In this tutorial, we’ll look at ways to check if a network port is open for connections in Linux. We’ll take a closer look at some of the Bash utilities that can be used for this purpose. We’ll also benchmark the performance of the methods offered to achieve a time-efficient solution.
2. Using the netstat Command
The netstat command-line utility in Linux prints the network statistics for a number of network protocols and interfaces. We can use it to investigate network sockets and test the status of network ports in Linux. First, let’s verify the default output of the netstat command:
$ netstat | head Active Internet connections (w/o servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:34702 localhost:8684 ESTABLISHED tcp 0 0 localhost:5747 localhost:55488 ESTABLISHED tcp 0 0 localhost:5161 localhost:44858 ESTABLISHED tcp 0 0 localhost:8580 localhost:42432 ESTABLISHED tcp 0 0 localhost:54090 localhost:3852 ESTABLISHED tcp 0 0 localhost:8186 localhost:52896 ESTABLISHED tcp 0 0 localhost:7367 localhost:43282 ESTABLISHED tcp 0 0 localhost:4441 localhost:35762 ESTABLISHED $
In this output, under the “Local Address” and “Foreign Address” columns, the port number is listed after the colon, “:“. The state of the network address (a combination of IP and port) is listed in the “State” column.
Let’s modify our command to check the status of a specific port:
$ netstat -nap | grep ":8001" (Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.) tcp 0 0 0.0.0.0:8001 0.0.0.0:* LISTEN 19135/nc tcp 0 0 127.0.0.1:8001 127.0.0.1:55368 ESTABLISHED 19135/nc tcp 0 0 127.0.0.1:55368 127.0.0.1:8001 ESTABLISHED 19138/nc
Here, we’re checking the status of port “8001” using the grep command.
Note that we’re using the -n option to force it to print numerical addresses rather than trying to determine hostnames. The –a option allows us to print all of the listening sockets as well as the non-listening sockets. The -p option is used to print the PID of the process to which this socket belongs.
When a port is in LISTEN mode, it’s open for a connection. Therefore, let’s grep for LISTEN. We’ll also take a look at the execution time of the command using the time command:
$ time (netstat -nap | grep ":8001" | grep LISTEN) (Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.) tcp 0 0 0.0.0.0:8001 0.0.0.0:* LISTEN 19135/nc real 0m0.471s user 0m0.199s sys 0m0.302s
It took around half a second (0.471 sec) to verify the port status using the above command. Let’s tweak the command options a bit and re-evaluate the execution time:
$ time (netstat -nl | grep ":8001") tcp 0 0 0.0.0.0:8001 0.0.0.0:* LISTEN real 0m0.072s user 0m0.039s sys 0m0.041s $
Great! We’re able to bring down the execution time from 0.471 sec to 0.072 sec. Here, we’re using the -l option to list only the sockets in LISTEN mode. In the previous approach, netstat was listing all the sockets and we were using the grep command to filter the sockets in the LISTEN mode, which caused additional overhead.
We also eliminated the -p option from our command, since getting the PID of the corresponding process isn’t of much value here.
Let’s also verify a negative test case:
$ time (netstat -ne | grep ":10001" ; echo $?) 1 real 0m0.088s user 0m0.079s sys 0m0.022s $
Here, as expected, netstat doesn’t give any output, and $? also holds a non-zero value since the port 10001 is not open for connections.
3.Using the ss Command
ss is a Linux utility for dumping socket statistics. Let’s take a look at its default output:
$ ss | head -10 Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port Process tcp ESTAB 0 0 127.0.0.1:34702 127.0.0.1:8684 tcp ESTAB 0 0 127.0.0.1:5747 127.0.0.1:55488 tcp ESTAB 0 0 127.0.0.1:5161 127.0.0.1:44858 tcp ESTAB 0 0 127.0.0.1:8580 127.0.0.1:42432 tcp ESTAB 0 0 127.0.0.1:54090 127.0.0.1:3852 tcp ESTAB 0 0 127.0.0.1:8186 127.0.0.1:52896 tcp ESTAB 0 0 127.0.0.1:7367 127.0.0.1:43282 tcp ESTAB 0 0 127.0.0.1:4441 127.0.0.1:35762 tcp ESTAB 0 0 127.0.0.1:7024 127.0.0.1:57206
Note that the output of the utility is similar to the netstat output. Interestingly, the argument options of this utility are also similar to the netstat command. Let’s verify the output with the -l option to only list the sockets in LISTEN mode:
$ time (ss -nl | grep ":8001") tcp LISTEN 0 1 0.0.0.0:8001 0.0.0.0:* real 0m0.029s user 0m0.039s sys 0m0.000s
This utility is faster as compared to netstat. With this utility, we were able to further reduce the timing to 0.029 secs, as compared to the execution time of 0.072 secs.
4. Using the lsof Command
We can also use the lsof utility with the -i option to verify if the port is open for connection:
$ lsof -i:8001 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME nc 19135 shubh 3u IPv4 326527 0t0 TCP *:8001 (LISTEN) nc 19135 shubh 4u IPv4 326528 0t0 TCP localhost:8001->localhost:55368 (ESTABLISHED) nc 19138 shubh 3u IPv4 326555 0t0 TCP localhost:55368->localhost:8001 (ESTABLISHED) $
Since we’re interested only in the LISTEN mode, let’s add a grep filter to our command:
$ time (lsof -i:8001 | grep "LISTEN") nc 19135 shubh 3u IPv4 326527 0t0 TCP *:8001 (LISTEN) real 0m0.625s user 0m0.133s sys 0m0.492s $
Notably, it takes more time than the netstat or the ss utility to display the relevant information. Here, we explicitly filtered the output using the grep command. Let’s also try to use the inbuilt flags -iTCP -sTCP:LISTEN to list the network files in LISTEN mode:
$ time (lsof -iTCP:8001 -sTCP:LISTEN) COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME nc 19135 shubh 3u IPv4 326527 0t0 TCP *:8001 (LISTEN) real 0m0.496s user 0m0.090s sys 0m0.407s
Although we were able to bring down the execution time, it’s still considerably large as compared to the netstat and ss utilities.
In this article, we discussed ways to test if a port is open for connections in Linux.
We started by discussing the solution using the netstat tool. Additionally, we also explored the various netstat options to achieve a time-efficient solution. Then, we checked the solution using the ss command.
Finally, we presented techniques to test the port state using the lsof command. From our benchmarking, we can conclude that using the ss utility with the best-suited options can be an efficient way to test the port state in Linux.