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12.6. Communications CommandsCertain of the following commands find use in chasing spammers, as well as in
network data transfer and analysis. Information and Statistics - host
Searches for information about an Internet host by name or
IP address, using DNS. bash$ host surfacemail.com
surfacemail.com. has address 202.92.42.236
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- ipcalc
Displays IP information for a host.
With the -h option,
ipcalc does a reverse DNS lookup, finding
the name of the host (server) from the IP address. bash$ ipcalc -h 202.92.42.236
HOSTNAME=surfacemail.com
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- nslookup
Do an Internet "name server lookup"
on a host by IP address. This is essentially equivalent
to ipcalc -h or dig -x
. The command may be run either interactively
or noninteractively, i.e., from within a script. The nslookup command has allegedly
been "deprecated," but it still has its
uses. bash$ nslookup -sil 66.97.104.180
nslookup kuhleersparnis.ch
Server: 135.116.137.2
Address: 135.116.137.2#53
Non-authoritative answer:
Name: kuhleersparnis.ch
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- dig
Similar to nslookup, do an Internet
"name server lookup" on a host. May be run
either interactively or noninteractively, i.e., from within
a script. Compare the output of dig -x with
ipcalc -h and
nslookup. bash$ dig -x 81.9.6.2
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;2.6.9.81.in-addr.arpa. IN PTR
;; AUTHORITY SECTION:
6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.
2002031705 900 600 86400 3600
;; Query time: 537 msec
;; SERVER: 135.116.137.2#53(135.116.137.2)
;; WHEN: Wed Jun 26 08:35:24 2002
;; MSG SIZE rcvd: 91
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- traceroute
Trace the route taken by packets sent to a remote host. This
command works within a LAN, WAN, or over the
Internet. The remote host may be specified by an IP
address. The output of this command may be filtered
by grep or sed in a pipe. bash$ traceroute 81.9.6.2
traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms
2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms
3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms *
...
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- ping
Broadcast an "ICMP ECHO_REQUEST" packet to
another machine, either on a local or remote network. This
is a diagnostic tool for testing network connections,
and it should be used with caution. A successful ping returns an exit status of
0. This can be tested for in a
script. bash$ ping localhost
PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
Warning: time of day goes back, taking countermeasures.
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec
--- localhost.localdomain ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms
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- whois
Perform a DNS (Domain Name System) lookup.
The -h option permits specifying which
particular whois server to query. See
Example 4-6. - finger
Retrieve information about users on a
network. Optionally, this command can display
a user's ~/.plan,
~/.project, and
~/.forward files, if present. bash$ finger
Login Name Tty Idle Login Time Office Office Phone
bozo Bozo Bozeman tty1 8 Jun 25 16:59
bozo Bozo Bozeman ttyp0 Jun 25 16:59
bozo Bozo Bozeman ttyp1 Jun 25 17:07
bash$ finger bozo
Login: bozo Name: Bozo Bozeman
Directory: /home/bozo Shell: /bin/bash
Office: 2355 Clown St., 543-1234
On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle
On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle
On since Fri Aug 31 20:13 (MST) on pts/1
On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle
No mail.
No Plan.
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Out of security considerations, many networks disable
finger and its associated daemon.
- chfn
Change information disclosed by the
finger command. - vrfy
Verify an Internet e-mail address.
Remote Host Access - sx, rx
The sx and rx
command set serves to transfer files to and from a remote
host using the xmodem protocol. These
are generally part of a communications package, such as
minicom. - sz, rz
The sz and rz
command set serves to transfer files to and from a remote
host using the zmodem protocol.
Zmodem has certain advantages over
xmodem, such as faster transmission
rate and resumption of interrupted file transfers.
Like sx and rx,
these are generally part of a communications package. - ftp
Utility and protocol for uploading / downloading
files to or from a remote host. An ftp session can be automated
in a script (see Example 17-6, Example A-5, and Example A-14). - uucp
Unix to Unix copy. This is a
communications package for transferring files between Unix
servers. A shell script is an effective way to handle a
uucp command sequence. Since the advent of the Internet and e-mail,
uucp seems to have faded into obscurity,
but it still exists and remains perfectly workable in
situations where an Internet connection is not available
or appropriate. - cu
Call Up
a remote system and connect as a simple terminal. This
command is part of the uucp package. It
is a sort of dumbed-down version of telnet. - telnet
Utility and protocol for connecting to a remote host.  | The telnet protocol contains security holes and
should therefore probably be avoided. |
- wget
The wget utility
non-interactively retrieves or
downloads files from a Web or ftp site. It works well in a
script.
wget -p http://www.xyz23.com/file01.html
wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -o $SAVEFILE |
- lynx
The lynx Web and file browser
can be used inside a script (with the
-dump option) to retrieve a file from a Web or
ftp site non-interactively.
lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE |
- rlogin
Remote login, initates a
session on a remote host. This command has security issues,
so use ssh instead. - rsh
Remote shell, executes
command(s) on a remote host. This has security issues,
so use ssh instead. - rcp
Remote copy, copies files
between two different networked machines. Using
rcp and similar utilities with
security implications in a shell script may not be
advisable. Consider, instead, using ssh
or an expect script. - ssh
Secure shell, logs onto
a remote host and executes commands there. This
secure replacement for telnet,
rlogin, rcp, and
rsh uses identity authentication
and encryption. See its manpage
for details.
Local Network - write
This is a utility for terminal-to-terminal communication.
It allows sending lines from your terminal (console
or xterm) to that of another user. The mesg command may, of course,
be used to disable write access to a terminal Since write is interactive, it
would not normally find use in a script.
Mail - mail
Send or read e-mail messages. This stripped-down command-line mail client
works fine as a command embedded in a script. Example 12-34. A script that mails itself #!/bin/sh
# self-mailer.sh: Self-mailing script
adr=${1:-`whoami`} # Default to current user, if not specified.
# Typing 'self-mailer.sh wiseguy@superdupergenius.com'
#+ sends this script to that addressee.
# Just 'self-mailer.sh' (no argument) sends the script
#+ to the person invoking it, for example, bozo@localhost.localdomain.
#
# For more on the ${parameter:-default} construct,
#+ see the "Parameter Substitution" section
#+ of the "Variables Revisited" chapter.
# ============================================================================
cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"
# ============================================================================
# --------------------------------------------
# Greetings from the self-mailing script.
# A mischievous person has run this script,
#+ which has caused it to mail itself to you.
# Apparently, some people have nothing better
#+ to do with their time.
# --------------------------------------------
echo "At `date`, script \"`basename $0`\" mailed to "$adr"."
exit 0 |
- mailto
Similar to the mail command,
mailto sends e-mail messages
from the command line or in a script. However,
mailto also permits sending MIME
(multimedia) messages. - vacation
This utility automatically replies to e-mails that
the intended recipient is on vacation and temporarily
unavailable. This runs on a network, in conjunction with
sendmail, and is not applicable to a
dial-up POPmail account.
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