Network_Troubleshooting

Netstat

sudo netstat -tlnp

t = tcp
l = listening sockets
n = do not resolve dns to hostnames
p = show process id

SS

sudo ss -lnmpO

l = Show listening
n = Numeric. Don't resolve DNS Names.
m = Memory. Show Socket Memory Usage.
p = Process. Show Socket Processes.
O = Print Socket's Data on a single line.

Misc Network Monitoring Tools:

bmon, iftop, gping, wavemon

DB9 Serial Cable Recovery:

Screen

screen -dmLS session_name /dev/ttyUSB0 115200,cs8
- -d -- detatch session.
- -m -- create new session.
- -L -- safe updating of background session.
- -S -- specify a session name.
screen -ls
- ls -- lists all screen sessions.
screen -r session_name
- -r -- reattach to a screen session.
screen -X -S session_name quit
- -X -- Execute a command on a session.

Minicom

sudo usermod -a -G dialout $(whoami)
- Users must be added to the dialout user group.
minicom -c on -C ~/minicom.log
- -c Color Output Toggle.
- -C file_path.log Capture a log of the session to the specified file path.

Either minicom or screen can be used to interact with equipment over a DB-9 Cable.

Edgerouter / Edgeswitch Serial Config

The specific settings to establish a connection with most Ubitiquiti equipment is below.

    Baud rate: 115200
    Data bits: 8
    Parity: NONE
    Stop bits: 1
    Flow control: NONE

OSI Model

Please (Physical)
Do (Data Link)
Not (Network)
Throw (Transport)
Sausage (Session)
Pizza (Presentation)
Away (Application)

CIDR Notation

A way of defining a subnet and its size with a "mask", a smaller mask = more address bits usable by the subnet & more IPs in the range. Most common ones:

10.0.0.1/32 (a single IP address, 10.0.0.1) netmask = 255.255.255.255
10.0.0.1/24 (255 ips from 10.0.0.1-255) netmask = 255.255.255.0
10.0.0.1/16 (65,536 ips from 10.0.0.0 - 10.0.255.255) netmask = 255.255.0.0
10.0.0.1/8 (16,777,216 ips from 10.0.0.0 - 10.255.255.255) netmask = 255.0.0.0
0.0.0.1/0 (4,294,967,296 ips from 0.0.0.0 - 255.255.255.255) netmask = 0.0.0.0

    CIDR |  Hosts | Netmask
-----------------------------------
    /30     4       255.255.255.252
    /29     8       255.255.255.248
    /28     16      255.255.255.240
    /27     32      255.255.255.224
    /26     64      255.255.255.192
    /25     128     255.255.255.128
    /24     256     255.255.255.0
    /23     512     255.255.254.0
    /22     1024    255.255.252.0
    /21     2048    255.255.248.0
    /20     4096    255.255.240.0
    /19     8192    255.255.224.0
    /18     16384   255.255.192.0
    /17     32768   255.255.128.0
    /16     65536   255.255.0.0

On most *nix OS - the packages ipcalc and sipcalc make for a quick-and-dirty subnet calculator.

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