Taking inventory of your own devices

Inventory is the part of reconnaissance you do on yourself. Before you can judge anything about a device you need to know what it has: its addresses, the services it runs, the tools it carries and how much room is left. In this lesson you will build that list for your own phone, with commands that need no root and no network connection. The same commands run on your own laptop or your own second phone, which is how one inventory becomes a picture of all the devices you own.

What you'll be able to do

• List the network interfaces on your device and the addresses they hold.
• Tell a private address from a public one by looking at it.
• Check which reconnaissance tools are installed and where they live.
• Read how much storage your home directory has left.

Step 1: List your interfaces and addresses

Every device that talks to a network has at least one interface with an address on it. The loopback interface is always there and only talks to the device itself.

ip -o addr show

1: lo    inet 127.0.0.1/8 scope global dynamic
1: lo    inet6 ::1/128 scope global dynamic
2: wlan0    inet 192.168.1.42/24 scope global dynamic
2: wlan0    inet6 fe80::2806:17ff:fe11:2dff/64 scope global dynamic

The -o flag prints each address on one line instead of spreading it over several, which is what makes this output easy to read and easy to pipe. Each line starts with the interface number and its name. lo is the loopback, and wlan0 is the wireless interface on a phone. The word inet marks an IPv4 address and inet6 marks an IPv6 one. The part after the slash, such as /24, is the prefix length, which is another way of writing the netmask.

Step 2: Pick out the private address

You rarely need every line. Asking for IPv4 only makes the list shorter.

ip -o -4 addr show

1: lo    inet 127.0.0.1/8 scope global dynamic
2: wlan0    inet 192.168.1.42/24 scope global dynamic

Two addresses are worth telling apart. 127.0.0.1 is the loopback and it always means this device, so nothing outside can reach it. 192.168.1.42 is your address on the local network, and it is a private address, because it falls in the 192.168.0.0/16 range. Addresses that begin with 10. or 172.16 through 172.31 are private too. A private address is only reachable inside your own network, which is why you can scan it and no one else's traffic is involved.

Step 3: Inventory your tools

The next list is what you have installed. command -v prints the full path of a command when it exists and prints nothing when it does not, so a list of names in gives a list of paths out.

command -v nmap curl wget openssl ssh ss netstat

/data/data/com.termux/files/usr/bin/nmap
/data/data/com.termux/files/usr/bin/curl
/data/data/com.termux/files/usr/bin/openssl
/data/data/com.termux/files/usr/bin/ss
/data/data/com.termux/files/usr/bin/netstat

A path on a line means the tool is present. A missing tool simply produces no line, so the output is shorter than the list you typed. Every one of these lives under the Termux prefix, /data/data/com.termux/files/usr, which is where packages are installed without root. If a line is missing and you want that tool, install it with pkg install and its name. Note that wget and ssh are absent above, which is exactly what a shorter list looks like.

Step 4: Inventory your storage

The last piece of the inventory is room. A device that is nearly full will fail to update a package or write a log, which can look like a security problem when it is only a space problem.

df -h ~

Filesystem      Size  Used Avail Use% Mounted on
/dev/block/dm-3  110G   42G   68G  39% /data

The -h flag prints sizes in units a person can read, and ~ asks about the filesystem that holds your home directory. Read the Use% column first. A figure near 100 per cent is the one to act on, because it means the next write may fail. The Avail column gives the same fact in gigabytes, which is often easier to plan against.

Quick reference

Command	What it does
ip -o addr show	Lists every interface and address on one line each
ip -o -4 addr show	The same, IPv4 addresses only
command -v NAME...	Prints the path of each tool that exists
df -h ~	Shows free space on the filesystem holding your home
pkg install NAME	Installs a missing tool in Termux

Next steps

• Reading a port scan properly turns the list of services into a scan you can read.
• Service versions and what they tell you shows how to ask each service what it is.
• Hardening your Termux setup uses the same lists to decide what to switch off.