Linux Networking: Troubleshooting and Diagnostics

ping -c 4 example.com

# Send ICMP echo requests to test basic reachability.

ping -i 0.2 -c 20 10.0.0.10

# Send pings at a faster interval.

ping -M do -s 1472 8.8.8.8

# Probe MTU problems by forbidding fragmentation.

mtr --report --report-cycles 20 example.com

# Run a report mode path-quality test.

curl -o /dev/null -s -w 'dns=%{time_namelookup} connect=%{time_connect} tls=%{time_appconnect} total=%{time_total}
' https://example.com

# Print DNS, connect, TLS, and total timings.

time dig +short example.com

# Measure how long a DNS query takes.

journalctl -u systemd-networkd --since -1h

# Inspect logs for systemd-networkd.

journalctl -u systemd-resolved --since -1h

# Inspect DNS resolver service logs.

journalctl -u NetworkManager --since -1h

# Inspect NetworkManager service logs.

dmesg --color=always | egrep -i 'eth|ens|eno|link|mtu|tcp|udp|icmp'

# Filter kernel ring buffer for common networking terms.

ip monitor all

# Watch interfaces, addresses, and routes in real time.

sar -n DEV 1 5

# Inspect sar network statistics if sysstat is installed.

env | grep -i proxy

# Inspect current shell proxy settings.

curl --noproxy '*' https://example.com

# Temporarily skip configured proxies for a single cURL request.

grep '^hosts:' /etc/nsswitch.conf

# Show how hostname resolution is ordered.

sudo resolvectl flush-caches

# Clear cached DNS results.

networkctl status eth0

# Inspect status for a systemd-networkd link.