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## Kernel Options in 1.1.14 and Higher

Starting with 1.1.14, which added alpha support for IPX, the configuration procedure changed slightly. The general options section now asks whether you want networking support in general. It is immediately followed by a couple of question on miscellaneous networking options.

To use TCP/IP networking, you must answer this question with y. If you answer with n, however, you will still be able to compile the kernel with IPX support.

You have to enable this option if your system acts as a gateway between two Ethernets, or between and Ethernet and a SLIP link, etc. Although it doesn't hurt to enable this by default, you may want to disable this to configure a host as a so-called firewall. Firewalls are hosts that are connected to two or more networks, but don't route traffic between them. They are commonly used to provide users from a company network with Internet access at a minimal risk to the internal network. Users will be allowed to log into the firewall and use Internet services, but the company's machines will be protected from outside attacks because any incoming connections can't cross the firewall.

This option works around an incompatibility with some versions of PC/TCP, a commercial TCP/IP implementation for DOS-based PCs. If you enable this option, you will still be able to communicate with normal machines, but performance may be hurt over slow links.

This function enables RARP, the Reverse Address Resolution Protocol. RARP is used by diskless clients and X-terminals to inquire their IP-address when booting. You should enable RARP only when you plan to serve this sort of clients. The latest package of network utilities (net-0.32d) contains a small utility named rarp that allows you to add systems to the RARP cache.

When sending data over TCP, the kernel has to break up the stream into several packets before giving it to IP. For hosts that can be reached over a local network such as an Ethernet, larger packets will be used than for hosts where data has to go through long-distance links. If you don't enable SNARL, the kernel will assume only those networks are local that it actually has an interface to. However, if you look at the class B network at Groucho Marx University, the whole class B network is local, but most hosts interface to only one or two subnets. If you enable SNARL, the kernel will assume all subnets are local and use large packets when talking to all hosts on campus.

If you do want to use smaller packet sizes for data sent to specific hosts (because, for instance, the data goes through a SLIP link), you can do so using the mtu option of route, which is briefly discussed at the end of this chapter.

Nagle's rule is a heuristic to avoid sending particularly small IP packets, also called tinygrams. Tinygrams are usually created by interactive networking tools that transmit single keystrokes, such as telnet or rsh. Tinygrams can become particularly wasteful on low-bandwidth links like SLIP. The Nagle algorithm attempts to avoid them by holding back transmission of TCP data briefly under some circumstances. You might only want to disable Nagle's algorithm if you have severe problems with packets getting dropped.

This enables support for IPX, the transport protocol used by Novell Networking. It is still under development, and isn't really useful yet. One benefit of this will be that you can exchange data with IPX-based DOS utilities one day, and route traffic between your Novell-based networks through a PPP link. Support for the high-level protocols of Novell networking is however not in sight, as the specifications for these are available only at horrendous cost and under a non-disclosure agreement.

Starting in the 1.1.16 kernel, supports another driver type, the dummy driver. The following question appears toward the start of the device driver section.

The dummy driver doesn't really do much, but is quite useful on standalone or SLIP hosts. It is basically a masqueraded loopback interface. The reason to have this sort of interface is that on hosts that do SLIP but have no Ethernet, you want to have an interface that bears your IP address all the time. This is discussed in a little more detail in section- in chapter-.

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