Master Network Connectivity Checks: From ethtool to nmap

Physical Connection Check

ethtool

Use ethtool <device_name> to view the physical link status of a network interface. The Link detected field shows yes when the connection is healthy.

Network Connectivity Check

ping

Ping tests reachability between hosts and reports fields such as icmp_seq (packet number), ttl (time‑to‑live), time (round‑trip latency), rtt , min , avg , max , and mdev (standard deviation). Use -I <interface> on Linux or -S <source_address> on Windows to specify the sending interface.

tracepath

Similar to traceroute, tracepath shows each hop a packet traverses. It is simpler and produces concise output.

Example:

tracepath -n 192.168.1.241

HTTP/HTTPS Connectivity Check

wget

Use wget --spider -S <url> to test a URL without downloading content and to display response headers.

curl

Use curl -I <url> to fetch only HTTP headers. Adding -f makes curl fail silently on HTTP errors.

TCP/UDP Connectivity Check

nc (Netcat)

Check TCP ports with nc -zv <remote_host> <port>. Use -u for UDP and -z to scan without sending data.

nc -zv remote_host port

For UDP, send a test payload:

echo test | nc -uv remote_host port

telnet

Simple TCP port check:

telnet remote_host port

nmap

Use nmap <target_ip> to scan common ports. Specify ports with -p, e.g., nmap -p 80,443 remote_host. The state open means a service is listening, closed means no listener, and filtered indicates a firewall blocked the probe. ACK scans ( -sA) reveal firewall presence.

nmap 192.168.0.104

Local Network Connection Inspection

netstat and ss

Both commands display active connections and listening sockets. Common options: -a: show all sockets -t: TCP only -u: UDP only -n: numeric addresses -l: listening sockets -p: show owning process

Examples:

netstat -a

netstat -tn

netstat -un

netstat -tulnp

The ss command uses the same options.

TCP Connection States

LISTEN – service waiting for connections

ESTABLISHED – successful three‑way handshake

SYN_SENT / SYN_RECV – intermediate handshake states

FIN_WAIT_1, FIN_WAIT_2, TIME_WAIT, CLOSE_WAIT, LAST_ACK, CLOSED – termination phases

Three‑Way Handshake Process

1. SYN : client sends a SYN with its initial sequence number. 2. SYN+ACK : server replies with its own sequence number and acknowledges the client. 3. ACK : client acknowledges the server, and the connection enters ESTABLISHED state.

DNS Availability Check

Use dig , host , or nslookup to verify DNS resolution. Example: dig www.baidu.com Query a specific DNS server:

dig www.baidu.com @180.76.76.76