Posts in category “Linux”

Fritz 7490: Root cause for Port mapping failure issue on a vagrant virtual machine


The root cause is that the default route was not set to the router's IP address.

My journey to resolve the issue:

The issue is that port mapping can work with my raspberry pi while it couldn't work with a virtual machine in the same LAN. I firstly think it must be a bug from the router. I upgrade the router to its latest firmware, but it still doesn't work. I google back and forth, I learned much from all kinds of answers, but they were just not my case. I almost decided to give up.

Then I found the following answer from E. van Putten, he answered this question and nobody gave his answer a "Like"!!!

In case you landed on this page because you can't reach a server running inside a Xen Guest from the internet (but can connect locally), then read on...

  1. The fritzbox can get confused by different OS'es appearing from the same MAC-address etc. (could happen while you are setting up / experimenting with Xen)
  2. The fritzbox has seemingly duplicate entries in the list, but with different settings, you need to delete the portmap from the incorrect entry, cleanup the list and reapply the portmap settings.
  3. It might be that your guest OS has no default gateway IP-address set. You'd expect a default gateway set to the local IP-address of your fritzbox. The symptoms of a missing default gateway is that your LAN PCs can access the server running inside the guest just fine, but external users from the internet cannot connect.

He gave three possible causes, and my case is the third one! here's my solution

opts = {
    :name => "yt-gateway",
    :ip => "",
    :mem => "1024",
    :cpu => "1"
Vagrant.configure("2") do |config| = "bento/ubuntu-20.04"
  config.vm.hostname = opts[:name] :public_network, ip: opts[:ip], bridge: "eno1"
  config.vm.provision "shell", run: "always", inline: "route add default gw || true"
  config.vm.provider "virtualbox" do |v| = opts[:name]
    v.customize ["modifyvm", :id, "--memory", opts[:mem]]
    v.customize ["modifyvm", :id, "--cpus", opts[:cpu]]
    v.customize ["modifyvm", :id, "--name", opts[:name]]

I love you E. van!

Setup PiVPN on a Vagrant virtual machine that is running Ubuntu 20.04

PiVPN should be easy to setup at any debian/ubuntu family distribution. However, my case wasn't.

  1. The first issue I met is Can't call method "set" on an undefined value at /usr/share/perl5/Debconf/ line 126, line 5., and I found a useful anwser from, which is
apt-get install --reinstall debconf

it worked! though I don't know the root cause.

  1. The second issue is rather weird, it simple stuck after showing ::: Backing up the openvpn folder... message. What's wrong with that on earth? Unfortunately, I couldn't find anything useful from google/stackoverflow.... I have to find out the root cause by myself.

ps -ef command shows there is a wget -qO- process running. Why it is running so long and looks never end? I copied the wget command and run it manually. aha, I found the cause!

--2021-10-10 23:01:15--
Resolving (
Connecting to (||:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: [following]
--2021-10-10 23:01:15--
Resolving ( 2606:50c0:8002::154, 2606:50c0:8003::154, 2606:50c0:8001::154, ...
Connecting to (|2606:50c0:8002::154|:443... ^C

You could see, all of the IP addresses wget used are in IPV6 format while my network doesn't support IPV6 . So it is easy to fix. According to this question from stackoverflow, I added the following line at the end of /etc/wgetrc and the issue has gone!

inet4_only = on

Solved: XPS 15 on Pop OS 20.04 gui got stuck after updating firmware

Previously, I have updated the firmware several times, so when I saw there's a new update, I clicked the "Update" button without too much thoughts. Yes, I got trouble this time. I got a black screen while the system is not dead. I can log into the system by pressing Alt+Ctrl+F2, no gui, but I can do something to fix it. It is good enough. I searched on my cellphone to try find something. And the following article helped me.

System76 login-loop-pop

In my case, I guess that the root cause could be the nvidia driver, so I followed the [Reinstall NVIDIA Driver] section in that article and it did save my life.

# remove current driver
sudo apt purge ~nnvidia
sudo apt autoremove
sudo apt clean
# reinstall it
sudo apt update
sudo apt full-upgrade
sudo apt install system76-driver-nvidia

In case someone else got trapped into the same situation, I wrote my solution here and hope it helps.

PopOS/Ubuntu 20.04 fcitx-rime setup

By my recommendation, my new workmate He (贺) moved to Ubuntu from windows, and he was annoyed of one thing: cannot input Chinese. And I have to google again to find out the answer. This is why I wrote this list for him like people and myself.

sudo apt install fcitx fcitx-rime
im-config  # and select fcitx as the input engine
sudo reboot # important!
fcitx-configtool # choose the correct input method you need

A simple nfs server on centos

At the server end

choose a server as the nfs-server, do things below in root role

dnf -y install nfs-utils
mkdir /mnt/nfs-share
chown -R nobody /mnt/nfs-share/

vim /etc/exports
add a line
save and exit

systemctl enable nfs-server
systemctl start nfs-server

At the client end

dnf -y install nfs-utils
mkdir /mnt/nfs-local
chown -R nobody /mnt/nfs-local/

vim /etc/hosts
add a line, replace to the IP addr of your NFS server

vim /etc/fstab
add a line
nfs-server:/mnt/nfs-share  /mnt/nfs-local   nfs defaults   0 0
save and exit
mount -a # to confirm what you have done is right

other utils:

You could use showmount -e to get a share list from the nfs server.