Network diagnostics

IP addresses and routes, DNS lookups, reachability, open ports and traffic capture.

Interfaces and routes

IP addresses of interfaces

ip -br addr

Routing table

ip route

Which route is used for an address

ip route get <ip>

Your public IP

curl -s https://ifconfig.me

Resolve a domain

dig +short <domain>

Specific record type (MX, TXT, CNAME…)

dig +short <domain> MX

Ask a specific DNS server

dig @8.8.8.8 <domain>

Full resolution path from the rootadvanced

dig +trace <domain>

Reverse lookup of an IP

dig -x <ip> +short

Ping 4 times

ping -c 4 <host>

Path to a host

traceroute <host>

Path with per-hop packet lossadvanced

mtr -rw <host>

Is a TCP port open?

nc -zv <host> <port>

Check a port without nc (bash only)advanced

timeout 3 bash -c '</dev/tcp/<host>/<port>' && echo open || echo closed

Listening ports with processes

sudo ss -tulpn

Established connections

ss -tn state established

Connections count per remote IPadvanced

ss -tn state established | awk 'NR>1 {split($4,a,":"); print a[1]}' | sort | uniq -c | sort -rn | head

Scan open ports of your own hostadvanced

nmap -sT <host>

Scan only hosts you own or are allowed to test.

Capture traffic on a portadvanced

sudo tcpdump -i any port <port> -nn

Save a capture for Wiresharkadvanced

sudo tcpdump -i any port <port> -w capture.pcap

Bandwidth between two hostsadvanced

iperf3 -c <host>

Start iperf3 -s on the other host first.