Basics of SSH Usage

SSH or secured shell is a network protocol to communicate between multiple machines securely. Any system administration service, data transfer and control operation can be performed over SSH. The access through a command-line interface to another machine's shell (I use bash on Ubuntu) is the most general mode of interaction. It is a secure channel that commnunicates using a client-server architecture. The default TCP port used is 22 but others can also be used.

This article is a location to aggregate all the methods commonly used for shell access, SSH-key deployment for repositories and account, SSH-tunneling, file transfer over SSH and using SSH in VS Code IDE for software development on other machines on the local network. All discussions in this article are made for SSH protocol version 2.

The home/user/.ssh. folder in Linux machines stores all the important configurations and security keys. The most common files configuration files are config, authorized_keys and known_hosts.

• The config file has configuration for different services like git that use the SSH credentials for various connections.
• The known_hosts file has a list of public SSH keys of all the machines currently or previously connected to.
• The authorized_keys file has the public SSH keys of the machines that it shall accept incoming SSH connections from. This is a highly safe access control feature that ensures no unwarranted client machine can access the server machine. The SSH software compares the public SSH against the private key from the incoming connection machine to establish the secure connection.

SSH Command Line Access

SSH command line access is the simplest method for machines to communiate. Common SSH client software include Putty (Windows & Linux), OpenSSH (I used this on Ubuntu), WinSCP (Windows) and CyberDuck (Mac) among others. The basic commands for command-line access are:

• ssh <machine_username>@<machine_ip_address>
  Access the other machine's shell directly using port 22
  
• ssh <machine_username>@<machine_ip_address> -p <port_number>
  Access the other machine's shell directly using any port number other than 22.
  
• ssh <machine_username>@<machine_ip_address> -i <identity_file>
  Access the other machine's shell directly using a specific identity file, a file with a private key. By default, a local network connection uses the .ssh/id_rsa private key of the machine (on Linux machines) for connections. SSH connection to online services generally uses a .pem file as an identity file.
  

Handling SSH Keys

SSH keys are primarily public and private keys with the default names id_rsa and id_rsa.pub. These keys are responsible for establishment of the identity and and credentials of the machine.

The basic commands to manage SSH keys are:

• ssh-keygen
  The keygen program is used to generate the SSH key pair - one public key and another private key. It uses the RSA algorithm by default and saves the id_rsa and id_rsa.pub private and public files in the location /home/user/.ssh. Unless specified, the software asks the user for a paraphrase to generate the keys from.
• ssh-keygen -t <rsa | dsa | ecdsa | ed25519> -b <number bits>
  The key can be generated using different algorithms like Rivest-Shamir-Adleman(RSA), Digital Signature Algorithm(DSA), Elliptic Curve Sigital Signature Algorithm(ECDSA)) and ED25519 (specially available on OpenSSH) among others. Usually, at least 2048 or 4096 bits should be used for the RSA algorithm, which is widely supported but very old.
• ssh-keygen -f <filename>
  Save the SSH keypair in a particular location of the system.
• ssh-keygen -f <filename>
  Save the SSH keypair in a particular location of the system.

GRAPHS

• Sequential Representation: Adjacency matrices are used for sequential representations of relationships between the nodes in a graph. Here, the N nodes are laid out along the rows and the columns of a N x N matrix adjacency_matrix. Now, for a pair of nodes that have an undirected edge, the value of adjacency_matrix_{(nodeA, nodeB)} and adjacency_matrix_{(nodeB, nodeA)}is non-zero and often used to represent the weight of the edge. For a directed graph, from node A to node B, only adjacency_matrix_{(nodeA, nodeB)} value is non zero and the other pair adjacency_matrix_{(nodeB, nodeA)} is zero (rows and column node represents the start and end node respectively).
• Linked Representation: For this representation adjacency lists are used. An adjacency list for a node represents the other nodes linked to it as a linked list. It terminates in a null.

MATRICES

Matrices in Robotics

STACK

Stacks in Robotics

When a Depth-First Search is implemented, the children of the node being explored are all pushed into a stack and popped when explored.

MORE ABOUT USE OF STACKS.

QUEUE