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netfilter.samba.org/ipchains www.openssh.org www.squid.org web.mit.edu/kerberos
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Outside users may also try to gain unauthorized access through any Internet services you may be hosting, such as a Web site. In such a case, you can set up a proxy to protect your site from attack. For Linux systems, use Squid proxy software to set up a proxy to protect your Web server (see 28).
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Firewalls: iptables and NAT
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A good foundation for your network's security is to set up a Linux system to operate as a firewall for your network, protecting it from unauthorized access. You can use a firewall to implement either packet filtering or proxies. Packet filtering is simply the process of deciding whether a packet received by the firewall host should be passed on into the local network. The
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packet-filtering software checks the source and destination addresses of the packet and sends the packet on, if it's allowed. Even if your system is not part of a network but connects directly to the Internet, you can still use the firewall feature to control access to your system. Of course, this also provides you with much more security. With proxies, you can control access to specific services, such as Web or FTP servers. You need a proxy for each service you want to control. The Web server has its own Web proxy, while an FTP server has an FTP proxy. Proxies can also be used to cache commonly used data, such as Web pages, so that users needn't constantly access the originating site. The proxy software commonly used on Linux systems is Squid, discussed in 27. An additional task performed by firewalls is Network Address Translation (NAT). Network Address Translation redirects packets to appropriate destinations. It performs tasks such as redirecting packets to certain hosts, forwarding packets to other networks, and changing the host source of packets to implement IP masquerading. Note ipchains was used to implement packet filtering and NAT tasks on firewalls for the Linux 2.2 kernel and earlier. The Netfilter software package implements both packet filtering and NAT tasks for the Linux 2.4 kernel and above. The Netfilter software is developed by the Netfilter Project, which you can find out more about at netfilter.samba.org. The command used to execute packet filtering and NAT tasks is iptables, and the software is commonly referred to as simply iptables. However, Netfilter implements packet filtering and NAT tasks separately using different tables and commands. A table will hold the set of commands for its application. This approach streamlines the packet-filtering task, letting iptables perform packet-filtering checks without the overhead of also having to address translations. NAT operations are also freed from being mixed in with packet-filtering checks. You use the iptables command for both packet filtering and NAT tasks, but for NAT you add the -nat option. The iptables software can be built directly into the 2.4 kernel or loaded as a kernel module, iptable_filter.o. Unlike its predecessor, ipchains, Netfilter is designed to be modularized and extensible. Capabilities can be added in the form of modules such as the state module, which adds connection tracking. Iptables includes backward-compatible modules for both ipfwadm and ipchains. In fact, iptables is very similar to ipchains. You can still use ipchains and the earlier ipfwadm commands by loading the ipchains.o or ipfwadm.o modules provided with the Netfilter software. These provide full backward compatibility. For the sake of older systems, ipchains is also discussed here.
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Netfilter is essentially a framework for packet management that can check packets for particular network protocols and notify parts of the kernel listening for them. Built on the Netfilter framework is the packet selection system implemented by IP Tables. With IP Tables, different tables of rules can be set up to select packets according to differing criteria. Packet filtering is implemented using a filter table that holds rules for dropping or accepting packets. Network Address Translation operations such as IP masquerading are implemented using the NAT table that holds IP masquerading rules. Preroute packet mangling uses the mangle table.
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This structure is extensible in that new modules can define their own tables with their own rules. It also greatly improves efficiency. Instead of all packets checking one large table, they only access the table of rules they need to. IP Tables rules are managed using the iptables command. For this command, you will need to specify the table you want to manage. The default is the filter table, which need not be specified. You can list the rules you have added at any time with the L and n options, as shown here. The n option says to use only numeric output for both IP addresses and ports, avoiding a DNS lookup for hostnames. You could, however, just use the L option to see the port labels and hostnames:
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