How to check if port open on linux

To determine if someone is listening, attempt to connect by loopback. If it fails, then the port is closed or we aren't allowed access. Afterwards, close the connection.

Modify this for your use case, such as sending an email, exiting the script on failure, or starting the required service.

@AmanJain cat waits for EOF or Ctrl-C to quit. You'll need to adjust this for your protocol. BTW are you running this to a remote server?

@AmanJain I've updated it for a local system. You do just want to check if it's listening correct? There isn't any protocol checking, such as requesting a page via http?

if a port is taken, it returns nothing, is there any way to make it say "port is taken by " or smth

This is not a reliable method since not all OS (e.g. ubuntu 16 as I discovered today) are shipped with bash compiled for building the /dev/tcp/IP/PORT tree

I use it with 127.0.0.1 as "remote" address.

this returns "0" if the port is open and "1" if the port is closed

-z Specifies that nc should just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunc- tion with the -l option.

That seems to be the easiest way, thanks. The sample script link is not working anymore though, yet it's quite self-explaining anyways.

Nice! This is much faster than the other answers on a server with many ports open. Returns in

The -z flag is not available in the nmap based ncat which most recent distros ship with: Fedora, Centos, etc. (nmap-ncat-6.01-9.fc18.x86_64)

@Sean unix commands typically return '0' to indicate success and non-zero for failure. So '0' indicates that it successfully connected and non-zero that it didn't connect for some reason. Note, however, that some versions of 'nc' don't support the '-z' argument so stackoverflow.com/a/25793128/6773916 is arguably a better solution.

You can use netstat this way for much faster results:

netstat -lnt | awk '$6 == "LISTEN" && $4 ~ /\.445$/' 

netstat -anp tcp | awk '$6 == "LISTEN" && $4 ~ /\.445$/' 

This will output a list of processes listening on the port (445 in this example) or it will output nothing if the port is free.

Actually it is correct syntax but probably you're using Linux and I am on Mac. For Linux use this: netstat -lnt | awk '$6 == "LISTEN" && $4 ~ ".445"'

In order to check for port 80 I needed to use awk '$6 == "LISTEN" && $4 ~ "80$"' . Instead of checking for the dot before the port number with \.80 , I used 80$ . Otherwise, this also matched IP addresses containing .80 and ports starting with 80 such as 8000 .

@PatrickOscity: Good point, but to make this robust to you need to combine both approaches: awk '$6 == "LISTEN" && $4 ~ "\.80$"'

You can use netcat for this.

connects to the server and directly closes the connection again. If netcat is not able to connect, it returns a non-zero exit code. The exit code is stored in the variable $?. As an example,

will return 0 if and only if netcat could successfully connect to the port.

This answer needs more upvotes. nc works perfectly for this case. the /dev/tcp trick is clever, but seems difficult to implement a script with signal interrupts.

nc has the -z flag for this purpose, which doesn't require taking input from /dev/null . There's already an answer using the -z flag above.

@AbeVoelker Not all versions of nc support the -z flag. I am on CentOS 7 and found Tony's solution to be what I needed.

Based on Spencer Rathbun's answer, using bash:

Good, it will suppress "Connection Refused" message. Auto-exits if the service accept the connection without wait forever.

Best solution for services that send no data after a new connection. About 20 times faster than calling netcat. Can be shortened to : &>/dev/null

I'd add a timeout to get a closed result faster: timeout 5s bash -c ': &>/dev/null ' && echo open || echo closed

@user1338062 This happens on an Ubuntu Bionic on Azure against external IP (not localhost or 127.0.0.1 ).

they're listed in /proc/net/tcp.

it's the second column, after the ":", in hex:

> cat /proc/net/tcp sl local_address rem_address st tx_queue rx_queue tr tm->when retrnsmt uid timeout inode 0: 00000000:0016 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0 0 10863 1 ffff88020c785400 99 0 0 10 -1 1: 0100007F:0277 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0 0 7983 1 ffff88020eb7b3c0 99 0 0 10 -1 2: 0500010A:948F 0900010A:2328 01 00000000:00000000 02:00000576 00000000 1000 0 10562454 2 ffff88010040f7c0 22 3 30 5 3 3: 0500010A:E077 5F2F7D4A:0050 01 00000000:00000000 02:00000176 00000000 1000 0 10701021 2 ffff880100474080 41 3 22 10 -1 4: 0500010A:8773 16EC97D1:0050 01 00000000:00000000 02:00000BDC 00000000 1000 0 10700849 2 ffff880104335440 57 3 18 10 -1 5: 0500010A:8772 16EC97D1:0050 01 00000000:00000000 02:00000BF5 00000000 1000 0 10698952 2 ffff88010040e440 46 3 0 10 -1 6: 0500010A:DD2C 0900010A:0016 01 00000000:00000000 02:0006E764 00000000 1000 0 9562907 2 ffff880104334740 22 3 30 5 4 7: 0500010A:AAA4 6A717D4A:0050 08 00000000:00000001 02:00000929 00000000 1000 0 10696677 2 ffff880106cc77c0 45 3 0 10 -1 

so i guess one of those :50 in the third column must be stackoverflow :o)

look in man 5 proc for more details. and picking that apart with sed etc is left as an exercise for the gentle reader.

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5 ways to check if a Port is open on a remote Linux PC

T here is an ample number of ways to check for any open ports on a remote Linux PC. Knowing open ports on a Linux machine helps system administrators to connect to the remote PC for troubleshooting system and cloud server issues.

TCP and UDP ports

TCP stands for Transmission Control Protocol. In this method, the computers get connected directly until the data transfer is taking place. Therefore, with this method, the data transfer is guaranteed and is reliable but puts a higher load on the server as it has to monitor the connection and the data transfer too.

UDP stands for User Datagram Protocol. Using this method, the data is sent in the form of little packages into the network with the hope that it reaches the final destination. It means the two computers are not connected directly to each other. This method does not provide any guarantee that the data you send will ever reach its destination. Load on the server is less, and so this method is used commonly by the system administrators first to try something that’s not so important.

Now that you know the types are ports on a Linux system, let’s get started with ways of finding the ones that are open.

Best ways to check if a Port is open on a Linux PC

There are multiple ways you can do it. However, the most reliable way to do this is by using the following commands:

• nc: netcat command
• nmap: network mapper tool
• telnet: telnet command
• echo > /dev/tcp/..
• netstat – tuplen

Let’s go through each method one by one.

1. netcat command

netcat is a simple Unix utility that can be used to write and read data using UDP and TCP protocol across network connections.

The primary reason for its design is to provide a back-end tool that works with the scripts and programs. It is also an exploration and network debugging tool that offers tons of features.

To use it, you need to install it in your distro using the respective installation commands.

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