I prefer red hat distros like centos or RL. (centos is EOL and it's sad).
Say i want to learn about mail servers (to send mails). i.e smtp. Now, I need to know about sendmail, postfix etc.
I want to understand all the TCP IP configuration for SMTP using sendmail, postfix etc. I want to send emails at the command line and trace the packets.
My goal is not to use postfix but configure all the things required in TCP IP protocol suite from scratch. But I'm not friendly with software development/network programming.
I'm wondering if you've any suggestions for me?
Try Rocky Linux or Alma Linux.
TCP/IP is OSI layer 4/3.
SMTP is application layer. You need a software like postfix to handle the server side. Or write your own software.
The client side is quite simple; many applications have a SMTP client built in.
You can use telnet for the SMTP dialogue. A little one follows:
telnet servername smtp
ehlo myhostname
mail from: mymail@myaddress
rcpt to: mail@address
data
line1
line2
.
quit
See also
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I second Rocky Linux. Rocky Linux was created with CentOS in its sights. Like CentOS, it's a downstream of RHEL and its binaries are compatible with other Red Hat products. They include a handy script "migrate2rocky" to help you make the move including changing all your repositories to Rocky. Good luck in your efforts!