Linux Unleashed, Third Edition:Networking





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rc Files
Linux reads the rc (run command) files when the system boots. The init program initiates the reading of these files, and they usually serve to start processes such as mail, printers, cron, and so on. They are also used to initiate TCP/IP connections. Most Linux systems have the rc command files in the directory /etc/rc.d.
The two files of interest to TCP/IP are rc.inet1, which sets the network parameters, and rc.inet2, which starts the daemons used by TCP/IP. On some systems, these two files are combined into one larger file called either rc.inet or rc.net.
To configure the rc.inet files to start up the TCP/IP network, you must first make sure that the files are actually read by the init program. This is handled by the /etc/inittab and /etc/rc.d/rc.M files, where there are one or more lines that control the reading of the rc.inet files.
Some Linux versions use only the /etc/inittab file to start the TCP/IP daemons. The Slackware version, though, has a line in the /etc/inittab file that tells the init program to read the file /etc/rc.d/rc.M when running multiuser mode. The TCP/IP daemons are not started when the system is in single-user mode.
Whichever file is involved, look for a line that refers to the rc.inet1, rc.inet, or rc.net file. In some cases, this line(s) is commented out (it has a pound sign as the first character) to prevent the system from trying to run TCP/IP when there is no requirement for it. If the line is commented out, remove the comment symbol. The Slackware release of Linux, for example, has an if loop within the rc.M file that has these lines in it:


/bin/hostname ‘cat /etc/HOSTNAME | cut -f1 -d .‘
/bin/sh /etc/rc.d/rc.inet1
/bin/sh /etc/rc.d/rc.inet1


Make sure that these lines (as well as the if loop that the lines are part of) are not commented out. You want these lines to execute each time the if condition is true. With the Slackware version, the if condition checks for the existence of the file rc.inet1.
If you can’t find a reference to the rc.inet files in /etc/inittab, or a pointer to another file that has these files referenced (as with Slackware’s /etc/rc.d/rc.M), you will have to add the lines to the /etc/inittab file manually. This can be scary for newcomers to Linux, so make copies of the files so you can always recover. An emergency boot floppy is also always handy to have.
Usually there is a good set of comments within the startup files to help you configure the system. There is a section for TCP/IP Ethernet support that often has a number of lines commented out. It consists of lines like this:


#IPADDR=”127.0.0.1”
#NETMASK=””
#NETWORK=”127.0.0”
#BROADCAST=””
#GATEWAY=””


Obviously, these correspond to the pieces of information you determined earlier. Therefore, uncomment the lines and type in the information about your machine. If you don’t have one of the pieces, such as a gateway address, leave that line commented out.

In the rc.inet1 file, please note several references to the programs ifconfig and route. These programs control TCP/IP communications. ifconfig configures network device interfaces, and route configures the routing table.
Near the top of the rc.inet1 file (or whichever file is used in your version of Linux) are a couple of lines that call both ifconfig and route for the loopback driver. The lines probably look like this:


/sbin/ifconfig lo 127.0.0.1
/sbin/route add -net 127.0.0.0


Neither of these lines should be commented out. They are necessary to set the loopback driver, which must exist on the system in order for TCP/IP to function properly.

There will probably be a number of lines that are commented out below the settings for your machine’s IP address with instructions to uncomment one of them. The differences between the lines is whether broadcast and netmask variables are included. To begin, try uncommenting the line that looks like this:


/etc/ifconfig eth0 ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}


If this causes problems later during system startup, switch the uncommented line to the one that reads



/etc/ifconfig eth0 ${IPADDR} netmask ${NETMASK}


eth0 is the first device for the first Ethernet card on your system, called eth0.
Finally, if you have a gateway machine on your network, a section of the rc.inet1 file lets you enter the IP address of the gateway. Again, these lines should be uncommented. You might want to try to get the system working properly before you set up the gateway because it is easier to debug when the number of potential problems is smaller.
The rc.inet2 file starts up the daemons used by TCP/IP. In most cases, you won’t have to make changes to this file because the most important daemons are usually started anyway. Look for a line that calls the inetd program, which is the most important TCP/IP daemon of all. There should be no comments on the line that starts it. It will probably look like this:


if [-f ${NET}/inetd
then
echo -n “ inetd”
${NET}/inetd
else
echo “no INETD found. INET cancelled.”
exit 1
fi


If you read Chapter 14, “Shell Programming,” or you know a little about programming in some other language, this short section might make sense. This routine checks for the existence of the inetd file and starts it if it’s there. If it’s not there, an error message is generated on the display (remember this is during the boot process), and the rc.inet2 file is exited.
More commented-out daemons are probably listed below inetd, such as named (the name server daemon that converts proper names to IP address), routed (used for routing), and several others. Unless you know that you want one of these daemons active, leave them commented out for now.



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