Rules, matches, targets, and chains in .NET Generating UPC-13 in .NET Rules, matches, targets, and chains

Rules, matches, targets, and chains using visual .net toencode european article number 13 for web,windows application Web app History Example rules Introduction to iptables Packet Destination = Port 23 Source = TCP stack Drop Alter destination Figure 25-15. Example of how rules in a chain work end of processing for that pa cket. If the answer is no, the kernel applies the second rule in the chain to see whether the packet came from the specified IP address. If the answer is yes, the destination in the packet s header is changed and the modified packet is sent on its way.

If the answer is no, the packet is sent on without being changed. Chains are collected in three tables: Filter, NAT, and Mangle. Each of these tables has builtin chains (described next).

You can create additional, user-defined chains in Filter, the default table.. Filter table The default table. This table is mostly used to DROP or ACCEPT packets based on their content; it does not alter packets. Builtin chains are INPUT, FORWARD, and OUTPUT.

All user-defined chains go in this table. The Network Address Translation table. Packets that create new connections are routed through this table, which is used exclusively to translate the source or destination fields of packets.

Builtin chains are PREROUTING, OUTPUT, and POSTROUTING. Use this table with DNAT, SNAT, and MASQUERADE targets only. DNAT (destination NAT) alters the destination IP address of the first inbound packet in a connection so it is rerouted to another host.

Subsequent packets in the connection are automatically DNATed. DNAT is useful for redirecting packets from the Internet that are bound for a firewall or a NATed server (page 896). SNAT (source NAT) alters the source IP address of the first outbound packet in a connection so it appears to come from a fixed IP address for example, a firewall or router.

Subsequent packets in the connection are automatically SNATed. Replies to SNATed packets are automatically de-SNATed so they go back to the original sender. SNAT is useful for hiding LAN addresses from systems outside the LAN and using a single IP address to serve multiple local hosts.

MASQUERADE differs from SNAT only in that it checks for an IP address to apply to each outbound packet, making it suitable for use with dynamic IP addresses such as those provided by DHCP (page 470). MASQUERADE is slightly slower than SNAT..

NAT table Mangle table Used exclusively to alter the EAN13 for .NET TOS (type of service), TTL (time to live), and MARK fields in a packet. Builtin chains are PREROUTING and OUTPUT.

. 882 25 firestarter, gufw, and iptables: Setting Up a Firewall Network PREROUTING Mangle, (D)NAT (Routing). FORWARD Mangle, Filter POSTROUTING Mangle, (S)NAT INPUT Filter, Mangle (Routing). OUTPUT Mangle, NAT, Filter Local system Figure 25-16. Network packets Filtering a packet in the kernel When a packet from the networ GS1-13 for .NET k enters the kernel s network protocol stack, it is given some basic sanity tests, including checksum verification. After passing these tests, the packet goes through the PREROUTING chain, where its destination address may be changed (Figure 25-16).

Next the packet is routed based on its destination address. If it is bound for the local system, it first goes through the INPUT chain, where it can be filtered (accepted, dropped, or sent to another chain) or altered. If the packet is not addressed to the local system (the local system is forwarding the packet), it goes through the FORWARD and POSTROUTING chains, where it can again be filtered or altered.

Packets created locally pass through the OUTPUT and POSTROUTING chains, where they can be filtered or altered before being sent to the network.. State The connection tracking machi visual .net EAN-13 Supplement 5 ne (also called the state machine) provides information on the state of a packet, allowing you to define rules that match criteria based on the state of the connection the packet is part of. For example, when a connection is opened, the first packet is part of a NEW connection, whereas subsequent packets are part of an ESTABLISHED connection.

Connection tracking is handled by the conntrack module. The OUTPUT chain handles connection tracking for locally generated packets. The PREROUTING chain handles connection tracking for all other packets.

For more information refer to State on page 889. Before the advent of connection tracking, it was sometimes necessary to open many or all nonprivileged ports to make sure that the system accepted all RETURN and RELATED traffic. Because connection tracking allows you to identify these kinds of traffic, you can keep many more ports closed to general traffic, thereby increasing system security.

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