What does the iptables command do in Linux?

What does the iptables command do in Linux?

Last Updated : 22 May, 2019 iptables is a command line interface used to set up and maintain tables for the Netfilter firewall for IPv4, included in the Linux kernel. The firewall matches packets with rules defined in these tables and then takes the specified action on a possible match. Tables is the name for a set of chains.

What are the rules in an iptables table?

Each table contains a number of built-in chains and may also contain user-defined chains. Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This is called a ‘target’, which may be a jump to a user-defined chain in the same table.

How to use iptables to configure multiple ports?

In both commands given below, you should replace “xxx” with your actual port, and if your requirement to block UDP traffic instead of TCP traffic change “TCP” into “UDP”. Iptables has functionalities to write command to configure multiple ports in a single command. To perform this use multiport as seen in below command.

What happens when a packet is sent to iptables?

When a packet is received, iptables finds the appropriate table, then runs it through the chain of rules until it finds a match. Rules: A rule is a statement that tells the system what to do with a packet. Rules can block one type of packet, or forward another type of packet. The outcome, where a packet is sent, is called a target.

Which is the input rule in iptables for incoming connections?

iptables -A INPUT: Append the new rule to the INPUT chain. For incoming connection request, this always has to be INPUT. -i eth0: This refers to the input interface. For incoming connections, this always has to be ‘-i’.

How to use iptables to drop UDP packets?

Syntax: iptables [-t table] -A [chain] -p {protocol_name} [target] Example: This command appends a rule in the INPUT chain to drop all udp packets. iptables -t filter -A INPUT -p udp -j DROP Output:-s, –source: is used to match with the source address of the packet. Syntax: iptables [-t table] -A [chain] -s {source_address} [target]

What does the chain table in iptables do?

It contains the built-in chains INPUT (for packets destined to local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets). This table is consulted when a packet that creates a new connection is encountered.

How does iptables insert rule at top of table?

The iptables allows you to APPEND or INSERT or REPLACE firewall rules as follows. The above command will insert rule in the INPUT chain as the given rule number. So, if the rule number is 1, the rule or rules are inserted at the head of the chain. This is also the default if no rule number is specified.

How to insert rule at top of tables in Linux?

Iptables insert rule at top of tables Linux syntax. The iptables allows you to APPEND or INSERT or REPLACE firewall rules as follows. Iptables append firewall rules to the end of the selected chain. The syntax is: For examples when you use the -A or –append switch you add rule to the end of the chain such as INPUT, FORWARD and more :

Which is an example of an iptables chain?

Tables is the name for a set of chains. Chain is a collection of rules. Rule is condition used to match packet. Target is action taken when a possible rule matches. Examples of the target are ACCEPT, DROP, QUEUE. Policy is the default action taken in case of no match with the inbuilt chains and can be ACCEPT or DROP.