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  • Iptables works inside Internet and Transport layers
  • Today, Linux can route on basically every single field or bit in the IP header, and even based on TCP, UDP or ICMP headers as well. This is called policy based routing, or advanced routing.
  • If the destination address is local, the packet is sent directly to that address via its hardware MAC address.
  • It does keep track of packets and see if they are of the same stream (via sequence numbers, port numbers, etc.) almost exactly the same way as the real TCP/IP stack. This is called connection tracking, and thanks to this we can do things such as Destination and Source Network Address Translation (generally called DNAT and SNAT),  as well as state matching of packets.
  • The second meaning is if a whole rule is a match
  • A jump instruction is written exactly the same as a target in iptables, with the exception that instead of writing a target name, you write the name of another chain.

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  • tribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1; with the Invariant Sections being "Introduction" and all sub-sections, with the Front-Cover Texts being "Original Author: Oskar Andreasson", and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License

  • hey will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. 

    You should have received a copy of the GNU General
  • pace in the HOWTO's out there lacking in information about the iptables and Netfilter functions in the new Linux 2.4.x kernels. Among other things, I'm going to try to answer questions that some might have about the new possibilities like state matching.  Most of this will be illustrated with an example rc.firewall.txt file that you  can use in your /etc/rc.d/ scripts. Yes, this file was  ori

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  • Iptables is an IP  filter,
  • But per definition an IP filter works in the second layer
  • most firewalls have default  behaviours

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  • what table to use, since by default >  iptables uses the filter table on >  which to implement all commands. Neither do you have to specify the table at >  just this point in the rule. It could be set pretty much anywhere along the >  line. However, it is m >ore or less standard to put the table specification at the begin
  • this  way. Hen
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  • Connection - This is generally referred to in this document as a series of packets relating to each other.
  • DNAT - Destination Network Address Translation.
  • PSEC - Internet Protocol Security is a protocol used to encrypt IPv4 packets and sending them securely over the Internet.
  • Packet - A singular unit sent over a network, containing a header and a data portion.
  • QoS - Quality of Service is a way of specifying how a packet should be handled and what kind of service quality it should receive while sending it.
  • Segment - A TCP segment is pretty much the same as an packet, but a formalized word for a TCP packet.
  • Stream - This term refers to a connection that sends and receives packets that are related to each other in some fashion.
  • SNAT - Source Network Address Translation. This refers to the techniques used to translate one source address to another in a packet.
  • It is very important to understand that iptables was and is specifically built  to work on the headers of the Internet and the Transport layers
  • The ICMP protocol is actually sort of a mix between the two layers. It runs in the Internet layer, but it has the exact same headers as the IP protocol, but also a few extra headers, and then directly inside that encapsulation, the data.
  • The IP protocol has a couple of basic functionalities that it must be able to handle. It must be able to define the datagram, which is the next building block created by the transport layer (this may in other words be TCP, UDP or ICMP for example)
  • The TCP protocol resides on top of the IP protocol.
  • second type of policy is the security policy that we may have written documentation on, for example for the whole company or for this specific network segment.
  • chain policies, which tells the firewall implementation the default behaviour to take on a packet if there was no rule that matched it.
  • Accept  - To accept a packet and to let it through the firewall rules.
  • Connection tracking - A firewall which implements connection tracking is able to track connections/streams simply put.
  • Jump  - The jump instruction is closely related to a target.
  • Ruleset  - A ruleset is the complete set of rules that are put into a whole IP filter implementation.
  • Rule  - A rule is a set of a match or several matches together with a single target in most implementations of IP filters, including the iptables implementation.
  • Targe
  • Target  - There is generally a target set for each rule in a ruleset. If the rule has matched fully, the target specification tells us what to do with the packet.
  • The TCP protocol looks at data as an continuous data stream with a start and a stop signal.
  • he first meaning would be a single match that tells a rule that  this header must contain this and this information.
  • Table  - Each table has a specific purpose, and in iptables there are 4 tables. The raw, nat, mangle and filter tables
  • Chain  - A chain contains a ruleset of rules that are applied on packets that traverses the chain.
  • State  - A specific state of a packet in comparison to a whole stream of packets.
  • Reject  - This is basically the same as a drop or deny target or policy, except that we also send a reply to the host sending the packet that was dropped.
  • Drop/Deny - When a packet is dropped or denied, it is simply deleted, and no further actions are taken.
  • Proxies  were developed to handle traffic in the higher layers, and are hence much better at fullfilling these requirements.
  • It does keep track of packets and see if they are of the same stream (via sequence numbers, port numbers, etc.) almost exactly the same way as the real TCP/IP stack. This is called connection tracking,
  • ilter packets based on their IP headers (Source and Destionation address, TOS/DSCP/ECN, TTL, Protocol, etc.
  • An IP filter operates mainly in layer 2, of the TCP/IP reference stack. Iptables however has the ability to also work in layer 3, which actually most IP filters of today have. But per definition an IP filter works in the second layer.
  • The second meaning is if a whole rule is a match. If the packet matches the whole rule, the jump or target instructions will be carried out (e.g., the packet will be dropped.)

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