Certain of the following commands find use in network data transfer and analysis, as well as in chasing spammers.
Searches for information about an Internet host by name or IP address, using DNS.
bash$host surfacemail.comsurfacemail.com. has address 202.92.42.236
Displays IP information for a host.
With the -h option,
ipcalc does a reverse DNS lookup, finding
the name of the host (server) from the IP address.
bash$ipcalc -h 202.92.42.236HOSTNAME=surfacemail.com
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 is still useful.
bash$nslookup -sil 66.97.104.180nslookup kuhleersparnis.ch Server: 135.116.137.2 Address: 135.116.137.2#53 Non-authoritative answer: Name: kuhleersparnis.ch
Domain Information Groper. Similar to nslookup, dig does an Internet name server lookup on a host. May be run from the command-line or from within a script.
Some interesting options to dig are
+time=N for setting a query timeout to
N seconds, +nofail for
continuing to query servers until a reply is received, and
-x for doing a reverse address lookup.
Compare the output of dig -x with ipcalc -h and nslookup.
bash$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
Example 16.40. Finding out where to report a spammer
#!/bin/bash # spam-lookup.sh: Look up abuse contact to report a spammer. # Thanks, Michael Zick. # Check for command-line arg. ARGCOUNT=1 E_WRONGARGS=85 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` domain-name" exit $E_WRONGARGS fi dig +short $1.contacts.abuse.net -c in -t txt # Also try: # dig +nssearch $1 # Tries to find "authoritative name servers" and display SOA records. # The following also works: # whois -h whois.abuse.net $1 # ^^ ^^^^^^^^^^^^^^^ Specify host. # Can even lookup multiple spammers with this, i.e." # whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . . # Exercise: # -------- # Expand the functionality of this script #+ so that it automatically e-mails a notification #+ to the responsible ISP's contact address(es). # Hint: use the "mail" command. exit $? # spam-lookup.sh chinatietong.com # A known spam domain. # "crnet_mgr@chinatietong.com" # "crnet_tec@chinatietong.com" # "postmaster@chinatietong.com" # For a more elaborate version of this script, #+ see the SpamViz home page, http://www.spamviz.net/index.html.
Example 16.41. Analyzing a spam domain
#! /bin/bash
# is-spammer.sh: Identifying spam domains
# $Id$
# Above line is RCS ID info.
#
# This is a simplified version of the "is_spammer.bash
#+ script in the Contributed Scripts appendix.
# is-spammer <domain.name>
# Uses an external program: 'dig'
# Tested with version: 9.2.4rc5
# Uses functions.
# Uses IFS to parse strings by assignment into arrays.
# And even does something useful: checks e-mail blacklists.
# Use the domain.name(s) from the text body:
# http://www.good_stuff.spammer.biz/just_ignore_everything_else
# ^^^^^^^^^^^
# Or the domain.name(s) from any e-mail address:
# Really_Good_Offer@spammer.biz
#
# as the only argument to this script.
#(PS: have your Inet connection running)
#
# So, to invoke this script in the above two instances:
# is-spammer.sh spammer.biz
# Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
# No Whitespace == Line Feed:Carriage Return
No_WSP=$'\x0A'$'\x0D'
# Field separator for dotted decimal ip addresses
ADR_IFS=${No_WSP}'.'
# Get the dns text resource record.
# get_txt <error_code> <list_query>
get_txt() {
# Parse $1 by assignment at the dots.
local -a dns
IFS=$ADR_IFS
dns=( $1 )
IFS=$WSP_IFS
if [ "${dns[0]}" == '127' ]
then
# See if there is a reason.
echo $(dig +short $2 -t txt)
fi
}
# Get the dns address resource record.
# chk_adr <rev_dns> <list_server>
chk_adr() {
local reply
local server
local reason
server=${1}${2}
reply=$( dig +short ${server} )
# If reply might be an error code . . .
if [ ${#reply} -gt 6 ]
then
reason=$(get_txt ${reply} ${server} )
reason=${reason:-${reply}}
fi
echo ${reason:-' not blacklisted.'}
}
# Need to get the IP address from the name.
echo 'Get address of: '$1
ip_adr=$(dig +short $1)
dns_reply=${ip_adr:-' no answer '}
echo ' Found address: '${dns_reply}
# A valid reply is at least 4 digits plus 3 dots.
if [ ${#ip_adr} -gt 6 ]
then
echo
declare query
# Parse by assignment at the dots.
declare -a dns
IFS=$ADR_IFS
dns=( ${ip_adr} )
IFS=$WSP_IFS
# Reorder octets into dns query order.
rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'
# See: http://www.spamhaus.org (Conservative, well maintained)
echo -n 'spamhaus.org says: '
echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')
# See: http://ordb.org (Open mail relays)
echo -n ' ordb.org says: '
echo $(chk_adr ${rev_dns} 'relays.ordb.org')
# See: http://www.spamcop.net/ (You can report spammers here)
echo -n ' spamcop.net says: '
echo $(chk_adr ${rev_dns} 'bl.spamcop.net')
# # # other blacklist operations # # #
# See: http://cbl.abuseat.org.
echo -n ' abuseat.org says: '
echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')
# See: http://dsbl.org/usage (Various mail relays)
echo
echo 'Distributed Server Listings'
echo -n ' list.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'list.dsbl.org')
echo -n ' multihop.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')
echo -n 'unconfirmed.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')
else
echo
echo 'Could not use that address.'
fi
exit 0
# Exercises:
# --------
# 1) Check arguments to script,
# and exit with appropriate error message if necessary.
# 2) Check if on-line at invocation of script,
# and exit with appropriate error message if necessary.
# 3) Substitute generic variables for "hard-coded" BHL domains.
# 4) Set a time-out for the script using the "+time=" option
to the 'dig' command.
For a much more elaborate version of the above script, see Example A.28, “Spammer Identification”.
Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe.
bash$traceroute 81.9.6.2traceroute 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 * ...
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.
bash$ping localhostPING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec --- localhost.localdomain ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms
A successful ping returns an exit status of 0. This can be tested for in a script.
HNAME=news-15.net # Notorious spammer. # HNAME=$HOST # Debug: test for localhost. count=2 # Send only two pings. if [[ `ping -c $count "$HNAME"` ]] then echo ""$HNAME" still up and broadcasting spam your way." else echo ""$HNAME" seems to be down. Pity." fi
Perform a DNS (Domain Name System) lookup.
The -h option permits specifying which
particular whois server to query. See
Example 4.6, “wh,
whois domain name lookup” and Example 16.40, “Finding out where to report a spammer”.
Retrieve information about users on a
network. Optionally, this command can display
a user's ~/.plan,
~/.project, and
~/.forward files, if present.
bash$fingerLogin Name Tty Idle Login Time Office Office Phone bozo Bozo Bozeman tty1 8 Jun 25 16:59 (:0) bozo Bozo Bozeman ttyp0 Jun 25 16:59 (:0.0) bozo Bozo Bozeman ttyp1 Jun 25 17:07 (:0.0)bash$finger bozoLogin: 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 Mail last read Tue Jul 3 10:08 2007 (MST) No Plan.
Out of security considerations, many networks disable finger and its associated daemon. [81]
Change information disclosed by the finger command.
Verify an Internet e-mail address.
This command seems to be missing from newer Linux distros.
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.
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.
Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see Example 19.6, “Upload a file pair to Sunsite incoming directory” and Example A.4, “encryptedpw: Uploading to an ftp site, using a locally encrypted password”).
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. The advantage of uucp is that it is fault-tolerant, so even if there is a service interruption the copy operation will resume where it left off when the connection is restored.
---
uux: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the uucp package.
---
cu: Call Up a remote system and connect as a simple terminal. It is a sort of dumbed-down version of telnet. This command is part of the uucp package.
Utility and protocol for connecting to a remote host.
The telnet protocol contains security holes and should therefore probably be avoided. Its use within a shell script is not recommended.
The wget utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in a script.
wget -p http://www.xyz23.com/file01.html # The -p or --page-requisite option causes wget to fetch all files #+ required to display the specified page. wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE # The -r option recursively follows and retrieves all links #+ on the specified site. wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2 # The -c option lets wget resume an interrupted download. # This works with ftp servers and many HTTP sites.
Example 16.42. Getting a stock quote
#!/bin/bash
# quote-fetch.sh: Download a stock quote.
E_NOPARAMS=86
if [ -z "$1" ] # Must specify a stock (symbol) to fetch.
then echo "Usage: `basename $0` stock-symbol"
exit $E_NOPARAMS
fi
stock_symbol=$1
file_suffix=.html
# Fetches an HTML file, so name it appropriately.
URL='http://finance.yahoo.com/q?s='
# Yahoo finance board, with stock query suffix.
# -----------------------------------------------------------
wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
# -----------------------------------------------------------
# To look up stuff on http://search.yahoo.com:
# -----------------------------------------------------------
# URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"
# wget -O "$savefilename" "${URL}"
# -----------------------------------------------------------
# Saves a list of relevant URLs.
exit $?
# Exercises:
# ---------
#
# 1) Add a test to ensure the user running the script is on-line.
# (Hint: parse the output of 'ps -ax' for "ppp" or "connect."
#
# 2) Modify this script to fetch the local weather report,
#+ taking the user's zip code as an argument.
See also Example A.30, “Making wget easier to use” and Example A.31, “A podcasting script”.
The lynx Web and file browser
can be used inside a script (with the
-dump option) to retrieve a file from a Web or
ftp site noninteractively.
lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE
With the -traversal option,
lynx starts at the HTTP URL specified
as an argument, then “crawls” through all
links located on that particular server. Used together
with the -crawl option, outputs page text
to a log file.
Remote login, initates a
session on a remote host. This command has security issues,
so use ssh instead.
Remote shell, executes
command(s) on a remote host. This has security issues,
so use ssh instead.
Remote copy, copies files
between two different networked machines.
Remote synchronize, updates
(synchronizes) files
between two different networked machines.
bash$rsync -a ~/sourcedir/*txt /node1/subdirectory/
Example 16.43. Updating FC4
#!/bin/bash
# fc4upd.sh
# Script author: Frank Wang.
# Slight stylistic modifications by ABS Guide author.
# Used in ABS Guide with permission.
# Download Fedora Core 4 update from mirror site using rsync.
# Should also work for newer Fedora Cores -- 5, 6, . . .
# Only download latest package if multiple versions exist,
#+ to save space.
URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/
# URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
# URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/
DEST=${1:-/var/www/html/fedora/updates/}
LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt
PID_FILE=/var/run/${0##*/}.pid
E_RETURN=85 # Something unexpected happened.
# General rsync options
# -r: recursive download
# -t: reserve time
# -v: verbose
OPTS="-rtv --delete-excluded --delete-after --partial"
# rsync include pattern
# Leading slash causes absolute path name match.
INCLUDE=(
"/4/i386/kde-i18n-Chinese*"
# ^ ^
# Quoting is necessary to prevent globbing.
)
# rsync exclude pattern
# Temporarily comment out unwanted pkgs using "#" . . .
EXCLUDE=(
/1
/2
/3
/testing
/4/SRPMS
/4/ppc
/4/x86_64
/4/i386/debug
"/4/i386/kde-i18n-*"
"/4/i386/openoffice.org-langpack-*"
"/4/i386/*i586.rpm"
"/4/i386/GFS-*"
"/4/i386/cman-*"
"/4/i386/dlm-*"
"/4/i386/gnbd-*"
"/4/i386/kernel-smp*"
# "/4/i386/kernel-xen*"
# "/4/i386/xen-*"
)
init () {
# Let pipe command return possible rsync error, e.g., stalled network.
set -o pipefail # Newly introduced in Bash, version 3.
TMP=${TMPDIR:-/tmp}/${0##*/}.$$ # Store refined download list.
trap "{
rm -f $TMP 2>/dev/null
}" EXIT # Clear temporary file on exit.
}
check_pid () {
# Check if process exists.
if [ -s "$PID_FILE" ]; then
echo "PID file exists. Checking ..."
PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
if /bin/ps --pid $PID &>/dev/null; then
echo "Process $PID found. ${0##*/} seems to be running!"
/usr/bin/logger -t ${0##*/} \
"Process $PID found. ${0##*/} seems to be running!"
exit $E_RETURN
fi
echo "Process $PID not found. Start new process . . ."
fi
}
# Set overall file update range starting from root or $URL,
#+ according to above patterns.
set_range () {
include=
exclude=
for p in "${INCLUDE[@]}"; do
include="$include --include \"$p\""
done
for p in "${EXCLUDE[@]}"; do
exclude="$exclude --exclude \"$p\""
done
}
# Retrieve and refine rsync update list.
get_list () {
echo $$ > $PID_FILE || {
echo "Can't write to pid file $PID_FILE"
exit $E_RETURN
}
echo -n "Retrieving and refining update list . . ."
# Retrieve list -- 'eval' is needed to run rsync as a single command.
# $3 and $4 is the date and time of file creation.
# $5 is the full package name.
previous=
pre_file=
pre_date=0
eval /bin/nice /usr/bin/rsync \
-r $include $exclude $URL | \
egrep '^dr.x|^-r' | \
awk '{print $3, $4, $5}' | \
sort -k3 | \
{ while read line; do
# Get seconds since epoch, to filter out obsolete pkgs.
cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
# echo $cur_date
# Get file name.
cur_file=$(echo $line | awk '{print $3}')
# echo $cur_file
# Get rpm pkg name from file name, if possible.
if [[ $cur_file == *rpm ]]; then
pkg_name=$(echo $cur_file | sed -r -e \
's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')
else
pkg_name=
fi
# echo $pkg_name
if [ -z "$pkg_name" ]; then # If not a rpm file,
echo $cur_file >> $TMP #+ then append to download list.
elif [ "$pkg_name" != "$previous" ]; then # A new pkg found.
echo $pre_file >> $TMP # Output latest file.
previous=$pkg_name # Save current.
pre_date=$cur_date
pre_file=$cur_file
elif [ "$cur_date" -gt "$pre_date" ]; then
# If same pkg, but newer,
pre_date=$cur_date #+ then update latest pointer.
pre_file=$cur_file
fi
done
echo $pre_file >> $TMP # TMP contains ALL
#+ of refined list now.
# echo "subshell=$BASH_SUBSHELL"
} # Bracket required here to let final "echo $pre_file >> $TMP"
# Remained in the same subshell ( 1 ) with the entire loop.
RET=$? # Get return code of the pipe command.
[ "$RET" -ne 0 ] && {
echo "List retrieving failed with code $RET"
exit $E_RETURN
}
echo "done"; echo
}
# Real rsync download part.
get_file () {
echo "Downloading..."
/bin/nice /usr/bin/rsync \
$OPTS \
--filter "merge,+/ $TMP" \
--exclude '*' \
$URL $DEST \
| /usr/bin/tee $LOG
RET=$?
# --filter merge,+/ is crucial for the intention.
# + modifier means include and / means absolute path.
# Then sorted list in $TMP will contain ascending dir name and
#+ prevent the following --exclude '*' from "shortcutting the circuit."
echo "Done"
rm -f $PID_FILE 2>/dev/null
return $RET
}
# -------
# Main
init
check_pid
set_range
get_list
get_file
RET=$?
# -------
if [ "$RET" -eq 0 ]; then
/usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
else
/usr/bin/logger -t ${0##*/} \
"Fedora update mirrored with failure code: $RET"
fi
exit $RET
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.
Example 16.44. Using ssh
#!/bin/bash
# remote.bash: Using ssh.
# This example by Michael Zick.
# Used with permission.
# Presumptions:
# ------------
# fd-2 isn't being captured ( '2>/dev/null' ).
# ssh/sshd presumes stderr ('2') will display to user.
#
# sshd is running on your machine.
# For any 'standard' distribution, it probably is,
#+ and without any funky ssh-keygen having been done.
# Try ssh to your machine from the command-line:
#
# $ ssh $HOSTNAME
# Without extra set-up you'll be asked for your password.
# enter password
# when done, $ exit
#
# Did that work? If so, you're ready for more fun.
# Try ssh to your machine as 'root':
#
# $ ssh -l root $HOSTNAME
# When asked for password, enter root's, not yours.
# Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
# Enter 'exit' when done.
# The above gives you an interactive shell.
# It is possible for sshd to be set up in a 'single command' mode,
#+ but that is beyond the scope of this example.
# The only thing to note is that the following will work in
#+ 'single command' mode.
# A basic, write stdout (local) command.
ls -l
# Now the same basic command on a remote machine.
# Pass a different 'USERNAME' 'HOSTNAME' if desired:
USER=${USERNAME:-$(whoami)}
HOST=${HOSTNAME:-$(hostname)}
# Now excute the above command-line on the remote host,
#+ with all transmissions encrypted.
ssh -l ${USER} ${HOST} " ls -l "
# The expected result is a listing of your username's home
#+ directory on the remote machine.
# To see any difference, run this script from somewhere
#+ other than your home directory.
# In other words, the Bash command is passed as a quoted line
#+ to the remote shell, which executes it on the remote machine.
# In this case, sshd does ' bash -c "ls -l" ' on your behalf.
# For information on topics such as not having to enter a
#+ password/passphrase for every command-line, see
#+ man ssh
#+ man ssh-keygen
#+ man sshd_config.
exit 0
Within a loop, ssh may cause
unexpected behavior. According to a
Usenet post in the comp.unix shell archives,
ssh inherits the loop's
stdin. To remedy this, pass
ssh either the -n
or -f option.
Thanks, Jason Bechtel, for pointing this out.
Secure copy, similar in
function to rcp, copies files between
two different networked machines, but does so using
authentication, and with a security level similar to
ssh.
This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal
Since write is interactive, it would not normally find use in a script.
A command-line utility for configuring a network adapter (using DHCP). This command is native to Red Hat centric Linux distros.
Send or read e-mail messages.
This stripped-down command-line mail client works fine as a command embedded in a script.
Example 16.45. A script that mails itself
#!/bin/sh
# self-mailer.sh: Self-mailing script
adr=${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
# Note that the "mailx" command (in "send" mode) may be substituted
#+ for "mail" ... but with somewhat different options.
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.
Show mail statistics. This command may be invoked only by root.
root#mailstatsStatistics from Tue Jan 1 20:32:08 2008 M msgsfr bytes_from msgsto bytes_to msgsrej msgsdis msgsqur Mailer 4 1682 24118K 0 0K 0 0 0 esmtp 9 212 640K 1894 25131K 0 0 0 local ===================================================================== T 1894 24758K 1894 25131K 0 0 0 C 414 0
This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. It runs on a network, in conjunction with sendmail, and is not applicable to a dial-up POPmail account.
A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events.
The word “daemon” means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way UNIX daemons wander about behind the scenes, silently carrying out their appointed tasks.