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}}'
/zerotier
/ntp                                               10.3.10.8
/nodered                                           10.3.10.11
/n8n                                               10.3.10.4
/ssh                                               10.3.10.9
/code                                              10.3.10.7
/semaphore                                         10.3.10.6
/rproxy                                            10.3.10.2
/homer                                             10.3.2.10.10
/pihole                                            10.3.10.27
/pihole_googledns                                  10.3.10.24
/pihole_opendns                                    10.3.10.23

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 (127.0.0.1/8) 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:127.0.0.1:<internal_port>
# binds only to an IP address
SOCAT_SOCKADDR=<interface_IP> socat TCP-LISTEN:<public_port>,fork TCP:127.0.0.1:<internal_port>

# examples:

# binds to all interfaces:
socat TCP-LISTEN:1880,fork TCP:127.0.0.1:1880
# just for an IP address of one interface:
SOCAT_SOCKADDR=10.2.0.110 socat TCP-LISTEN:1880,fork TCP:127.0.0.1:1880

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
cfdisk
# change partition size using the UI
opkg install losetup resize2fs
BOOT="$(sed -n -e "/\s\/boot\s.*$/{s///p;q}" /etc/mtab)"
DISK="${BOOT%%[0-9]*}"
PART="$((${BOOT##*[^0-9]}+1))"
ROOT="${DISK}0p${PART}"
LOOP="$(losetup -f)"
losetup ${LOOP} ${ROOT}
fsck.ext4 -y ${LOOP}
resize2fs ${LOOP}
reboot

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@192.168.4.74 "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@10.2.0.1 "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 -

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 ( https://nmap.org ) at 2021-05-19 15:07 CEST
Pre-scan script results:
| broadcast-dhcp-discover:
|   Response 1 of 1:
|     IP Offered: 192.168.1.127
|     DHCP Message Type: DHCPOFFER
|     Subnet Mask: 255.255.255.0
|     Renewal Time Value: 4d00h00m00s
|     Rebinding Time Value: 7d00h00m00s
|     IP Address Lease Time: 8d00h00m00s
|     Server Identifier: 192.168.1.1
|     Router: 192.168.1.1
|_    Domain Name Server: 8.8.8.8, 8.8.4.4
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 1.43 seconds

May 26

Alive: shell script for alive monitoring using PING

Reading time: < 1 minute

Simple shell script based on bash which monitor a host with command line ping. Just bash and ping are the unique dependencies. Only state change are going to be printed:

#!/bin/bash

IP="THE_IP_TO_MONITOR"
STATE="offline"

show_state()
{
  echo "$(date '+%Y-%m-%dT%H:%M:%S') - " + $STATE;
}

while true;
do
  ping -c 4 $IP > /dev/null 2>&1
  if [ "$?" = "0" ]; then
    if [ "$STATE" = "offline" ];
    then
      STATE="online"
      show_state
    fi
  else
    if [ "$STATE" = "online" ];
    then
      STATE="offline"
      show_state
    fi
  fi
  sleep 10
done

Nov 27

Get Linux system process list without ‘ps’ command

Reading time: < 1 minute

When you work with embedded systems sometimes you would feel happy to have a Linux box until you discover there are plenty of basic things that you don’t have available, the extreme of that could be the ‘ps’ command which is used most of the time for checking if any process is running . Maybe you know that thanks the /proc filesystem there is access to the source of the information.

Keep next command close for solving this inconvenience he next time:

find /proc -mindepth 2 -maxdepth 2 -name exe -exec ls -lh {} \; 2>/dev/null

Jun 11

Nested byobu, re-assigning shortcuts

Reading time: 2 – 2 minutes

I’m a byobu user for a long time, I love it for many reasons. But this is just a quick tip for extreme users like me. I mean people who use byobu for local consoles with remote byobu sessions running on top of SSH, for instance.

When prefix key combinations has to be sent to the remote host we have to press “Control + a + a” and finally the command that we want to send to the remote systems. This is not comfortable many times. So, I modified my configuration file for changing the prefix when I want to send remote commands to the nested byobu.

This is going to work this way:

Control + a

    • as a prefix for local byobu session.

Control + b

    as a prefix for remote byobu session

Take a look on this screen capture where you can see byobu status bars stacked.

If you find useful the configuration that I described the only thing that you have to do is modify the configuration file: ~/.byobu/keybindings.tmux

unbind-key -n C-a 
set -g prefix C-a
set -g prefix2 F12
unbind-key -n C-b 
bind-key -n C-b send-prefix

I hope this is useful as it is for me.