Send 4 ICMP packets to check reachability and latency; failure is not proof of downtime
ping -c 4 example.comMany hosts and firewalls drop ICMP by default; confirm with port tests.
Network Troubleshooting Commands - CodeSwordsman
A full troubleshooting toolkit for connectivity, DNS, port usage, routing and packet capture, with practical ss, lsof, dig and tcpdump combinations for timeouts and refused connections.
iproute2 / iputils / bind-utils / tcpdumpUpdated 2026-10-3021 commands
Connectivity
Show the hop-by-hop path to find where packets start dropping
traceroute -n example.comContinuous probing with per-hop loss summary, better than traceroute for link quality
mtr -rw -c 20 example.comQuick check whether a target port accepts connections
telnet 10.0.0.12 3306Port probe with timeout; succeeded means the TCP handshake completed
nc -vz -w 3 10.0.0.12 3306DNS Resolution
Print the resolved records only for a fast answer check
dig +short api.example.comQuery a public resolver to compare with local results and spot cache issues
dig @8.8.8.8 api.example.com +shortWalk from the root to find authoritative gaps or stale caches
dig +trace api.example.comInspect local DNS settings; container failures often trace back to nameserver here
cat /etc/resolv.confPorts & Connections
List all listening TCP/UDP ports with processes to verify the service is bound
ss -tunlpSummarize connections by state to spot TIME-WAIT buildup or backlog
ss -sShow established connections to a specific target to verify pool size
ss -tn state established dst 10.0.0.12:3306Find the process holding a port, useful for port conflicts
lsof -i:8080 -P -nFallback for systems without ss; output semantics are similar
netstat -anp | grep 8080Capture & Analysis
Capture 50 packets on port 8080; -nn skips name resolution for speed
sudo tcpdump -i any -nn port 8080 -c 50Save full packets to pcap for Wireshark analysis of handshakes and retransmits
sudo tcpdump -i eth0 -s 0 -w /tmp/app.pcap host 10.0.0.12Read a pcap in ASCII to confirm whether HTTP requests arrived
sudo tcpdump -r /tmp/app.pcap -nn -A | lessRequest with timeouts and verbose timings to isolate DNS, connect, TLS or response slowness
curl -v --connect-timeout 3 --max-time 10 https://api.example.com/healthRouting & Interfaces
Show which interface and gateway will be used for a destination
ip route get 8.8.8.8Inspect the ARP table for same-subnet reachability and IP conflicts
ip neighCheck link speed, duplex and state to rule out physical layer issues
ethtool eth0 | head -20