iptables can be used to manage firewalls in Linux.

Chain management

A chain contains a list of rules.

Creating a new chain

# iptables --new USER-FOO

The -N switch can be used instead of --new

Listing rules in a chain

# iptables --list USER-FOO

The -L switch can be used instead of --list

It will return 1 if the chain does not exist.

iptables: No chain/target/match by that name.
# echo $?
1

If the chain is empty it will return 0.

Chain USER-FOO (0 references)
target     prot opt source               destination
# echo $?
0

To display more details, add --numeric or -n for numeric IPs, add --verbose or -v to show packet and byte counts.

# iptables --list USER-FOO --numeric --verbose

Deleting a chain’s rules

To delete all rules in a chain, flush the chain.

# iptables --flush USER-FOO 

The -F switch can be used instead of --flush

Deleting a chain

# iptables --delete-chain USER-FOO

The -X switch can be used instead of --delete-chain

Rules

A chain consists of an ordered list of rules. Each rule can be referred by its ordinal or rulenum within a chain. Rules are numbered starting from 1.

Each rule has a specification.

For a packet to match a rule and perform an action, the packet is matched against the rule-specification.

Rule-specification can include tests such as

• -p, --protocol to match a protocol. Examples include tcp and udp.
• -s, --source for source address/mask.
• -d, --destination for destination address/mask.
• --source-port, --sport source port.
• --destination-port, --dport destination port.

Rule-specifications can include an action such as

• -j, --jump to jump to a target. Valid targets include
  • RETURN to exit this chain and continue the previous / calling chain
  • DROP to silently drop the packet. The caller will not see a response.
  • REJECT to actively reject the packet.
  • ACCEPT allow the packet through.
  • a chain. Processing will move to the specified chain, and continue in the current chain if a RETURN is matched.
• -g, --goto a chain. Processing will move to the specified chain, but continue in the previous chain if a RETURN is matched. This effectively stops any further processing in the current chain.

conntrack: connection tracking module

The conntrack module is useful for packets that have already been NATed, such as those seen in DOCKER-USER after docker is installed.

To enable this module on a rule, -m conntrack must be added to the rule specification.

Rule-specification now has additional options available when using this module.

• --ctstate connection statuses to match, e.g. NEW,RELATED,ESTABLISHED
• --ctorigdst original destination address/mask

To exit the chain when a packet matches 333.333.333.333:25

iptables -A DOCKER-USER -j RETURN -p tcp -m conntrack --ctstate NEW,RELATED,ESTABLISHED --ctorigdst 333.333.333.333 --dport 25

See the man page iptables-extensions for more details.

reply

Packets flow in the original or reply direction. Rules usually block packets in the original direction only.

To append a rule allowing reply packets in the DOCKER-USER chain, run:

iptables -A DOCKER-USER -j RETURN -p tcp -m conntrack --ctdir REPLY

Chain and Rule Ordering - Whitelist

To configure a chain to behave like a whitelist

1. Add rules that jump to RETURN if the packet matches. The previous chain should be configured to ACCEPT the packets.
2. Add a final rule in the chain that DROPs the packet

Port Redirection

In this example we have a tomcat web server running port on port 8080 and 8443, and would like it to also appear on port 80 and 443.

iptables -t nat -A OUTPUT -d localhost -p tcp --dport 80 -j REDIRECT --to-ports 8080
iptables -t nat -A OUTPUT -d 10.0.0.20 -p tcp --dport 80 -j REDIRECT --to-ports 8080
iptables -t nat -A PREROUTING -d 10.0.0.20 -p tcp --dport 80 -j REDIRECT --to-ports 8080

iptables -t nat -A OUTPUT -d localhost -p tcp --dport 443 -j REDIRECT --to-ports 8443
iptables -t nat -A OUTPUT -d 10.0.0.20 -p tcp --dport 443 -j REDIRECT --to-ports 8443
iptables -t nat -A PREROUTING -d 10.0.0.20 -p tcp --dport 443 -j REDIRECT --to-ports 8443

References

• iptables --help
• man iptables
• man iptables-extensions