2:80
 +
 +
#Allow forwarded traffic through firewall
 +
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT
 +
 +
#Tillad DNS (udp port 53)
 +
$FW -A FORWARD -p udp -o $EXT_IF --dport 53 -j ACCEPT
 +
 +
 +
#TCP open ports
 +
$FW -A FORWARD -i $INT_IF -p tcp --dport 80 -d 0.0.0.0/0 -j ACCEPT
 +
$FW -A FORWARD -i $INT_IF -p tcp --dport 443 -d 0.0.0.0/0 -j ACCEPT
 +
 +
#Accept forwarding. Start routing
 +
echo "1" > /proc/sys/net/ipv4/ip_forward
 
</source>
 
</source>
 
<!--
 
<!--
Line 404: Line 471:
  
 
= Links =
 
= Links =
 +
*[https://netfilter.org/projects/nftables/ nft - iptables replacement]
 
*[http://www.netfilter.org/documentation/index.html#documentation-howto Dokumentation]
 
*[http://www.netfilter.org/documentation/index.html#documentation-howto Dokumentation]
 
== Software ==
 
== Software ==
Line 409: Line 477:
 
*[http://www.squid-cache.org/ squid]
 
*[http://www.squid-cache.org/ squid]
 
*[http://www.webmin.com/ Webmin]
 
*[http://www.webmin.com/ Webmin]
−
[[Category:Linux]][[Category:DatateknikerV7]][[Category:Linux Command]]
+
[[Category:Linux]][[Category:DatateknikerV7]][[Category:Linux Command]][[category:Firewall]]

Latest revision as of 12:38, 28 October 2023

NOTE: IP tables has been replaced with nftables. (iptables command converts into nft rulesets) iptables is a packet filtering, NAT/PAT and packet mangling tool for Linux. Used by a variety of Linux distributions including Ubuntu, Redhat and CentOS. Iptables is highly configurable and are used in many firewall solutions.

Using IPTABLES

Iptables is a commandline command with numerous options, and need some training to master. There are three tables you can use filter(default) nat and mangle. Only filter and nat shown below.

iptables flowchart showing chains

The filter chains

Basically iptables has three filters/chains

  • INPUT  : Packets from a Interface to a local process on the machine. A packet from outside to the machine.
  • FORWARD: Packets traversing from one Interface to another Interface
  • OUTPUT : Packets from a local process - the machine itself - to the outside world.

See the tables with the command 

[root@bkshost sysconfig]# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

The nat chains

When you use the nat table or mangle tables

  • PREROUTING : Incoming packets before Routing decision. Used fx. for Destination nat
  • POSTROUTING: Incoming packets after Routing decision. Used fx. for Source nat
  • OUTPUT  : Packets from a local process - the machine itself - to the outside world.

See the tables with the command 

[root@bkshost sysconfig]# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

IPTABLES firewall solutions

iptables syntax

Format: iptables [-t tabel] kommando regel handling


Tabel
filter Regler, der bruges til at filtrere uønskede pakker fra.

Virker på INPUT, OUTPUT og FORWARD kæde

mangle Regler, der bruges til at ændre i indholdet af pakker.

Virker på PREROUTING og OUTPUT kæde

nat Regler, der bruges til at ændre adresser i pakker.

Viker på PREROUTING, OUTPUT og POSTROUTING kæde

Kommando
-A kæde Tilføj (append) for kæde
-I kæde [nummer] Indsæt for kæde. Angives nummer ikke, antages 1 (dvs. først i tabellen)
-D kæde Slet for kæde
-F [kæde] Slet alle regler. Eventuelt i kæde
-P kæde politik Sæt handling for kæde, hvis ingen regler findes. Politik er ACCEPT, DROP og RETURN
-L [kæde] Vis en liste over regler, eventuelt for bestemt kæde
Regel
-p [!]protokol Match, hvis pakke bruger protokol. Protokol kan være tcp, udp, icmp eller andre
-s [!]adresse[/maske] Match, hvis kilde adresse på pakke passer. Der kan angives en maske til et subnet (både 24 og 255.255.255.0 kan bruges)
-d [!]adresse[/maske] Match, hvis destinatinsadresse på pakke passer. Der kan angives en maske til et subnet (både 24 og 255.255.255.0 kan bruges)
-i [!]interface Match, hvis input interface er anvendt til at modtage pakken
-o [!]interface Match, hvis output interface skal anvendes til at sende pakken
--sport [!] [port[:port]] Match, hvis kildeport eller kildeport interval (f.eks. 22:80) anvendes i pakken. Bruges med tcp og udp (se -p regel)
--dport [!] [port[:port]] Match, hvis destinationsport eller destinationsport interval (f.eks. 22:80) anvendes i pakken. Bruges med tcp og udp (se -p regel)
--icmp-type [!]typenavn Match, hvis ICMP meddelelse er af typenavn, f.eks. echo-request, echo-reply, source-quench, time-exceeded, destination-unreachable, network-unreachable, host-unreachable, protocol-unreachable og port-unreachable. Bruges med icmp (se -p regel)
Bruges ! i ovenstående regler, virker match modsat. F.eks. vil -p !tcp matche alle pakker, der ikke bruger tcp.
Handling
-j ACCEPT Accepter pakker, dvs. lad dem gå videre
-j DROP Drop pakker, dvs. smid dem væk
-j SNAT --to-source adresse[:port] Lav ændring i pakkens source IP, hvor adresse bliver den nye source IP og port den nye source port
-j DNAT

--to-destination adresse[:port]

Lav ændring i pakkens destination IP, hvor adresse bliver den nye destination IP og port den nye destination port

Very basic firewall

Accepts only ping from inside to the outside. The machine is open to the Internet. Could be started in /etc/rc.local. 

#!/bin/bash
#
# Start firewall
# Diverse erklæringer
FW=iptables
INT_NET="192.168.54.0/24"
EXT_NET="172.16.0.0/16"
EXT_IP="172.16.4.4"
EXT_IF="eth0"

# Fjern alle tidligere regler og ikke indbyggede kæder
$FW –F
$FW -F -t nat
$FW –X
$FW -X -t nat

# Sæt default politik til afvisning
$FW –P FORWARD DROP

# Tillad ping indefra
$FW –A FORWARD –s $INT_NET –p icmp --icmp-type echo-request –j ACCEPT

# Tillad pong udefra
$FW –A FORWARD –d $INT_NET –p icmp --icmp-type echo-reply –j ACCEPT

# Source NAT på udgående pakker
$FW -t nat –A POSTROUTING –o $EXT_IF –s $INT_NET –j SNAT --to-source $EXT_IP

#Accept forwarding. Start routing
echo "1" > /proc/sys/net/ipv4/ip_forward

Basic firewall

iptables-save udskriver en liste over regler. Kommandoen 

iptables-save > /etc/sysconfig/iptables

Gør nuværende opsætning af iptables permanent dvs. den loades under opstart af iptables scriptet /etc/init.d/iptables.


Her er et eksempel på et simpelt firewall script, hvor der åbnet for ping fra det interne net og ud. Samtidig ændres alle interne IP til ekstern IP 172.16.4.4: 

#!/bin/bash
#
# Start firewall
#
# Tillader ping indefra og ud.
# Lukker for al anden trafik.
#
# OBS! Denne firewall rører udelukkende ved FORWARD
#

# Diverse erklæringer
FW=iptables
INT_NET="192.168.54.0/24"
EXT_NET="172.16.0.0/16"
EXT_IP="172.16.4.4"
EXT_IF="eth0"

# Fjern alle tidligere regler
$FW –F

# Sæt default politik til afvisning
$FW –P FORWARD DROP

# Tillad ping indefra
$FW –A FORWARD –s $INT_NET –p icmp --icmp-type echo-request –j ACCEPT

# Tillad pong udefra
$FW –A FORWARD –d $INT_NET –p icmp --icmp-type echo-reply –j ACCEPT

# Source NAT på udgående pakker
$FW -t nat –A POSTROUTING –o $EXT_IF –s $INT_NET –j SNAT --to-source $EXT_IP

NAT Masqerading

Masqerading anvendes hvis det eksterne netkort får tildelt en IP adresse med DHCP. 

#  Enable IP Forwading between Interfaces (Routing)
echo "1" > /proc/sys/net/ipv4/ip_forward
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
iptables -t nat -A POSTROUTING -s 172.20.0.0/24 -d 0.0.0.0/0 -o eth0 -j MASQUERADE

TCP

established

iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT

Port forwarding

#!/bin/bash
#
# Start firewall
#
# Tillader ping indefra og ud.
# Port forwarder port 8080 til intern server port 80
# Lukker for al anden trafik.
#
# OBS! Denne firewall rører udelukkende ved FORWARD
#

# Diverse erklæringer
FW=iptables
INT_NET="192.168.1.0/24"           # Internal network behind firewall
EXT_NET="192.168.138.0/24"         # External public network
EXT_IP="192.168.138.250"           # External public IP address
EXT_IF="ens160"                    # External interface
INT_IF="ens192"                    # Internal interface

# Fjern alle tidligere regler og ikke indbyggede kæder
$FW –F
$FW -F -t nat
$FW –X
$FW -X -t nat

# Sæt default politik til afvisning
$FW -P FORWARD DROP

# Tillad ping indefra
$FW -A FORWARD -s $INT_NET -p icmp --icmp-type echo-request -j ACCEPT

# Tillad pong udefra
$FW -A FORWARD -d $INT_NET -p icmp --icmp-type echo-reply -j ACCEPT

# Source NAT på udgående pakker
$FW -t nat -A POSTROUTING -o $EXT_IF -s $INT_NET -j SNAT --to-source $EXT_IP
#Accept forwarding. Start routing
echo "1" > /proc/sys/net/ipv4/ip_forward


#Forward port 8080 on $EXT_IP to internal webserver at 192.168.1.2 port 80

#Port forward i PREROUTING kæde
$FW -t nat -A PREROUTING -p tcp -i $EXT_IF  -d $EXT_IP --dport 8080 \
        -j DNAT --to 192.168.1.2:80
#Tillad trafik fra $EXT_IF til intern server til port 80
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT
#Tillad returtrafik fra intern server fra port 80
$FW -A FORWARD -p tcp -i $INT_IF -d 0.0.0.0/0 -s 192.168.1.2 --sport 80 -j ACCEPT

#Tillad DNS (udp port 53)
iptables -A FORWARD -p udp -o ens160 --dport 53 -j ACCEPT
iptables -A FORWARD -p udp -i ens160 --sport 53 -j ACCEPT

Complete firewall examples

Using SNAT (Source NAT)

#!/bin/bash
#
# Start firewall
#
# Tillader ping indefra og ud.
# Lukker for al anden trafik.
#
# OBS! Denne firewall rører udelukkende ved FORWARD
#

# Diverse erklæringer
FW=iptables
INT_NET="192.168.1.0/24"           # Internal network behind firewall
EXT_NET="192.168.138.0/24"         # External public network
EXT_IP="192.168.138.250"           # External public IP address
EXT_IF="ens160"                    # External interface
INT_IF="ens192"                    # Internal interface
# Fjern alle tidligere regler og ikke indbyggede kæder
$FW –F
$FW -F -t nat
$FW –X
$FW -X -t nat
# Sæt default politik til afvisning
$FW -P FORWARD DROP

#Accepting established and relatec TCP/UDP and ICMP connections
## Accept return traffic from inside users.
$FW -A FORWARD -i $EXT_IF -o $INT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
## Accept return traffic from insode servers (port forwarding)
$FW -A FORWARD -i $INT_IF -o $EXT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT

# Tillad ping indefra
$FW -A FORWARD -s $INT_NET -p icmp --icmp-type echo-request -j ACCEPT

# Source NAT på udgående pakker
$FW -t nat -A POSTROUTING -o $EXT_IF -s $INT_NET -j SNAT --to-source $EXT_IP

#Forward port 8080 on $EXT_IP to internal webserver at 192.168.1.2 port 80
$FW -t nat -A PREROUTING -p tcp -i $EXT_IF  -d $EXT_IP --dport 8080 \
        -j DNAT --to 192.168.1.2:80
#Allow forwarded traffic through firewall
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT

#Tillad DNS (udp port 53)
$FW -A FORWARD -p udp -o $EXT_IF --dport 53 -j ACCEPT


#TCP open ports
$FW -A FORWARD -i $INT_IF -p tcp --dport 80 -d 0.0.0.0/0 -j ACCEPT
$FW -A FORWARD -i $INT_IF -p tcp --dport 443 -d 0.0.0.0/0 -j ACCEPT

#Accept forwarding. Start routing
echo "1" > /proc/sys/net/ipv4/ip_forward

Using Masqerading

Use masqerading when the external IP address is unknown. For example when using DHCP. 

#!/bin/bash
#
# Start firewall
#
# Tillader ping indefra og ud.
# Lukker for al anden trafik.
#
# OBS! Denne firewall rører udelukkende ved FORWARD
#

# Diverse erklæringer
FW=iptables
INT_NET="192.168.1.0/24"           # Internal network behind firewall
EXT_IF="ens160"                    # External interface
INT_IF="ens192"                    # Internal interface

# Fjern alle tidligere regler og ikke indbyggede kæder
$FW –F
$FW -F -t nat
$FW –X
$FW -X -t nat

# Sæt default politik til afvisning
$FW -P FORWARD DROP

#Accepting established and relatec TCP/UDP and ICMP connections
## Accept return traffic from inside users.
$FW -A FORWARD -i $EXT_IF -o $INT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT
## Accept return traffic from insode servers (port forwarding)
$FW -A FORWARD -i $INT_IF -o $EXT_IF -m state --state ESTABLISHED,RELATED -j ACCEPT

# Tillad ping indefra
$FW -A FORWARD -s $INT_NET -p icmp --icmp-type echo-request -j ACCEPT

iptables -t nat -A POSTROUTING -s $INT_NET -d 0.0.0.0/0 -o $EXT_IF -j MASQUERADE
#Forward port 8080 on $EXT_IP to internal webserver at 192.168.1.2 port 80
iptables -t nat -A PREROUTING -p tcp -i $EXT_IF --dport 8080 -j DNAT --to-destination 192.168.1.2:80

#Allow forwarded traffic through firewall
$FW -A FORWARD -p tcp -i $EXT_IF -d 192.168.1.2 --dport 80 -j ACCEPT

#Tillad DNS (udp port 53)
$FW -A FORWARD -p udp -o $EXT_IF --dport 53 -j ACCEPT


#TCP open ports
$FW -A FORWARD -i $INT_IF -p tcp --dport 80 -d 0.0.0.0/0 -j ACCEPT
$FW -A FORWARD -i $INT_IF -p tcp --dport 443 -d 0.0.0.0/0 -j ACCEPT

#Accept forwarding. Start routing
echo "1" > /proc/sys/net/ipv4/ip_forward

Links

Software

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