Preliminaries:

1. WiFi is running
2. No ethernet
3. External Monitor plugged in
4. In  a small terminal window, start the nmcli monitor
5. A terminal window in sudo
6. A terminal window not in sudo

See Troubleshooting Home Local Area Networks using Linux command line network tools rev B (on Google Slides) .

Verify that the network is working properly (11:40)

http http://google.com
http https://amazon.com
ssh -4 -T git@github.com
ssh -6 -T git@gitlab.com

Is there a generic link problem? (11:43)

ip --stats link show dev wlp2s0         # GOOD!
ip --stats link show dev eno1           # bad!

Does Network Manager think there is a problem? (11:45)

nmcli g
rfkill
rfkill block wlan
nmcli g
rfkill unblock wlan
nmcli g

Diagnose the NIC: hardware problem? (11:50)

sudo lshw -C network
lspci -nnk | grep -i -A 2 net
find /sys/devices/ -iname "*net*"
lsusb | sort -k 6

The physical layer (Wires) (See slide 13 to see a bad ethernet, see slide 14 to see a good ethernet, and slide 15 to see a flakey ethernet ).

ip link show dev eno1

The Physical Layer (Radio, WiFi) 

sudo iw phy phy0 info

sudo iw dev wlp2s0 info
sudo iw dev wlp2s0 link
sudo iw dev wlp2s0 scan | fgrep SSID:               # look at SSIDs

The Physical Layer (Radio, WiFi)

ip --stats link show dev wlp2s0

Any SSIDs? Use iw if NetworkManager not helpful

sudo iw dev wlp2s0 scan | fgrep SSID:

Media Access Control (MAC)

ip neigh list
ip -6 neigh list
ip -4 neigh list
arp -an

Routing

ip -4 route list
ip -6 route list

Who is my local default router?

ip -4 route show default
ip -6 route show default

Look at IPv6 routing announcements

tcpdump -n -i eth0 icmp6
tcpdump -n -i eth0 icmp6 and ip6[40] == 134

Check your ISPs routing.  f5 is a short name.

tracepath -4 f5.com
tracepath -6 f5.com
ping -4 f5.com
ping -6 f5.com

Why IPv6?

python3 -c 'print(f"{2**128:,}")'
python3 -c 'print(f"{2**32:,}")'

Checking DNS

dig google.com
dig -t aaaa google.com
dig @8.8.8.8 jeffsilverm.ddns.net
dig @2001:4860:4860::8888 jeffsilverm.ddns.net
dig +short usps.gov

nmcli

Is network manager even running?

systemctl status NetworkManager

nmcli general and nmcli network and nmcli radio

nmcli g
nmcli n
nmcli radio

nmcli monitor

nmcli monitor

nmcli connections

nmcli c s --active
nmcli c s Wire and hit the TAB button

nmcli device

nmcli d

nmcli device disconnect and connect a device

nmcli d disconnect wlp2s0
nmcli d disconnect wlp2s0

Maybe you are using the wrong WiFi password?  Use nmcli

nmcli --show-secrets c s "ohiawaikiki" | fgrep psk:

For a good time, try nmcli --show-secrets c show and hit the TAB button

How to find all of the SSIDs using NetworkManager.

sudo nmcli dev wifi list --rescan yes

Other command line networking tools

Transport issues: nmap. This is a borderline hacking tools.  If you don't want to attempt OS detection, then do not use -O and you won't need root privs.

sudo nmap -O -sT jeffsilverm.ddns.net -p 20,21,22,23,25,53,80,135-139,161,162,443,445,3306 -T4
nmap -sT jeffsilverm.ddns.net -p 20,21,22,23,25,53,80,135-139,161,162,443,445,3306 -T4

egrep "22/tcp|80/tcp|3306/tcp" /etc/services
nmap -sT jeffsilverm.ddns.net -p ftp-data,ftp,ssh,telnet,smtp,domain,http,epmap,netbios-ns,netbios-dgm,netbios-dgm,snmp,snmp-trap,https,microsoft-ds,mysql -T4

Find this file at http://jeffsilverm.ddns.net/Troubleshooting_Home_Local_Area_Networks_using_Linux_command_Linux_network_tools_rev_B.html .

Find the presentation on Google Slides.