Running Linux, 5th Edition - Matthias Kalle Dalheimer [265]
This is the setup for a standalone Linux workstation. If you plan to run a small network behind the Linux machine, you will have to use masquerading, as described earlier in this chapter; you can find help for this in a book that covers Linux firewalling, such as the Linux Network Administrator's Guide and the Linux iptables Pocket Reference (O'Reilly). Some Linux distributions, such as Slackware, turn off IP forwarding by default, which means that masquerading will not work. If this is the case, add the following line to your startup script:
echo 1 >/proc/sys/net/ipv4/ip_forward
All cable modems can be configured remotely. If you are unlucky and have an ISP that does not configure the cable modem for you, you have to configure it from scratch; this will require more than average knowledge about how TCP/IP works, and you should probably seek assistance from your ISP (or switch to one that does configure the modem for you).
In some cases, the cable modem is configured such that it is works only with one particular Ethernet card, and you have to give the MAC address of your card to your ISP for configuration purposes. If this is the case, you need to ask your ISP to reconfigure the modem in case you should switch Ethernet cards (or computers).
Network Diagnostics Tools
There are a number of useful tools that can help you diagnose network problems. We discuss three of them here that are generally helpful; a host of others for diagnosing particular problems are available as well.
ping
The first tool we look at is called ping. ping sends so-called ICMP packets to the server that you specify, the server returns them, and the ping determines the time the round trip took. This is useful to get an idea of the quality of your Internet connection, but we most often use it to see whether we can get a connection somewhere at all. For example, to see whether you have an Internet connection, just ping any computer on the Internet. For example:
kalle@tigger:~> ping www.oreilly.com
PING www.oreilly.com (208.201.239.36) 56(84) bytes of data.
64 bytes from www.oreillynet.com (208.201.239.36): icmp_seq=1 ttl=46 time=280 ms
64 bytes from www.oreillynet.com (208.201.239.36): icmp_seq=2 ttl=46 time=250 ms
64 bytes from www.oreillynet.com (208.201.239.36): icmp_seq=3 ttl=46 time=244 ms
--- www.oreilly.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2001ms
rtt min/avg/max/mdev = 244.976/258.624/280.430/15.586 ms
Notice that we pressed Ctrl-C here after a few seconds—it is not very nice to use the opposite server for this purpose for too long. What can you see from this? Well, first of all, you can see that you are actually able to contact a computer on the Internet. Since you did not type in the numerical IP address, but rather the hostname, you can also see that DNS name resolution worked. The first line of the output shows you the IP address that belonged to www.oreilly.com. In the following lines, you can see for each packet sent how long the trip to the server and back took. Of course, the times reported here are going to differ greatly depending on how far that server is away from you network-wise. Also notice the icmp_seq information. Each packet gets a sequence number, and you should receive all of them back. If you don't, if there are gaps in the sequence, then your connection to that host is flakey, or maybe the host is overloaded and drops packets.
It should also be said that ping is not completely reliable for diagnosing network problems. Getting no ping response may also be due to the server not responding to the ICMP packets—no server is obliged to do so, and some actually don't, in order to reduce their server load and in order to increase security (if you cannot really know that somebody is there, it is difficult to attack that somebody). It is considered good networking practice, though, to answer ping requests.
ping is also interesting to see what does not work. If ping does not answer at all, or only answers with network not