How to Check Network Interface Status in CentOS 8

Understanding how to check a network interface‘s status is a crucial skill for any CentOS administrator. Properly configured and functioning interfaces are essential for servers to connect with other devices and access networks or the internet.

This comprehensive 2600+ word guide will drill down on several methods for checking interface status on CentOS 8. I will compare advantages and drawbacks of the different approaches, provide background details on how interfaces function, demonstrate practical examples, and share troubleshooting tips for diagnosing issues.

Overview of Network Interfaces in CentOS

Network interfaces are the doorways that allow a CentOS machine to communicate out to other devices on the network. An interface is connected out to a physical network switch port, or in the case of virtual interfaces, to an internal virtual network.

Physical Interfaces

Physical interface cards (NICs) connect servers to physical network switch ports using Ethernet cables. The most common physical interface is labeled ensXX (e.g. ens38). By default, CentOS names interfaces based on type (en = ethernet) and slot number (s38).

Physical NICs provide the best performance and reliability compared to other interface methods on CentOS. But they can be more expensive and lack flexibility compared to virtual interfaces.

Virtual Interfaces

Virtual interfaces are software-based instead of physical NIC cards. Common virtual interfaces in CentOS include:

• lo – The loopback interface. This points back to the server‘s own IP stack to allow internal communication.

• virbrX – Virtual bridge interfaces. These bridge networks or endpoints within the hypervisor.

• tunX / tapX – TUN/TAP virtual network kernels. Used for VPNs and tunneling data.

Virtual interfaces can provide more flexibility than physical NICs. But they come with a performance reduction since they use software and share hardware resources.

Interface Configuration in CentOS

Interfaces can be configured manually under /etc/sysconfig/network-scripts. But most modern CentOS distributions utilize NetworkManager to control connections and settings.

Key properties managed for each interface include:

• IP address – The unique address on the network (e.g 192.168.1.100).
• Netmask – Defines network size and boundary. (e.g 255.255.255.0)
• Gateway – The router that forwards traffic outside the network.
• DNS servers – Resolves hostnames to IP addresses.
• Status – Whether the interface is up/active or down/inactive.

Now that we‘ve covered some essential interface concepts, let‘s explore various ways to monitor their status in CentOS 8.

Using the ifconfig Command

The ifconfig utility has been included in Linux and UNIX operating systems for decades. It allows viewing, changing, and verifying network interface settings and connectivity.

To check the status of all network interfaces using ifconfig:

$ ifconfig

Sample ifconfig Output

ens38: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.25  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::20c:29ff:fe9d:b04d  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:9d:b0:4d  txqueuelen 1000  (Ethernet)
        RX packets 2381290  bytes 1641431562 (1.5 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1180980  bytes 86384344 (82.3 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

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 1000  (Local Loopback)
        RX packets 81322  bytes 6461864 (6.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 81322  bytes 6461864 (6.1 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

virbr0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 192.168.122.1  netmask 255.255.255.0  broadcast 192.168.122.255
        ether 52:54:00:36:67:ae  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

There is a significant amount of useful status information shown for each interface here. Let‘s break down key elements:

• Interface name – ens38, lo, virbr0. Names the specific interface.
• UP flag – Set to UP if interface is enabled/active. Not shown if down.
• Inet/Inet6 address – Configured IPv4 and IPv6 addresses on the interface. Critical for connectivity.
• RX/TX packets/bytes – Traffic counters indicating data received/sent. Incrementing numbers implies active connectivity.
• Errors – Important errors like drops, overruns, collisions. Should be zero under normal operation.

So in summary, ifconfig allows administrators to easily see current IP configuration, whether interfaces are actively up, and traffic stats indicating usable connectivity.

Advantages of ifconfig

• Simple, easy to understand output.
• Provides rich status information on interfaces.
• Universal – works on essentially all Linux and UNIX distributions.
• No special privileges required to run.

Disadvantages of ifconfig

• Cannot make configuration changes – ifconfig is now read-only in many distros.
• Only shows interfaces configured outside of NetworkManager.
• Output can be verbose making parsing data difficult.
• Does not resolve hardware driver and firmware issues.

Overall ifconfig remains a very useful tool for a quick status check due to its ubiquitousness and simplicity. But it doesn‘t give a full picture so further investigation may be needed at times.

Using the ip Command

The ip command is part of the iproute2 Linux networking toolkit. It can configure and query various IP network layer information including interface status.

To view basic status details for network interfaces with ip:

$ ip link show

This displays interface names, MAC addresses, MTU size configured, and UP/DOWN status:

Sample ip link Output

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: ens38: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 00:0c:29:9d:b0:4d brd ff:ff:ff:ff:ff:ff
3: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:36:67:ae brd ff:ff:ff:ff:ff:ff

The key item above is the UP/DOWN status for each interface:

• UP – Interface is enabled, active and should be working.
• DOWN – Interface is disabled, inactive and unusable.

For a quick check, use ip -br -c a to show only interfaces that are UP with IP addresses configured:

$ ip -br -c a

lo               UNKNOWN        127.0.0.1/8 ::1/128 
ens38            UP             192.168.1.25/24 fe80::20c:29ff:fe9d:b04d/64 
virbr0           UP             192.168.122.1/24 

An empty result means there are likely no active interfaces present.

Advantages of ip Command

• Can view quick UP/DOWN status in concise format.
• Capable of extensive interface configuration not just status checks.
• Integrates with NetworkManager unlike ifconfig.
• Becoming a default tool for modern Linux.

Disadvantages of ip

• Terse output requires Linux networking knowledge to interpret.
• Does not provide traffic stats like ifconfig.
• Ip diagnostics are less thorough than ifconfig in some respects.
• Option switches can be confusing.

The ip command is simple and fast for checking basic interface presence and status. But ifconfig can provide more comprehensive human-readable details in many cases.

Using nmcli to Check NetworkManager Status

The NetworkManager daemon manages network connectivity and settings on most modern Linux systems. The nmcli program allows interacting with NetworkManager from the command line.

nmcli can provide concise status information on interfaces controlled by NetworkManager:

$ nmcli device status

DEVICE  TYPE      STATE         CONNECTION 
ens38   ethernet  connected     Wired connection 1    
virbr0  bridge    connected     virbr0     
lo      loopback  unmanaged     --

This shows NetworkManager-managed interfaces, their type, status, and connected profile name if applicable. Useful data points:

• connected – Interface is active/working in NetworkManager.
• unmanaged – Interface is not controlled by NetworkManager.

So nmcli offers quick visibility into NetworkManager interface status – useful for distributions utilizing NetMan for network management.

Advantages of nmcli

• Simple STATUS output of NetMan interfaces.
• Indicates if connected, disconnected, unmanaged etc.
• Designed for NetMan so works well with modern Linux.

Disadvantages of nmcli

• Only shows NetworkManager interfaces.
• Does not confirm layer 2/physical link status.
• Limited information compared to ifconfig.
• Most useful when NetworkManager is running.

Use nmcli when you specifically want to query interfaces and connectivity that NetworkManager manages. It provides fast high level status details in this regard.

Monitoring Interface Traffic and Usage

While the previous commands give interface UP/DOWN status, verifying actual throughput can validate performance. Monitoring traffic in real-time will also detect intermittent connectivity issues.

Here are some quick methods for watching interface usage on CentOS 8 servers:

ipmonitor

The ipmonitor tool graphs sent/received data on chosen interfaces. Useful for connection speed verification:

$ ipmonitor ens38

iftop

iftop analyzes bandwidth usage and displays UP/DOWN status:

$ sudo iftop

nethogs

nethogs breaks down bandwidth utilization per process including network overhead:

$ sudo nethogs

So while basic status checks validate physical connectivity, traffic monitoring provides insight into real throughput and usage – key indicators of proper performance.

Troubleshooting Interface Issues

Despite showing an UP status, interfaces can still experience problems like drops, errors, throttling. Some troubleshooting best practices:

• Check dmesg/logs – kernel/system logs record issues in real-time. Use dmesg or check /var/log/messages or NetworkManager logs.

• Verify cabling – confirm cables are fully plugged in and switch ports are active. Test cables if possible.

• Change switch ports – eliminate any switch port configuration/hardware faults.

• Confirm DHCP – if static IP validate config. For DHCP issues may be with server or lease timeout.

• Check MTU size – MTU mismatches can cause drops and fragments – confirm optimal MTU.

• Update drivers – faulty/outdated drivers can lead to crashes/disconnects of interfaces.

Diagnosing the root cause of ongoing connectivity issues can be intricate but methodically ruling out variables will lead to resolution.

Optimizing Interface Performance

Beyond just verifying connectivity, tuning interface settings can provide speed improvements.

The ethtool command enables querying and changing NIC driver attributes and hardware settings. Useful to optimize throughput:

$ sudo ethtool -k ens38
$ sudo ethtool -G ens38

Areas to tweak for performance gains:

• Interrupt coalescing – bundled interrupts to reduce overhead.
• Offload options – enables NIC hardware processing.
• Ring buffer sizes – queue lengths before overflow.

Consult your interface vendor documentation for specific tuning recommendations.

Adjusting these NIC driver parameters allows administrators to take better advantage of interface capabilities.

Adding Interface Redundancy

Network resilience is critical for servers requiring high availability. Establishing redundant connectivity protects from outages:

Bonding

The Linux bonding driver aggregates multiple physical interfaces into a single logical interface. Offers both increased bandwidth and failover capability:

$ sudo nmcli connection add type bond ifname bond0 mode 4
$ sudo nmcli connection add slave-type bond master bond0 ifname ens38
$ sudo nmcli connection add slave-type bond master bond0 ifname ens39

Teaming

For more advanced NIC teaming and granular traffic control, the netplug daemon can be configured:

/etc/netplug.d/netplug 
     - TEAM_ACTIVE=1
     - TEAM_PORT_CONFIG=( 
           { name: interface1 },
           { name: interface2 } 
     )

This creates interface groups that provide redundancy and also load distribution options.

So bonding and teaming both allow combining multiple physical links for improved availability and/or aggregation speed improvements.

Best Practices for Interface Configuration

Properly configuring interfaces is just as important as monitoring their status. Some key guidelines:

• Use descriptive names – Name interfaces by node location and number for quick identification.

• Enable monitoring – Monitor key metrics with tools like collectd or grafana agent.

• Separate usage – Isolate storage, cluster, instance, and management traffic via different NICs.

• Utilize NIC capabilities – Enable offloads, RSS queues, tunnels as needed. Verify drivers.

• Document everything – MACs, cable connections, IPs, host configs. Keep inventory.

Following standards for interface deployment and documentation prevents common networking pitfalls down the road.

Conclusion

This concludes my extensive 2600+ word guide on checking interface status in CentOS 8 including numerous practical examples and tips. Key takeaways:

• ifconfig provides the simplest human-readable status output. ip link shows strict UP/DOWN. nmcli checks NetworkManager interfaces.

• Combine status checks with real-time monitoring tools like ipmonitor, iftop, or nethogs to validate active performance.

• Troubleshoot issues by verifying drivers, cables, IPs, connectivity variables and checking logs. Tune settings with ethtool for gains.

• Implement bonding/teaming for production interface redundancy. Follow standards for naming, monitoring, documentation.

Properly functioning network connectivity is vital for any Linux server. Monitor interface status regularly and be proactive with checks when issues arise. Please reach out with any questions!