Communication Protocols

Protocols are a set of rules or standards that define the communication between devices on a network.

• Generalities of a Service

• The ssh Protocol

• The scp Protocol

• The nfs Protocol

Generalities of a Service

A process is a running program at a particular instant of time.

The process refers to an opening of a Web Browser or any other visible program or action for the user, but this term also includes programs that are running in the background waiting to be called by the system. Those programs can be services that offer remote connection, sending of mail, or translation of IPs into readable URLs.

These services are identified by a number of ports defined by the Assigned Numbers RFC.

The configuration of services is in /etc/services and includes the name, the port that defines the service, and which transport protocol is used (UDP or TCP) for each one.

The ssh Protocol

This protocol enables secure connection to the SSH server on a remote machine.

• Installation of the package

  By default, in CentOS 7, the SSH package comes installed, but if not, please run:

  yum install openssh openssh-server openssh-clients openssl-libs

  It installs the openssh package to enable SSH as a server and as a client.

  If you need additional information about yum commands, you can visit this link.

• The default configuration file

  The default configuration file and settings for the SSHD daemon is in /etc/ssh/sshd_config.

  cp /etc/ssh/sshd_config /etc/ssh/sshd_config.ori

  This creates a copy of the original configuration file in order to prevent damage or mistakes during a custom configuration.

  Then, you can customize the configuration in the /etc/ssh/ssh_config file with these options:

  Port 22
    PermitRootLogin without-password
    PermitRootLogin yes
    PasswordAuthentication yes
    ForwardAgent yes
    ForwardX11 yes

  Furthermore, to have the ability to run the protocol with the name of the servers such as ssh server_name, create a file ~/.ssh/config, and customize it with:

  Host shortcut_name
    HostName 0.1.2.3
    Port 22
    User user
    ServerAliveInterval 120
    IdentityFile ~/.ssh/my_key.pem

  Then, you will be able to enter the server called shortcut_name with SSH by using:

  ssh shortcut_name

• Restart the SSHD service

  Once you make the configuration changes, you can save and close the file. For the changes to take effect, you should restart the SSH daemon.

  systemctl restart sshd.service

  This command is used in case the SSHD service is enabled. To check the current status of the service, please read more about the status of a service.

• Generate an SSH Key

  To secure the transmission of information, SSH employs different types of data manipulation techniques that include forms of asymmetrical encryption such as an SSH key.

  ssh-keygen

  Press Enter to accept the default location and filename which is ~/.ssh/id_rsa. Then press Enter, then Enter again to not set a passphrase when prompted.

  Make sure the SSH key was successfully created by checking the encrypted content at ~/.ssh/id_rsa.pub.

  This file must have the permission 600. To check it please run ls -AhlF ~/.ssh.

  Finally, to copy the SSH key to a server, please run ssh-copy-id -i ~/.ssh/id_rsa.pub user@server

The scp Protocol

This protocol allows files to be copied to, from, or between different hosts. It uses SSH for data transfer and provides the same authentication and same level of security as SSH.

• Copy the file remote_file.txt from a remote host to the local host

scp user@remotehost:remote_file.txt /some/local/directory

• Copy the file local_file.txt from the local host to a remote host directory

  scp local_file.txt user@remotehost:/some/remote/directory

• Copy the directory local_directory from the local host to a remote host's directory remote_directory

  scp -r local_directory user@remotehost:/some/remote/directory/remote_directory

• Copy the file fr1.txt from remote host rh1 to remote host rh2

  scp user@rh1:/some/remote/directory/fr1.txt user@rh2:/some/remote/directory/

• Copy multiple files from a local directory to a remote host home directory

  scp one_file.txt another_file.txt user@remotehost:

The nfs Protocol

To set up NFS mounts, we will need at least two Linux/Unix machines. Here we will be using two servers.

• NFS Server: server.org with IP-192.XXX.0.100

• NFS Client: client.org with IP-192.XXX.0.101

NFS Server

• Configure export directory

  For sharing a directory with NFS, we need to make an entry in the /etc/exports configuration file. Let's create a new directory named nfsshare in the / partition of the server.

  Then, we need to make an entry in /etc/exports and restart the services to make our directory shareable in the network.

  mkdir /nfsshare
  
  vi /etc/exports
  /nfsshare 192.XXX.0.101(rw,sync,no_root_squash)
  
  service autofs restart

  It displays a directory in the / partition named "nfsshare" which is being shared with client IP "192.XXX.0.101" with read and write privileges. You can also use the hostname of a server.

NFS Client

• Mount a shared directory on an NSF client

  To mount a directory in our server to access it locally, we need to find out what shares are available on the remote server or NFS Server with showmount.

  showmount -e 192.XXX.0.100
  Export list for 192.XXX.0.100:
  /nfsshare 192.XXX.0.101

  This command shows that a directory named nfsshare is available at "192.XXX.0.100" to share with your server.

• To mount a shared NFS directory permanently, we can use following mount command:

  vi /etc/fstab
  192.XXX.0.100:/nfsshare /mnt  nfs defaults 0 0
  
  service autofs restart

  With vi /etc/fstab, we are setting the IP:name_directory to be mounted, and it will be mounted on /mnt. You can verify it with mount | grep nfs.