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To debug your routing tables, you can either edit rc.inet1 and reboot, or use the route command by hand to add or delete entries. The manual page for route(8) describes the full syntax of this command. Note that simply editing rc.inet1 and re-executing it will not clear out old entries in the routing table; you must either reboot or use route del to delete the entries.

If absolutely nothing seems to work, there may be a problem with your Ethernet device configuration. First, be sure that your Ethernet card was detected at the appropriate address and/or IRQ at boot time. The kernel boot messages will give you this information; if you are using syslogd, kernel boot-time messages are also saved in a file, such as /var/log/messages.

A good way to determine whether it was really the Ethernet card that created the trouble is to use the command ifconfig interface_name, as in the following example:

ifconfig eth0

This will output statistics about the interface. If it has received or sent any packets, it must have been recognized by the kernel, and there cannot be a general hardware problem. If your card is not listed when issuing:

ifconfig

it wasn't even recognized by the kernel.

If detection of your Ethernet card is faulty, you may have to modify kernel parameters to fix it. The Linux Ethernet HOWTO includes much information on debugging Ethernet card configurations. In many cases, the fix is as simple as specifying the appropriate IRQ and port address at the LILO (or Grub, or whichever bootloader you are using) boot prompt. For example, booting via LILO with the command:

lilo: linux ether=9,0x300,0,1,eth0

will select IRQ 9, base address 0x300, and the external transceiver (the fourth value of 1) for the eth0 device. To use the internal transceiver (if your card supports both types), change the fourth value of the ether option to 0.

Also, don't overlook the possibility that your Ethernet card is damaged or incorrectly connected to your machine or the network. A bad Ethernet card or cable can cause no end of trouble, including intermittent network failures, system crashes, and so forth. When you're at the end of your rope, consider replacing the Ethernet card or cable, or both, to determine if this is the source of the problem.[*]

If your Ethernet card is detected but the system is still having problems talking to the network, the device configuration with ifconfig may be to blame. Be sure you have specified the appropriate IP address, broadcast address, and subnet mask for your machine. Invoking ifconfig with no arguments displays information on your Ethernet device configuration.

* * *

[*] Linux supports IPv6, but since most local networks and ISPs do not use it yet, it is not very relevant at this time, unfortunately.

[†] Why not 65,536 instead? For reasons to be discussed later, a host address of 0 or 255 is invalid.

[*] On many systems, sshd is not always listening to port 22; the Internet services daemon inetd is listening on its behalf. For now, let's sweep that detail under the carpet.

[*] One of the authors once spent three hours trying to determine why the kernel wouldn't recognize an Ethernet card at boot time. As it turned out, the 16-bit card was plugged into an 8-bit slot—mea culpa.

Dial-Up PPP

To communicate over TCP/IP using a modem (such as through a dial-up account to an Internet service provider) or through some other serial device (such as a "null modem" serial cable between two machines), Linux provides the Point-to-Point Protocol software suite, commonly known as PPP. PPP is a protocol that takes packets sent over a network (such as TCP/IP) and converts them to a format that can be easily sent over a modem or serial wire. Chances are, if you have an Internet account with an ISP, the ISP's server uses PPP to communicate with dial-up accounts. By configuring PPP under Linux, you can directly connect to your ISP account in this way.

SLIP is an earlier protocol that has the same basic features as PPP. However, it lacks certain important qualities, such as the ability