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Linux networking and routing overview

Network packets walk through linux kernel following two main chains and traversing some specific points. An incoming packet must be inspected by the kernel to know its destination: it could be locally destined or could have to be forwarded to another internet node. Anyway, a packet which arrives at the network card reaches the so called pre routing hook. At this point a routing decision is made and the packet can reach one of the two possible hooks: the input or forward one. If a packet has to be delivered to a local application, it will hit the input hook, if it's to be routed to another machine, it will go through the forward one.
On the other hand, a packet going out might be locally generated or forwarded. Both the local packets and those to be forwarded hit the post routing hook, passing first through the output one in the first case, or coming from the forward hook in the second.
In IPFIRE-wall, the direction of a packet to be filtered or network/port translated must always be specified. Directions can be INPUT (in), OUTPUT (out), FORWARD (fwd), PRE-ROUTING (pre) or POST-ROUTING (post).
Filtering is done in INPUT, OUTPUT or FORWARD directions, while the translation is usually done in the PRE (destination nat) or POST directions (snat -not tested!- or MASQUERADING). Destination NAT can be done in OUTPUT direction too.

IPFIRE rules and TCP/IP protocol.

IPFIREwall can be told to filter or translate the network packets taking care of many packet fields each of those having a meaning related to internet protocol, called the TCP/IP (Transmission Control Protocol / Internet Protocol). In this page we will give a little explanation of the packet fields and their meanings, leaving to the reader the chance to deepen the argument on the specialized books.

Identification of a machine inside the network.

To identify a machine in a computer network, we need an address, just like when we want to send a postcard to a friend of ours we have to know his address. Identified our friend's house, we need to know the letterbox on which to leave the postcard as well. The aim of identifying a particular machine on a network is carried out by the Internet Protocol, the one of identifying the service in which we are interested in is achieved by means of the Transmission Control Protocol (TCP).

Connection reliability.

The Internet communication can be reliable or not: some kinds of messages need to arrive at the destination in the same order they have been sent, and it is important that no packet is lost on the way. It's the case of the web traffic, of the commercial transactions, of the secure shell sessions... This kind of communication is carried out by the TCP protocol. It takes care of setting up a new connection and verifying that every packet reaches the end communication point in the right order. Sometimes it is instead convenient that the message exchange is faster and in such cases it is not fundamental that the packet order is respected or that some packets get lost. For such communications, the User Datagram Protocol (UDP) can be used, and it is faster and lighter, but connectionless: a packet is sent on the network and no guarantee is given on its arrival at the right destination. This kind of protocol is used for instance when we need to in resolve Internet names into addresses (i.e. the Domain Name Service) or in audio/viedo streams, where it is better not to receive a packet of the stream (which would mean some noise) instead of waiting for a lost packet to be retrasmitted (that would cause interruptions during playback). Another protocol often used in the network communication is the ICMP, Internet Control Message Protocol, which is adopted for control messages. An example of a program which makes use of the ICMP is ping. Some firewalls also respond with ICMP messages when they want to communicate some error code or the unavailability of some service.
IPFIRE supports TCP, UDP and ICMP protocols only. It is a good idea to specify the kind of protocol of each rule when creating a new one. Then an Internet address can be specified for source and destination. Internet addresses can be expressed in the classical ipv4 dotted decimal form, such as
IPFIREwall acceptsa also intervals of addresses: intervals can be expressed in the form - or or
All the forms above are well accepted and assume the same meaning.
Internet addresses can be expressed also in a negative form, prepending a ! mark before an address or interval (not well tested..). If we send a packet from our machine with address to, the packet will have the first as source address and the second as destination. The response to this packet will have the address fields inverted.
For this reason, if we specify a permission rule towards, we should put a rule to allow packets from, that is with such address as the source.
This double rule for a single communication flow can be avoided specifying that the rule itself has to be stateful. As well as the addresses and the protocol kind, two other fields can be setup: the Type of Service (TOS) and the Total length. The first field concerns the type of desired service in which speed or reliability can result privileged, one at the expence of the other. The total length specifies the length of the whole IP datagram (max 65535 bytes). See specialized literature for further information.

Identifying a service on a host.

Each service is identified in the TCP and UDP protocols by a number called port. Each application on a host listens for a connection on a specific port. When a machine has been reached by a remote packet by means of its address, information carried in packet must be delivered to the right application. A port is an integer between 0 and 65535 (a 16 bit unsigned integer), and, as goes for the addresses, there is a source and a destination port. When we are interested in a remote service, we specify a port to which to connect. Such port is the destination one. The source port is normally chosen by the operating system at the beginning of a new communication.
For example, the web service is associated to the tcp port number 80, the Domain Name Resolution (DNS) with the udp port number 53. Thus, blocking the TCP destination port 80 means blocking the access of the workstation to the world wide web. The ports are treated by IPFIRE as the addresses, with the exception for the fact that the intervals cannot be specified in the negative form, that is !6699 or 1025-10000 are alright, but the syntax !1025-10000 is not recognized.
See the file /etc/services for a list of port numbers and their associated meanings.
IPFIRE user interface can resolve the port numbers into the service names as well, if -services is given on command line or if b si pressed or again if RESOLVE_SERVICES is specified in the configuration file. A high performance in the service name resolution is assured by a list dynamically allocated in the main memory. See the chapters above for further details.

TCP specific options.

Being a connection oriented service, the TCP protocol stores in its packets the information about the connection state in some flags. They can be specified in a firewall rule, to add a match pattern upon them. A list of flags follows, together with a short explanation of their meaning and a possible usage in a firewall context.

SYN flag is used to create new connections.
When a host wants to initiate a new connection, it sends a packet with this flag set to active (flags are single bit fields, so they can assume 0 or 1 as possible values) and with the ACK flag set to 0. The response to an accepted connection is instead characterized by both SYN and ACK flags set to 1. In practise, SYN is used to denote messages of type connection request or connection accepted. An interesting scenario is the one in which rules are made up of stateful ones to allow output traffic (generated by the local machine) and a denial rule is setup for the packets with the SYN flag set. In this situation, it would be possible to have all attempts to create new connections towards our machine logged in red color, while accepting all the traffic (in both directions) generated and leaving our node.

In addition to the function described above, the ACK flag indicates that the field acknowledgement number in TCP segment is valid. If ACK is 0, such field is ignored.

PSH (Push) flag indicates data of push type. In this way, the receiver is asked to deliver the packet to the application at the arrival moment, without first saving it in a temporary buffer while waiting for this to be filed in. Buffer saving would be otherwise probable for efficiency reasons.

RST (Reset) flag is used to reinitialize a connection become unstable for a host fault or for some other trouble. It is used to refuse an invalid segment or a new connection. If a reset is received, it means that somewhere a problem happened.

FIN flag is set active to close a connection and specifies that the sender has no more data to transmit.

URG, urgent pointer field, indicates there is an offset, in bytes, from the sequence number, where receiver should find urgent data. This choice allows interruption delivery to application leyer without involving TCP protocol in the reason of interruption itself.

Other rule fields of interest.

Network device

IPFIREwall can filter packets also depending on the interface they are arriving or they are leaving from. In the INPUT or PRE-ROUTING or FORWARD direction an input device can be specified, while in OUTPUT, POST ROUTING and again FORWARD directions an output device can be specified. If no device is indicated in a rule, then kernel won't check device name against that rule.

Give a name to a rule

At the moment of inserting a new rule in the firewall, it is possible to give it a simple name, which will be printed out when a match with a packet occurs. This is useful for the user to remember the rules that are being applied without having to inspect all the network information provided by the logging function.
Support for rule names must be compiled in both in kernel and in user modules. See compiling instructions for userspace program and for kernel modules to know how to do this.
If you are installing IPFIREwall as is, there is no need to worry about the activation of the rule naming: it is enabled by default.

Andrew S. Tanenbaum Reti di Computer (Computer Networks), Third Edition.

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