Basic Networking Utilities
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.
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.
host
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.
dig
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.
finger
finger(1) isn't exactly a network diagnostic
tool as much as it is a network-user diagnostic tool. Using
finger, you can gather a handful of useful
information about users on servers running the
fingerd(8) daemon. Today very few servers
still offer fingerd, but for those that do
it can be a useful tool for keeping track of your friends and
co-workers.
darkstar:~$ finger alan@cardinal.lizella.net
[cardinal.lizella.net]
Login: alan Name: Alan Hicks
Directory: /home/alan Shell: /bin/bash
Office: 478 808 9919, 478 935 8133
On since Wed Apr 13 17:43 (UTC) on pts/9 from
75-150-12-113-atlanta.hfc.comcastbusiness.net
32 minutes 24 seconds idle
(messages off)
On since Wed Apr 13 17:45 (UTC) on pts/10 from :pts/9:S.0
48 minutes 56 seconds idle
Mail forwarded to alan@lizella.net
No mail.
No Plan.
Web Browsers
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.
lynx
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.
links
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.
wget
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]
Mail Clients
pine
pine is one of the oldest command-line
interface mail clients still in existance and remains one of the most
user-friendly. pine was created by the
University of Washington and carries with it both a trademark and a
copyright license that are difficult to work with. Thankfully back in
2005, the university saw fit to re-write it without the trademark and
with a more open license, so alpine(1) was
born. This is the pine-clone distributed with Slackware. To start using
it, simply type pine at the command line.
Using it is very simple due to its menu-driven system as well as the
command referance neatly located at the bottom of the screen. See for
yourself.
ALPINE 2.00 MAIN MENU Folder: INBOX No
Messages
? HELP - Get help using Alpine
C COMPOSE MESSAGE - Compose and send a message
I MESSAGE INDEX - View messages in current folder
L FOLDER LIST - Select a folder to view
A ADDRESS BOOK - Update address book
S SETUP - Configure Alpine Options
Q QUIT - Leave the Alpine program
Copyright 2006-2008 University of Washington
[Folder "INBOX" opened with 0 messages]
? Help P PrevCmd R RelNotes
O OTHER CMDS > [ListFldrs] N NextCmd K KBLock
mutt
Some people don't like pine. Some people
want more control. Some people want a fully-configurable mail client
with plugin support and a no-nonsense attitude. Those people use
mutt(1). mutt
isn't as user friendly as pine, but makes up
for it with power. You won't find the user-friendly command referance
at the bottom of the screen, mutt uses every
last inch of real-estate for mail processing duty. It's feature support
is extensive: threaded displays are no problem for the mighty
mixed-breed! You can configure mutt with a
.muttrc file in your home directory. With all the
many different possible configuration options, there's even a man page
for that, muttrc(5). You might want to read up on it.
3172 N Jan 17 Thomas Morper (2.8K) │ └─>Re: [Slackbuilds-users] Exim Sbo
3173 N Jan 17 TuxaneMedia (2.5K) └─>Re: [Slackbuilds-users] Exim Sbo
3174 N Jan 06 Uli Sch?fer (4.6K) [Slackbuilds-users] mrtg-2.15.2 breaks wi
3175 N Jan 06 Willy Sudiarto (4.0K) └─>
3176 N Jan 06 Audrius Ka??uka (3.9K) └─>
3177 N Jan 06 Niels Horn (4.4K) └─>
3178 N * Jan 09 King Beowulf (6.5K) [Slackbuilds-users] libreoffice (3.3.0rc
3179 N * Jan 09 Niels Horn (4.8K) └─>Re: [Slackbuilds-users] libreoffice (3
3180 * Jan 09 Niels Horn ( 73) └─>
3181 N * Jan 09 King Beowulf (5.9K) └─>
3182 N Jan 10 Robby Workman (7.8K) [Slackbuilds-users] Updates - 20110110
3183 N Jan 10 B Watson (3.6K) [Slackbuilds-users] Fwd: nted slackbuild
3184 N Jan 11 SlackBuilds@cat (2.5K) [Slackbuilds-users] /usr/share/ package s
3185 N Jan 11 Robby Workman (3.4K) └─>
3186 N Jan 12 SlackBuilds@cat (3.4K) └─>
3187 N Jan 11 Robby Workman (4.5K) └─>
3188 N Jan 12 Robby Workman (2.3K) [Slackbuilds-users] Unavailability of mai
3189 N Jan 13 Mr. B-o-B (3.7K) └─>
3190 N Jan 13 Josiah Boothby (3.2K) [Slackbuilds-users] Ted's Word Processor
3191 N Jan 14 Adam Swift (7.4K) [Slackbuilds-users] Web submission failur
3192 N Jan 14 Rob McGee (3.5K) └─>
mailx
So those are great and everything, but what if you just want a mail
client that isn't menu-driven? Thankfully
mailx is here to save you.
FTP Clients
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.
ftp
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
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
lftp
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
rsync
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.
rsync Arguments
Argument
Explaination
-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