Basic Network Troubleshooting command:
- ping: Ping sends ICMP “echo” type packets that request a corresponding ICMP “echo-reply” response from the device at the target address.
- Using Telnet To Test Network Connectivity: An easy way to tell if a remote server is listening on a specific TCP port is to use the telnet command. By default, telnet will try to connect on TCP port 23, but you can specify other TCP ports by typing them in after the target IP address. HTTP uses TCP port 80, HTTPS uses port 443.
Example of testing server 192.168.1.102 on the TCP port 22 reserved for SSH:
[[email protected] ~]#telnet 192.168.1.102 22
3. traceroute : It gives a listing of all the router hops between your server and the target server. This helps you verify that routing over the networks in between is correct.
- Traceroute works by sending a UDP packet destined to the target with a TTL of “0”.
- The first router on the route recognizes that the TTL has already been exceeded and discards or “drops” the packet, but also sends an ICMP “time exceeded” message back to the source.
- The traceroute program records the IP address of the router that sent the message and knows that that is the first hop on the path to the final destination.
- The traceroute program tries again, with a TTL of “1”. The first hop, sees nothing wrong with the packet, decrements the TTL to 0 as expected, and forwards the packet to the second hop on the path. Router 2, sees the TTL of “0”, drops the packet and replies with an ICMP time exceeded message.
- Traceroute now knows the IP address of the second router. This continues around and around until the final destination is reached.
Note: In Linux the traceroute command is “traceroute“. In Windows it is “tracert“.