Using tar and gzip

Picture from flickr by Windwirral

Here are the basic steps to use tar and gzip to package up some files in a compressed archive, so that you can move them to another computer or back them up.

First you want to create and add files to the archive. Go to the target directory and enter individual filenames and directories you want to have included.

tar -vzcf mystuff.tar.gz file1.txt file2.doc mypictures/

Now you can test if that worked by listing the files in the archive.

tar -tvf mystuff.tar.gz

When you want to extract the files, go to the folder where you want to extract them, in this case 'targetfolder'.

cd targetfolder

tar -vxf mystuff.tar.gz

Securing an ssh server with fail2ban

There are all kinds of ways to secure an ssh server, with varying degrees of increasing security and decreasing flexibility. For example by limiting your server to only accept connections from certain known IP's, you are secure from random hackers on the internet, but you lose the ability to connect to your machine from anywhere you want to. Perhaps while travelling, for example.

Port knocking and listening on a high numbered non-standard port. make it harder for an attacker to even start trying to hack your connection. But this also requires you connect with a machine that you have the knock program installed on. Again, less convenient, more secure.

However, once an attacker does find your port there's nothing to stop brute force password hacking. If you look in your log file, you should see people connecting to your ssh port quite frequently and trying password attacks.

cat /var/log/auth.log

If you have a secure password then it would require days of brute force hacking to gain access to your ssh account, but even so, if you don't watch your logs then it's perfectly possible somebody will gain access eventually.

Brute force attacks can be limited using fail2ban. There's a great article on setting it up here. This program will scan your auth.log for you, using a regular expression to find failed password attempts. On a specified number of failures from a given IP, it will then modify the iptables on your machine (the firewall), to lock that IP out for a specified time.

Now instead of watching your auth.log fill up with reams of failed passwords, you'll see a greatly reduced amount of brute force attacks, and you can watch your fail2ban log file fill up with the IP addresses of hackers.

Using screen

When running a linux app, perhaps on a remote box, you don't want it to terminate when you close the terminal window. You can run some applications as daemons, or run them with the nohup command (no hang up).

For example:

nohup ./myapp > output.txt

will run the program myapp and send the output to output.txt.

You can then follow the output.txt with a command

tail -f output.txt

but if you hit Ctrl-C or close the terminal (or have a power cut), the application will still be running on the remote machine.

A more powerful solution is the Gnu screen application. This lets you run multiple shell sessions and switch between them. Using screen you can be logged into a server in your office, then detach from the screen session, go home and reattach to it there (assuming you have network access to the computer).

Here's a cheat sheet for using screen.

Running it:

screen

Learning to use it:

Ctrl-a help

Important commands:

Ctrl-a c (open a new window)

Ctrl-a p (prev window)

Ctrl-a n (next window)

exit or C-a q to exit

How to reattach to the screen session:

screen -ls

That shows screen sessions on the machine your logged on to, and then you reattach using:

screen -r name

That's all folks!

Linux: Looking at files changed recently

This is really handy (on Linux and Cygwin) when you're working on a project that may not be in source control and you want to see what you changed recently.

I made this blog post, when it seems a trivial use of find, because I don't think the man page covers it well.

This command finds all files modified in the last 60 minutes:

find . -type f -cmin -60