Communication Protocols

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

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
    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.


    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/

    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/ [email protected]

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 [email protected]: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 [email protected]:/some/remote/directory
  • Copy the directory local_directory from the local host to a remote host's directory remote_directory

    scp -r local_directory [email protected]:/some/remote/directory/remote_directory
  • Copy the file fr1.txt from remote host rh1 to remote host rh2

    scp [email protected]:/some/remote/directory/fr1.txt [email protected]:/some/remote/directory/
  • Copy multiple files from a local directory to a remote host home directory

    scp one_file.txt another_file.txt [email protected]:

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: with IP-192.XXX.0.100

  • NFS Client: 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.