Quick Fixes for Your Linux Network Issue Troubleshoot with Ease

Linux Network Troubleshooting

Linux Network : Stuck with network issues on your Linux system? No worries use commands like ip a, netstat, ping, and traceroute to figure out what’s going wrong. Whether it’s a wonky interface, a service not playing nice, or DNS acting up, these steps will help you troubleshoot and get your network back on track.

1. Check Network Interface Status:

Command:

ip a

Interpretation:

• Look for the UP status under the interface name (e.g., eth0, wlan0). If the interface is marked as DOWN, the network interface might be disabled or disconnected.
• Ensure the IP address assigned to the interface is correct and belongs to the expected network.
• If there is no IP address, there may be an issue with DHCP or static IP configuration.

Example Output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 192.168.1.2/24 brd 192.168.1.255 scope global eth0

• UP means the interface is operational.
• The inet line shows the assigned IP (192.168.1.2/24). If the IP is not present, check DHCP settings.

2. Check for Active Connections:

Command:

netstat -tuln

Interpretation:

• This command shows a list of active ports your system is listening on. This helps verify if services (like HTTP, SSH) are running and accepting connections.
• Look for services like ssh (port 22), http (port 80), or https (port 443). If the service you expect to be running is not listed, it may not be started or there could be a firewall blocking the port.

Example Output:

Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp6       0      0 :::80                   :::*                    LISTEN

• If LISTEN appears, it means the service is ready to accept connections.
• If you don’t see LISTEN for a service, check if the service is running or check firewall rules.

3. Verify DNS Resolution:

Command:

nslookup google.com

Interpretation:

• If the nslookup command successfully resolves a domain (e.g., google.com), it means DNS resolution is working. If you see an error like “Non-existent domain,” it means DNS isn’t resolving correctly.
• If DNS resolution fails, check the contents of /etc/resolv.conf to ensure valid DNS servers are listed.

Example Output:

Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
Name:   google.com
Address: 172.217.5.78

• If the DNS server is 8.8.8.8 (Google’s public DNS), it means the DNS query was successful.
• If no address is returned or a different DNS server is listed, investigate your DNS configuration.

4. Check Routing Table:

Command:

ip route show

Interpretation:

• This shows the current routing table. The key route to look for is the default route, which directs packets to the internet.
• If the default route is missing or misconfigured, your system won’t be able to reach external networks.

Example Output:

default via 192.168.1.1 dev eth0
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.2

• default via 192.168.1.1 indicates that the default gateway is 192.168.1.1 (usually your router).
• If the default route is missing or incorrect, you may need to configure it or fix the network gateway.

5. Ping the Gateway and Remote Hosts:

Command:

ping 192.168.1.1

Interpretation:

• If you can successfully ping your gateway (router), the issue is likely with your external connection or DNS configuration.
• If you can’t ping the gateway, check the network connection or verify if the network interface is configured correctly.

Example 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=0.242 ms

• If the latency is low (in ms), the local network is fine.
• High latency or timeouts indicate a problem in the local network.

Ping External Host (e.g., 8.8.8.8):

ping 8.8.8.8

Interpretation:

• If pinging an external IP (like Google’s DNS server 8.8.8.8) works, it indicates that the system can reach the internet, but there may be a DNS issue.
• If pinging 8.8.8.8 fails, there is likely a network routing issue, a firewall issue, or an ISP connectivity problem.

6. Traceroute to Diagnose Routing Issues:

Command:

traceroute google.com

Interpretation:

• This command helps identify where packets are getting delayed or dropped in the network path.
• Each line in the output represents a router (hop) along the route, and the times show how long it took to reach that router. If there’s a large delay at a certain hop, the issue may be with that router or network segment.

Example Output:

1  192.168.1.1 (192.168.1.1)  1.428 ms  1.408 ms  1.387 ms
2  10.0.0.1 (10.0.0.1)  4.213 ms  4.194 ms  4.190 ms
3  172.16.1.1 (172.16.1.1)  20.370 ms  20.340 ms  20.317 ms
4  203.0.113.1 (203.0.113.1)  35.781 ms  35.748 ms  35.725 ms
5  google.com (172.217.5.78)  48.251 ms  48.214 ms  48.183 ms

• If there is a large delay (e.g., over 100ms) at any hop, the problem could be with that specific hop or the router’s network.
• If the trace stops (shows * * *), it indicates that the router at that hop did not respond to ICMP packets (not necessarily a problem).

7. Test Bandwidth and Speed:

Command:

iperf3 -s  # Run this on the server
iperf3 -c <server_IP>  # Run this on the client

Interpretation:

• iperf3 measures the bandwidth between two systems. You should check for the average throughput reported by iperf3.
• If the throughput is low, it could indicate congestion, packet loss, or hardware limitations in the network.

Example Output:

[  5]  0.0-10.0 sec  100 MBytes  83.3 Mbits/sec

• If the bandwidth is much lower than expected, check for network congestion, faulty cables, or poor-quality network devices.

8. Check Firewall and Security Settings:

Command:

sudo ufw status

Interpretation:

• If the firewall is blocking essential ports (e.g., SSH on port 22, HTTP on port 80), services may be inaccessible.
• Ensure the firewall allows inbound and outbound traffic for the services you’re trying to access.

Example Output:

Status: active
To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       Anywhere
80/tcp                     ALLOW       Anywhere

Command:

sudo iptables -L

Interpretation:

• Check for any rules that might block traffic. For example, if a rule exists that blocks INPUT or OUTPUT for certain ports, this could explain network connectivity issues.

9. Check for Network Congestion or Errors:

Command:

netstat -i

Interpretation:

• Check for any packet errors, dropped packets, or collisions on the network interface. These can indicate network congestion, a faulty cable, or misconfigured hardware.

Example Output:

Iface   MTU  RX-OK  RX-ERR  RX-DROP  TX-OK  TX-ERR  TX-DROP  Coll
eth0    1500  100000  0       0        100000 0       0        0

• If RX-ERR or TX-ERR is non-zero, it indicates packet loss or hardware issues.

10. Review Logs for Errors:

Command:

sudo journalctl -xe

Interpretation:

• Look for any error messages related to networking, such as interface down, DHCP issues, or DNS resolution errors.
• This log can provide clues if a service failed to start or if a network-related error was logged by the kernel.

FAQ : Linux Network Issue Troubleshoot

1. How do I check if my network interface is up?

Command:
ip a

What to look for:

• Look for the interface status. If it shows UP under the interface (e.g., eth0, wlan0), then the network interface is active.
• If it shows DOWN, the interface may be disabled or disconnected. You can bring it up using: sudo ip link set eth0 up

2. How can I see which ports are open on my system?

Command:
netstat -tuln

What to look for:

• This will list active listening ports and the services bound to them. For instance, port 22 for SSH, port 80 for HTTP, and port 443 for HTTPS.
• If the service you expect is missing, ensure that it’s running, or check if a firewall is blocking the port.

3. How do I check if my DNS is working properly?

Command:
nslookup google.com

What to look for:

• If the DNS query returns an IP address (e.g., 172.217.5.78), it means DNS is working.
• If it fails, check your /etc/resolv.conf file for correct DNS server entries, or test with different public DNS servers like 8.8.8.8 (Google’s DNS).

4. What do I do if I can’t reach the internet?

Commands:
ping 192.168.1.1 (ping your router)
ping 8.8.8.8 (ping an external address)

What to look for:

• If you can ping your router (192.168.1.1), but not external addresses (like 8.8.8.8), the issue is likely with your gateway or routing.
• If you can’t ping the router, it’s a local network issue (wrong IP settings or physical connection problems).

5. How do I check if my routing table is correct?

Command:
ip route show

What to look for:

• Ensure there is a valid default route pointing to your gateway, usually shown as default via 192.168.1.1.
• If the default route is missing or incorrect, add it using: sudo ip route add default via 192.168.1.1

6. How can I troubleshoot network latency or slow connections?

Command:
traceroute google.com

What to look for:

• The output will show each hop (router) your data takes on its way to google.com. If there’s a delay at any particular hop, it could indicate a network congestion or issue at that point.
• High latency or timeouts could suggest problems with that hop or a broader network issue.

7. How can I test network performance or bandwidth?

Command:
iperf3 -s (on the server)
iperf3 -c <server_IP> (on the client)

What to look for:

• This will provide throughput stats, including bandwidth, latency, and packet loss.
• If throughput is lower than expected, check for congestion or hardware limitations.

8. How do I check if my firewall is blocking network traffic?

Commands:
sudo ufw status (for UFW firewall)
sudo iptables -L (for iptables firewall)

What to look for:

• Check for any rules that may be blocking important ports like 22 (SSH), 80 (HTTP), or 443 (HTTPS).
• If you need to open a port, you can do so with: sudo ufw allow 22/tcp

9. How do I identify network errors or congestion?

Command:
netstat -i

What to look for:

• Look for any RX-ERR or TX-ERR values that are greater than 0. These could indicate packet errors or hardware issues.
• Network congestion can also show as dropped packets, which could mean faulty cables or misconfigured network hardware.

10. How do I review system logs for network issues?

Command:
sudo journalctl -xe

What to look for:

• Search for any networking-related errors such as interface failures, DHCP issues, or DNS resolution errors. Logs can often provide more context on what went wrong.