Efficiently test if a port is open on Linux?
From a bash script how can I quickly find out whether a port 445 is open/listening on a server. I have tried a couple of options, but I want something quick:
1. lsof -i :445 (Takes seconds)
2. netstat -an |grep 445 |grep LISTEN (Takes seconds)
3. telnet (it doesn’t return)
4. nmap , netcat are not available on the server It will be nice to know of a way that doesn’t enumerate first and greps after that.
netstat -lnt (with -t and without -a ) will limit output to listening TCP connections only. It may speed-up a little bit. You can add -4 for IPv4 only if you don’t need IPv6.
I don’t know why lsof is slow for you, but normally it is the best of the solutions you listed. Your netstat solution is not very reliable (you can guess it whenever you use grep ; anyway it returns true if someone is listening on e.g. 4450). telnet and netcat actually attempt to create a connection, which may not always be what you want.
14 Answers 14
A surprise I found out recently is that Bash natively supports tcp connections as file descriptors. To use:
exec 6<>/dev/tcp/ip.addr.of.server/445 echo -e "GET / HTTP/1.0\n" >&6 cat I'm using 6 as the file descriptor because 0,1,2 are stdin, stdout, and stderr. 5 is sometimes used by Bash for child processes, so 3,4,6,7,8, and 9 should be safe.
As per the comment below, to test for listening on a local server in a script:
exec 6<>/dev/tcp/127.0.0.1/445 || echo "No one is listening!" exec 6>&- # close output connection exec 6 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
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.
3 Ways to Find Out Which Process Listening on a Particular Port
A port is a logical entity that represents an endpoint of communication and is associated with a given process or service in an operating system. In previous articles, we explained how to find out the list of all open ports in Linux and how to check if remote ports are reachable using the Netcat command.
In this short guide, we will show different ways of finding the process/service listening on a particular port in Linux.
1. Using netstat Command
netstat (network statistics) command is used to display information concerning network connections, routing tables, interface stats, and beyond. It is available on all Unix-like operating systems including Linux and also on Windows OS.
In case you do not have it installed by default, use the following command to install it.
$ sudo apt-get install net-tools [On Debian/Ubuntu & Mint] $ sudo dnf install net-tools [On CentOS/RHEL/Fedora and Rocky Linux/AlmaLinux] $ pacman -S netstat-nat [On Arch Linux] $ emerge sys-apps/net-tools [On Gentoo] $ sudo dnf install net-tools [On Fedora] $ sudo zypper install net-tools [On openSUSE]
Once installed, you can use it with the grep command to find the process or service listening on a particular port in Linux as follows (specify the port).
In the above command, the flags.
- l – tells netstat to only show listening sockets.
- t – tells it to display tcp connections.
- n – instructs it to show numerical addresses.
- p – enables showing of the process ID and the process name.
- grep -w – shows matching of exact string (:80).
Note: The netstat command is deprecated and replaced by the modern ss command in Linux.
2. Using lsof Command
lsof command (List Open Files) is used to list all open files on a Linux system.
To install it on your system, type the command below.
$ sudo apt-get install lsof [On Debian, Ubuntu and Mint] $ sudo yum install lsof [On RHEL/CentOS/Fedora and Rocky Linux/AlmaLinux] $ sudo emerge -a sys-apps/lsof [On Gentoo Linux] $ sudo pacman -S lsof [On Arch Linux] $ sudo zypper install lsof [On OpenSUSE]
To find the process/service listening on a particular port, type (specify the port).
3. Using fuser Command
fuser command shows the PIDs of processes using the specified files or file systems in Linux.
You can install it as follows:
$ sudo apt-get install psmisc [On Debian, Ubuntu and Mint] $ sudo yum install psmisc [On RHEL/CentOS/Fedora and Rocky Linux/AlmaLinux] $ sudo emerge -a sys-apps/psmisc [On Gentoo Linux] $ sudo pacman -S psmisc [On Arch Linux] $ sudo zypper install psmisc [On OpenSUSE]
You can find the process/service listening on a particular port by running the command below (specify the port).
Then find the process name using PID number with the ps command like so.
$ ps -p 2053 -o comm= $ ps -p 2381 -o comm=
You can also check out these useful guides about processes in Linux.
You might also like:
That’s all! Do you know of any other ways of finding the process/service listening on a particular port in Linux, let us know via the comment form below.