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About sshd_config file

                               In my earlier post " SSH workings and key-exchange procedure"  we came across a file responsible for SSH configuration and default settings. It is one file for system-wide configuration of SSH, file initial comment section mentions, all options are present with default values but commented. Any uncommented options will override the default value.      We verify today what each options means. I am using  sshd_config,v 1.93.          # Port 22 : Default port to be used for making ssh connection. Can be changed           # AddressFamily any : Specifies which IP address family sshd should use.                 Valid arguments are: any, inet (IPv4 only), inet6 (IPv6 only).           #ListenAddress 0.0.0.0 : Take host:port or host as valid arguments. 0.0.0.0 means                 any host IP.          #Protocol : Support both SSH protocol 1 and 2.  Protocol 2 supports both RSA                 and DSA keys;   protocol 1 only supports RSA keys.         
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SSH workings and key-exchange procedure

                     Reachability is a less problem compared to security of the route to target. Even though we had many tools to access a remote server yet,  Secure Shell(SSH)  is ubiquitous the reason is security. The motivation for SSH ,  created in 1995 by Finland native Tatu Ylönen, came in response to a password-sniffing attack at his university. Emphasis on security and encryption.        How does SSH work? When we execute ssh user@Host3 from Host2, there is exchange of keys between them. In case, connecting first time SSH will prompt to verify the key and store it. Then based on authentication type defined SSH will prompt user for password or allow session to be created without password. The key transferred keep the communication encrypted and SSH supports various encryption methods.        Few important question that comes forward, what are these keys ?, Where are the keys get stored?  Where to define authentication  type? What are various method of encryption? I try to answer
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Unix and Linux with common boot stages

                                  UNIX and LINUX are identified as multitasking, multiuser and timesharing operating system. Unix was developed in 1987, which started from an University, further worked by AT&T, BSD and other organizations, each developed their own OSes keeping same platform of UNIX. Eventually different variant of UNIX was merged into and combined to form System V release 4 which became a common base for all future UNIX OSes. These releases were proprietary.                                     Richard Stallman started free UNIX-like OS and worked upon by his students, one of them was Linus Torvalds, who helped to release the first version of Linux kernel.  Both system have same elements which are Kernel, File System, Process and shell . Kernel is an interface between computer's hardware and its processes. Kernel allocates itself a space in memory called as kernel space and it uses this to manage process, memory usage, device drivers. system calls, and security.
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