bash netcat 超时问题
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/24198456/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Issue with netcat timeout
提问by anditpainsme
Why does the following netcat command not time out if the attempt to connect takes longer than 3 seconds (ie: when the port isn't open)? I assumed that the -w flag would be what I needed. OS is OSX 10.9.
如果尝试连接的时间超过 3 秒（即：端口未打开时），为什么以下 netcat 命令不会超时？我认为 -w 标志将是我所需要的。操作系统是 OSX 10.9。
nc -v -z -w 3 127.0.0.1 5050
nc -v -z -w 3 127.0.0.1 5050
Assuming that worked, I planned to implement like this (unsure if this will work, total bash noob)
假设有效，我计划像这样实施（不确定这是否有效，完全是 bash noob）
nc -v -z -w 3 127.0.0.1 5050 | /dev/null && echo "Online" || echo "Offline"
nc -v -z -w 3 127.0.0.1 5050 | /dev/null && echo "Online" || echo "Offline"
回答by CDahn
You need to redirect to /dev/null, not pipe to it. Try the following:
您需要重定向到 /dev/null，而不是通过管道连接到它。请尝试以下操作：
nc -v -z -w 3 127.0.0.1 5050 &> /dev/null && echo "Online" || echo "Offline"
On my machine, port 5050 isn't open, and I get the following:
在我的机器上，端口 5050 未打开，我得到以下信息：
$ nc -v -z -w 3 localhost 5050 &> /dev/null && echo "Online" || echo "Offline"
Offline
回答by Thiago Ruiz
There is an old bug report about this on debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=97583) and still having the same behavior in Debian GNU/Linux 7.7 (wheezy)
在 debian ( https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=97583)上有一个关于此的旧错误报告，并且在 Debian GNU/Linux 7.7 中仍然具有相同的行为 (wheezy)
I found a "solution" to this: installing the openbsd version of nc:
我找到了一个“解决方案”：安装 nc 的 openbsd 版本：
apt-get install netcat-openbsd
回答by Mike Q
Nc:nc is usually installed already , however on some systems such as Mac OS X, the command hangs on unreachable systems without the -G option. If that does not work use the Workaround.
Nc：nc 通常已经安装，但是在某些系统（例如 Mac OS X）上，如果没有 -G 选项，该命令会在无法访问的系统上挂起。如果这不起作用，请使用解决方法。
nc -v -z -w 3 127.0.0.1 22 &> /dev/null && echo "Online" || echo "Offline"
Mac OSX:
Mac OSX：
nc -z -G 3 127.0.0.1 22 &> /dev/null && echo "Online" || echo "Offline"
Alternative workaround option:
替代解决方法选项：
bash -c '(sleep 3; kill $$) & exec nc -z 127.0.0.1 22' &> /dev/null
echo $?
0
bash -c '(sleep 3; kill $$) & exec nc -z 1.2.3.4 22' &> /dev/null
echo $?
143
(examples illustrate connecting to port 22 ssh over a good and bad host example, use the $? to determine if it reached the host with the sleep time of 3 seconds)
（示例说明通过一个好的和坏的主机示例连接到端口 22 ssh，使用 $? 确定它是否以 3 秒的睡眠时间到达主机）
For Mac Users (mainly) etc, you can use the command in the script like so :
对于 Mac 用户（主要是）等，您可以在脚本中使用命令，如下所示：
# -- use NMAP, if not avail. go with nc --
if command -v nmap | grep -iq nmap ; then
nmap ${ip} -PN -p ${ssh_port} | grep -iq "open"
res=$?
elif command -v nc | grep -iq nc ; then
# -- run command if fails to complete in 3 secs assume host unreachable --
( nc -z ${ip} ${ssh_port} ) & pid=$!
( sleep 3 && kill -HUP $pid ) 2>/dev/null & watcher=$!
if wait $pid 2>/dev/null; then
pkill -HUP -P $watcher
wait $watcher
# -- command finished (we have connection) --
res=0
else
# -- command failed (no connection) --
res=1
fi
else
echo "Error: You must have NC or NMAP installed"
fi
if [[ ${res} -lt 1 ]] ;then
success=1
echo "testing => $ip SUCCESS connection over port ${ssh_port}"
break;
else
echo "testing => $ip FAILED connection over port ${ssh_port}"
fi
回答by Pierfrancesco PierQR Aiello
On Mac OS X (10.14.6) the -w 3parameter is somehow ignored.
在 Mac OS X (10.14.6) 上，该-w 3参数以某种方式被忽略。
The workaround I found is: timeout 3 nc -vz 10.18.50.134 23
我找到的解决方法是： timeout 3 nc -vz 10.18.50.134 23
回答by MacTroy
Since Mac OS X 10.8.x, nchas used the -Goption to set the timeout for initiating a connection. This is separate from the -woption, which sets the timeout for a connection that has been made but has gone idle.
从 Mac OS X 10.8.x 开始，nc已使用该-G选项来设置启动连接的超时时间。这与-w设置已建立但已空闲的连接的超时的选项不同。
If you are trying to use ncfor port scanning, i.e. nc -v -z 10.0.1.1 1-1023, it will spend over a minute trying to connect to each non-responding port unless you add a -Gtimeout value:
如果您尝试nc用于端口扫描，即nc -v -z 10.0.1.1 1-1023，除非您添加-G超时值，否则它将花费超过一分钟的时间尝试连接到每个无响应的端口：
nc -v -z -G 1 10.0.1.1 1-1023
nc -v -z -G 1 10.0.1.1 1-1023
That's one second per port scanned — much more useful.
这是每个端口扫描一秒钟 - 更有用。
