telnet - 如何减少 unix telnet 连接超时

Question

我有一个 unix shell 脚本，可以测试文件中列出的多个主机的 ftp 端口。

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

一般来说，这个脚本可以工作，但是如果我遇到一个没有连接的主机，即 telnet 是“正在尝试...”，我怎样才能减少这个等待时间，以便它可以测试下一个主机？

Answer 1

您是否尝试过使用 netcat ( nc) 而不是 telnet？它具有更大的灵活性，包括能够设置超时：

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

该-w 5选项将在 5 秒后使连接超时。

Answer 2

尝试使用 timeout3 脚本非常健壮，我在不同情况下使用了很多没有问题。示例仅等待 3 秒以尝试检查 ssh 端口是否打开。

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

输出：您可以使用 grep 搜索“已连接”或“已终止”

timeout3 文件内容：

#

#!/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 <Dmitry.Golovashkin@sas.com>

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 "$@"

#

Answer 3

使用启动一个进程来休眠并终止 telnet 进程。大致：

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.

我避免了回声 QUIT | telnet 管道在涉及第一个作业的退出代码时不会有歧义。

此代码未经测试。

Answer 4

如果你有 nmap

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

Answer 5

使用timeout以便在 x 秒内退出，无论操作成功还是失败：

timeout 运行一个有时间限制的命令，启动 COMMAND，如果在 DURATION 之后仍在运行，则将其杀死。

公式：

timeout <seconds> <operation>

例子：

timeout 5 ping google.com

你的例子：

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