LPIC-1 109.3 — Basic Network Troubleshooting

Exam weight: 4 — LPIC-1 v5, Exam 102

What You Need to Know

From the official LPIC-1 objectives:

• Troubleshoot networking configuration issues on client hosts.
• Use system utilities to identify networking configuration and troubleshoot network problems.

Key files and commands: ip, ifconfig, route, ping, ping6, traceroute, traceroute6, tracepath, tracepath6, netcat, netstat, ss.

Interface Configuration

ip — Modern Tool

# Show all interfaces
ip addr show
ip addr show eth0

# Add / remove an IP address
ip addr add 192.168.1.10/24 dev eth0
ip addr del 192.168.1.10/24 dev eth0

# Bring interface up / down
ip link set eth0 up
ip link set eth0 down

ifconfig — Legacy Tool

ifconfig                              # show active interfaces
ifconfig -a                           # show all, including inactive
ifconfig eth0 192.168.1.10 netmask 255.255.255.0
ifconfig eth0 down
ifconfig eth0 up

Routing

Viewing the Routing Table

ip route show           # modern
route -n                # legacy (numeric output)
netstat -r              # legacy (routing table)

Managing Routes

# Add a route
ip route add 192.168.2.0/24 via 192.168.1.1
ip route add default via 192.168.1.1

# Remove a route
ip route del 192.168.2.0/24

# Legacy route command
route add -net 192.168.2.0/24 gw 192.168.1.1 dev eth0
route del -net 192.168.2.0/24

IPv6 Routing

ip -6 route show
ip -6 route add 2001:db8::/32 via fe80::1 dev eth0

ping and ping6

ping sends ICMP echo request packets to test host reachability.

ping 8.8.8.8               # continuous
ping -c 4 8.8.8.8          # send exactly 4 packets
ping6 ::1                  # IPv6 ping
ping -6 ::1                # alternative IPv6 syntax

Common reasons ping can fail:

• Remote host is down.
• Firewall (local or remote) is blocking ICMP.
• Incorrect hostname or DNS resolution failure.
• Incorrect local network configuration.
• Interface is disconnected.
• Router ACL blocks ICMP.

traceroute and traceroute6

traceroute traces the path packets take to a destination by sending packets with incrementing TTL values and recording each router that sends back an ICMP Time Exceeded message.

traceroute 8.8.8.8
traceroute6 2001:4860:4860::8888

By default, traceroute uses UDP packets. The -I and -T options require root.

tracepath and tracepath6

tracepath is similar to traceroute but does not require root privileges and automatically discovers the Path MTU.

tracepath 8.8.8.8
tracepath6 ::1
tracepath -6 ::1

Unlike traceroute, tracepath reports the MTU at each hop and does not require elevated privileges.

nc — netcat

nc (netcat) creates arbitrary TCP or UDP connections for testing and data transfer.

# Connect to a TCP port
nc hostname 80

# Listen on a port
nc -l 1234

# UDP mode
nc -u hostname 53

# Check if a port is open (no data transfer)
nc -z hostname 22

Manual HTTP request with nc

nc learning.lpi.org 80
GET /index.html HTTP/1.1
HOST: learning.lpi.org

(Press Enter twice after the last header.)

netstat and ss

Both commands show open network connections, routing tables, and interface statistics.

ss — Modern Tool

ss -tulnp          # TCP and UDP listening, numeric, with process names
ss -a              # all sockets (listening and established)
ss -ln | grep ":80"  # check if port 80 is in use

netstat — Legacy Tool

netstat -tulnp     # TCP and UDP listening, numeric, with process names
netstat -r         # routing table

Common Options (both tools)

ss is the modern replacement for netstat. Both accept the same options.

Quick Reference

Interface config:
  ip addr show / ip addr add IP/PREFIX dev IFACE / ip addr del IP/PREFIX dev IFACE
  ip link set IFACE up/down
  ifconfig IFACE IP netmask MASK   (legacy)

Routing:
  ip route show / ip route add / ip route del
  ip route add default via GW
  route -n / netstat -r            (legacy)
  ip -6 route show

ping / ping6:
  ping -c N HOST    (N packets)
  ping6 HOST  or  ping -6 HOST

traceroute:
  traceroute HOST   (UDP default)
  -I ICMP  -T TCP  -i IFACE  --mtu
  traceroute6 HOST  or  traceroute -6 HOST

tracepath HOST / tracepath6 HOST
  no root needed; discovers MTU along path

nc HOST PORT              connect TCP
nc -l PORT                listen
nc -u HOST PORT           UDP
nc -z HOST PORT           check if port open

ss -tulnp    netstat -tulnp
  -a all  -l listening  -t TCP  -u UDP  -n numeric  -p process

Exam Questions

1. What command adds a default gateway using the ip suite? → ip route add default via GATEWAY
2. What option sends exactly 5 ICMP echo requests with ping? → -c 5
3. What is the IPv6 equivalent of ping? → ping6 or ping -6
4. By default, what protocol does traceroute use to probe hops? → UDP
5. What traceroute option switches probes to ICMP echo requests? → -I
6. What is the key advantage of tracepath over traceroute? → tracepath does not require root privileges and automatically discovers the Path MTU.
7. What nc option switches to UDP mode? → -u
8. What nc command checks whether port 443 is open on a host? → nc -z hostname 443
9. What is the modern replacement for netstat? → ss
10. What ss command shows all TCP listening sockets with process names? → ss -tlnp
11. What legacy command shows the routing table? → route -n or netstat -r
12. What command brings up interface ens3 with ip? → ip link set ens3 up
13. What command removes IP 10.0.0.5/24 from interface eth0? → ip addr del 10.0.0.5/24 dev eth0
14. What tools can be used to capture network packets on a Linux host? → tcpdump and wireshark
15. What option to traceroute forces it to use a specific network interface? → -i IFACE
16. How do you view the IPv6 routing table? → ip -6 route show
17. What does ss -ln | grep ":80" do? → Shows whether any socket is listening on port 80.
18. What is the difference between traceroute and tracepath? → traceroute is more flexible (protocol choice, flags) but requires root for some modes; tracepath requires no root and reports MTU discovery automatically.

Exercises

Exercise 1 — Add and Verify a Route

Add a static route to network 10.5.0.0/24 via gateway 192.168.1.254, then verify it appears in the routing table.

ip route add 10.5.0.0/24 via 192.168.1.254
ip route show

Exercise 2 — Test Reachability

Send exactly 3 ICMP packets to 8.8.8.8 and then trace the path to it.

ping -c 3 8.8.8.8
tracepath 8.8.8.8

Exercise 3 — Find Listening Services

Show all TCP and UDP sockets that are currently listening, including process names, using the modern tool.

ss -tulnp

Exercise 4 — Test a TCP Port

Use nc to check whether port 22 is open on 192.168.1.50, then connect to port 80 and manually send an HTTP request.

nc -z 192.168.1.50 22

nc 192.168.1.50 80
GET / HTTP/1.1
HOST: 192.168.1.50

Exercise 5 — Trace MTU Path

Determine the maximum MTU along the path to 8.8.8.8 without requiring root.

tracepath 8.8.8.8

LPIC-1 Study Notes | Topic 109: Networking Fundamentals