Tag: networking

Check Your SSH Server Configuration

Today, I’m going to show you how to check your SSH server configuration. It’s a simple process, but not one many people seem to know about. It’s also a pretty handy tool if you’re having SSH issues. Once again, this one isn’t all that complicated, I think… Read on!

So, why would you want to check your SSH server configuration?

Your SSH server might not be working. You may have made some changes and want to test it before moving it to production. An upgrade to the SSH application may have made some of the options different or even removed the options entirely.

There are all sorts of reasons why you’d want to check your SSH server configuration. Those are just a few of them. Not only will the article show you how to check your SSH configuration files – it’ll show you how to test alternative configurations. So, you can test your changes before making them – potentially saving you a physical trip to the server.

Check Your SSH Server Configuration:

This article requires an open terminal, like many other articles on this site. If you don’t know how to open the terminal, you can do so with your keyboard – just press CTRL + ALT + T and your default terminal should open.

By the way, I would test/learn this on a local system. You’re potentially going to break things. In fact, let’s start by breaking them! Well, let’s create a backup first, and then we’ll break stuff.

Okay. Now let’s break something! Run this command:

Find a line that has a command and doesn’t start with a #. You can also remove the # from an option and it’ll be work. Find a one line option that has a “no” option field and change it to “oh_no” *sans quotes, though that probably won’t matter) and then save the file. 

(Also, to save the file in nano, press CTRL + X, then Y, and then ENTER and that should do it.)

Now, let’s check that SSH server configuration with the following command:

If things go according to plan, it will tell you that you have an error. On top of that, it will tell you on which line you have the error. If it doesn’t throw an error, it means your configuration is fine – or that you may need to restart your SSH service for it to see the new configuration.

If you do somehow need to restart SSH server (you shouldn’t have to), restart it with the following command:

Run the command again and that should definitely show the error, which you can easily fix by simply undoing what you did in the steps above and saving it. You almost certainly shouldn’t need to restart SSH to show the error, though you may want to restart it after you’re done playing around in the config file. Of course, if you did have to restart the SSH server, you’ll need to do so again after fixing the error you intentionally introduced.

BONUS: If you want, you can list the path and check a configuration file that’s not actually in use. So, you can check the configuration file before putting it into production. That’s just:

Again, under normal circumstances, it won’t show any output if it finds no errors. It only outputs information if there’s actually an error. So, a null response is considered normal and good.

Closure:

See? Nice and easy. Now you can check your SSH server configuration for errors – even doing so before putting the config into production. It’s a pretty handy tool to have. Also, you’ll need SSH installed and running on the machine you’ll be testing with. I figure that’s obvious, but I better mention it somewhere or someone will point it out or ask about it. Then again, people seldom read this far down in an article.

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Change Your Hostname

In today’s article, we’re going to learn how to change your hostname in Linux. It’s a pretty easy article to follow along with, so even if you’re a beginner you can follow along. If you’re an advanced user, you probably already know how to do this. If not,  you will now.

So, what is your hostname? Basically, it’s the name of your computer (generally speaking). You login as <your_username> to a computer referred to by its <hostname>. So, in this case, I’m currently kgiii@kgiii-lmde. The ‘kgiii-lmde’ is the hostname. When you open your terminal, in all likelihood it shows you the <user>@<hostname> at the start of your regular prompt.

You can do some neat stuff with the hostname. For instance, and depending on the distro, it’s often local or localhost, you can connect to your LAN devices (without knowing the IP address) by connecting to <hostname>.local. This is handy for using SSH around the house, or even for using FTP or whatever.

If you don’t know your hostname, you can use one of the following commands to figure it out:

Or you can try:

There are other ways to show the hostname, but those two should be enough to get you sorted out. Either of them will happily spit out the hostname. Now that you know your hostname, it’s time to learn how to …

Change Your Hostname:

Like so many other articles on this site, this one requires an open terminal to continue. So, just press CTRL + ALT + T and your default terminal should open.

With your terminal open, enter the following command:

That should permanently change your hostname to the new hostname you chose to use in the command. You can actually change it temporarily, it will not be persistent between reboots, with the hostname command. to do that you’d just sudo hostname <new_hostname> and it’ll change it for the current session only.

To verify that you’ve changed your hostname, run one of the commands listed in the preamble section of this article. (Just type hostname and to verify you know how to change your hostname.)

Anyhow, that’s all there is to it. You really don’t need anything more than that if you want to change your hostname in Linux. Anyone should be able to follow the few directions needed.

Closure:

There you have it, another article. To think, we’re over 200 articles now. This is just one more among many, and this one is easy enough to follow. If you ever wanted to change your hostname, now you know…

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Let’s Use ‘netstat’ To Find Out What Process Is Listening On A Specific Port

Today’s article is going to teach you how to use ‘netstat’ to find out what process is listening on a specific port. If you have open ports and don’t know why – and what’s listening on that port, you’re not making good security choices.

Let’s say you followed an article about how to monitor TCP/UDP in real time. If you’re new, or even just not all that advanced with Linux and networking, you might not know why there are all those ports and all that activity. Well, one of the things you should know is how to identify what process is listening on a specific port.

There are a number of ways to do this, but we’ll be using ‘netstat’. The ‘netstat’ application can be pretty advanced, but what we’ll be doing is pretty straightforward. If you’re curious, ‘netstat’ defines itself thus:

Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships

As you can see, it’s a pretty advanced application. It has a rather expansive man page, and we’ll largely ignore that as we really only need some limited functionality. All we really need to do, for this exercise, is find out what process is listening on a specific port.

With that in mind, let’s leap into the meat of the article…

What Process Is Listening On A Specific Port:

This article requires an open terminal. If you don’t know how to open the terminal, you can do so with your keyboard – just press CTRL + ALT + T and your default terminal should open.

Once you have that open, you’ll need to find out if you have ‘netstat’ already installed. There are many ways to do this, I prefer:

If you do not have ‘netstat’ installed, it’s certainly available for all mainstream distros and easily installed from your default repositories. Here’s how to install on a few distros:

Debian/Ubuntu/Mint/etc:

RHEL/CentOS/Fedora/Rocky Linux/etc:

SUSE/OpenSUSE/etc:

If you’re using a different distro, root through the default repositories. It’s a pretty common tool and I’d like to imagine it’s easily available to anyone.

Once you have ‘netstat’ installed, the command we’re going to use is really, really, simple. In fact, I wrote this whole darned article mostly for just one command. I probably could have made it shorter. Still, it might as well be long enough to give some extra information along the way.

Anyhow, the command you run is this:

For example, you might have an open port 22 and want to know what process is listening on that specific port. So, you’re command would look like this:

The end result will look something like this:

netstat being used to find out which process is listening at a specific port
While not completely clear, you can deduce that it’s just the SSH daemon listening on port 22.

It may not be completely clear, but you can use this to deduce what process is listening on a specific port. If it’s not completely clear, you can get actually dig a little deeper. See the “1100” in there? Well, that’s the PID (Process ID) and you can use the following command to get more informtion. It looks like this:

The output from that in this case is:

Which, as we know, is the daemon for SSH and thus nothing unexpected is running on port 22. See? Security!

Closure:

And there you have it, another article. This one is a pretty handy one, especially for when you want to know what process is listening on a specific port. If you have things running that you can’t identify, you can always stop by Linux.org and ask for help. Someone there will try to find out what’s going on for you.

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Change Your DNS Servers To Google’s In Lubuntu

This article will show you how to change your DNS servers from your default servers to the DNS servers run by Google, specifically while you’re using Lubuntu. If  you’re not using Lubuntu, the process is likely fairly similar. Either way, it should be a nice and fun article, and we’ll even do it in the GUI instead of the terminal.

So, what is DNS? DNS stands for ‘Domain Name System’. As you know by now, machines are identified by their IP address. It’d suck to have to remember numbers instead of names. It’s also possible to route multiple domain names to the same IP address. So, we have domain names and use those domain names to resolve to IP addresses.

If you’re like most people, right now the DNS servers you’re using have come from your ISP – the folks who provide your internet service. This means that they can see which sites you visit, based on the requests you make to the DNS service.

Some folks don’t like this and prefer to find another DNS provider. (There’s also Secure DNS which this article will not be touching on.) One of those companies that provides free DNS servers is Google. Like them or not, their DNS servers are robust and consistently updated, often making domain propagation quicker for you.

This article is for Lubuntu, as stated above, but you may very well be able to follow the same exact steps with your distro of choice. And, now that you have a general idea of what’s going on, let’s learn how to…

Change Your DNS Servers:

To get started with changing your DNS servers, you need to find your networking icon in your system tray. It’ll be down on the right, not far from the clock. Once you have found it, right click on it so that it brings up the menu to let you “Edit Connections”. It will look something like this:

change network settings
Of course, your version won’t have the nifty arrow.

You’ll want to click the gear icon. That’s why I put the arrow there! 

That will open another window. This window will have tabs  you need to worry about – or a tab you need to know about. You probably shouldn’t need both. The tabs you’re interested in will look like this:

changing the network connections
You should need one of those, probably not both of those…

Now,  you should only need to edit one of those. If you’re still using IPv4 then you use that tab. If you’re using IPv6 then you’ll obviously want to use the appropriate tab. For example, the IPv4 would look like this:

screen to edit dns servers
This would be the tab you’re looking for, pretty much…

Now, where that arrow is is where you want to enter the new DNS server information. You separate them with a comma, though you can use a comma and a space – there will need to be a comma.

For Google’s IPv4 addresses, your choices for 8.8.8.8 and 8.8.4.4.

For Google’s IPv6 addresses, your choices are 2001:4860:4860::8888 and 2001:4860:4860::8844.

Note: The ifconfig or ip addr will help you tell if you have IPv4 or IPv6.

When you’re done, be sure to click the save button to ensure your new settings take effect. Remember the screen and changes, should things go pear shaped. You can undo this easily enough.

This will, of course, work with any set of DNS servers out there. You can use it with other servers if you aren’t a fan of Google. This can serve as a general guideline for other servers, should you wish.

Closure:

Yup… There it is. You have another article. This time, it tells you how to change your DNS servers if you use Lubuntu. Again, it’ll work for other distros, but I’m only including pictures/vouching for it with Lubuntu.

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FTP From The Linux Terminal

Why would you want to use FTP from the Linux terminal? It’s quick, easy, consistent, and works well enough as an FTP client. If it’s not already installed, it’s just a quick installation command away. In my experience, it’s usually already there.

If I’m going to be transferring a bunch of files or editing files, I’ll just use a GUI FTP client. My choice FTP client has been Filezilla for quite a long time now. It does the trick and I just keep moving my profile/config to new computers as I get them. Being easy to migrate is a definite selling point for me.

So, what is FTP? It stands for File Transfer Protocol. It’s not encrypted, so many folks have moved to SFTP – which is actually not related to FTP at all. It just shares similar functionality. Even though FTP traffic isn’t secure, it’s still fairly widely used. 

An FTP server is probably not installed by default on your desktop Linux system. However, I’ve written how to Install An FTP Server With VSFTPD which should get you sorted if you need one.

The application will be using is “FTP” which describes itself like:

ftp — Internet file transfer program

And you may find that your version of terminal-based FTP is maintained by your distro maintainers. The FTP command and flagsr should still be pretty universal throughout. Still, be sure to check the man page – especially if things seem weird.

FTP From The Linux Terminal:

Obviously, this requires an open terminal.

Press CTRL + ALT + T and your terminal should pop right open.

Once you have the terminal open the command is basically this:

You can connect via domain name, hostname, or even IP address. It’ll look like this, or pretty close:

ftp from the terminal
See: You can use FTP from the Linux terminal.

There are many commands available, so you should probably check man ftp to see what’s on offer. We’re just going to cover a few of the more useful commands.

You’ll find that ‘ls’ works to list files, just like you’re used to, as does ‘cd’. If you want to change your local directory, that is your working directory on your machine and not the working directory on the server, you need to use the ‘lcd’ command – like so:

Note: Read More About Paths

After you’ve logged in and learned to navigate, it’s time to do something. If you want to download something, you use the ‘get’ command:

That will download the file from the server to the local directory. To do the opposite, that is upload something from your current local directory, you would use the ‘put’ command, like so:

If you want to get or put more than one file, the commands are ‘mget’ and ‘mput’ respectively. They look like this:

Note: It may give a warning about using passive mode. If that’s the case, add the -p flag to your connection command – like ftp -p hostname.

Just like with your regular terminal file management, you can ‘mkdir’, ‘rmdir’, and even use ‘pwd’ to be reminded of your present working directory. Files can also be deleted with the delete <file_name> command.

Closure:

And there you have it, an article that explains how to use FTP from the Linux terminal. It is important to note that the FTP command transmits data without encryption. If security is a concern, use SCP or SFTP.

Thanks for reading! If you want to help, or if the site has helped you, you can donate, register to help, write an article, or buy inexpensive hosting to start your own site. If you scroll down, you can sign up for the newsletter, vote for the article, and comment.

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