CBT Nugget: Linux Server Administration - 9. Testing Network Connectivity

Testing Network Connectivity

Network connectivity testing is essential for diagnosing and resolving network issues. Various tools and commands can help verify the status and performance of network connections. Below is a comprehensive guide to testing network connectivity.

Basic Connectivity Tests

1. ping:

   • Used to check if a remote host is reachable and measure round-trip time for packets.

   Example:

   sh
   ping -c 4 example.com
   

   Output:

   plaintext
   PING example.com (93.184.216.34): 56 data bytes
   64 bytes from 93.184.216.34: icmp_seq=0 ttl=56 time=15.2 ms
   64 bytes from 93.184.216.34: icmp_seq=1 ttl=56 time=14.9 ms
   64 bytes from 93.184.216.34: icmp_seq=2 ttl=56 time=15.0 ms
   64 bytes from 93.184.216.34: icmp_seq=3 ttl=56 time=15.1 ms
   
   --- example.com ping statistics ---
   4 packets transmitted, 4 packets received, 0.0% packet loss
   

2. traceroute:

   • Traces the path packets take to reach a remote host.

   Example:

   sh
   traceroute example.com
   

   Output:

   plaintext
   traceroute to example.com (93.184.216.34), 30 hops max, 60 byte packets
    1  192.168.1.1 (192.168.1.1)  1.153 ms  1.166 ms  1.159 ms
    2  10.0.0.1 (10.0.0.1)  9.254 ms  9.270 ms  9.264 ms
    3  example.com (93.184.216.34)  15.265 ms  15.278 ms  15.272 ms
   

3. nslookup and dig:

   • Query DNS to resolve domain names to IP addresses and vice versa.

   nslookup Example:

   sh
   nslookup example.com
   

   Output:

   plaintext
   Server:		192.168.1.1
   Address:	192.168.1.1#53
   
   Non-authoritative answer:
   Name:	example.com
   Address: 93.184.216.34
   

   dig Example:

   sh
   dig example.com
   

   Output:

   plaintext
   ; <<>> DiG 9.11.3-1ubuntu1.13-Ubuntu <<>> example.com
   ;; global options: +cmd
   ;; Got answer:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 22606
   ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
   
   ;; QUESTION SECTION:
   ;example.com.			IN	A
   
   ;; ANSWER SECTION:
   example.com.		299	IN	A	93.184.216.34
   
   ;; Query time: 23 msec
   ;; SERVER: 192.168.1.1#53(192.168.1.1)
   ;; WHEN: Wed Jun 12 10:20:23 UTC 2024
   ;; MSG SIZE  rcvd: 56
   

Advanced Connectivity Tests

1. telnet and nc (netcat):

   • Check if specific ports on a remote host are open.

   telnet Example:

   sh
   telnet example.com 80
   

   Output:

   plaintext
   Trying 93.184.216.34...
   Connected to example.com.
   Escape character is '^]'.
   

   netcat (nc) Example:

   sh
   nc -zv example.com 80
   

   Output:

   plaintext
   Connection to example.com 80 port [tcp/http] succeeded!
   

2. ifconfig and ip:

   • Display and configure network interfaces.

   ifconfig Example:

   sh
   ifconfig
   

   Output:

   plaintext
   eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
           inet 192.168.1.100  netmask 255.255.255.0  broadcast 192.168.1.255
           inet6 fe80::a00:27ff:fe4e:66a  prefixlen 64  scopeid 0x20<link>
           ether 08:00:27:4e:66:6a  txqueuelen 1000  (Ethernet)
           RX packets 156  bytes 20526 (20.5 KB)
           RX errors 0  dropped 0  overruns 0  frame 0
           TX packets 148  bytes 19582 (19.5 KB)
           TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
   

   ip Example:

   sh
   ip addr
   

   Output:

   plaintext
   2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
       link/ether 08:00:27:4e:66:6a brd ff:ff:ff:ff:ff:ff
       inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0
          valid_lft forever preferred_lft forever
       inet6 fe80::a00:27ff:fe4e:66a/64 scope link
          valid_lft forever preferred_lft forever
   

3. ethtool:

   • Display or change Ethernet device settings.

   Example:

   sh
   sudo ethtool eth0
   

   Output:

   plaintext
   Settings for eth0:
           Supported ports: [ TP ]
           Supported link modes:   10baseT/Half 10baseT/Full
                                   100baseT/Half 100baseT/Full
           Supported pause frame use: No
           Supports auto-negotiation: Yes
           Advertised link modes:  10baseT/Half 10baseT/Full
                                   100baseT/Half 100baseT/Full
           Advertised pause frame use: No
           Advertised auto-negotiation: Yes
           Speed: 100Mb/s
           Duplex: Full
           Port: Twisted Pair
           PHYAD: 1
           Transceiver: internal
           Auto-negotiation: on
           MDI-X: Unknown
   

4. mtr:

   • Combines the functionality of ping and traceroute to diagnose network issues.

   Example:

   sh
   mtr example.com
   

   Output:

   plaintext
   Start: 2024-06-12T10:20:23+0000
   HOST: hostname                    Loss%   Snt   Last   Avg  Best  Wrst StDev
     1. 192.168.1.1                   0.0%    10    0.4   0.4   0.4   0.5   0.0
     2. 10.0.0.1                      0.0%    10    9.2   9.3   9.2   9.4   0.0
     3. example.com                   0.0%    10   15.1  15.0  14.9  15.1   0.0
   

Example Scenario

Diagnosing Network Connectivity Issue:

1. Ping the Gateway:

   sh
   ping -c 4 192.168.1.1
   

2. Traceroute to an External Host:

   sh
   traceroute 8.8.8.8
   

3. Check DNS Resolution:

   sh
   nslookup example.com
   

4. Verify Interface Configuration:

   sh
   ip addr show eth0
   

5. Check Ethernet Device Settings:

   sh
   sudo ethtool eth0
   

6. Diagnose with mtr:

   sh
   mtr 8.8.8.8
   

Summary

Testing network connectivity involves using a combination of tools to diagnose and resolve issues. Basic commands like ping, traceroute, nslookup, and dig help verify connectivity and DNS resolution. Advanced tools like telnet, netcat, ifconfig, ip, ethtool, and mtr provide deeper insights into network configurations and performance. By systematically using these tools, you.