Networking Network Devices and Network Connections

Hostname

“-”, “_ “, and “. " characters are allowed.
Up to 253 characters.
Stored in /etc/hostname.
Can be viewed with several different commands, such as hostname, hostnamectl, uname, and nmcli, as well as by displaying the content of the /etc/hostname file.

View the hostname:

hostnamectl --static

hostname

uname -n

cat /etc/hostname

Lab: Change the Hostname

Server1

Open /etc/hostname and change the entry to server10.example.com
Restart the systemd-hostnamed service daemon

sudo systemctl restart systemd-hostnamed

confirm

hostname

server2

Change the hostname with hostnamectl:

sudo hostnamectl set-hostname server21.example.com

Log out and back in for the prompt to update
Change the hostname using nmcli

nmcli general hostname server20.example.com

Hardware and IP Addressing

Ethernet Address

48-bit address that is used to identify the correct destination node for data packets transmitted from the source node.
The data packets include hardware addresses for the source and the destination node.
Also referred to as the hardware, physical, link layer, or MAC address.

List all network interfaces with their ethernet addresses:

ip addr | grep ether

Subnetting

Network address space is divided into several smaller and more manageable logical subnetworks (subnets).
Benefits:
- Reduced network traffic
- Improved network performance
- de-centralized and easier administration
- uses the node bits only
Results in the reduction of usable addresses.
All nodes in a given subnet have the same subnet mask.
Each subnet acts as an isolated network and requires a router to talk to other subnets.
The first and the last IP address in a subnet are reserved. The first address points to the subnet itself, and the last address is the broadcast address.

IPv4

View current ipv4 address:

ip addr

Classful Network Addressing

See Classful ipv4

IPv6 Address

See ipv6

The ip addr command also shows IPv6 addresses for the interfaces:

[root@server200 ~]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:b9:4e:ef brd ff:ff:ff:ff:ff:ff
    inet 172.16.1.155/20 brd 172.16.15.255 scope global dynamic noprefixroute enp0s3
       valid_lft 79061sec preferred_lft 79061sec
    inet6 fe80::a00:27ff:feb9:4eef/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

Tools:

ping6
traceroute6
tracepath6

Protocols

Defined in /etc/protocols
Well known ports are defined in /etc/services

cat /etc/protocols

TCP and UDP Protocols

See IP Transport and Applications and tcp_ip_basic

ICMP

Send two pings to server 20

ping -c2 192.168.0.120

Ping the server’s loopback interface:

ping 127.0.0.1

Send a traceroute to server 20

traceroute 192.168.0.120

Or:

tracepath 192.168.0.120

ICMPv6

IPv6 version of ICMP
enabled by default

Ping and ipv6 address:

ping6

Trace a route to an IPv6 address:

tracepath6

traceroute6

Show IPv6 addresses:

ip addr | grep inet6

Network Manager Service

Default service in RHEL for network:

interface and connection configuration.
Administration.
Monitoring.

NetworkManager daemon

Responsible for keeping interfaces and connection up and active.
Includes:
- nmcli
- nmtui (text-based)
- nm-connection-editor (GUI)
Does not manage loopback interfaces.

Interface Connection Profiles

Configuration file on each interface that defines IP assignments and other relevant parameters for it.
The networking subsystem reads this file and applies the settings at the time the connection is activated.
Connection configuration files (or connection profiles) are stored in a central location under the /etc/NetworkManager/system-connections directory.
The filenames are identified by the interface connection names with nmconnection as the extension.
Some instances of connection profiles are: enp0s3.nmconnection, ens160.nmconnection, and em1.nmconnection.

On server10 and server20, the device name for the first interface is enp0s3 with connection name enp0s3 and relevant connection information stored in the enp0s3.nmconnection file.

This connection was established at the time of RHEL installation. The current content of the file from server10 are presented below:

[root@server200 system-connections]# cat /etc/NetworkManager/system-connections/enp0s3.nmconnection 
[connection]
id=enp0s3
uuid=45d6a8ea-6bd7-38e0-8219-8c7a1b90afde
type=ethernet
autoconnect-priority=-999
interface-name=enp0s3
timestamp=1710367323

[ethernet]

[ipv4]
method=auto

[ipv6]
addr-gen-mode=eui64
method=auto

[proxy]

Each section defines a set of networking properties for the connection.

Directives

Any description given to this connection. The default matches the interface name.

uuid

The UUID associated with this connection

type

Specifies the type of this connection

autoconnect-priority

If the connection is set to autoconnect, connections with higher priority will be preferred. A higher number means higher priority. The range is between -999 and 999 with 0 being the default.

interface_name

Specifies the device name for the network interface

timestamp

The time, in seconds since the Unix Epoch that the connection was last activated successfully. This field is automatically populated each time the connection is activated.

address1/method

Specifies the static IP for the connection if the method property is set to manual. /24 represents the subnet mask.

addr-gen-mode/method

Generates an IPv6 address based on the hardware address of the interface.

View additional directives:

man nm-settings

Naming rules for devices are governed by udevd service based on:

Device location
Topology
setting in firmware
virtualization layer

Understanding Hosts Table

See DNS and Time Synchronization

/etc/hosts file

Table used to maintain hostname to IP mapping for systems on the local network, allowing us to access a system by simply employing its hostname.

Each row in the file contains an IP address in column 1 followed by the official (or canonical) hostname in column 2, and one or more optional aliases thereafter.

EXAM TIP: In the presence of an active DNS with all hostnames resolvable, there is no need to worry about updating the hosts file.

As expressed above, the use of the hosts file is common on small networks, and it should be updated on each individual system to reflect any changes for best inter-system connectivity experience.

Networking DIY Challenge Labs

Lab: Update Hosts Table and Test Connectivity.

Add both server10 and server20’s interfaces to both server’s /etc/host files:

192.168.0.110  server10.example.com  server10 <-- This is an alias
192.168.0.120  server20.example.com  server20   
172.10.10.110  server10s8.example.com   server10s8
172.10.10.120  server20s8.example.com   server20s8

Send 2 packets from server10 to server20’s IP address:

ping -c2 192.168.0.120

Send 2 pings from server10 to server20’s hostname:

ping -c2 server20

Lab 15-1: Add New Interface and Configure Connection Profile with nmcli

Add a third network interface to rhel9server40 in VirtualBox.
As user1 with sudo on server40, run ip a and verify the addition of the new interface.
Use the nmcli command and assign IP 192.168.0.40/24 and gateway 192.168.0.1

[root@server40 ~]# nmcli c a type Ethernet ifname enp0s8 con-name enp0s8 ip4 192.168.0.40/24 gw4 192.168.0.1

Deactivate and reactivate this connection manually.

[root@server40 ~]# nmcli c d enp0s8
Connection 'enp0s8' successfully deactivated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

[root@server40 ~]# nmcli c s
NAME    UUID                                  TYPE      DEVICE 
enp0s3  6e75a5e4-869b-3ed1-bdc4-c55d2d268285  ethernet  enp0s3 
lo      66809437-d3fa-4104-9777-7c3364b943a9  loopback  lo     
enp0s8  9a32e279-84c2-4bba-b5c5-82a04f40a7df  ethernet  --     
[root@server40 ~]# nmcli c u enp0s8
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/4)

[root@server40 ~]# nmcli c s
NAME    UUID                                  TYPE      DEVICE 
enp0s3  6e75a5e4-869b-3ed1-bdc4-c55d2d268285  ethernet  enp0s3 
enp0s8  9a32e279-84c2-4bba-b5c5-82a04f40a7df  ethernet  enp0s8 
lo      66809437-d3fa-4104-9777-7c3364b943a9  loopback  lo

Add entry server40 to server30’s hosts table.

[root@server30 ~]# vim /etc/hosts
[root@server30 ~]# ping server40
PING server40.example.com (192.168.0.40) 56(84) bytes of data.
64 bytes from server40.example.com (192.168.0.40): icmp_seq=1 ttl=64 time=3.20 ms
64 bytes from server40.example.com (192.168.0.40): icmp_seq=2 ttl=64 time=0.628 ms
64 bytes from server40.example.com (192.168.0.40): icmp_seq=3 ttl=64 time=0.717 ms
^C
--- server40.example.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2009ms
rtt min/avg/max/mdev = 0.628/1.516/3.204/1.193 ms

Lab: Add New Interface and Configure Connection Profile Manually (server30)

Add a third network interface to RHEL9server30 in VirtualBox.

run ip a and verify the addition of the new interface.

Use the nmcli command and assign IP 192.168.0.30/24 and gateway 192.168.0.1

nmcli c a type Ethernet ifname enp0s8 con-name enp0s8 ip4 192.168.0.30/24 gw4 192.168.0.1

Deactivate and reactivate this connection manually. Add entry server30 to the hosts table of server 40

[root@server30 system-connections]# nmcli c d enp0s8
Connection 'enp0s8' successfully deactivated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)

[root@server30 system-connections]# nmcli c u enp0s8
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/4)

/etc/hosts

192.168.0.30 server30.example.com server30

ping tests to server30 from server 40

[root@server40 ~]# ping server30
PING server30.example.com (192.168.0.30) 56(84) bytes of data.
64 bytes from server30.example.com (192.168.0.30): icmp_seq=1 ttl=64 time=1.59 ms
64 bytes from server30.example.com (192.168.0.30): icmp_seq=2 ttl=64 time=0.474 ms
^C
--- server30.example.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.474/1.032/1.590/0.558 ms

Or create the profile manually and restart network manager:

[connection]
id=enp0s8
type=ethernet
interface-name=enp0s8
uuid=92db4c65-2f13-4952-b81f-2779b1d24a49

[ethernet]

[ipv4]
method=manual
address1=10.1.13.3/24,10.1.13.1

[ipv6]
addr-gen-mode=default
method=auto

[proxy]

Administration Tools

ip

Display monitor and manage network interfaces, routing, connections, traffic, etc.

ifup

Brings up an interface

ifdown

Brings down an interface

nmcli

Creates, updates, deletes, activates, and deactivates a connection profile.

`nmcli` command

Create, view, modify, remove, activate, and deactivate network connections.
Control and report network device status.
Supports abbreviation of commands.

Operates on 7 different object categories.

general
networking
connection (c)(con)
device (d)(dev)
radio
monitor
agent

[root@server200 system-connections]# nmcli --help
Usage: nmcli [OPTIONS] OBJECT { COMMAND | help }

OPTIONS
  -a, --ask                                ask for missing parameters
  -c, --colors auto|yes|no                 whether to use colors in output
  -e, --escape yes|no                      escape columns separators in values
  -f, --fields <field,...>|all|common      specify fields to output
  -g, --get-values <field,...>|all|common  shortcut for -m tabular -t -f
  -h, --help                               print this help
  -m, --mode tabular|multiline             output mode
  -o, --overview                           overview mode
  -p, --pretty                             pretty output
  -s, --show-secrets                       allow displaying passwords
  -t, --terse                              terse output
  -v, --version                            show program version
  -w, --wait <seconds>                     set timeout waiting for finishing operations

OBJECT
  g[eneral]       NetworkManager\'s general status and operations
  n[etworking]    overall networking control
  r[adio]         NetworkManager radio switches
  c[onnection]    NetworkManager\'s connections
  d[evice]        devices managed by NetworkManager
  a[gent]         NetworkManager secret agent or polkit agent
  m[onitor]       monitor NetworkManager changes

3. connection

Activates, deactivates, and administers network connections.

Options:

show (list connections)
up/down (Brings connection up or down)
add(a) (adds a connection)
edit (edit connection or add a new one)
modify (modify properties of a connection)
delete(d) (delete a connection)
reload (re-read all connection profiles)
load (re-read a connection profile)

4. Device