Ubuntu release/renew DHCP IP of a specific interface from CLI

To renew the DHCP lease on the wlp0s20f3 interface in Ubuntu Desktop, you can use the dhclient command. Here’s how:

Open a terminal and type the following command:

sudo dhclient -r wlp0s20f3

This command sends a DHCP release message to the DHCP server, effectively releasing the current DHCP lease on the specified interface.

Then, to obtain a new DHCP lease, type the following command:

sudo dhclient wlp0s20f3

This command requests a new DHCP lease for the specified interface.

After running these commands, your network interface should have a renewed DHCP lease, and it should be able to connect to the network using the new lease.

After changing hostname in ubuntu 22.04 desktop OS unable to launch google-chrome browser

I’ll provide specific steps tailored to your Ubuntu 22.04 setup:

1. Check and Terminate Chrome Processes:
   • Open a terminal window (Ctrl+Alt+T).
   • Use the command ps aux | grep chrome to list Chrome processes.
   • If any are running, terminate them using kill -9 <process_id>, replacing <process_id> with the actual process ID.
2. Remove the Lock File:
   • Navigate to the Chrome profile folder: cd ~/.config/google-chrome
   • Delete the lock file: rm -rf SingletonLock
3. Relaunch Chrome:
   • Type google-chrome in the terminal to launch Chrome.

The above steps worked for me.

If the issue persists:

• Consider Reverting Hostname Change: If possible, temporarily revert to the previous hostname to see if it resolves the issue.
• Reset Chrome Profile (if necessary): As a last resort, create a new Chrome profile to start fresh

nmap command to scan TCP/UDP ports

Nmap, short for Network Mapper, emerges as a command-line tool capable of scanning networks by sending packets and analyzing the responses. It’s particularly adept at identifying open ports and services running on a target system.

Scanning TCP Ports

Nmap’s TCP port scanning is robust. For instance, scanning ports 1 to 100 on a target:

nmap -p 1-100 <target>

To focus on specific ports, say 80, 443, and 8080:

nmap -p 80,443,8080 <target>

Or a comprehensive scan across all TCP ports (1 to 65535):

nmap -p- <target>

Scanning UDP Ports

UDP port scanning differs due to the protocol’s connectionless nature. Scanning UDP ports 1 to 100:

nmap -sU -p 1-100 <target>

For specific UDP ports, e.g., 53 and 161:

nmap -sU -p 53,161 <target>

Scanning Both TCP & UDP ports

nmap -sU -sT -p 53 <target>

or

`nmap -sUT -p 53 <target>`

Validate SSL certificates from CLI using openssl command

The following steps are used to validate the SSL certificates with openssl command

Check the Certificate Chain: To check the certificate chain and ensure that it’s valid, you can use the openssl verify command. This command will check if the certificate chain is valid up to a trusted root certificate.

openssl verify -CAfile gd_bundle-g2-g1.crt abc.crt

In this command:

• gd_bundle-g2-g1.crt is the file containing the trusted root certificates (the certificate authority bundle).
• abc.crt is the certificate you want to verify.

If the certificate chain is valid, you’ll see a message like: abc.crt: OK.

Check Certificate Details:

To view detailed information about a certificate, you can use the openssl x509 command. For example, to view the details of the abc.crt certificate:

openssl x509 -in abc.crt -text

This will display all the information about the certificate, including its subject, issuer, validity dates, and more.

Check the Private Key and Certificate Match:

To verify if a private key (abc.key) matches a certificate (abc.crt), you can use the openssl rsa and openssl x509 commands together:

openssl rsa -noout -modulus -in abc.key | openssl md5

openssl x509 -noout -modulus -in abc.crt | openssl md5

If the modulus values printed by these commands match, it indicates that the private key and certificate match.

Check Certificate Expiry Date:

To check the expiry date of a certificate, you can use the openssl x509 command:

openssl x509 -enddate -noout -in abc.crt

This will display the certificate’s expiry date.

These OpenSSL commands provide various ways to validate SSL certificates and perform different checks. Adjust the commands based on your specific requirements for certificate validation.

Keepalive ssh sessions for longer durations

 

In general most of the ISP providers will terminate idle sessions as early as possible(maybe in a couple of minutes).

This will be an irritating thing if you work on a remote server with ssh. I had a similar issue with my ISP(Act Fibernet). To fix this issue I have experimented in multiple ways and I am sharing the easiest way to that works.

Add following lines in your /etc/ssh/sshd_config file:

ClientAliveInterval 60
ClientAliveCountMax 5

where ClientAliveInterval 60 seconds will send a null request from your node(client) to server every 60 seconds. ClientAliveCountMax 5 is to give up if it doesn’t receive any response after 5 retries.

After adding the above configurations restart ssh with the following command:

sudo service ssh restart

You can try with different values for ClientAliveInterval based on your ISP. In general most of the ISP’s will persist idle sessions for a couple of minutes. In my case Act Fibernet sessions are not responding after 2 minutes(approximately), so I used 60 seconds.

 

Scheduling conditional statements with crontab

 

Recently while trying to auto-restart a daemon which is down by identifying with ps aux and if condition, it is not worked as intended with crontab.

I used following command which is perfectly running in from command line but not through crontab.

if [ `ps aux | grep nrpe | grep -v grep | wc -l` -eq 0 ]; then service nagios-nrpe-server restart ;fi

After trying with different commands following thing worked for me:

pgrep nrpe; [ $? != 0 ] && /etc/init.d/nagios-nrpe-server restart

where pgrep returns non zero exit code if the process nrep is not running and with $? is used to get the exit code of previous command (in this case pgrep) and start the process.

ImportError: No module named ‘boto3’

 

I got this error while I am trying to use boto3 module in python.

The main reason for this error is, there is no boto3 module installed.

To install boto3 user following command:

sudo pip3 install boto3

One thing you need to observe here is to use pip3 to install boto3.

If pip3 not installed in your node user following command to install it:

sudo apt-get install python3-pip

Nginx – Host/serve an Android apk file

 

To host/serve Android apk with Nginx following changes are required:

• Adding mime type in /etc/nginx/mime.types
• Explicitly adding header *.apk in site conf

Adding mime type in /etc/nginx/mime.types:

Add following line in mime.types file

application/vnd.android.package-archive apk;

Explicitly adding header *.apk in site conf: 

Update following configuration in your site conf file

location ~* \.(apk)$ {
  ......
  add_header Content-Type application/vnd.android.package-archive;
  ......
}

Finally restart Nginx.

sudo service nginx restart

 

Lists/Remove all empty files in a directory

 

To list all empty files in a directory use follwoing command:

for file in `ls`; do if [[ ! -s $file ]]; then echo $file; fi; done

To remove all empty files in a directory use following commad:

for file in `ls`; do if [[ ! -s $file ]]; then echo $file; rm $file; fi; done

ftp – sync data from remote ftp server

 

To sync data from remote ftp server install ncftp with following command:

apt-get install ncftp

Now use following command to sync data from remote ftp server:

ncftpget -R -T -v -u userName -p 'password' ftp_hostname "directoryPathInFtpServer" "directoryPathInLocalMachine"