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Linux Networking Interfaces

Links: Linux Networking Index

Commands

Interfaces

  • List the interfaces on the host: ip link
  • See the IP address and other details of the interfaces: ip addr
  • See the IP address and other detail of a particular interface: ip addr show <interface-name>
  • Only see interfaces of particular type: ip addr show type <interface-type>
    • Example to only show bridge interface: ip addr show type bridge
  • Set IP addresses on the interface: ip addr add 192.168.1.12/24 dev eth0
    • Here dev stands for device.
    • These are valid only till a restart.
    • If we want to persist these changes we have to change the network interfaces file in the /etc directory.
  • Get statistics of each interface: ip -s link
    • To see the statistics of a particular interface: ip -s link show <interface-name>
  • We can bring interfaces up/down using the following commands:
    • ip link set <interface-name> up
    • ip link set <interface-name> down

Routes

  • View the routing table: ip route or route (gives output in a tabular format)
  • Add entries into the routing tables
    • ip route add 192.168.1.0/24 via 192.168.2.1
  • Delete entries from the routing table
    • ip route delete 192.168.2.1/23 via 10.11.12.3
  • Adding and deleting a route can also be performed using the route command.

Understanding the output of route command

attachments/Pasted image 20230525155117.jpg
- Here gateway is the router. - The 4 0's is a special value, it is like a * which means it matches any value. - The routing tables uses the longest prefix match so the closest match of the destination from these entries helps decide where to go. - NOT dependent on the order. - If you want to go to any destination within the network 192.168.1.0 you can find this within this network, 0.0.0.0 in the gateway means no need to use the router send directly using the lan connection. - You can also verify this using the arp -n command to see the computers to which you can send packets without going through the router.

Miscellaneous

  • View the arp cache: arp
  • Check if IP forwarding is enabled on the host:
    • cat /proc/sys/net/ipv4/ip_forward
    • 1 means it is enabled.

Interfaces

  • A network interface is the point of connection between a computer and a network.
    • A network interface is how the kernel links up the software side of networking to the hardware side.
  • The Linux system distinguishes two types of network interfaces – the physical network interface and the virtual network interface.
    • A physical network interface represents a network hardware device such as NIC (Network Interface Card), WNIC (Wireless Network Interface Card), or a modem.
    • A virtual network interface DOES NOT represent a hardware device but it is usually linked to one.
      • There are different kinds of virtual interfaces e.g. Loopback, bridges, VLANs, tunnel interfaces and so on
  • Linux systems use two different styles of naming the network interfaces.
    • The first style is the old-style name, such as eth0, eth1, and wlan0. 
    • The new ones are based on hardware locations like enp3s0 and wlp2s0.
  • A machine can have multiple interfaces which means it can have multiple IP addresses.
    • This means the machine can participate in many networks and have an IP address for each network.
      • attachments/Pasted image 20230525080401.jpg
  • Generally multiple interface means that the computer has an multiple NIC (network interface cards a.k.a network adapters).
    • NICs are integrated in the laptop but incase of a PC it might not come with a NIC and we would have to buy one and attach to the motherboard.
    • NICs use PCIE slots.
  • When you run a server or a program by default it runs on a particular network interface.
    • This means the program or server will be able to accept connections from only that network.
    • Example if we have a nodejs server running on the blue network then it won't be accessible from the green network or the yellow network.
      • attachments/Pasted image 20230525080720.jpg
    • If we choose 0.0.0.0 then our app will listen to all the interfaces.
    • We can find the interface our app is listening to using netstat -tpln
  • ip link command also shows us which interfaces are up (kernel level).
    • attachments/Pasted image 20230525081513.jpg
    • LOWER_UP means that we have established connection at the physical level.
  • The second line for each interface shows the link layer address associated with the device.
    • attachments/Pasted image 20230525091151.jpg

References

Last updated: 2023-05-29