Weight: 4
Candidates should be able to view, change and verify configuration settings on client hosts.
Key Knowledge Areas
- Understand basic TCP/IP host configuration
- Configure ethernet and wi-fi network configuration using NetworkManager
- Awareness of systemd-networkd
Terms and Utilities
/etc/hostname/etc/hosts/etc/nsswitch.conf/etc/resolv.confnmclihostnamectlifupifdown
Intro
As we saw in the previous section, every PC, server, laptop, phone, .. should have an IP configuration (IP, Netmask, Default gateway, DNS, ..) to work properly in the network. This can be done in various ways. Some devices like laptops are changing their network all the time and should be able to keep up with the changes. Some servers remain in the same location (physical and network wise) all their life and should persist this configuration after restarts, outages, upgrade and HW changes.
In this section we will see how this can be achieved in modern GNU/Linux systems.
Network Interface
The NIC (or Network Interface Card) is the physical network hardware in your computer. This can be the chip+antenna in your mobile phone or an Ethernet Card connected to a network cable on your PC.
In older systems, these were called things like eth0, eth1, eth2, ... where 0, 1 & 2 were decided by the kernel - mostly based on the order of loading the drivers. In recent Linux machines the NICs are called by wlan0, eno1, ens1, enp3s2 and such. This is based on some more concrete data like being an wireless or ethernet, PCI (ens) or bus like (enp).
The ip command can show these:
➜ ~ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: wlp108s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DORMANT group default qlen 1000
link/ether 00:bb:60:97:6b:07 brd ff:ff:ff:ff:ff:ff
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
link/ether 02:42:75:d3:e6:ff brd ff:ff:ff:ff:ff:ff
The
lois a virtual network adapter called the loopback device. It is always there and points to "this device or 127.0.0.1 as IPv4 calls it".
Configuring NICs
In older distributions, the ifconfig was used to check / configure the IP settings on NICs. Have a look:
$ ifconfig
enp0s25: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether f0:de:f1:62:c5:73 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
device interrupt 20 memory 0xd1500000-d1520000
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1 (Local Loopback)
RX packets 560719 bytes 339937974 (324.1 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 560719 bytes 339937974 (324.1 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.35 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::8ea9:82ff:fe7b:8906 prefixlen 64 scopeid 0x20<link>
ether 8c:a9:82:7b:89:06 txqueuelen 1000 (Ethernet)
RX packets 2325385 bytes 2629859900 (2.4 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 2023796 bytes 510997240 (487.3 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
It is also possible to use ifconfig to change the network configurations, but you should have root access:
$ sudo ifconfig enp0s25 192.168.42.42
password for jadi:
$ ifconfig enp0s25
enp0s25: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 192.168.42.42 netmask 255.255.255.0 broadcast 192.168.42.255
ether f0:de:f1:62:c5:73 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
device interrupt 20 memory 0xd1500000-d1520000
$
In case you want to change the netmask of an interface, do ifconfig eth0 netmask 255.255.0.0 or as most of us used to do, issue both in one command:
# ifconfig eth0 192.168.42.42 netmask 255.255.255.0
It is also possible to turn the interfaces up and down (on and off) using predefined configurations by:
$ sudo ifconfig enp0s25 down
[sudo] password for jadi:
$ ifconfig
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1 (Local Loopback)
RX packets 562273 bytes 340257228 (324.4 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 562273 bytes 340257228 (324.4 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.35 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fe80::8ea9:82ff:fe7b:8906 prefixlen 64 scopeid 0x20<link>
ether 8c:a9:82:7b:89:06 txqueuelen 1000 (Ethernet)
RX packets 2330388 bytes 2634026235 (2.4 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 2027352 bytes 511549072 (487.8 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
As you can see, downing the interface removed it from the list of active interfaces, using switch -a will tell the ifconfig to show ALL interfaces, even if they are down.
In many systems there are ifup and ifdown commands directly to up and down interfaces easily. They work just like ifup eth0.
These predefined configs are located at /etc/network/interfaces on Debian based machines and at /etc/sysconfig/network-scripts/ in RPM based distro.
This is a sample of such file on a RedHat based distro:
$ cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
ONBOOT=yes
TYPE=Ethernet
IPADDR=192.168.1.10
NETMASK=255.255.255.0
DNS1=4.2.2.4
On RPM systems, the default gateway is configured via the below file:
cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=lpictest
GATEWAY=192.168.1.1
On Debian based systems (including Ubuntu) the main configuration file for network interfaces is /etc/network/interfaces. This one file has the configuration for all the interfaces. Have a look:
auto lo
iface lo inte loopback
auto eth0
#ifconfig eth0 inet dhcp
iface eth0 inet static
address 192.168.1.10
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 4.2.2.4
ip
Recent distributions are mostly using the ip command. This command can do lots of things including and not limited to showing and configuring the IP addresses, netmasks, default gateways & routing rules.
ip addr add 172.19.1.10/24 dev eth2 # temporary adding an IP
ip addr show eth2
ip addr del 172.19.1.10/24 dev eth2 # deleting an IP address
ip link set eth2 up # brining a NIC up
ip route show
ip route add default via 192.168.1.1 # add default gateway
Please note that above commands do temporary changes which will be lost after restarting the NetworkManager service. If you need permanent changes, it should be done using the configuration files or Network Manager interfaces.
NetworkManager & nmcli
In recent years, NetworkManager services has gained a lot of popularity. This service can "watch" the status of network and various configuration and configure the network cards (specially the wifi ones) accordingly. This is what makes our laptop connected whenever we open it in an area with a known WiFi or ask about the password if we want to connect to a new network or assign IP addresses as soon as we connect the cable to our Ethernet card. This IP assignment might happen via the "permanent IP configuration" on your device or a protocol called DHCP. When using DHCP (Dynamic Host Configuration Protocol), your computer asks a DHCP server (say your home's wifi router) about the IP, Netmask, Default gateway, DNS and other stuff and sets them.
By default, NetworkManager daemon controls the networks which are not mentioned in /etc/network/interfaces. This service is running in the background and controls the NICs which are not configured there. Various frontend GUI (graphical user interface) or TUI (textual user interface. try nmtui ) or CLI (command line interfaces) programs exists to control or configure the NetworkManager daemon. If you are using a Desktop Linux, you've probably already used / know one (say the network manager applet). Here I will show you how to use the nmcli from the command line.
We always call the nmcli with one of it various commands, here is a list:
| Command | Usage |
|---|---|
| general | NetworkManager’s general status and operations. |
| networking | Overall networking control. |
| radio | NetworkManager radio switches. |
| connection | Controlling the connection |
| device | Devices controlled by NetworkManager |
| agent | secret or polkit agent |
| monitor | Montor the changes |
For example, we can check the current status with the general command:
➜ ~ nmcli general
STATE CONNECTIVITY WIFI-HW WIFI WWAN-HW WWAN
connected full enabled enabled missing enabled
Or if you want to check the devices or list of Wi-Fi connections:
➜ ~ nmcli device
DEVICE TYPE STATE CONNECTION
wlp108s0 wifi connected Sharm Bar Sansoor 5
docker0 bridge connected (externally) docker0
lo loopback connected (externally) lo
p2p-dev-wlp108s0 wifi-p2p disconnected --
➜ ~ nmcli device wifi
IN-USE BSSID SSID MODE CHAN RATE SIGNAL BARS SECURITY
6C:AD:EF:38:13:38 AxLTE Infra 3 270 Mbit/s 84 ▂▄▆█ WPA2
00:E0:4C:93:1D:B8 Lanat Be Sansoorchi Infra 6 130 Mbit/s 59 ▂▄▆_ WPA2
* 24:F5:A2:42:DE:CE Sharm Bar Sansoor 5 Infra 36 540 Mbit/s 47 ▂▄__ WPA2
30:A2:20:DD:8B:54 AvinaAmin Infra 7 270 Mbit/s 29 ▂___ WPA1 WPA2
30:85:A9:8C:71:2C bahram Infra 11 65 Mbit/s 29 ▂___ WPA2
To connect to a Wi-Fi network, you're a do:
nmcli device wifi connect AxLTE password AFunkyPassword
Fancy Names for Computers
hostname
Remembering IP addresses are easy for robots but not for humans. Thats why we have "hostname"s. A hostname is a like a contact list where you just tell "call Jadi" and the system known my phone number. If you check your /etc/hostname, you will see your machines name there. Although you can change it temporarily (or permanently). The command is hostnamectl.
[funlap ~]# hostnamectl set-hostname mycoolmachine
[funlap ~]# hostname
mycoolmachine
[funlap ~]# cat /etc/hostname
mycoolmachine
[funlap ~]# bash
[mycoolmachine ~]#
Or you can change it as transient, which is a temporary change using the --transient switch.
It is also possible to define a "pretty" name for your computer so other systems might show it in their interfaces more nicely:
[mycoolmachine ~]# hostnamectl --pretty set-hostname "LAN Shared Storage"
[mycoolmachine ~]# hostnamectl status
Static hostname: mycoolmachine
Pretty hostname: LAN Shared Storage
Icon name: computer-convertible
Chassis: convertible
Machine ID: 0b126c4b6f4347168140eaa6202ce8be
Boot ID: 675eff37f42648c6bdea31177596557f
Operating System: Manjaro Linux
Kernel: Linux 6.1.38-1-MANJARO
Architecture: x86-64
Hardware Vendor: Dell Inc.
Hardware Model: Latitude 7390 2-in-1
Firmware Version: 1.30.0
/etc/hosts
This file contains a list of IPs and their corresponding names, including your own computers.
[mycoolmachine ~]# head -20 /etc/hosts
##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost funlife db
255.255.255.255 broadcasthost
::1 localhost
##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
198.74.56.50 jobs.jadi.net
192.168.1.22 amoledtesting
67.217.170.72 vps
So when you need to reach a machine by its name, your OS will now which IP to reach.
DNS configuration
DNS (which stands for Domain Name System) is a server which translates human-readable domain names (or more technically, text based domain names) to the corresponding IP addresses. You have to configure your computer to use a DNS so it will know which IP to contacted if you wanted to reach linux1st.com (and donate maybe).
This configuration can be found in /etc/resolve.conf.
nameserver 192.168.1.1
nameserver 4.2.2.4
domain jadi.net
search jadi.net company.com
Here I'm telling my computer to contact the DNS on my home network (192.168.1.1) or a DNS located at 4.2.2.4 if it needed to translate an address to an IP.
The domain configuration sets a local domain name so the machines in this domain will be able to use a short name (tv, instead of tv.jadi.net) and the search config does kind the same and tells the resolver to search for tv.jadi.net and tv.company.com if it was trying to resolve tv.
nsswitch
The /etc/nsswitch.conf file is used to configure which services are to be used to determine information such as hostnames, password files, and group files. Mine is
# cat /etc/nsswitch.conf
# Begin /etc/nsswitch.conf
passwd: files
group: files
shadow: files
publickey: files
hosts: files dns myhostname
networks: files
protocols: files
services: files
ethers: files
rpc: files
netgroup: files
# End /etc/nsswitch.conf
So if someone wants to check a password, the system will try the password file on the system. Or if they want to check an ip address of a hostname, my config says hosts: files dns myhostname so the computer first tries the files (/etc/hosts) and then goes for DNS. If I reverse these and change the line to
hosts: dns files
Any resolve request will be sent to a DNS server first and the /etc/hosts will be used only if the DNS servers answers "I don't know!"
| ← 109.1 Fundamentals of internet protocols | 109.3 Basic network troubleshooting → |