Basic Networking Commands

Network Diagnostic Tools Slackware Linux includes a great many networking tools for troubleshooting and diagnosing network connection troubles, or just for seeing what's out there on the network. Most of these tools are command-line tools, so you can run them from a virtual terminal or in a console window on your graphical desktop. A few of them even have graphical front-ends, but we're going to deal almost exclusively with command-line tools for now.

ping ping(8) is a handy tool for determining if a computer is operational on your network or on the Internet at large. You can think of as a type of sonar for computers. By using it, you send out a "ping" and listen for an echo to determine if another computer or network device is listening. By default, ping checks for the remote computer once per second indefinitely, but you can change the interval between checks and the total number of checks easily, just check the man page. You can terminate the application at any time with CTRL-c. When ping is finished, it displays a handy summary of its activity. ping is very useful for determining if a computer on your network or the Internet is available, but some systems block the packets ping sends, so sometimes a system may be functioning properly, but still not send replies. darkstar:~# ping -c 3 www.slackware.com 64 bytes from slackware.com (64.57.102.34): icmp_seq=1 ttl=47 time=87.1 ms 64 bytes from slackware.com (64.57.102.34): icmp_seq=2 ttl=47 time=86.2 ms 64 bytes from slackware.com (64.57.102.34): icmp_seq=3 ttl=47 time=86.7 ms --- slackware.com ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2004ms rtt min/avg/max/mdev = 86.282/86.718/87.127/0.345 ms

traceroute traceroute(8) is a handy tool for determining what route your packets take to reach some other computer. It's mainly of use for determining which computers are "near" or "far" from you. This distance isn't strictly geographical, as your Internet Service Provider may route traffic from your computer in strange ways. traceroute shows you each router between your computer and any other machine you wish to connect to. Unfortunately, many providers, firewalls, and routers will block traceroute so you might not get a complete picture when using it. Still, it remains a handy tool for network troubleshooting. darkstar:~# traceroute www.slackware.com traceroute to slackware.com (64.57.102.34), 30 hops max, 46 byte packets 1 gw.ctsmacon.com (192.168.1.254) 1.468 ms 2.045 ms 1.387 ms 2 10.0.0.1 (10.0.0.1) 7.642 ms 8.019 ms 6.006 ms 3 68.1.8.49 (68.1.8.49) 10.446 ms 9.739 ms 7.003 ms 4 68.1.8.69 (68.1.8.69) 11.564 ms 6.235 ms 7.971 ms 5 dalsbbrj01-ae0.r2.dl.cox.net (68.1.0.142) 43.859 ms 43.287 ms 44.125 ms 6 dpr1-ge-2-0-0.dallasequinix.savvis.net (204.70.204.146) 41.927 ms 58.247 ms 44.989 ms 7 cr2-tengige0-7-5-0.dallas.savvis.net (204.70.196.29) 42.577 ms 46.110 ms 43.977 ms 8 cr1-pos-0-3-3-0.losangeles.savvis.net (204.70.194.53) 78.070 ms 76.735 ms 76.145 ms 9 bpr1-ge-3-0-0.LosAngeles.savvis.net (204.70.192.222) 77.533 ms 108.328 ms 120.096 ms 10 wiltel-communications-group-inc.LosAngeles.savvis.net (208.173.55.186) 79.607 ms 76.847 ms 75.998 ms 11 tg9-4.cr01.lsancarc.integra.net (209.63.113.57) 84.789 ms 85.436 ms 85.575 ms 12 tg13-1.cr01.sntdcabl.integra.net (209.63.113.106) 87.608 ms 84.278 ms 86.922 ms 13 tg13-4.cr02.sntdcabl.integra.net (209.63.113.134) 87.284 ms 85.924 ms 86.102 ms 14 tg13-1.cr02.rcrdcauu.integra.net (209.63.114.169) 85.578 ms 85.285 ms 84.148 ms 15 209.63.99.166 (209.63.99.166) 84.515 ms 85.424 ms 85.956 ms 16 208.186.199.158 (208.186.199.158) 86.557 ms 85.822 ms 86.072 ms 17 sac-main.cwo.com (209.210.78.20) 88.105 ms 87.467 ms 87.526 ms 18 slackware.com (64.57.102.34) 85.682 ms 86.322 ms 85.594 ms

telnet Once upon a time, telnet(1) was the greatest thing since sliced bread. Basically, telnet opens an unencrypted network connection between two computers and hands control of the session to the user rather than some other application. Using telnet, people could connect to shells on other computers and execute commands as if they were physically present. Due to its unencrypted nature this is no longer recommended; however, telnet is still used for this purpose by many devices. Today, telnet is put to better use as a network diagnostic tool. Because it passes control of the session directly to the user, it can be used for a great variety of testing purposes. As long as you know what ASCII commands to send to the receiving computer, you can do any number of activies, such as read web pages or check your e-mail. Simply inform telnet what network port to use, and you're all set. darkstar:~# telnet www.slackware.com 80 Trying 64.57.102.34... Connected to www.slackware.com. Escape character is '^]'. HEAD / HTTP/1.1 Host: www.slackware.com HTTP/1.1 200 OK Date: Thu, 04 Feb 2010 18:01:35 GMT Server: Apache/1.3.27 (Unix) PHP/4.3.1 Last-Modified: Fri, 28 Aug 2009 01:30:27 GMT ETag: "61dc2-5374-4a973333" Accept-Ranges: bytes Content-Length: 21364 Content-Type: text/html

ssh As we mentioned, telnet may be useful as a diagnostic tool, but its unencrypted nature makes it a security concern for shell access. Thankfully, there's the secure shell protocol. Nearly every Linux, UNIX, and BSD distribution today makes use of OpenSSH, or ssh(1) for short. It is one of the most commonly used network tools today and makes use of the strongest cryptographic techniques. ssh has many features, configuration options, and neat hacks, enough to fill its own book, so we'll only go into the basics here. Simply run ssh with the user name and the host and you'll be connected to it quickly and safely. If this is the first time you are connecting to this computer, ssh will ask you to confirm your desire, and make a local copy of the encryption key to use. Should this key later change, ssh will warn you and refuse to connect because it is possible that some one is attempting to hijack the connection using what is known as a man-in-the-middle attack. darkstar:~# ssh alan@slackware.com alan@slackware.com's password: secret alan@slackware.com:~$ The user and hostname are in the same form used by e-mail addresses. If you leave off the username part, ssh will use your current username when establishing the connection.

tcpdump So far all the tools we've looked at have focused on making connections to other computers, but now we're going to look at the traffic itself. tcpdump(1) (which must be run as root) allows us to view all or part of the network traffic originating or received by our computer. tcpdump displays the raw data packets in a variety of ways with all the network headers intact. Don't be alarmed if you don't understand everything it displays, tcpdump is a tool for professional network engineers and system administrators. By default, it probes the first network card it finds, but if you have multiple interfaces, simply use the -i argument to specify which one you're interested in. You can also limit the data displayed using expressions and change the manner in which it is displayed, but that is best explained by the man page and other reference material. darkstar:~# tcpdump -i wlan0 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on wlan0, link-type EN10MB (Ethernet), capture size 96 bytes 13:22:28.221985 IP gw.ctsmacon.com.microsoft-ds > 192.168.1.198.59387: Flags [P.], ack 838190560, win 3079, options [nop,nop,TS val 1382697489 ecr 339048583], length 164WARNING: Short packet. Try increasing the snap length by 140 SMB PACKET: SMBtrans2 (REPLY) 13:22:28.222392 IP 192.168.1.198.59387 > gw.ctsmacon.com.microsoft-ds: Flags [P.], ack 164, win 775, options [nop,nop,TS val 339048667 ecr 1382697489], length 134WARNING: Short packet. Try increasing the snap length by 110 SMB PACKET: SMBtrans2 (REQUEST)

nmap Suppose you need to know what network services are running on a machine, or multiple machines, or you wish to determine if multiple machines are responsive? You could ping each one individually, telnet to each port you're interested in, and note every detail, but that's very tedious and time consuming. A much easier alternative is to use a port scanner, and nmap(1) is just the tool for the job. nmap is capable of scanning TCP and UDP ports, determining the operating system of a network device, probing each located service to determine its specific type, and much much more. Perhaps the simplist way to use nmap is to "ping" multiple computers at once. You can use network address notation (CIDR) or specify a range of addresses and nmap will scan every one and return the results to you when it's finished. You can even specify host names as you like. In order to "ping" hosts, you'll have to use the -sP argument. The following command instructs nmap to "ping" www.slackware.com and the 16 IP addresses starting at 72.168.24.0 and ending at 72.168.24.15. darkstar:~# nmap -sP www.slackware.com 72.168.24.0/28 Should you need to perform a port scan, nmap has many options for doing just that. When run without any arguments, nmap performs a standard TCP port scan on all hosts specified. There are also options to make nmap more or less aggressive with its scanning to return results quicker or fool intrusion detection services. For a full discussion, you should refer to the rather exhaustive man page. The following three commands perform a regular port scan, a SYN scan, and a "Christmas tree" scan. darkstar:~# nmap www.example.com darkstar:~# nmap -sS www.example.com darkstar:~# nmap -sX www.example.com Be warned! Some Internet Service Providers frown heavily on port scanning and may take measures to prevent you from doing it. nmap and applications like it are best used on your own systems for maintenance and security purposes, not as general purpose Internet scanners.