So you've finally managed to setup your network connection, now what? How do you know that it's working? How do you know that you set it up correctly? And just what do you do now that it's setup? Well this chapter is for you.

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(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(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

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

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.

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)

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.

Often network problems stem from a failure of DNS (Domain Name Service) which maps domain names to IP addresses. An easy way to perform quick DNS lookups is the host(1) command. When this is run, your computer will perform a few common DNS lookups and return the results. darkstar:~# host www.slackware.com www.slackware.com is an alias for slackware.com. slackware.com has address 64.57.102.34 slackware.com mail is handled by 1 mail-mx.cwo.com.

More complex DNS lookups can be manually performed with the dig(1) tool. dig is "the meanest dog in the pound" when it comes to troubleshooting DNS issues. With this tool, you can perform virtually any DNS lookup from reverse lookups to A, CNAME, MX, SP, TXT records and more. There are far too many command-line options and lookup types to go into depth here, but the man page lists all the common use cases. darkstar:~# dig @207.69.188.185 www.slackware.com a ; <<>> DiG 9.4.3-P4 <<>> @207.69.188.185 www.slackware.com a ; (1 server found) ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57965 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 0 ;; QUESTION SECTION: ;www.slackware.com. IN A ;; ANSWER SECTION: www.slackware.com. 86400 IN CNAME slackware.com. slackware.com. 86400 IN A 64.57.102.34 ;; AUTHORITY SECTION: slackware.com. 86400 IN NS ns2.cwo.com. slackware.com. 86400 IN NS ns1.cwo.com. ;; Query time: 348 msec ;; SERVER: 207.69.188.185#53(207.69.188.185) ;; WHEN: Sat Jul 3 16:25:10 2010 ;; MSG SIZE rcvd: 105 Let's take a took at the command-line options used above. The @207.69.188.185 argument tells dig what DNS server to query. If it is not specified, dig will simply use whatever servers are listed in /etc/resolv.conf. The a argument at the end is the type of DNS record to lookup. In this case we looked for an "A" record which returned an IPv4 address.

Slackware includes a variety of web browsers. If you're using a graphical desktop, you'll find Firefox, Seamonkey, and others you may already be familiar with, but what about console access? Fortunately, there are a number of capable web browsers here as well.

The oldest console-based web browser included with Slackware is definitely lynx(1), a very capable if somewhat limited web browser. lynx does not support frames, javascript, or pictures; it is strictly a text web browser. Navigation is performed using your keyboard's arrow keys and optionally, a mouse. While it lacks many features that other browsers support, lynx is one of the fastest web browsers you'll ever use for gathering information. For example, the -dump argument sends the formatted web page directly to the console, which can then be piped to other programs.

A more feature-rich alternative is the popular links(1), a console-based web browser that supports frames and has better table rendering than lynx. Like its predecessor, links is navigated with the arrow keys, and the use of a mouse is supported. Unlike, lynx it also includes a handy menu (simply click on the top line with your mouse to activate) and generally formats web pages better.

Unlike the other browsers we've looked at, wget(1) is non-interactive. Rather than display HTTP content, wget downloads it. This takes the "browsing" out of the web browser. Unlike the dump modes of other browsers, wget does not format its downloads; rather it copies the content in its exact form on the web server with all tags and binary data in place. It also supports several recursive options that can effectively mirror online content to your local computer. wget need not operate exclusively on HTTP content; it also supports FTP and several other protocols. darkstar:~# wget ftp://ftp.osuosl.org/pub/slackware/slackware-current/ChangeLog.txt --2010-05-01 13:51:19-- ftp://ftp.osuosl.org/pub/slackware/slackware-current/ChangeLog.txt => `ChangeLog.txt' Resolving ftp.osuosl.org... 64.50.236.52 Connecting to ftp.osuosl.org|64.50.236.52|:21... connected. Logging in as anonymous ... Logged in! ==> SYST ... done. ==> PWD ... done. ==> TYPE I ... done. ==> CWD /pub/slackware/slackware-current ... done. ==> SIZE ChangeLog.txt ... 75306 ==> PASV ... done. ==> RETR ChangeLog.txt ... done. Length: 75306 (74K) 100%[======================================>] 75,306 110K/s in 0.7s 2010-05-01 13:51:22 (110 KB/s) - `ChangeLog.txt' saved [75306]

Lots of data is stored on FTP servers the world over. In fact, Slackware Linux was first publically offered via FTP and continues to be distributed in this fashion today. Most open source software can be downloaded in source code or binary form via FTP, so knowing how to retrieve this information is a handy skill.

The simplest FTP client included with Slackware is named simply, ftp(1) and is a reliable if somewhat simple means of sending and retrieving data. ftp connects to an FTP server, asks for your username and password, and then allows you to put or get data to and from that server. ftp has fallen out of favor with more experienced users do to a lack of features, but remains a handy tool, and much of the documentation you see online will refer you to it. Once an FTP session has been initialized, you'll be placed at a prompt somewhat like a shell. From here you can change and list directories using the "cd" and "ls" commands, just like a shell. Additionally, you may issue the "put" command to send a file to the server, or a "get" command to retrieve data from the server. If you're connecting to a public FTP server, you'll want to use the "anonymous" username and simply enter your e-mail address (or a fake one) for the password. darkstar:~# ftp ftp.osuosl.org Name (ftp.osuosl.org:alan): anonymous 331 Please specify the password. Password: secret 230 Login successful. Remote system type is UNIX. Using binary mode to transfer files. ftp> cd pub/slackware/slackware-current/ 250 Directory successfully changed. ftp> get ChangeLog.txt local: ChangeLog.txt remote: ChangeLog.txt 200 PORT command successful. Consider using PASV. 150 Opening BINARY mode data connection for ChangeLog.txt (33967 bytes). 226 File send OK. 33967 bytes received in 0.351 secs (94 Kbytes/sec) ftp> bye 221 Goodbye.

ncftp(1) (pronounced nick-f-t-p), is a more feature rich successor to ftp, supporting tab completion and recursive retrieval. It automatically connects to a server as the anonymous user, unless you specify a different username on the commandline with the -u argument. The primary advantage over ftp is the ability to send and retrieve multiple files at once with the "mput" and "mget" commands. If you pass the -R argument to either of them, they will recursively put or get data from directories. darkstar:~# ncftp ftp.osuosl.org Logging in... Login successful. Logged in to ftp.osuosl.org. ncftp / > cd pub/slackware/slackware-current Directory successfully changed. ncftp ...ware/slackware-current > mget -R isolinux isolinux/README.TXT: 4.63 kB 16.77 kB/s isolinux/README_SPLIT.TXT: 788.00 B 5.43 kB/s isolinux/f2.txt: 793.00 B 5.68 kB/s isolinux/initrd.img: 13.75 MB 837.91 kB/s isolinux/iso.sort: 50.00 B 354.50 B/s isolinux/isolinux.bin: 14.00 kB 33.99 kB/s isolinux/isolinux.cfg: 487.00 B 3.30 kB/s isolinux/message.txt: 760.00 B 5.32 kB/s isolinux/setpkg: 2.76 kB 19.11 kB/s ncftp ...ware/slackware-current > bye

The last client we're going to look at is lftp(1). Like ncftp, it supports tab completion and recursive activity, but has a more friendly license. Rather than user "mget" and "mput", all recursive operations are handled with the "mirror" command. "mirror" has many different options available, so I'll have to refer you to the man page and the built-in "help" command for complete details. darkstar:~# lftp ftp.osuosl.org lftp ftp.osuosl.org:~> cd /pub/slackware/slackware-current cd ok, cwd=/pub/slackware/slackware-current lftp ftp.osuosl.org:/pub/slackware/slackware-current> mirror isolinux Total: 2 directories, 16 files, 1 symlink New: 16 files, 1 symlink 14636789 bytes transferred in 20 seconds (703.7K/s) lftp ftp.osuosl.org:/pub/slackware/slackware-current> bye

Ready to see something cool? Have you ever found yourself needing just a handful of files from a large directory, but you're not entirely sure which files you already have and which ones you need? You can download the entire directory again, but that's duplicating a lot of work. You can pick and chose, manually check everything, but that's very tedious. Perhaps you've downloaded a large file such as an ISO, but something went wrong with the download? It doesn't make sense that you should have to pull down the entire file again if only a few bits have been corrupted. Enter rsync(1), a fast and versatile copying tool for local and remote files. rsync uses a handful of simple, but very effective techniques to determine what needs to be changed. By checking file size and time stamps, it can determine if two files are different. If something has changed, it can determine what bytes are different, and simply download that handfull of data rather than an entire file. It is truly a marvel of modern technology. In its simplist form, rsync connects to an rsync protocol server and downloads a list of files and directories, along with their sizes, timestamps, and other information. It then compares this to the local files (if any) to determine what it needs to transfer. Only files that are different will be synced. Additionally, it breaks up large files into smaller chunks and compares those chunks using a quick and simple hash function. Any chunks that match are not transferred, so the amount of data that must be copied can be dramatically reduced. rsync also supports compression, verbose output, file deletion, permission handling, and many other options. For a complete list, you'll need to refer to the man page, but I've included a small table of some of the more common options. ArgumentExplaination -v Increased verbosity -c Checksum all files rather than relying on file size and timestamp -a Archive mode (equivilant to -rlptgoD) -e Specify a remote shell to use -r Recursive mode -u Update - skip files that are newer on the receiving end -p Preserve permissions -n Dry-run - perform a trial run without making any changes -z Compress - handy for slow network connections Due to the power and versatility of rsync, it can be invoked in a number of ways. The following two examples connect to an rsync protocol server to retrieve some information and to another server via ssh to encrypt the transmission. darkstar:~# rsync -avz rsync://ftp.osuosl.org/pub/slackware/slackware-current/ \ /src/slackware-current/ darkstar:~# rsync -e ssh ftp.slackware.com:/home/alan/foo /tmp/foo