‘traceroute’ in Linux, Let’s Take a Closer Look!

This article will explain a little about ‘traceroute’ as a network diagnostic application. We’ll give a few traceroute examples, enabling you to figure out some networking issues.

Whenever I find myself unable to connect to a networked device, including websites online via the internet, one of the first tools I reach for is called ‘traceroute’. It’s not exclusive to Linux and you may know of the tool ‘tracert’ in Windows that does the same thing.

traceroute defines itself as this:

traceroute – print the route packets trace to network host

More realistically, it shows you the hops (devices) you go through in order to make a connection. See, when you connect to a different computer over the network, you don’t generally do so without going through other devices. Your data will travel through multiple devices to reach the source device and all those hops along the way are potential points of failure.

Sometimes those devices are under your control and you can actually do something about it. Other times, it’s just informative and all you can do is wait, or inform someone else and hope they fix it. If nothing else, you’ll know where your packets stopped or slowed down to the point of uselessness.

For example, there 13 hops (devices) between me and linux-tips.us.

traceroute in action
See? There are 13 hops to reach my destination. 

So, while that picture should explain it well enough, let’s get a little deeper.

Using traceroute:

You may find that traceroute isn’t already installed. If it isn’t, it’s absolutely in your default repositories. However you would normally install software is how you install this. If you look, traceroute is sure to be in there. So, go ahead and install it if it’s not already installed. For example:

Just adjust that to your package management system and it’ll be in there. It’s that important a tool that I’m sure it’s in there. In fact, I’m a bit surprised that it’s not always installed by default, but it isn’t.

Now, the most basic usage is just like you saw in the image above.

So long as you’re within 30 hops and use 60 or fewer packets, that’s going to work well enough. The information it spits out is what devices it has traveled through (their hostname and IP address) and RTT – Round Trip Times. There are three of them because three packets are sent. Ideally, you’ll see your destination listed last. If not, you’ll see the closest you got to your destination.

If you see an asterisk, that means the device didn’t respond as expected. Frequently, this means the device is blocking ICMP. You can try to get around this by using ICMP ECHO (-I) or TCP (-T) packets. However, both of those will require elevated permissions, or the use of sudo.

All of this is mostly informative – unless you’re in control of the network and devices.

When it’s a network and devices under your control, you can use this information to troubleshoot. You can see the device names and time taken for packet transit, narrowing down your choices for troubleshooting.

When you’re using this over the public internet, you’re subject to other people who control the devices. If you find a break along the way, about all you can do is wait – or maybe use the data to contact your ISP (or hosting provider, if it’s your site that you’re trying to reach).

There are other options with traceroute. You can change the port you use, you can send more or fewer packets, you can not resolve hostnames, and more. To see the rest of the traceroute options:

That will fill you in with the many other choices you have. I find I don’t really need the advanced options, but system admins may need some of the features. As a regular user, I just use it to troubleshoot my own connections on my private network or when I am having web hosting/connectivity issues.

Closure:

And there you have it. Another article is in the books, and this time it’s just a nice easy article about the venerable traceroute. If you don’t already have this tool in your toolbox, it’d be worth adding and adding a basic familiarity to your mental toolbox.

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Dealing With Duplicates In Your Bash History

Your Bash history can be a valuable tool when you mess up and need to retrace your steps to find out what went wrong. It’s in your best interest to keep it from getting cluttered.

Some of my readers are fairly new to Linux, so you may not know that you have a Bash history. Well, you probably do. It’s helpfully called .bash_history and it’s a hidden file in your home directory. You can open it with a text editor if you want.

See, every time you write a command into the terminal (unless you take steps to avoid it) that command is stored in a hidden file called .bash_history. The period at the start of the file name means that it’s a hidden file and you’ll need to enable hidden file viewing in your file manager. (It’s quite frequently F4 or under the ‘View’ menu.)

If you open your terminal (press CTRL + ALT + T) and push the up arrow on your keyboard, you’ll see the last command you used in your terminal. Well, it’s a bit more complicated than that, but you’ll see the bottom-most entry in your .bash_history.

If you use your terminal a lot, you’ll have duplicates. It’s just a fact of life. Many commands will be run multiple times, such as performing updates. These clutter up your history and just waste space. You don’t have to keep the duplicates, or even allow duplicates.

De-dupe .bash_history

First, let’s disable duplicates and delete them if they do somehow occur. To do this, you’re going to edit your .bashrc file. As you can see by the . at the start of the file name, the file is hidden by default. You can use the GUI file manager and favorite text editor. Enable viewing hidden files, and just add the following line to the very bottom of that file.

Save it, of course.

If you want, you can do it right in the terminal. Open the terminal with the directions above and enter the following command:

Notice how there’s no use of ‘sudo’? That’s because you already own these files and you don’t need elevated permissions to edit them. 

Anyhow, use the down arrow and then paste in the text above, just as though you were editing it in your favorite text editor. Then, go ahead and save it by pressing CTRL + X, then Y, and then ENTER.

There… Now Bash should ignore duplicates and erase any new duplicates that it finds.

All well and good?

Maybe…

I mean, it’s not like you just installed your OS today and have a sparkling clean bash history. No, if you’re like me, you’ve probably got a bunch of clutter in there and all sorts of duplicated content. So, you’re going to want to clear those already existing duplicates out as well.

Seeing as you’re already in the terminal, you can use the following command that I once found online and saved to my notes:

That right there will go ahead and remove any duplicates that it finds in your bash history. You might want to make a backup of the file before trying that command – but I’ve used it so many times that I have complete faith in it. So far, it has only removed duplicates and not broken anything. (Pay most of your attention to the use of awk in the command if you’re wondering how it works.)

And, there you have it. You should have a nice .bash_history that isn’t full of duplicates. I’ve been known to open the file manually and run through it to remove stuff I’m never going to use again, commands that I’ve gotten wrong, or commands that are no longer of any use. 

Addendum

I should also mention that I have been using the (mostly) same history file for a long time. You save it and import it on new computers that will run the same operating system. Personally, I use my bash history fairly often and I like the consistency. I also do the same with my aliases, but that’s a topic for another day.

By the way, if you don’t want to have your command stored in the history, the easiest way to avoid it (on a command-by-command basis) is just to put a space in front of it. The system doesn’t care that there’s a leading space, except to use it as a cue to not store the command in your history. If you’re entering sensitive data into your terminal, you might want to consider that. If it’s just a junk command that you’re never going to see again, you might want to consider that.

Finally, thanks for reading. I appreciate the audience and I enjoy getting my notes online. The new domain is going well and we’ll soon be getting some of the older content transferred. It’s just a bunch of tedious work. I’d ask for help, but there’s not a whole lot anyone can do to help. I’ll just have to find the time to do it. Don’t forget the newsletter! 

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