Networking Review

• OSI Layered Architecture review:

• For list of supported protocols in your Linux OS see /etc/protocols
• IETF is the sole publisher of protocols since 1986
  • TCP/IP was started by US Department of Defense in the 1960’s
  • TCP/IP is an open standard (anyone can use it)
  • There were other “commercial” network communications that did not last:
    • Appletalk, NCP, IPX, SPX, NetBEUI, UUCP

Networking on Server vs. Client

• Servers:
  • Servers can have many interfaces depending on their roles
  • Servers generally want to have static network settings (stay the same) after each boot
  • May have to deal with many protocols and complex configurations (WAN, LAN, WLAN, vlans, bgp, static routes, vpn, etc.)
• Clients
  • Clients (Laptop or Desktop) have one or two interfaces (ex. LAN and WLAN)
  • The clients usually have dynamic configurations and may/should not retain that config after each boot-up
    • May have to dynamically or automatically adjust networking configuration to deal with changing conditions (DHCP, WIFI, etc)

Network interfaces and Naming in Linux

• Virtual and Physical interfaces:
  • Physical Interface: Ethernet, WLAN, FC (Fiber Channel)
  • Virtual interfaces: VPN, Bridge, VLAN, Loopback, etc
  • Virtual interfaces can be linked to existing physical interface
    • Overlaid interface (assigning additional IP addresses to the same physical interface)
    • VLAN (using IEEE 802.1q tagging)
• Typical interface naming
  • Physical Interface naming formula: Short name (from the device driver) + Instance number(s):
    • eth0, ens23, enp0s3, ens1p1, wlan0, etc…
  • Virtual Interfaces: intf0:0 (numbers between 0 to 255)
    • Example: eth0.2 = vlan 2 on eth0
  • Naming varies between vendors, protocols, and various GNU/Linux OS distros. Read the documentation for your distro, and documentation from the device and driver vendors for naming convention

Network Renderer in Linux

• NIC device driver in the Linux Kernel provides communication to the network hardware via a device file
  • For example your browser reads and writes to a /dev/eth0 file when communicating to the internet
• Network Renderers are daemons that are used to manage network configurations
  • Linux systemd (systemd-networkd)
  • Network Manager daemon
• You can configure or use scripts to work directly with the network renderer on the system
  • Learning all the network renderer interworking is complicated
  • Therefore usually we use a front-end utility to do this instead

Systemd-networkd

• Networkd is a network renderer in Linux that comes bundled with systemd
• It uses ini-style files to initiate network settings
• Directly reads configurations files on Startup
• Configuration files may be (depending on the distro) located at:
  • /etc/network/ (check out these directories in your VM!)
  • /etc/systemd/network
  • /usr/lib/systemd/network
  • /usr/local/lib/systemd/network
    

Network Manager

• Developed by Red Hat in 2004 to deal with “modern” networking needs.
• Default config file:
  • /etc/NetworkManager/NetworkManager.conf (is this present in your Ubuntu Server?)
• Many desktop distributions include:
  • Network Manager Daemon
  • GUI front end
    

Network Config in GUI

• Different distros use different GUI environments that come with different network configuration tools
  • SUSE has YaST, Others (including Ubuntu) may use GNOME, KDE, or other GUI environments
  • GUI tools are ideal for configuring Wifi on a laptop on the go, may not be ideal for configuring complex settings and protocols on servers
• Different GUI tools may/can use different network renderers on the back-end. If used incorrectly, settings may conflict with, or override the configuration files and commands that were entered by you!
• Read the Distro documentation!
  • For Ubuntu Desktop see here: https://help.ubuntu.com/stable/ubuntu-help/net.html.en

Network Configuration in CLI

• Instead of working with Network Renderer directly we may use various utilities to achieve the configurations we need
  • Ifconfig, ip, netplan, ethtool, etc…
• Different distros may come with different tools, but for the most part these utilities have been around for a long time and CLI commands are standard
• Note: Ubuntu 20.04 and many distros have deprecated ifconfig and have switched to more modern, more powerful ip and netplan utilities
  • You can choose to use networkd or Network Manager in Ubuntu

ip Command

Note: ifconfig is deprecated. You can still install ifconfig utility manually in Ubuntu via ethtools package

• ip utility is designed to be one command to rule them all! Functionally organized on Layer 2 and 3 of the network stack
• ip is capable of almost all networking-related tasks:
  • Displaying or Modifying Interface properties such as IP settings (L3), Link settings (L2)
  • Adding, Removing ARP Cache entries along creating new Static ARP entry for a host.
  • Displaying MAC addresses, and logical names associated with all the interfaces.
  • Displaying and modifying kernel routing tables (route).
• Read the Server Guide under IP Addressing section: https://ubuntu.com/server/docs/network-configuration
• Configuration changes using IP commands take effect immediately
• The configurations are NOT persistent in Ubuntu server (They are lost after a reboot). For persistent settings use Netplan instead!

ip Command Structure (how to use)

1. Call the IP command (i.e. ip or sudo ip)
2. Then use sub-commands to target protocol/layer:
   • address (Layer 3)
   • link (layer 2, physical interface itself)
   • maddr (layer 3, Multi-cast address)
   • neigh (layer 2, ARP tables)
   • route (layer 3, route tables)
3. Then use show or modifying sub-commands:
   • show
   • add, del, set
4. Target specific device or interface
   • dev <interface>
     

Here is a nice IP Command Cheat Sheet from RedHat: https://access.redhat.com/sites/default/files/attachments/rh_ip_command_cheatsheet_1214_jcs_print.pdf

Netplan for Persistent Configuration

• Visit Netplan website for introduction and documentation: https://netplan.io/
• Netplan uses YAML format for configuration files
• Reads configurations from files located at /etc/netplan/*.yaml
• Applies the setting from the YAML files one after another (numerically in order of file names) to the network renderer in use by the system during boot
• Currently Netplan supports Network Manager and system-networkd (you can choose via .yaml)
  

YAML Config File

# <- this is a comment
# Blank space indicate indentation (use space bar to insert appropriate number of them)!
network:
    version: 2      # We only use version: 2 
    renderer: <renderer name>     # use “networkd” or “networkmanager”
    ethernets:
       <device_name>:      # Use `ip addr show` to get the logical name of the interface (examples: eth1, ens33, or ens34) Or use `lshw –class network`
         dhcp4: <true/false>
         addresses: [<IP>/<mask>]
         gateway: <Gateway_IP>
         nameservers:
            addresses: [<DNS_IP_1>,<DNS_IP_1>]

# <- this is a comment
# Blank space indicate indentation (use space bar to insert appropriate number of them)!
network:
    version: 2      # We only use version: 2 
    renderer: <renderer name>     # use “networkd” or “networkmanager”
    ethernets:
       <device_name>:      # Use `ip addr show` to get the logical name of the interface (examples: eth1, ens33, or ens34) Or use `lshw –class network`
         dhcp4: <true/false>
         addresses: [<IP>/<mask>]
         gateway: <Gateway_IP>
         nameservers:
            addresses: [<DNS_IP_1>,<DNS_IP_1>]

• In the following examples we have two NICs on the server. Since our configuration was more complex we decided to break the configuration into two separate files (one for each NIC)
  • 50-comp1071.yaml
  • 80-comp1071.yaml
• Netplan reads through each .yaml file numerically and applies the settings
  • If an interface is repeated, then the configuration in the last file will override the previous configuration
    
    
• To have Netplan read your configuration use the following commands:
  • sudo netplan try: Try out the configuration for 120 Seconds to make sure everything is fine
  • sudo netplan apply: Apply configuration permanently
  • In case of a syntax error, this might help to debug your YAML files: sudo netplan --debug generate

Interesting Files, Utilities, and Commands

• ip
• netplan
• lshw –C network
• ping
• /etc/protocols
• /etc/netplan/*.yaml
• /etc/network/
• /etc/systemd/network
• /usr/lib/systemd/network
• /usr/local/lib/systemd/network