Linux Network Troubleshooting | Diagnosing and Fixing Connectivity Issues

Network issues can be frustrating, whether you’re trying to connect to the internet, access a remote server, or troubleshoot a local network problem. At first, networking might seem straight forward just making sure devices can connect and communicate with each other. But, as you dive deeper into it, especially when you’re trying to send data over the Internet or expose a server application to external traffic, things can get pretty tricky. Issues may arise unexpectedly, and figuring out why something isn’t working can get complicated.

When networking doesn’t behave as expected, pinpointing the cause can be difficult. This requires an understanding of networking concepts and familiarity with the tools available on Linux to diagnose and resolve problems. This is true across all operating systems, but with Linux, it’s especially important to grasp the network stack and use the right tools to fix issues.

Here are some tools you’ll likely use when troubleshooting networking problems on Linux. Most of these tools come pre-installed with many distributions, or they can be easily added via packages. These tools allow you to check network configurations, modify settings, and monitor network activity. While there are many more tools available, this list focuses on the most commonly used ones to help you diagnose and fix networking issues effectively.

1. Check Your Network Interface

Problem: You can’t connect to the internet, and you’re not sure if your network interface is active.

Solution: Use the ip command to check the status of your network interfaces.

Example:

ip a

Output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.100/24 brd 192.168.1.255 scope global dynamic eth0
       valid_lft 86388sec preferred_lft 86388sec

If eth0 is DOWN, bring it up:

sudo ip link set eth0 up

2. Verify IP Address Configuration

Problem: Your interface is up, but you still can’t connect to the internet.

Solution: Check if your interface has an IP address.

Example:

ip a show eth0

Output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.100/24 brd 192.168.1.255 scope global dynamic eth0
       valid_lft 86388sec preferred_lft 86388sec

If there’s no IP address, renew it using DHCP:

sudo dhclient eth0

3. Test Connectivity with ping

Problem: You’re not sure if your system can reach the router or external servers.

Solution: Use ping to test connectivity.

Example:

ping 192.168.1.1

Output:

PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=1.23 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=1.12 ms

If you can’t ping the gateway, check your physical connections or Wi-Fi settings.

4. Check DNS Resolution

Problem: You can ping IP addresses but not domain names (e.g., google.com).

Solution: Test DNS resolution with nslookup.

Example:

nslookup google.com

Output:

Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   google.com
Address: 142.250.72.206

If DNS resolution fails, check /etc/resolv.conf:

cat /etc/resolv.conf

Add a public DNS server if needed:

echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf

5. Inspect Routing Tables

Problem: You can’t reach external networks.

Solution: Check your routing table.

Example:

ip route

Output:

default via 192.168.1.1 dev eth0 proto dhcp src 192.168.1.100 metric 100
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100

If there’s no default route, add it manually:

sudo ip route add default via 192.168.1.1

6. Analyze Network Traffic with tcpdump

Problem: You suspect packet loss or unusual traffic.

Solution: Use tcpdump to capture and analyze packets.

Example:

sudo tcpdump -i eth0

Output:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:34:56.789012 IP 192.168.1.100.22 > 192.168.1.1.12345: Flags [P.], seq 1:100, ack 1, win 256, length 99

Use filters to narrow down the output:

sudo tcpdump -i eth0 port 80

7. Check Firewall Rules

Problem: A service (e.g., a web server) isn’t accessible.

Solution: Check firewall rules with ufw.

Example:

sudo ufw status

Output:

Status: active

To                         Action      From
--                         ------      ----
80/tcp                     ALLOW       Anywhere

If the port isn’t allowed, add a rule:

sudo ufw allow 80/tcp

8. Test Port Connectivity with nc

Problem: You want to check if a specific port is open.

Solution: Use nc (Netcat) to test port connectivity.

Example:

nc -zv 192.168.1.100 80

Output:

Connection to 192.168.1.100 80 port [tcp/http] succeeded!

If the port is closed, the service might be down or blocked.

9. Restart Network Services

Problem: Network issues persist after configuration changes.

Solution: Restart network services.

Example:

sudo systemctl restart networking

For systems using NetworkManager:

sudo systemctl restart NetworkManager

10. Check Logs for Clues

Problem: You’re unsure what’s causing the issue.

Solution: Check system logs for errors.

Example:

sudo journalctl -xe

Look for network-related errors or warnings.

11. Test Hardware and Drivers

Problem: You suspect a hardware issue.

Solution: Check if your network adapter is recognized.

Example:

lspci | grep -i network

Output:

00:19.0 Ethernet controller: Intel Corporation Ethernet Connection (2) I219-V

Ensure the correct driver is loaded:

lsmod | grep e1000e

12. Use mtr for Advanced Diagnostics

Problem: You’re experiencing packet loss or high latency.

Solution: Use mtr to diagnose the issue.

Example:

mtr google.com

Output:

Start: 2023-10-01T12:34:56+0000
HOST: localhost                   Loss%   Snt   Last   Avg  Best  Wrst StDev
  1.|-- 192.168.1.1                0.0%    10    1.2   1.1   1.0   1.3   0.1
  2.|-- 10.0.0.1                   0.0%    10    5.6   5.5   5.4   5.7   0.1
  3.|-- 142.250.72.206             0.0%    10   20.1  20.0  19.8  20.2   0.1

Understanding how to troubleshoot network issues in Linux is essential for maintaining a reliable and efficient system. By familiarizing yourself with the key networking tools and concepts, you’ll be better equipped to identify problems and apply solutions quickly. While networking can be complex, the right knowledge and approach will empower you to tackle even the most challenging issues with confidence. Stay proactive, keep exploring new tools, and soon enough, you’ll be troubleshooting like a pro.