Dec 14

Deploying gotop with Ansible

Reading time: 1 – 2 minutes

Gotop is a terminal based graphical activity monitor inspired by gtop and vtop; it’s available at:

I published a role in Ansible Galaxy for deploying gotop in Linux servers. The role page in Ansible Galaxy is at:

Role installation command and deployment command:

ansible-galaxy install oriolrius.install_gotop

# change SERVER_IP, for the IP address where you want to deploy gotop
ansible -i SERVER_IP, -u root -m include_role -a name=oriolrius.install_gotop all

Nov 16

Get the IP addresses of local Docker containers

Reading time: < 1 minute

We have docker running and the containers have their own private network, thanks to this command we’re going to get the private IP address of all of them:

$ sudo docker inspect $(docker ps -q ) \
--format='{{ printf "%-50s" .Name}} {{range .NetworkSettings.Networks}}{{.IPAddress}} {{end}}'

Oct 25

OpenSSH public key fingerprint

Reading time: < 1 minute

Quick and easy, how to get the fingerprint of your SSH RSA key.

# syntax:
openssl pkey -in PATH/PRIVATE_RSA_KEY -pubout -outform DER | openssl md5 -c

# example:
$ openssl pkey -in ~/.ssh/id_rsa -pubout -outform DER | openssl md5 -c
MD5(stdin)= a6:26:23:d9:c1:d3:d5:e5:c0:38:ab:3c:c1:6a:3f:ea

Aug 25

socat: publish a port only available in localhost

Reading time: 1 – 2 minutes

Assume that we have a service only available in localhost ( and we want to expose this port temporarily. Of course, you can use iptables for redirecting the port. But take care, this is not a simple DNAT because packets will not be evaluated by PREROUTING (-t nat) rules.

Another option is using an old-powerful Swiss knife tool: socat (github).

# binds public port to any local interface
socat TCP-LISTEN:<public_port>,fork TCP:<internal_port>
# binds only to an IP address
SOCAT_SOCKADDR=<interface_IP> socat TCP-LISTEN:<public_port>,fork TCP:<internal_port>

# examples:

# binds to all interfaces:
socat TCP-LISTEN:1880,fork TCP:
# just for an IP address of one interface:

Aug 24

WSL2: upgrade from Ubuntu 18.04 to 20.04

Reading time: 2 – 4 minutes

I was afraid for missing my files, configurations and much other staff that I had in Linux filesystem of WSL2 (Windows 10). But I had to upgrade because of compatibility with an application that I need. So, finally, I decided to upgrade the Ubuntu 18.04 that I installed in WSL2 to a newer version 20.04. Yes, I know that there is 22.04 available nowadays, but I wasn’t comfortable jumping to many versions.

I followed simple steps that I found at:

How To Upgrade Existing WSL/WSL2 Ubuntu 18.04 to 20.04

The steps in a nutshell were:

sudo apt update
sudo apt upgrade -y
sudo apt --purge autoremove
sudo apt install update-manager-core
sudo do-release-upgrade

When I answered all the questions and after stopping the WSL2 VM with:

# command that I ran from 'cmd.exe' (Windows console)
wsl --shutdown 

I had and issue rebooting, WSL2 didn’t boot and it gave this message:

wsl/usr/sbin/enter-systemd-namespace: line 10: /usr/sbin/daemonize: No such file or directory

I resolved the problem, shutting down the WSL and running the command:

wsl -u root -e bash --noprofile --norc

It gave me a root console where I could modify the file:


specifically, in line 10, I changed:


And when I accessed to WSL2 everything worked perfectly. I used to access WSL2 from Windows Terminal which uses the command:

C:\WINDOWS\system32\wsl.exe -d Ubuntu-18.04

But it has small issues with bybou, finally I changed this command to:

C:\WINDOWS\system32\bash.exe ~ -login

I love to use a console with byobu enabled when access the shell, but with my former command, the command ‘byobu-enable’ was ignored. I didn’t find why, in the end the solution was to change the command that I use for accessing Linux console (WSL2).

Likewise, I hope these notes can help someone.

Aug 04

Resize Squashfs/Ext4 partition of OpenWRT in a Raspberry PI

Reading time: < 1 minute

Quick cookbook for just doing what the subject of the post says:

opkg update
opkg install cfdisk
# change partition size using the UI
opkg install losetup resize2fs
BOOT="$(sed -n -e "/\s\/boot\s.*$/{s///p;q}" /etc/mtab)"
LOOP="$(losetup -f)"
losetup ${LOOP} ${ROOT}
fsck.ext4 -y ${LOOP}
resize2fs ${LOOP}

Aug 16

Network traffic sniffing: tcpdump on Linux + Wireshark on Windows

Reading time: 2 – 2 minutes

From the Windows box using the CLI console (cmd):

ssh USER@HOST "tcpdump -s 0 -U -n -w - -i NETIF FILTER" | "c:\Program Files\Wireshark\Wireshark.exe" -k -i -

# USER - remote user of the linux box
# HOST - host address of the remote linux box
# NETIF - network interface to snif in the remote linux box
# FILTER - (optional) rules for filtering traffic to capture

Use case:

C:\Windows\System32\OpenSSH>ssh root@ "tcpdump -s 0 -U -n -w - -i eno2 udp and not port 53" | "c:\Program Files\Wireshark\Wireshark.exe" -k -i -

Let me present a rare use case of this useful trick. I use a QNAP NAS as a gateway in my home network, where I have 5 NICs. So it’s really useful to snif traffic remotly but I have no tcpdump packet in the system. What I did is use tcpdump as Docker container and finally the commands is like that.

# sniffing SIP traffic (port 5060) on interface eth0
# remote linux host (QNAP NAS) use SSH port 55222
# docker container is created and when work is done is removed
C:\Windows\System32\OpenSSH>ssh -p 55222 admin@ "cd /share/Container/tcpdump && docker run --rm --net=host corfr/tcpdump -s 0 -U -n -w - -i eth0 not port 22 and port 5060" | "c:\Program Files\Wireshark\Wireshark.exe" -k -i -

Jul 07

rp_filter Linux kernel feature

Reading time: 1 – 2 minutes

It’s just an IP spoofing protection which is by default enabled on Linux kernels. When it’s value is ‘1’ means that all IP address which are not directly routable and received form a network interface they are directly discarded.

So, if you want to scan a range of IP address in your LAN which not belong to that interface address space when packets from IP addresses are received they are going to be discarded by the kernel. So, take that into account when you have those “unusual” requirements.

It can be enable/disabled by all interfaces or just one:

root@mini9:/proc/sys/net/ipv4# cat ./conf/all/rp_filter
root@mini9:/proc/sys/net/ipv4# cat ./conf/ztly5q4n37/rp_filter

May 27

DRY DHCP Client: request and IP address to the DHCP server without a DHCP Client

Reading time: < 1 minute

When you want to discover LAN metadata without being part of that network. So, when you want to discover network address range, gateway, DNS IPs, DHCP server IPs, etc. this simple nmap parameter will help you so much.

# nmap --script broadcast-dhcp-discover

Starting Nmap 7.60 ( ) at 2021-05-19 15:07 CEST
Pre-scan script results:
| broadcast-dhcp-discover:
|   Response 1 of 1:
|     IP Offered:
|     DHCP Message Type: DHCPOFFER
|     Subnet Mask:
|     Renewal Time Value: 4d00h00m00s
|     Rebinding Time Value: 7d00h00m00s
|     IP Address Lease Time: 8d00h00m00s
|     Server Identifier:
|     Router:
|_    Domain Name Server:,
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 1.43 seconds