Linux network

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Linux network issues can come from a lot of places—misconfiguration, hardware problems, DNS failures, firewall rules, or even upstream network outages. The trick is to diagnose step-by-step instead of guessing. Here’s a practical workflow you can follow.

1. Check Basic Connectivity

Start simple—make sure the system is even connected.

ip a
  • Look for an IP address (not just 127.0.0.1)
  • Check if the interface is UP
ip link
  • Verify your interface (e.g., eth0, wlan0) is not DOWN

If it’s down:

sudo ip link set eth0 up

2. Test Network Reachability

Try pinging known addresses:

ping -c 4 8.8.8.8
  • If this fails → no internet or routing issue

Then test DNS:

ping -c 4 google.com
  • If IP ping works but domain fails → DNS problem

3. Check Routing Table

ip route

Look for a default route:

default via 192.168.1.1 dev eth0

If missing, add it:

sudo ip route add default via <gateway-ip>

4. Verify DNS Configuration

Check:

cat /etc/resolv.conf

You should see something like:

nameserver 8.8.8.8

If not, temporarily add:

sudo nano /etc/resolv.conf

Or if using systemd:

systemd-resolve --status

5. Check Network Manager / Services

Depending on your system:

Using NetworkManager

nmcli device status
nmcli connection show

Restart:

sudo systemctl restart NetworkManager

Using systemd-networkd

networkctl status
sudo systemctl restart systemd-networkd

6. Inspect Firewall Rules

Firewall can silently block traffic.

sudo iptables -L -n

Or if using UFW:

sudo ufw status

Temporarily disable (for testing):

sudo ufw disable

7. Check Open Ports

ss -tulnp

This shows:

  • Listening services
  • Ports in use

8. Test with traceroute

See where packets stop:

traceroute 8.8.8.8

If it stops early → local network issue
If it goes far but fails → ISP or remote issue

9. Check Logs

Logs often reveal the real problem:

journalctl -u NetworkManager
journalctl -xe
dmesg | grep -i network

10. Wireless-Specific Checks

If using Wi-Fi:

iwconfig

Scan networks:

iw dev wlan0 scan

Reconnect:

nmcli device wifi connect "SSID" password "PASSWORD"

11. Hardware / Driver Issues

Check drivers:

lspci | grep -i network
lsmod | grep <driver>

If missing drivers, you may need to install firmware packages.

Common Problems & Quick Fixes

  • No IP address → DHCP issue → restart network service
  • Can ping IP but not domain → DNS misconfigured
  • Interface down → enable with ip link set
  • Connected but no internet → gateway missing
  • Wi-Fi connected but slow → interference or driver issue

On Ubuntu, networking is usually managed by NetworkManager, so troubleshooting is a bit more standardized. Let’s narrow this down quickly with a focused Ubuntu workflow.

1. Check if NetworkManager is running

sudo systemctl status NetworkManager

If it’s not active:

sudo systemctl restart NetworkManager

2. See connection status

nmcli device status

You’ll see something like:

  • connected → good
  • disconnected → problem
  • unmanaged → config issue

To reconnect:

nmcli connection up <connection-name>

3. Check IP address

ip a

Look for something like:

inet 192.168.x.x

❌ No IP? Try renewing DHCP:

sudo dhclient -v

4. Test connectivity

ping -c 4 8.8.8.8
  • ❌ fails → network/gateway issue
  • ✅ works → move to DNS test:
ping -c 4 google.com
  • ❌ fails → DNS problem

5. Check default gateway

ip route

You should see:

default via 192.168.x.1

If missing:

sudo ip route add default via 192.168.1.1

6. Fix DNS (very common on Ubuntu)

Check:

cat /etc/resolv.conf

If it’s broken or empty, try:

sudo nano /etc/systemd/resolved.conf

Set:

DNS=8.8.8.8 1.1.1.1

Then restart:

sudo systemctl restart systemd-resolved

7. Wi-Fi specific (if applicable)

Check:

nmcli device wifi list

Reconnect:

nmcli device wifi connect "SSID" password "PASSWORD"

If Wi-Fi adapter missing:

lspci | grep -i network

8. Firewall check

Ubuntu often uses UFW:

sudo ufw status

Temporarily disable:

sudo ufw disable

9. Look at logs (very useful)

journalctl -u NetworkManager --no-pager | tail -50

Fast “reset everything” fix (works surprisingly often)

sudo systemctl restart NetworkManager
sudo systemctl restart systemd-resolved
sudo dhclient

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