SSH, TLS, Mosh, Tmux, Eternal Terminal

I was using iTerm2 and connecting to my devservers through “ssh address_of_server”, however i realized that if i try to open more than one connection for the same devserver, my previous connections are dropped.

It turns out that, ssh itself has some problems, like maintaining the connection by plain ssh is problematic, if your computer goes to sleep or if your connection changes from company proxy to home wifi. And it is also slow for the commands that require high bandwidth.

Eternal Terminal

You can connect to eternal terminal by et hostname command.

Tmux

Tmux is defined as terminal multiplexer in its official Github website [2].

Mosh

Starting from macOS Catalina (3 June 2019) WWDC 2019, support only 64 bit applications. Mac uses zsh as the default login shell and interactive shell.

zsh is highly compatible with the Bourne shell (sh) and mostly compatible with bash, with some differences. For more about zsh and its comprehensive command-line completion system, enter man zsh in Terminal.

What is Login Shell?

Login shells are similar to other shells by one or more initial setup scripts are invoked. Their names generally includes ‘profile’ word. Login shell is the place we login, it is shown with a hyphen ‘-‘ if you do ps -f

A user must have at least one login shell per host, this is required to specify actions that you want to happen only once.

There are different files sourced when login shell is invoked:

if /etc/profile exists, source (run) it if ~/.bash_profile exists, source (run) it if ~/.bash_login exists, source (run) it if ~/.profile exists, source (run) it

Ideally, login shell files, .login or .profile should set the inheritable settings and then, .bashrc should handle the settings that are not inheritable like set.

Another thing is that login shells should do the heavy lifting jobs, like resource-incentive actions like running certain processes after each login, not after each time you open a terminal.

What is Non-Login Shell?

/etc/bash.bashrc and ~/.bashrc is the order of the file.

• The rc at the end of bashrc, zshrc, or vimrc is the abbreviation for runcom, which is the run commands.

.zsh_history file stores all the command that you have used previously

What is Interactive Shell?

Reads input from standard-input

.bashrc file is run for interactive shell invocations.

/etc/bashrc can be used by both the ~/.bash_profile and ~/.bashrc file. This means that it is sourced whether this is a login or non-login shell.

Kernel vs. Shell

Kernel is basically the main program in the OS, it talks to your CPU, Memory and IO devices. Kernel has the complete control over anything in your system. Basically, it is the central module in the OS. It is loaded first (before the boot loader) and stays in the RAM, e.g. primary storage. It is generally loaded into the protected area of the RAM, to avoid it being overwritten or attacked.

Kernel is like a bridge between software and hardware components.

Example kernels used in OS’s:

• Linux kernel is used in Linux, FreeBSD, Android.

Monolithic Kernel

As the name implies, it is one piece of software which includes less lines of code and thus faster. However, the size of the monolithic kernel is generally bigger, because both kernel services and user service resides in the kernel address space. The reason why the monolith kernel is faster although it is bigger in size is that, the communication between the applications and hardware is done by system calls. All the code resides in the same area of the memory. There are some cons of this approach, which are; since the system in coupled, a bug can crash the whole system, and maintaining a single piece of software is harder as in the other type of applications.

Most famous monolith kernel OS’s are Linux, FreeBSD, Android, Microsoft Windows, Solaris.

MicroKernel

MicroKernel is a more decouple kernel system, where user services and kernel services are separated and they are located in different address space. Therefore the size of the microkernel is lesser, since only kernel services take place in the kernel address space. There are more lines of code in this kernel and its execution is slower than monolith. The communication between applications and hardware are done via message passing (IPC - Inter Process Communication) and thus the microkernels are slower compared to monolith kernels. Since, this is a decoupled system (which means it can easily be extended), if a services crashes system continues to operate. Thus, microkernel is more secure and reliable than monolith kernel.

Most famous microkernel OS’s are Mac OS X and Symbian.

The following image from Wikipedia is a good visualization of the differences between two.

Structure of monolithic and microkernel-based operating systems, respectively

Shell is basically the terminal that you use to communicate with the Kernel.

First Shell is creted by Stephen Bourne, the Bourne Shell (sh 1977) developed in C. [7]

C Shell (csh 1978) Bill Joy, scripting language similar to C.

Korn Shell (ksh 1983) David Korn, combined Bourne and C shell, compiles with POSIX. Backward compatible with Bourne Shell.

Bourne-Again Shell (bash 1989) Brian Fox for GNU project, replacement of the Bourne Shell. Filename globbing, piping, command substitution and control structures for conditional testing and iteration.

zsh 1990

POSIX 1992

sh vs bash?

Shell script, Shell Command Language, is a programming language which is compatible to POSIX standards. So, Shell

/bin/sh symlink or hardlink to POSIX system.

Bash was shell compatible but, it is not a POSIX shell, it is a dialect of POSIX shell.

Bash is one of the implementations of the Shell Command Language.

Boot Loader

BootLoaders are code pieces that run before any OS or software runs. Their main purpose is to boot the system, in other words bootloader put OS into the memory, RAM from the harddisk. They are very hardware (CPU and board) specific. They are usually stored in ROM, but since ROM size is not sufficient and it is slower, the bootloader code is moved to RAM and executed from there. Then, BootLoader loads the kernel and it can do that from different locations, like memory, file, and network. [5]

The two common boot loaders for Linux are: LILO (LInux LOader) and LOADLIN (LOAD LINux). The GRUB (GRand Unified Bootloader) is popular among RedHat Linux since it is the default bootloader for that.

Apple Login Shell Options

Linux vs Unix

Unix

Linux is written by Linus Torvalds in 1991 with C programming language and released under GNU licence. It is nothing but a clone of the UNIX.

Library functions Vs System calls

Library functions are nothing but a functions written in some kind of software library, like a strlen() in C. There are two types of library functions: functions which do not call a system call and and the functions which do call a system call. For instance, fopen() is a library function, where is calls the open() system call underneath.

The main difference of system calls is that, they reach to the system kernel. For example socket() is a system call, it reaches to the kernel to be able to create a network socket. They are the entry points to the OS. [6]

You can debug a library function whereas, it is impossible to debug the system calls.

The following figure is really great explaning the flow between syscalls and lib functions.

System calls vs Library functions

Sandboxing vs VM vs Container

VM:

• Bigger security, isolated with the OS
• A security vulnerability in one VM cannot spread to another VM or even the host machine.
• Kernel and processes are consuming memory
• Has its own disk

Container:

• Security is not good, container escape vulnerability
• Containers does not have kernel and services so less size
• chroot jails

Sandbox: is an isolated environment on a network, they are used to execute a suspicious in a safe manner. A code execution in sandbox cannot harm the host machine or the network. The goal of sandboxing is to take unknown files and detonate them within one of the VMs to determine if the file is safe for installation. This is mostly related to security. [8], [9]

• What happens in sandbox, stays in sandbox :)
• OS X is supporting sandboxing since OS X Lion released in 2011. The App Store requires applications to be sandboxed since March 2012. Microsoft Windows does not support app sandboxing by default, but it allows some of apps to be sandboxed.

POSIX

References

[1]

[2] https://github.com/tmux/tmux/wiki

[3] https://techdifferences.com/difference-between-microkernel-and-monolithic-kernel.html

[4] https://en.wikipedia.org/wiki/Microkernel

[5] https://www.cs.tau.ac.il/telux/lin-club_files/linux-boot/slide0002.htm

[6] https://www.thegeekstuff.com/2012/07/system-calls-library-functions/

[7] https://www.youtube.com/watch?v=aLjFxiWP5uQ

[8] https://searchsecurity.techtarget.com/answer/Whats-the-difference-between-software-containers-and-sandboxing

[9] https://en.wikipedia.org/wiki/Sandbox_(computer_security)

[10] https://stackoverflow.com/questions/18186929/what-are-the-differences-between-a-login-shell-and-interactive-shell