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Networking.md

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Networking

Notes

Do not use ifconfig anymore. It's been deprecated for over 20 years now. Ifconfig does not support secondary ip addresses for example.

See information for your NIC

Display IP addresses

ip ad or nmcli

Display route/gateway information

ip -4 route ip -6 route

Legacy command to see route information. netstat -rn or route -n

Display ARP table

arp -a

Display the ARP default timeout

cat cat /proc/sys/net/ipv4/neigh/default/gc_stale_time

Change settings for your NIC

Assign IP address

You can use the "graphical" nmtui tool to configure your network connection. If you use nmtui, remember to set the subnet mask when you enter the ip address. Using nmtui for the exam is better, it will save time.

nmtui

Another way is to use the nmcli tool. Remember to swap out the name for the correct name you see when you execute nmcli connection show nmcli connection edit enp1s0

Now you are in the nmcli interface. Type print to see detailed information for the connection named "enp1s0". To see the name of all your connection, nmcli connection show and nmcli device show

Find the connection name, nmcli connection show Make sure that the bash-completion package is installed when working with nmcli.

Assign the IP address to the correct connection name. nmcli connection modify enp0s3 ipv4.addresses 192.168.1.21/24

ip is an excellent command for troubleshooting but using the ip command only changes runtime environment, it does not change anything in the configuration files.

Here are a few nmcli examples:

nmcli device status
nmcli connection show -active
If the connection is unmanaged or not connecting, try this command.
sudo nmcli connection mod <connection-name> connection.autoconnect yes

Activate Changes
nmcli connection reload
This only makes the NM aware of the changes.
You have to take the connection down and then up (nmcli con down NAME; nmcli con up NAME) or most changes can be applied directly with nmcli dev reapply NAME

Change the gateway

nmcli connection modify enp0s3 ipv4.gateway 192.168.1.254

Use static instead of DHCP

nmcli connection modify enp0s3 ipv4.method static

Disabling and enabling an interface

ip link set ens33 down ip link set ens33 up

Changing the MTU for e.g., iSCSI

nmcli con mod ensp92 802-3-ethernet.mtu 9000

Firewall (netfilter/nftables)

On the exam it's better to restart services than to reload them.

The netfilter framework in the Linux kernel manages firewall operations, and it forwards specific operations to kernel modules.

  • Packet filtering
  • Network address translation
  • Port forwarding

Firewalld

Firewalld is a good interface to create and manage a simple firewall but the framework behind it is the Netfilter (nftables) firewall.

To see config files for services, check out. "/usr/lib/firewalld/"

General commands

firewall-cmd --list-all

firewall-cmd --get-services

firewall-cmd --add-service squid --permanent
Remember to use the permanent switch, otherwise the rule is written only to the runtime and is lost if you restart firewalld or the server!

firewall-cmd --reload

Add IP address to the trusted zone. firewall-cmd --zone=trusted --add-source=192.168.124.1 --permanent

List configuration for all zones. firewall-cmd --list-all-zones

Zones

A zone is a default configuration to which network cards can be assigned to apply specific settings.

Service

You only need to know the service part for the RHCSA exam!

Ports

Optional elements to allow access to specific ports.

Network Sockets

Use ss to show socket information. This will show all connections.

ss -tu shows connected TCP and UDP sockets.

ss -tua shows connected TCP and UDP sockets + sockets in a listening state.

ss -tulpn Shows TCP and UDP sockets in a listening state, it also adds process names or PID to the output.

Debug

  • Use nm-connection-editor if you have issues with certs for you network cards.
  • Make sure your subnet mask is correct.
  • Ping your gateway to see if you can reach the router.
  • DNS not working, check "/etc/resolv.conf" to make sure it's correct.