Network Configuration Troubleshooting
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Video Transcription
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>> Hey, there Cybrarians and welcome back to
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the Linux plus course here at Cybrary.
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I'm your instructor Rob Gills,
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and in today's lesson, we're going to be
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covering network configuration troubleshooting.
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Upon completion of today's lesson,
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you're going to be able to understand
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common network interface locations
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as well as work with the IP,
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arp, and nmcli commands.
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Network interface management was covered in Module 3,
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but we're going to recap some of the content here,
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and that's because we need
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this information to talk about network troubleshooting.
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It's just a good idea to refresh at this point.
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We're also going to talk a little
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bit more about some of the commands that we
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saw in Module 3 because we're going to use
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these when we're troubleshooting
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network connections and network connectivity.
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First off, we need to know where to
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look for interface configuration information,
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because we're going to use the interface configuration
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when doing our troubleshooting
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>> to answer questions like,
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>> well, what's the MAC address for
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this interface and what subnet is in use.
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Now when we're on
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a Red Hat-based distribution like CentOS,
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we can use the /etc/sysconfig/network-scripts directory
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because that holds persistent
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network connection information,
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and each interface is going to have
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its own configuration file inside of this directory.
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There are a few important configuration items or
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lines that we'll see in
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these interface configuration files,
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for example, we might see the boot protocol or boot
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protoline being set to either static or DHCP.
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If you see that it's set to static,
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then it also gives us the ability to
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specify a static IP address and that set
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with the IPADDR or netmask or the gateway.
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Next up, if we were looking at in
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Ubuntu or Debian-based system,
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the /etc/netplan directory is where we're starting to
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see IP address management use in Ubuntu.
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This replace the /etc/network/interfaces
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and ifup and ifdown
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directories that were used previously.
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Nowadays, network configuration is being set in
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a single net plan YAML file by default.
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There are some syntax items to
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consider when you're looking at a net plane YAML file,
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for example, the interface name will be
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the interface under which all configs live.
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We might see DHCPv4.
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This will be set to DHCP for yes.
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Yes will be set for DHCP or you might see no on
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DHCP4 if we're doing static IP addressing.
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Then if we have static IP addressing turned on,
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we can specify IP addresses that we want to use,
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as well as the gateway.
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Now most modern Linux distros
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are actually starting to use
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network manager for the detection
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and management of network connections.
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The nmcli command is a command-line tool
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that you use to configure network manager.
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This is a little bit easier to use than trying to open
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a configuration file and
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parse through it when you're doing troubleshooting,
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and one of the really nice things about nmcli
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is that it also allows you to do tab completion.
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If you get lost, if you're trying to
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figure out what to do or what command to run,
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just a tab a few times and it'll give you a list of
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options that you can use at that point that you're at.
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Now nmcli can be used to manage the network connection,
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but we can also look at a specific device
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or view the network device status,
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for example, we could do nmcli connection show,
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that'll show us all of the network connections,
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we can do nmcli device show to
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get statistics about that device,
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and we can get the status of
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the device by doing an nmcli device status,
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they'll tell us if that device
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is connected or disconnected.
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Now the IP command,
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this is really helpful when you just
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need to find the IP address info.
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The IP command superseded
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the ifconfig command for configuring IP addressing.
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For example, to list all the IP
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>> addresses on the system.
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>> We can just do ip addr show,
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that's just short for address ip addr show.
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We can also do ip addr add IP dev device name,
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and we're going to assign that IP to
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that specified network interface
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>> that's specified device.
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>> Then if we wanted to enable or disable an interface,
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we can do that with the ip link set command.
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We say ip link
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set and we provide it with the interface name,
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and we say up to enable it,
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or ip link set interface down
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>> to disable that interface.
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>> Now the arp command or
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Address Resolution Protocol command is
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used on a local network to
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translate an IP address to a MAC address.
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This is important because if we don't
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have IP to MAC mapping,
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we can have a lot of issues.
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We need to have that low-level mapping setup
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or it causes a lot of network problems,
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things like arp cache poisoning, all that stuff.
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Now the arp command is what we
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use to view the arp table,
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but this command is now more or less deprecated.
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What is recommended now is using
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another function of the ip command,
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which is the ip neigh command.
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Now, ip neigh is short for neighbor,
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and that replace the arp command.
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Just like we saw with arp ip neigh
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will display the arp table
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and the network discovery or
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neighborhood discovery NDISC table.
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NDISC is what is used for IPv6 addresses,
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whereas the arp table response is
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used for IPv4 addresses.
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In this lesson, we covered
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common network interface locations as well as the ip,
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arp, and nmcli commands.
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Thank you so much for being here and I look
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forward to seeing you in the next lesson.
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