Related
When I run docker-compose up in my Docker project it fails with the following message:
Error starting userland proxy: listen tcp 0.0.0.0:3000: bind: address already in use
netstat -pna | grep 3000
shows this:
tcp 0 0 0.0.0.0:3000 0.0.0.0:* LISTEN -
I've already tried docker-compose down, but it doesn't help.
In your case it was some other process that was using the port and as indicated in the comments, sudo netstat -pna | grep 3000 helped you in solving the problem.
While in other cases (I myself encountered it many times) it mostly is the same container running at some other instance. In that case docker ps was very helpful as often I left the same containers running in other directories and then tried running again at other places, where same container names were used.
How docker ps helped me:
docker rm -f $(docker ps -aq) is a short command which I use to remove all containers.
Edit: Added how docker ps helped me.
This helped me:
docker-compose down # Stop container on current dir if there is a docker-compose.yml
docker rm -fv $(docker ps -aq) # Remove all containers
sudo lsof -i -P -n | grep <port number> # List who's using the port
and then:
kill -9 <process id> (macOS) or sudo kill <process id> (Linux).
Source: comment by user Rub21.
I had the same problem. I fixed this by stopping the Apache2 service on my host.
You can kill the process listening on that port easily with one command below :
kill -9 $(lsof -t -i tcp:<port#>)
ex :
kill -9 $(lsof -t -i tcp:<port#>)
or for ubuntu:
sudo kill -9 `sudo lsof -t -i:8000`
Man page for lsof : https://man7.org/linux/man-pages/man8/lsof.8.html
-9 is for hard kill without checking any deps.
(Not related, but might be useful if its PORT 5000 mystery) - the culprit process is due to Mac OS monterery.
The port 5000 is commonly used to serve local development servers. When updating to the latest macOS operating system, I was unable the docker to bind to port 5000, because it was already in use. (You may find a message along the lines of Port 5000 already in use.)
By running lsof -i :5000, I found out the process using the port was named ControlCenter, which is a native macOS application. If this is happening to you, even if you use brute force (and kill) the application, it will restart itself. In my laptop, lsof -i :5000 returns that Control Center is being used by process id 433. I could do killall -p 433, but macOS keeps restarting the process.
The process running on this port turns out to be an AirPlay server. You can deactivate it in
System Preferences › Sharing, and unchecking AirPlay Receiver to release port 5000.
I had same problem,
docker-compose down --rmi all (in the same directory where you run docker-compose up)
helps
UPD: CAUTION - this will also delete the local docker images you've pulled (from comment)
For Linux/Unix:
Simple search for linux utility using following command
netstat -nlp | grep 8888
It'll show processing running at this port, then kill that process using PID (look for a PID in row) of that process.
kill PID
In some cases it is critical to perform a more in-depth debugging to the problem before stopping a container or killing a process.
Consider following the checklist below:
1) Check you current docker compose environment
Run docker-compose ps. If port is in use by another container, stop it with docker-compose stop <service-name-in-compose-file> or remove it by replacing stop with rm.
2) Check the containers running outside your current workspace
Run docker ps to see list of all containers running under your host.
If you find the port is in use by another container, you can stop it with docker stop <container-id>.
(*) Because you're not under the scope of the origin compose environment - it is a good practice first to use docker inspect to gather more information about the container that you're about to stop.
3) Check if port is used by other processes running on the host
For example if the port is 6379 run:
$ sudo netstat -ltnp | grep ':6379'
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN 915/redis-server 12
tcp6 0 0 ::1:6379 :::* LISTEN 915/redis-server 12
(*) You can also use the lsof command which is mainly used to retrieve information about files that are opened by various processes (I suggest running netstat before that).
So, In case of the output above the PID is 915. Now you can run:
$ ps j 915
PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND
1 915 915 915 ? -1 Ssl 123 0:11 /usr/bin/redis-server 127.0.0.1:6379
And see the ID of the parent process (PPID) and the execution command.
You can also run: $ pstree -s <PID> to a visual display of the process and its related processes.
In our case we can see that the process probably is a daemon (PPID is 1) - In that case consider running: A) $ cat /proc/<PID>/status in order to get a more in-depth information about the process like the number of threads spawned by the process, its capabilities, etc'.
B) $ systemctl status <PID> in order to see the systemd unit that caused the creation of a specific process. If the service is not critical - you can stop and disable the service.
4) Restart Docker service
Run: sudo service docker restart.
5) You reached this point and..
Only if its not placing your system at risk - consider restarting the server.
In my case it was
Error starting userland proxy: listen tcp 0.0.0.0:9000: bind: address already in use
And all that I need is turn off debug listening in php storm
Most probably this is because you are already running a web server on your host OS, so it conflicts with the web server that Docker is attempting to start.
So try this one-liner before trying anything else:
sudo service apache2 stop; sudo service nginx stop; sudo nginx -s stop;
I had apache running on my ubuntu machine. I used this command to kill it!
sudo /etc/init.d/apache2 stop
I was getting the below error when i was trying to launch a new container -
listen tcp 0.0.0.0:8080: bind: address already in use.
To check which process is running on port 8080, run below command:
netstat -tulnp | grep 8080
i got the output below
[root#ip-112-x6x-2x-xxx.xxxxx.compute.internal (aws_main) ~]# netstat -tulnp | grep 8080 tcp 0 0 0.0.0.0:8080 0.0.0.0:* LISTEN **12749**/java [root#ip-112-x6x-2x-xxx.xxxxx.compute.internal (aws_main) ~]#
run
kill -9 12749
Then try to relaunch the container it should work
If redis server is started as a service, it will restart itself when you using kill -9 <process_id> or sudo kill -9 `sudo lsof -t -i:<port_number>` . In that case you will need to stop the redis service using following command.
sudo service redis-server stop
I upgraded my docker this afternoon and ran into the same problem. I tried restarting docker but no luck.
Finally, I had to restart my computer and it worked. Definitely a bug.
Check docker-compose.yml, it might be the case that the port is specified twice.
version: '3'
services:
registry:
image: mysql:5.7
ports:
- "3306:3306" <--- remove either this line or next
- "127.0.0.1:3306:3306"
Changing network_mode: "bridge" to "host" did it for me.
This with
version: '2.2'
services:
bind:
image: sameersbn/bind:latest
dns: 127.0.0.1
ports:
- 172.17.42.1:53:53/udp
- 172.17.42.1:10000:10000
volumes:
- "/srv/docker/bind:/data"
environment:
- 'ROOT_PASSWORD=secret'
network_mode: "host"
I ran into the same issue several times. Restarting docker seems to do the trick
A variation of #DmitrySandalov's answer: I had tomcat/java running on 8080, which needed to keep going. Looked at the docker-compose.yml file and altered the entry for 8080 to another of my choosing.
nginx:
build: nginx
ports:
#- '8080:80' <-- original entry
- '8880:80'
- '8443:443'
Worked perfectly. (The only wrinkle is the change will be wiped if I ever update the project, since it's coming from an external repo.)
At first, make sure which service you are running in your specific port. In your case, you are already using port number 3000.
netstat -aof | findstr :3000
now stop that process which is running on specific port
lsof -i tcp:3000
I resolve the issue by restarting Docker.
It makes more sense to change the port of the docker update instead of shutting down other services that use port 80.
Just a side note if you have the same issue and is with Windows:
In my case the process in my way is just grafana-server.exe. Because I first downloaded the binary version and double click the executable, and it now starts as a service by user SYSTEM which I cannot taskkill (no permission)
I have to go to "Service manager" of Windows and search for service "Grafana", and stop it. After that port 3000 is no longer occupied.
Hope that helps.
The one that was using the port 8888 was Jupiter and I had to change the configuration file of Jupiter notebook to run on another port.
to list who is using that specific port.
sudo lsof -i -P -n | grep 9
You can specify the port you want Jupyter to run uncommenting/editing the following line in ~/.jupyter/jupyter_notebook_config.py:
c.NotebookApp.port = 9999
In case you don't have a jupyter_notebook_config.py try running jupyter notebook --generate-config. See this for further details on Jupyter configuration.
Before it was running on :docker run -d --name oracle -p 1521:1521 -p 5500:5500 qa/oracle
I just changed the port to docker run -d --name oracle -p 1522:1522 -p 5500:5500 qa/oracle
it worked fine for me !
On my machine a PID was not being shown from this command netstat -tulpn for the in-use port (8080), so i could not kill it, killing the containers and restarting the computer did not work. So service docker restart command restarted docker for me (ubuntu) and the port was no longer in use and i am a happy chap and off to lunch.
maybe it is too rude, but works for me. restart docker service itself
sudo service docker restart
hope it works for you also!
I have run the container with another port, like... 8082 :-)
I came across this problem. My simple solution is to remove the mongodb from the system
Commands to remove mongodb in Ubuntu:
sudo apt-get purge mongodb mongodb-clients mongodb-server mongodb-dev
sudo apt-get purge mongodb-10gen
sudo apt-get autoremove
Let me add one more case, because I had the same error and none of the solutions listed so far works:
serv1:
...
networks:
privnet:
ipv4_address: 10.10.100.2
...
serv2:
...
# no IP assignment, no dependencies
networks:
privnet:
ipam:
driver: default
config:
- subnet: 10.10.100.0/24
depending on the init order, serv2 may get assigned the IP 10.10.100.2 before serv1 is started, so I just assign IPs manually for all containers to avoid the error. Maybe there are other more elegant ways.
I have the same problem and by stopping docker container it was resolved.
sudo docker container stop <container-name>
i solved with this sudo service redis-server stop
When I run docker-compose up in my Docker project it fails with the following message:
Error starting userland proxy: listen tcp 0.0.0.0:3000: bind: address already in use
netstat -pna | grep 3000
shows this:
tcp 0 0 0.0.0.0:3000 0.0.0.0:* LISTEN -
I've already tried docker-compose down, but it doesn't help.
In your case it was some other process that was using the port and as indicated in the comments, sudo netstat -pna | grep 3000 helped you in solving the problem.
While in other cases (I myself encountered it many times) it mostly is the same container running at some other instance. In that case docker ps was very helpful as often I left the same containers running in other directories and then tried running again at other places, where same container names were used.
How docker ps helped me:
docker rm -f $(docker ps -aq) is a short command which I use to remove all containers.
Edit: Added how docker ps helped me.
This helped me:
docker-compose down # Stop container on current dir if there is a docker-compose.yml
docker rm -fv $(docker ps -aq) # Remove all containers
sudo lsof -i -P -n | grep <port number> # List who's using the port
and then:
kill -9 <process id> (macOS) or sudo kill <process id> (Linux).
Source: comment by user Rub21.
I had the same problem. I fixed this by stopping the Apache2 service on my host.
You can kill the process listening on that port easily with one command below :
kill -9 $(lsof -t -i tcp:<port#>)
ex :
kill -9 $(lsof -t -i tcp:<port#>)
or for ubuntu:
sudo kill -9 `sudo lsof -t -i:8000`
Man page for lsof : https://man7.org/linux/man-pages/man8/lsof.8.html
-9 is for hard kill without checking any deps.
(Not related, but might be useful if its PORT 5000 mystery) - the culprit process is due to Mac OS monterery.
The port 5000 is commonly used to serve local development servers. When updating to the latest macOS operating system, I was unable the docker to bind to port 5000, because it was already in use. (You may find a message along the lines of Port 5000 already in use.)
By running lsof -i :5000, I found out the process using the port was named ControlCenter, which is a native macOS application. If this is happening to you, even if you use brute force (and kill) the application, it will restart itself. In my laptop, lsof -i :5000 returns that Control Center is being used by process id 433. I could do killall -p 433, but macOS keeps restarting the process.
The process running on this port turns out to be an AirPlay server. You can deactivate it in
System Preferences › Sharing, and unchecking AirPlay Receiver to release port 5000.
I had same problem,
docker-compose down --rmi all (in the same directory where you run docker-compose up)
helps
UPD: CAUTION - this will also delete the local docker images you've pulled (from comment)
For Linux/Unix:
Simple search for linux utility using following command
netstat -nlp | grep 8888
It'll show processing running at this port, then kill that process using PID (look for a PID in row) of that process.
kill PID
In some cases it is critical to perform a more in-depth debugging to the problem before stopping a container or killing a process.
Consider following the checklist below:
1) Check you current docker compose environment
Run docker-compose ps. If port is in use by another container, stop it with docker-compose stop <service-name-in-compose-file> or remove it by replacing stop with rm.
2) Check the containers running outside your current workspace
Run docker ps to see list of all containers running under your host.
If you find the port is in use by another container, you can stop it with docker stop <container-id>.
(*) Because you're not under the scope of the origin compose environment - it is a good practice first to use docker inspect to gather more information about the container that you're about to stop.
3) Check if port is used by other processes running on the host
For example if the port is 6379 run:
$ sudo netstat -ltnp | grep ':6379'
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN 915/redis-server 12
tcp6 0 0 ::1:6379 :::* LISTEN 915/redis-server 12
(*) You can also use the lsof command which is mainly used to retrieve information about files that are opened by various processes (I suggest running netstat before that).
So, In case of the output above the PID is 915. Now you can run:
$ ps j 915
PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND
1 915 915 915 ? -1 Ssl 123 0:11 /usr/bin/redis-server 127.0.0.1:6379
And see the ID of the parent process (PPID) and the execution command.
You can also run: $ pstree -s <PID> to a visual display of the process and its related processes.
In our case we can see that the process probably is a daemon (PPID is 1) - In that case consider running: A) $ cat /proc/<PID>/status in order to get a more in-depth information about the process like the number of threads spawned by the process, its capabilities, etc'.
B) $ systemctl status <PID> in order to see the systemd unit that caused the creation of a specific process. If the service is not critical - you can stop and disable the service.
4) Restart Docker service
Run: sudo service docker restart.
5) You reached this point and..
Only if its not placing your system at risk - consider restarting the server.
In my case it was
Error starting userland proxy: listen tcp 0.0.0.0:9000: bind: address already in use
And all that I need is turn off debug listening in php storm
Most probably this is because you are already running a web server on your host OS, so it conflicts with the web server that Docker is attempting to start.
So try this one-liner before trying anything else:
sudo service apache2 stop; sudo service nginx stop; sudo nginx -s stop;
I had apache running on my ubuntu machine. I used this command to kill it!
sudo /etc/init.d/apache2 stop
I was getting the below error when i was trying to launch a new container -
listen tcp 0.0.0.0:8080: bind: address already in use.
To check which process is running on port 8080, run below command:
netstat -tulnp | grep 8080
i got the output below
[root#ip-112-x6x-2x-xxx.xxxxx.compute.internal (aws_main) ~]# netstat -tulnp | grep 8080 tcp 0 0 0.0.0.0:8080 0.0.0.0:* LISTEN **12749**/java [root#ip-112-x6x-2x-xxx.xxxxx.compute.internal (aws_main) ~]#
run
kill -9 12749
Then try to relaunch the container it should work
If redis server is started as a service, it will restart itself when you using kill -9 <process_id> or sudo kill -9 `sudo lsof -t -i:<port_number>` . In that case you will need to stop the redis service using following command.
sudo service redis-server stop
I upgraded my docker this afternoon and ran into the same problem. I tried restarting docker but no luck.
Finally, I had to restart my computer and it worked. Definitely a bug.
Check docker-compose.yml, it might be the case that the port is specified twice.
version: '3'
services:
registry:
image: mysql:5.7
ports:
- "3306:3306" <--- remove either this line or next
- "127.0.0.1:3306:3306"
Changing network_mode: "bridge" to "host" did it for me.
This with
version: '2.2'
services:
bind:
image: sameersbn/bind:latest
dns: 127.0.0.1
ports:
- 172.17.42.1:53:53/udp
- 172.17.42.1:10000:10000
volumes:
- "/srv/docker/bind:/data"
environment:
- 'ROOT_PASSWORD=secret'
network_mode: "host"
I ran into the same issue several times. Restarting docker seems to do the trick
A variation of #DmitrySandalov's answer: I had tomcat/java running on 8080, which needed to keep going. Looked at the docker-compose.yml file and altered the entry for 8080 to another of my choosing.
nginx:
build: nginx
ports:
#- '8080:80' <-- original entry
- '8880:80'
- '8443:443'
Worked perfectly. (The only wrinkle is the change will be wiped if I ever update the project, since it's coming from an external repo.)
At first, make sure which service you are running in your specific port. In your case, you are already using port number 3000.
netstat -aof | findstr :3000
now stop that process which is running on specific port
lsof -i tcp:3000
I resolve the issue by restarting Docker.
It makes more sense to change the port of the docker update instead of shutting down other services that use port 80.
Just a side note if you have the same issue and is with Windows:
In my case the process in my way is just grafana-server.exe. Because I first downloaded the binary version and double click the executable, and it now starts as a service by user SYSTEM which I cannot taskkill (no permission)
I have to go to "Service manager" of Windows and search for service "Grafana", and stop it. After that port 3000 is no longer occupied.
Hope that helps.
The one that was using the port 8888 was Jupiter and I had to change the configuration file of Jupiter notebook to run on another port.
to list who is using that specific port.
sudo lsof -i -P -n | grep 9
You can specify the port you want Jupyter to run uncommenting/editing the following line in ~/.jupyter/jupyter_notebook_config.py:
c.NotebookApp.port = 9999
In case you don't have a jupyter_notebook_config.py try running jupyter notebook --generate-config. See this for further details on Jupyter configuration.
Before it was running on :docker run -d --name oracle -p 1521:1521 -p 5500:5500 qa/oracle
I just changed the port to docker run -d --name oracle -p 1522:1522 -p 5500:5500 qa/oracle
it worked fine for me !
On my machine a PID was not being shown from this command netstat -tulpn for the in-use port (8080), so i could not kill it, killing the containers and restarting the computer did not work. So service docker restart command restarted docker for me (ubuntu) and the port was no longer in use and i am a happy chap and off to lunch.
maybe it is too rude, but works for me. restart docker service itself
sudo service docker restart
hope it works for you also!
I have run the container with another port, like... 8082 :-)
I came across this problem. My simple solution is to remove the mongodb from the system
Commands to remove mongodb in Ubuntu:
sudo apt-get purge mongodb mongodb-clients mongodb-server mongodb-dev
sudo apt-get purge mongodb-10gen
sudo apt-get autoremove
Let me add one more case, because I had the same error and none of the solutions listed so far works:
serv1:
...
networks:
privnet:
ipv4_address: 10.10.100.2
...
serv2:
...
# no IP assignment, no dependencies
networks:
privnet:
ipam:
driver: default
config:
- subnet: 10.10.100.0/24
depending on the init order, serv2 may get assigned the IP 10.10.100.2 before serv1 is started, so I just assign IPs manually for all containers to avoid the error. Maybe there are other more elegant ways.
I have the same problem and by stopping docker container it was resolved.
sudo docker container stop <container-name>
i solved with this sudo service redis-server stop
The above one seems like an easy question, but believe me I have tried multiple methods but all seems to be in vain
For example : Port is 8080
1st - I followed many oracle docs and tried opening port 8080, but failed miserably
2nd - I followed another stack overflow posts - Opening port 80 on Oracle Cloud Infrastructure Compute node
Opening port 19132 on an Oracle compute instance (ubuntu-20.04)
still no success
3rd - I followed these oracle instructions and tried to open up the port - https://docs.cloud.oracle.com/en-us/iaas/developer-tutorials/tutorials/apache-on-ubuntu/01oci-ubuntu-apache-summary.htm
but not working -> to my surprise, when I tried port no 80 with the same method,it worked well..but not working for any other port at all
In short : ** I enabled port 8080 in Security rules in VNC - didnt work
** I tried ,installing firewalld and allowing through that -> didnt work for me
** Tried this -> didnt work
iptables -I INPUT 5 -i ens3 -p tcp --dport 8080 -m state --state NEW,ESTABLISHED -j ACCEPT
So I was hoping someone else can find me a solution Please to open up a port, or is there any way to completely disable my firewall so that I can use any port at all - currently I am able to listen to only port 80 from outside
Thank you
Ubuntu images in Oracle Cloud seem to have this 'strange' (compared to other clouds) behavior by default where you need to explicitly enable incoming traffic in the VM:
sudo iptables -I INPUT -j ACCEPT
Then you need to make this permanent:
sudo iptables-save -f /etc/iptables/rules.v4
To open a certain port it may require creating a security application specific to the port before creating the security rule. Please refer to this walkthru on it if you haven't already - https://www.oracle.com/webfolder/technetwork/tutorials/obe/cloud/compute/permitting_public_tcp_traffic_to_compute_instances/permitting_public_tcp_traffic_to_compute_instances.html
I am using Oracle Linux and this worked for me.
First add a new rule inside the default security list(see the link below)
firewall-cmd --permanent --zone=public --add-port=8080/tcp
firewall-cmd --reload
Reference : OCI: Amend Firewall Rules
In your case, you have Ubuntu instead of Oracle Linux, so you could try the above two commands as per Ubuntu and check if it works.
I'm in a network and i wanna capture ftp packets from another server in the network but i have a problem with tcpdump about this.
I've used this command :
tcpdump -i eth0 dst X.X.X.X -A and port 21
But it doesn't shows anything! ( i tested and sure that ftp port is 21 )
But if i use this on my server it works properly.
tcpdump -i eth0 -A and port 21
I've this problem when i enter " port " in the command. but if i enter a command without specific port it works and captures properly.
What is the problem?
Thanks.
I don't have enough reputation to ask a question, so this is part question and part insight.
Is the IP you're filtering on the client or the server for the FTP connection?
For the first command, try using src x.x.x.x or just host x.x.x.x and port 21.
For the second command, the "and" is not necessary with the -A flag. This should look more like this:
tcpdump -A -i eth0 port 21
tcpdump -Ai eth0 port 21
Another thing I've seen is if there are vlan tags, normal filtering won't work without adding "vlan and " to your filter. For example:
tcpdump -A -i eth0 "vlan and host x.x.x.x and port 21"
Also keep in mind that FTP uses a control and data connection. The control is over port 21, but the data can vary depending on whether you're using active or passive FTP.
I am new to Jekyll blogging and trying to view blog locally on
http://localhost:4000
but failed.
➜ my-awesome-site > jekyll serve
Notice: for 10x faster LSI support, please install http://rb-gsl.rubyforge.org/
Configuration file: /home/Git/my-awesome-site/_config.yml
Source: /home/Git/my-awesome-site
Destination: /home/Git/my-awesome-site/_site
Generating...
done.
Configuration file: /home/Git/my-awesome-site/_config.yml
jekyll 2.2.0 | Error: Address already in use - bind(2)
I tried
$ lsof -wni tcp:3000
$ lsof -wni tcp:4000
but both of them return nothing.
My Ruby version is:
➜ my-awesome-site > ruby --version
ruby 2.0.0p451 (2014-02-24 revision 45167) [universal.x86_64-darwin13]
What should I do next? I've re-installed jekyll already but the same problem remains.
See the comments in http://jekyllrb.com/docs/usage/, should help you:
If you need to kill the server, you can kill -9 1234 where "1234" is
the PID.
If you cannot find the PID, then do, ps aux | grep jekyll
and kill the instance. Read more.
Steps here fixed it for me. I had to append 'sudo' along with the commands.
$> sudo lsof -wni tcp:4000
It will give you information of process running on tcp port 4000 which also contains PID (Process ID). Now use command below to kill the process.
$> sudo kill -9 PID
Now you can execute jekyll serve command to start your site
Try to see which process is using that port, kill it and run again or try running jekyll on different port.
If #Matifou's answer here doesn't work, do the following instead:
The fix for anyone: run jekyll serve on an unused port:
Two ways:
In your _config.yml file, specify a port other than 4000 like this, for example:
port: 4001
OR (my preferred choice), add --port 4001 to your jekyll serve command, like this, for example:
bundle exec jekyll serve --livereload --port 4001
From: https://jekyllrb.com/docs/configuration/options/#serve-command-options
See my answer here: Is it possible to serve multiple Jekyll sites locally?
My particular problem: NoMachine is interfering:
When I run:
bundle exec jekyll serve --livereload --drafts --unpublished
I get these errors:
jekyll 3.9.0 | Error: Address already in use - bind(2) for 127.0.0.1:4000
.
.
.
/usr/lib/ruby/2.7.0/socket.rb:201:in `bind': Address already in use - bind(2) for 127.0.0.1:4000 (Errno::EADDRINUSE)
ps aux | grep jekyll doesn't show any processes running except this grep command itself. So, that doesn't help.
sudo lsof -wni tcp:4000, however, shows a running nxd nx daemon process:
$ sudo lsof -wni tcp:4000
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
nxd 914803 nx 3u IPv4 7606783 0t0 TCP *:4000 (LISTEN)
nxd 914803 nx 4u IPv6 7599664 0t0 TCP *:4000 (LISTEN)
I discovered this is due to my NoMachine remote login server.
If running NoMachine, click on the NoMachine icon in the top-right of your task bar. Ex: this is on Ubuntu 20.04:
Then click on "Show server status" --> Ports, and you'll see that NoMachine is running nx on Port 4000, which is interfering:
So, use the fix above to serve jekyll on a different port, such as 4001 instead of 4000. I recommend leaving the NoMachine port settings as-is, on port 4000, because NoMachine says:
Automatic updates require that hosts with NoMachine client or server installed have access to the NoMachine update server on port 4000 and use the TCP protocol.
See also:
Is it possible to serve multiple Jekyll sites locally?
my answer