As we all know, iptables is a flexible firewall utility built for Linux operating systems. It uses a set of tables with chains. And, these chains contains a set of built-in or user-defined rules. But why we use these rules? Actually, not all the packets that reach the firewall are safe. So, these rules filter the packets effectively.
Iptables provide five tables (filter, nat, mangle, security, raw), but the most commonly used are the filter table and the nat table. Tables are organized as chains, and there are five predefined chains, PREROUTING, POSTROUTING, INPUT, FORWARD, and OUTPUT. Here we focus only on the nat table.
But, the REJECT command sends an error message back to the source indicating a connection failure. For TCP connections, iptables will send RST/ACK segments. But, with UDP, It returns an “ICMP destination port unreachable” message.
And, no indication is sent to the client or server. But, the REJECT command sends an error message back to the source indicating a connection failure. For TCP connections, iptables will send RST/ACK segments. But, with UDP, It returns an “ICMP destination port unreachable” message.
@Dagelf, you're partially correct, but made quite a mess of things. DROP will NOT reply anything!A closed port will return with RST/ACK, an open port will ACK or SYN/ACK. Now a REJECT rule will default to "reject-with icmp-port-unreachable" (replying with an icmp packet), and is thereby different from a closed port response and a DROPped (timeout) response both.
I've always taken it for granted that with firewalls, the safest bet is to unceremoniously 'DROP' everything by default. But recently in my scripts …
The -A flag tells iptables to append the rule to the chain, meaning it ends up under your REJECT rule, and since the first rule matches, it's never used.. You list your rules with iptables -L FORWARD and you will see this yourself. To get rid of the rule you added, run. iptables -D FORWARD -j REJECT Until there are no more such rules in the chain.
Although already over a year old, I stumbled across this question a couple of times on other Google search and I believe I can improve on the previous answer for the benefit of others.
The REJECT target rejects the packet. If you do not specify which ICMP message to reject with, the server by default will send back ICMP port unreachable (type 3, code 3). It would be wise to read the iptables documentation to understand what each action does (-j DROP vs. -j REJECT).
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. 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.
Iptables defines five hook points in the kernels packet processing pathways : PREROUTING, INPUT, FORWARD, POSTROUTING and OUTPUT. Built-in chains are attached to these hook points you can add a sequence of rules for each hook points each rule represents an opportunity to affect or monitor packet flow.
The Internet Protocol (IP) is a data-oriented protocol that allows multiple hosts to talk to each other across network connections. Data in an IP network are sent in blocks referred to as packets or datagrams. They typically have a source host, destination host, and source and destination ports associated with the communication. Layered on top of the IP protocol are other protocols. These are typically transport layers. There are two main transport protocols that are heavily used. The transmission control protocol (TCP) is a stateful delivery mechanism that makes a best effort to deliver the packets requested. If the first attempt fails, multiple attempts are made to route and deliver the packet. This protocol is very good at delivering text files that can not tolerate data corruption. With this protocol, clients have the ability to request redelivery of packets that were not properly received and can handle our of order delivery of packets. This protocol is very good for applications like patch delivery, email, network file shares, and web pages. It is not very good for delivery of streaming video or voice over IP applications.
The TCP extensions are automatically loaded if `-p tcp is specified. It provides the following options (none of which match fragments).
Protocol can be a number (if you know the numeric protocol values for IP) or a name for the special cases of `TCP, `UDP or ` ICMP. The case doesn’t matter, so `TCP works as well as `TCP.
When a packet matches a rule whose target is a user-defined chain, the packet begins traversing the rules in that user-defined chain. If that chain doesn’t decide the fate of the packet, then once traversal on that chain has finished, traversal resumes on the next rule in the current chain.
Packets traversing the INPUT chain don’t have an output interface, so any rule using `-o in this chain will never match. Similarly, packets traversing the OUTPUT chain don’t have an input interface, so any rule using `-i in this chain will never match.
Iptables provide five tables (filter, nat, mangle, security, raw), but the most commonly used are the filter table and the nat table. Tables are organized as chains, and there are five predefined chains, PREROUTING, POSTROUTING, INPUT, FORWARD, and OUTPUT.
It helps to break the diagram down. An incoming packet goes through the PREROUTING chain, and then the kernel makes the routing decision based on the routing table . If the packet is destined for the machine itself, it’s put into a local process listening on the machine. If it’s destined for another machine (so this machine serves as a router), the kernel will put it through the POSTROUTING chain and then send it to an output interface.
Note that we use the port number 80 here because the port number has already been translated from 8080 to 80 by the DNAT rule in PREROUTING chain.
For a packet generated by a local process, it goes to the routing step first. The kernel matches the destination address against the route table, selects an output interface, and takes the IP address of the chosen interface as its source address. For example, when PC1 sends a packet to 127.0.0.1 (or other localhost addresses), the packet will be routed to the loopback interface and thus take the IP address 127.0.0.1. In contrast, if the packet is sent to the Internet, it will be routed by the routing rule default via 192.168.1.1 dev eth0 to interface eth0 and take the IP address 192.168.1.2. After the routing process, the packet is then put into the OUTPUT chain. As a result, we have access to both the packet’s source and destination address in the OUTPUT chain.
MASQUERADE can only be used in the POSTROUTING chain because the source IP address of the packet will be obtained from the network interface it gets routed to, so it should be placed after the routing decision.
The option -p tcp ( p stands for protocol ) matches for TCP connections (so this rule doesn’t apply to UDP), while --dport 8080 ( --dport stands for destination port) matches for TCP traffic destining port 8080. This rule will replace the destination address of any IP packets going to port 8080 with 192.168.1.2, the IP address of the service. The port is also replaced with 80, the port of the service. For any response packets, the reverse operation is also automatically applied, just like in SNAT. The process works like the diagram below.
The filter table is also essential, but it’s mainly used for firewalls, so we do not discuss it here. The nat tables are used for network address translation, and it’s available in PREROUTING, POSTROUTING, and OUTPUT chains.
Iptables have a set of rules to manage incoming and outgoing traffic. Drop and reject belongs to the actions against each unsafe packets.
As we all know, iptables is a flexible firewall utility built for Linux operating systems. It uses a set of tables with chains.
Incase of a DDOS attack, the server needs to respond to each request with a response.
For TCP connections, iptables will send RST/ACK segments. But, with UDP, It returns an “ICMP destination port unreachable” message.
Actually , not all the packets that reach the firewall are safe. So, these rules filter the packets effectively.
Actually, when we use the DROP command, it will not forward the packet or answer it. But, simply drops the packet silently.
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 won't accept your mail otherwise).
The REJECT target rejects the packet. If you do not specify which ICMP message to reject with, the server by default will send back ICMP port unreachable (type 3, code 3).
Iptables have a set of rules to manage incoming and outgoing traffic. Drop and reject belongs to the actions against each unsafe packets.
As we all know, iptables is a flexible firewall utility built for Linux operating systems. It uses a set of tables with chains.
Incase of a DDOS attack, the server needs to respond to each request with a response.
For TCP connections, iptables will send RST/ACK segments. But, with UDP, It returns an “ICMP destination port unreachable” message.
Actually , not all the packets that reach the firewall are safe. So, these rules filter the packets effectively.
Actually, when we use the DROP command, it will not forward the packet or answer it. But, simply drops the packet silently.