how do i reduce timeout on unix telnet on connecti

问题:

I have a unix shell script which test ftp ports of multiple hosts listed in a file.

for i in `cat ftp-hosts.txt`
        do
        echo "QUIT" | telnet $i 21
done

In general this scripts works, however if i encounter a host which does not connect, i.e telnet is "Trying...", how can I reduce this wait time so it can test the next host ?

回答1:

Have you tried using netcat (nc) instead of telnet? It has more flexibility, including being able to set the timeout:

echo 'QUIT' | nc -w SECONDS YOUR_HOST PORT
# e.g.
echo "QUIT" | nc -w 5       localhost 21

The -w 5 option will timeout the connection after 5 seconds.

回答2:

Try using timeout3 script is very robust and I used a lot without problems on different situations. Example to wait just 3 seconds trying to check if ssh port is open.

> echo QUIT > quit.txt
> ./timeout3 -t 3 telnet HOST 22 < quit.txt 

outputs: you can grep for "Connected" or "Terminated"

timeout3 file contents:

#

#!/bin/bash
#
# The Bash shell script executes a command with a time-out.
# Upon time-out expiration SIGTERM (15) is sent to the process. If the signal
# is blocked, then the subsequent SIGKILL (9) terminates it.
#
# Based on the Bash documentation example.
# If you find it suitable, feel free to include
# anywhere: the very same logic as in the original examples/scripts, a
# little more transparent implementation to my taste.
#
# Dmitry V Golovashkin <[e-mail]>

scriptName="${0##*/}"
declare -i DEFAULT_TIMEOUT=9
declare -i DEFAULT_INTERVAL=1
declare -i DEFAULT_DELAY=1
# Timeout.
declare -i timeout=DEFAULT_TIMEOUT

# Interval between checks if the process is still alive.
declare -i interval=DEFAULT_INTERVAL

# Delay between posting the SIGTERM signal and destroying the process by SIGKILL.
declare -i delay=DEFAULT_DELAY

function printUsage() {
    cat <<EOF

Synopsis
    $scriptName [-t timeout] [-i interval] [-d delay] command
    Execute a command with a time-out.
    Upon time-out expiration SIGTERM (15) is sent to the process. If SIGTERM
    signal is blocked, then the subsequent SIGKILL (9) terminates it.

    -t timeout
        Number of seconds to wait for command completion.
        Default value: $DEFAULT_TIMEOUT seconds.

    -i interval
        Interval between checks if the process is still alive.
        Positive integer, default value: $DEFAULT_INTERVAL seconds.

    -d delay
        Delay between posting the SIGTERM signal and destroying the
        process by SIGKILL. Default value: $DEFAULT_DELAY seconds.

As of today, Bash does not support floating point arithmetic (sleep does),
therefore all delay/time values must be integers.
EOF
}

# Options.
while getopts ":t:i:d:" option; do  
    case "$option" in
        t) timeout=$OPTARG ;;
        i) interval=$OPTARG ;;
        d) delay=$OPTARG ;;
        *) printUsage; exit 1 ;;
    esac
done
shift $((OPTIND - 1))

# $# should be at least 1 (the command to execute), however it may be strictly
# greater than 1 if the command itself has options.

if (($# == 0 || interval <= 0)); then 
    printUsage
    exit 1
fi

# kill -0 pid   Exit code indicates if a signal may be sent to $pid process.
(
    ((t = timeout))

    while ((t > 0)); do
        sleep $interval
        kill -0 $$ || exit 0
        ((t -= interval))
    done
    # Be nice, post SIGTERM first.
    # The 'exit 0' below will be executed if any preceeding command fails.
    kill -s SIGTERM $$ && kill -0 $$ || exit 0
    sleep $delay
    kill -s SIGKILL $$
) 2> /dev/null &

exec "$@"

#

回答3:

Use start a process to sleep and kill the telnet process. Roughly:

echo QUIT >quit.txt
telnet $i 21 < quit.txt &
sleep 10 && kill -9 %1 &
ex=wait %1
kill %2
# Now check $ex for exit status of telnet.  Note: 127 inidicates success as the
# telnet process completed before we got to the wait.

I avoided the echo QUIT | telnet pipeline to leave no ambiguity when it comes to the exit code of the first job.

This code has not been tested.

回答4:

if you have nmap

 nmap -iL hostfile -p21  | awk '/Interesting/{ip=$NF}/ftp/&&/open/{print "ftp port opened for: "ip}'

回答5:

Use timeout in order to quit in x seconds whether the operation succeed or fails:

timeout runs a command with a time limit , Start COMMAND, and kill it if still running after DURATION.

Formula:

timeout <seconds> <operation>

example:

timeout 5 ping google.com

your example:

for i in `cat ftp-hosts.txt`
do
    timeout 5 telnet $i 21
done