Suricata IDS/IPS on Ubuntu 20.04

No network security setup would be complete without a network-based Intrusion Detection System and Intrusion Prevention System (IPS/IDS). For many businesses, network-based IDS/IPS is a key component of maintaining a PCI-compliant environment. PCI compliance is mandated for any business that handles credit card-holder data in any way. Other businesses handle such sensitive information that literally every packet needs to be inspected for any kind of malicious activity. Financial institutions, healthcare providers and government bodies usually fall into this category.

Whatever the business need, IDS/IPS (from this point forward, I mean network-based. Host-based is another topic entirely) is yet another necessary security tool for keeping a network environment safe. Today we’ll take a look at suricata, an open-source IDS/IPS system that you can install on Ubuntu 20.04. You can even get free, regularly-updated rule lists for staying up-to-date on the latest threats. Some ruleset providers charge for their lists, but many are provided without charge. Let’s get the basics setup and see how it works!


Topology in GNS3

We’ll install Suricata on Ubuntu20.04-1, which is acting as a router between the hapless workstation and the Internet. We’ll configure Suricata to listen for threats on the “outside”, or Internet-facing interface, ens3.


Installation requires setting up the Ubuntu PPA for recent, prebuilt packages. We can add it with this command:

add-apt-repository ppa:oisf/suricata-stable

Then update apt, and install:

apt-get update
apt-get install suricata


Now that it’s installed, we need to make a couple of configurations in text files. The first is in /etc/default/suricata, which holds the configuration mode and default interface. You’ll want to change the following lines:



LISTENMODE sets whether you want suricata to be in IDS or IPS mode. IDS will passively listen and write an alert to a log file. IPS does that too, but with the option to drop detected threats. In order to perform the drop, packets needs to be sent to the nfqueue processing queue. IFACE is whatever network interface you want suricata to listen on.

Next we need to update our rules with the suricata-update command. You can see what lists are enabled by running suricata-update list-sources, but the default ones will do just fine.


Now we should have a set of rules at /var/lib/suricata/rules/suricata.rules. We’ll use the sed tool to mark some of the rules to drop traffic, with these commands:

sed -i -e '/ATTACK_RESPONSE/ s/^alert/drop/' /var/lib/suricata/rules/suricata.rules 
sed -i -e '/USER_AGENTS/ s/^alert/drop/' /var/lib/suricata/rules/suricata.rules 

Finally, we need to send all packets to the NFQUEUE processing queue so they can be dropped if determined to be malicious. These iptables commands will send all packets to that queue with the exception of port 22, which is usually SSH traffic. That way our SSH session to the server isn’t interrupted:

iptables -I INPUT 1 -p tcp --dport 22 -j NFQUEUE --queue-bypass
iptables -I OUTPUT 1 -p tcp --sport 22 -j NFQUEUE --queue-bypass
iptables -I FORWARD -j NFQUEUE
iptables -I INPUT 2 -j NFQUEUE
iptables -I OUTPUT 2 -j NFQUEUE

And finally reload suricata:

systemctl restart suricata


Quick note: With the way we set suricata up, you can run these tests either from the Ubuntu server where suricata is installed, or from anything behind it. Either will work. Also please rest assured neither of these tests actually does anything close to malicious.

If we send a normal HTTP request to from the hapless workstation at, it should work:

<a bunch of web nonsense output here>

But if we change the HTTP user-agent string to “BlackSun”, suricata will detect this as a malware attack. Apparently BlackSun is some kind of ransomware, but just changing the user-agent string to “BlackSun” isn’t going to do anything at all, other than test whether suricata is working or not. The below command will achieve this goal:

curl -A "BlackSun"
<no response>

Suricata is dropping these packets! Our IPS is working. Let’s check if IDS is working too. Activity is logged at /var/log/suricata/fast.log:

cat /var/log/suricata/fast.log
01/20/2022-13:40:00.339281  [Drop] [**] [1:2008983:8] ET USER_AGENTS Suspicious User Agent (BlackSun) [**] [Classification: A Network Trojan was detected] [Priority: 1] {TCP} ->

Success! We’ve been notified of a network attack. Let’s try another one. The website has a nice test we can run with this command:

<no response>

This well send an index.html file containing the output of the id root command, a common way for remote command-and-control servers to execute attacks. Let’s check the log:

cat /var/log/suricata/fast.log
01/20/2022-13:44:40.778197  [Drop] [**] [1:2100498:7] GPL ATTACK_RESPONSE id check returned root [**] [Classification: Potentially Bad Traffic] [Priority: 2] {TCP} ->


How To Ping Sweep With Python in GNS3

I debated for a long time whether to include coding in my networking blog. But since it seems the future of networking lies in code and automation, I believe it is time for some code.

Today we’ll look at how we can quickly ping sweep a subnet using python. If you are looking for some resources on learning python, you might check out this free ebook on python for network engineers or getting a course on Udemy for python. If you’d like me to create a python learning resource in the future, please let me know in the comments or contact form!


Topology in GNS3

We’ll use a simple IP subnet of to sweep. Connected devices have been placed randomly at, and We’ll write the sweep python code on the Ubuntu 20.04 server at Let’s get started!

Simple python ping sweep script

The basic logic of the code will be to loop through all hosts in the subnet, from to (.0 and .255 are the network and broadcast addresses, so no need to ping them) and ping each IP address once. If it responds, we’ll print to the CLI that it worked.

We’ll use two modules, ipaddress and subprocess. ipaddress is a handy network tool for working with IP addresses. Knowing where to start and stop in the loop is relatively simple with a /24 subnet, what if it were Just incrementing the fourth octet by 1 each time won’t work. You’ll go to, which isn’t a valid IP address. That’s where ipaddress helps out. subprocess lets us call the ping command from python.

Here’s our code:

import ipaddress
import subprocess

mynet = ipaddress.ip_network('') #create an ipaddress object for

for host in mynet.hosts():                    #loop through each host of
    host = str(host)                          #change from ipaddress object to string to hand to ping command
    proc =                    #use subprocess to call ping command, split to multiple lines cuz its long
        ['ping', host, '-c', '1'],            #calling ping here, putting in host to ping
        stderr=subprocess.DEVNULL,            #silence ping command errors
        stdout=subprocess.DEVNULL             #silence ping command output
    if  proc.returncode == 0:                 #return code of 0 from ping command means it got a reply
        print(f'{host} is alive!')            #say this host is alive if we got a reply

Hopefully this is pretty straightforward. The magic is happening in a couple spots. The first is with this line:

mynet = ipaddress.ip_network('')

This creates an “object” that holds the network we’re working with. This object has super powers, one of them is visible in this line:

for host in mynet.hosts():

It lets us move through the hosts of the subnet, from to, each time the host IP is assigned to host. We can then hand host to the ping command.

The second spot with magic is here:

    proc = 
        ['ping', host, '-c', '1'],

This is just spawning another process, ping, from python.

When we run the script, we should see all the IP addresses that are alive!

python3 #might be python or python3 based on your OS is alive! is alive! is alive! is alive!

There’s only one problem – the script takes quite a while to run. The issue is that each time the ping command runs, python waits until it finishes before moving to the next. This is known as “blocking”, which basically means the script comes to a halt while it’s waiting for a ping process to finish. A common /24 size subnet of 254 hosts takes a good while to complete.

What if we could ping them all at the same time, or close to it? Well, this leads us into the dark, dangerous world of multi-threading, parallel processing, multiprocessing, and asynchronous processing. Even the words are ominous-sounding. But don’t worry, it’s not so bad with the help of a handy module called asyncio.

Super-charged ping sweep with asyncio

The issue we’re faced with is that we need to do multiple things at once. There are many ways to solve this problem, and some people spend their whole careers in this complex field. Recently though, the python asyncio is getting popular because it’s relatively easy to work with and not so terribly complicated compared to others. As of python 3.6, it’s part of the standard library.

Here’s the same ping sweep, this time written using the python asyncio module:

import ipaddress
import asyncio

async def ping(host):                              #add the "async" keyword to make a function asynchronous
    host = str(host)                               #turn ip address object to string
    proc = await asyncio.create_subprocess_shell(  #asyncio can smoothly call subprocess for you
            f'ping {host} -c 1',                   #ping command
            stderr=asyncio.subprocess.DEVNULL,     #silence ping errors
            stdout=asyncio.subprocess.DEVNULL      #silence ping output
    stdout,stderr = await proc.communicate()       #get info from ping process
    if  proc.returncode == 0:                      #if process code was 0
        print(f'{host} is alive!')                 #say it's alive!

loop = asyncio.get_event_loop()                    #create an async loop
tasks = []                                         #list to hold ping tasks

mynet = ipaddress.ip_network('')      #ip address module
for host in mynet.hosts():                         #loop through subnet hosts
    task = ping(host)                              #create async task from function we defined above
    tasks.append(task)                             #add task to list of tasks

tasks = asyncio.gather(*tasks)                     #some magic to assemble the tasks
loop.run_until_complete(tasks)                     #run all tasks (basically) at once

No denying it, this is more complicated. Might be a bit foreign even if you’re familiar with python. The key thing here is that we define a single ping task in an async function. Then when we loop through the subnet hosts, we create a task from that function, instead of running it on the spot. Then we call the asyncio module at the end to gather up the tasks and run them all asynchronously, which has the effect of appearing to run them all at once.

Also note that there’s no subprocess module here. Asyncio has built-in subprocess management (check the documentation here), so no need for the standard subprocess module.

While the output is the same, you’ll notice this takes about a second to run compared to minutes for the first script:

python3 is alive! is alive! is alive! is alive!

Please let me know in the comments if you want to see more content like this!

IPv6: IPsec Not Mandatory to Use or Exist

One thing is clear, there is a serious amount of misinformation about IPv6. There are many blog posts and even official documents from respected sources that have blatantly incorrect information. Many blog posts that start out with an introduction such as this one, but end up spreading misinformation about IPv6 anyway. To avoid spreading misinformation myself, I’m going to cite the only sources of truth in the network world: the IETF RFC’s. If you’re looking for a clear and simple one-liner to sum up this article here it is:

Since the IETF issued RFC 6434 in 2011, the IPv6 standard does not require devices to implement (to be capable of) IPsec, nor does it require the enablement or use of IPsec for IPv6 nodes that do implement (are capable of) IPsec.

Yes, you read that right. You cannot count on IPsec even existing in nodes and devices running IPv6. For many years now a great number of sources have been trumpeting that IPv6 will herald an era of not having to think about network security anymore. Most respectable sources reject this notion, saying that it’s required to be implemented (to exist) in all IPv6 nodes, but not required to be turned on. Unfortunately that is also false.

RFC 1883 – where it all started

In December 1995, the IETF (Internet Engineering Task Force, a standards body accepted as the source of truth in the networking world) released RFC 1883, which defined IPv6 as a successor to IPv4.

Page 36 of that RFC briefly and quietly stated that Authentication Header (AH) and Encapsulating Security Payload (ESP) be required to secure IPv6 traffic:

This document specifies that the IP Authentication Header [RFC-1826] and the IP Encapsulating Security Payload [RFC-1827] be used with IPv6, in conformance with the Security Architecture for the Internet Protocol [RFC-1825].

This lead to a lot of folks being really excited about the ramifications of mandated secure connections in IPv6. While I can’t find any articles from 1995, this blog post from Bitdefender (a respected name in security) from 2012 is a really good example of an article making really misleading statements about how IPsec works with IPv6:

IPv6 comes with built-in IPSec , a technology that ensures secure host-to-host communication. This means that two clients communicating over IPv6 can automatically do authentication, message integrity and encryption or any combination of those.

While the implementation (the capability) of IPsec was indeed mandatory in IPv6 for the first 16 years after RFC 1883 was released, no one ever said that the use or enablement of it would be mandatory.

RFC 6434 – IPsec from MUST to SHOULD

In December 2011, the IETF updated the IPv6 Node Requirements RFC with RFC 6434. In section 11 (Security), they make this unmistakable statement:

Previously, IPv6 mandated implementation of IPsec and recommended the key management approach of IKE. This document updates that recommendation by making support of the IPsec Architecture [RFC4301] a SHOULD for all IPv6 nodes.

While IPv6 IPsec is implemented (the capability exists) in major desktop/laptop OS’s such as Windows and macOS, the Internet is made up of much more than that. Internet of Things comes to mind.

It seems the IETF realized that it doesn’t make sense (or in many cases it’s not possible) to include the complexity of IPsec in an IPv6 implementation. They went on to say in RFC 6434:

This document recognizes that there exists a range of device types and environments where approaches to security other than IPsec can be justified.

Loads of misinformation

So yes, starting in 2011, IPsec even existing in a device running IPv6 is a big maybe. Despite this, some of the most respected companies in the world write documentation that describe the mandatory-ness of IPsec in IPv6. Take this IOS configuration guide from Cisco, written in August 2012:

IPsec is a mandatory component of IPv6 specification.

Or this article from Microsoft written in 2020:

Internet Protocol Security (IPsec) is a set of security protocols used to transfer IP packets confidentially across the Internet. IPsec is mandatory for all IPv6 implementations and optional for IPv4.

Or this article from Redhat written in 2019:

In What you need to know about IPv6, we mentioned that Internet Protocol Security (IPSec) is incorporated into IPv6. This statement simply means that communication between the two endpoints is either authenticated, encrypted, or both, via the extension headers. There is a long-running discussion on the internet regarding whether the interpretation of “IPSec being mandatory” in IPv6 is correct or not. If you need to know more about this topic, see RFC 6434.

This last one from Redhat is particularly confusing. They mention the very RFC where IPsec became optional and even mention the ongoing debate, but continue to say that communications in IPv6 are either authenticated or encrypted, or both.

What’s actually happening? Not IPv6 IPsec.

All of this RFC and protocol stuff is a bunch of theoretical pie in the sky. What’s actually happening out in the wild? Well, I can’t speak to the whole Internet. Maybe folks are using IPsec like they do in IPv4 – for site-to-site tunnels. Or maybe something else. But to test typical web traffic, I have native IPv6 capability in my home. I took some Wireshark captures to various websites.


IPv6 capture of


IPv6 capture of


IPv6 capture of

Where’s the IPsec? I don’t see any IKE, AH or ESP. Looks like typical TLS traffic to me. So… no change in security from IPv4, then.

iperf2 vs iperf3: What’s the difference?

At first glance, you might be tempted to use iperf3 simply because it is one more than iperf2 (don’t worry, I’m guilty of this crime as well). It’s not an unfair assumption to think that iperf3 is the most recent version of the software, because of the name. It’s common to have two different versions of software in parallel existence, so the new one can take hold while the older version slowly dies away. Python2 and Python3 come to mind. This is not the case with iperf, however.

I recently wrote a post on how to use iperf3 to test bandwidth. Shortly after that one of the authors of iperf2, Bob McMahon, reached out to me. He pointed out that iperf2 is very much actively developed with some cool new features having been added recently. Under the surface they are very different projects, maintained by different teams with different goals.

Today we’ll take a look at some of the differences between the two.


Ubuntu 20.04 and Rocky Linux 8.5 VM’s in GNS3

We have a really basic topology here, Ubuntu 20.04 and Rocky Linux 8.5 connected on a single link with IP subnet Both VM’s have iperf2 and iperf3 installed.

Bandwidth Test

For a bandwidth test, the two are almost identical. You can perform a bandwidth test using either with the same commands. For this test, the Ubuntu VM will be the client, and Rocky the server. Start the server on Rocky like this:

iperf -s

Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)

And from Ubuntu perform a test like this:

iperf -c
Client connecting to, TCP port 5001
TCP window size:  238 KByte (default)
[  3] local port 36528 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  1.90 GBytes  1.63 Gbits/sec

These commands will work using iperf2 or iperf3, however it should be noted you can’t use an iperf2 client with an iperf3 server, and vice-versa. Also, they use different TCP ports by default. Even if you used an iperf3 client with an iperf2 server and manually set the TCP port to be the same, you will get an error. They are not compatible:

iperf3 -c -p 5001
iperf3: error - received an unknown control message

Supported Operating Systems

iperf2 is the clear winner here, primarily because it has up-to-date Windows packages available for easy download right on the sourceforge page. I avoid Windows when I can, but it has a tendency to be unavoidable due to it’s sheer installation base. iperf3 apparently had some unofficial builds a while back but nothing officially supported. You’ll need to compile it yourself to work on Windows which can be an inconvenience at best.

iperf2 downloads page

For Linux, many operating systems come with iperf2 preinstalled, Ubuntu 20.04 is one such example. iperf3 is just a command away though, with package managers. For macOS, the Homebrew package manager can quickly get you iperf2 or iperf3.

Feature: iperf3 authentication (not encryption)

Description of authentication features in iperf3

iperf3 supports authenticating clients to the server using public key/private key as well as a users file. I decided to try it out. To avoid a hassle I just used the exact commands they provided in the man file. You first generate a public key and private key on the server:

openssl genrsa -des3 -out private.pem 2048
openssl rsa -in private.pem -outform PEM -pubout -out public.pem
openssl rsa -in private.pem -out private_not_protected.pem -outform PEM

Then create a “credentials.csv” file with hashed passwords. The following commands will get a hashed password for you:

S_USER=james S_PASSWD=james
echo -n "{$S_USER}$S_PASSWD" | sha256sum | awk '{ print $1 }'
0b0c98028105e9e4d3f100280eac29bba90af614d1c75612729228e4d160c601 #This is the hash of "james"

Then create a “credentials.csv” file that looks like this:


Now start the server:

iperf3 -s --rsa-private-key-path ./private_not_protected.pem --authorized-users-path ./credentials.csv

Then from the client, copy the public key over:

scp james@ .

Then run the client:

iperf3 -c --rsa-public-key-path ./public.pem --username james

You’ll be asked for the password. If you get it right, the server will display a message that authentication succeeded:

Server listening on 5201
Authentication successed for user 'james' ts 1639396545
Accepted connection from, port 32784
[  5] local port 5201 connected to port 32786
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   194 MBytes  1.63 Gbits/sec                  
[  5]   1.00-2.00   sec   204 MBytes  1.71 Gbits/sec

Feature: iperf2 isochronous mode

One of the coolest features of iperf2 is its “isochronous” option. This option is designed to simulate video streaming network traffic. You can hear Bob McMahon explain it himself on his youtube video on this feature.

Using the parameters and commands he describes in his video, we’ll run on a test. The Ubuntu server will be the iperf2 server:

iperf -s -e -i 1

Then on Rocky Linux we’ll run the client test:

[james@localhost ~]$ iperf -c -i 1 --isochronous=60:40m,10m
Client connecting to, TCP port 5001 with pid 1640
UDP isochronous: 60 frames/sec mean=40.0 Mbit/s, stddev=10.0 Mbit/s, Period/IPG=16.67/0.005 ms
TCP window size:  340 KByte (default)
[  3] local port 49150 connected with port 5001 (ct=1.44 ms)
[ ID] Interval        Transfer    Bandwidth       Write/Err  Rtry     Cwnd/RTT        NetPwr
[  3] 0.00-1.00 sec   214 MBytes  1.79 Gbits/sec  1708/0          0       67K/562 us  398346.93
[  3] 1.00-2.00 sec   217 MBytes  1.82 Gbits/sec  1738/0        230      145K/608 us  374676.21
[  3] 2.00-3.00 sec   205 MBytes  1.72 Gbits/sec  1640/0        427      142K/583 us  368710.26
[  3] 3.00-4.00 sec   212 MBytes  1.78 Gbits/sec  1697/0        575      118K/920 us  241770.85
[  3] 4.00-5.00 sec   200 MBytes  1.68 Gbits/sec  1599/0        371      134K/853 us  245702.38
[  3] 5.00-6.00 sec   200 MBytes  1.68 Gbits/sec  1598/0        423      117K/529 us  395941.50

On the server we get our output:

james@u20vm:~$ iperf -s -e -i 1
Server listening on TCP port 5001 with pid 3045
Read buffer size:  128 KByte
TCP window size:  128 KByte (default)
[  4] local port 5001 connected with port 49150
[ ID] Interval            Transfer    Bandwidth       Reads   Dist(bin=16.0K)
[  4] 0.0000-1.0000 sec   213 MBytes  1.79 Gbits/sec  4631    503:1500:1008:577:276:191:138:438
[  4] 1.0000-2.0000 sec   217 MBytes  1.82 Gbits/sec  4018    570:838:812:502:255:231:164:646
[  4] 2.0000-3.0000 sec   204 MBytes  1.71 Gbits/sec  5074    590:1537:1637:511:316:152:115:216
[  4] 3.0000-4.0000 sec   212 MBytes  1.78 Gbits/sec  3924    599:805:717:464:266:264:246:563
[  4] 4.0000-5.0000 sec   200 MBytes  1.68 Gbits/sec  3876    575:953:672:462:258:242:188:526
[  4] 5.0000-6.0000 sec   200 MBytes  1.68 Gbits/sec  4046    656:1040:687:476:258:242:238:449

Unfortunately the version of iperf that is available in Ubuntu 20.04 repositories (2.0.13) doesn’t support isochronous TCP mode mentioned in the video. You would need to compile from source or use Windows for that. A newer version will be included (probably already has been by the time you’re reading this) in Ubuntu 22.04 LTS.

Various smaller differences

There are many other spots that iperf2 and iperf3 are different.

  • iperf2 supports an “enhanced output mode” using -e that is totally revamped (used it above in the isochronous section).
  • iperf3 supports json output using the -j option.
  • iperf2 supports a bidirectional test which performs tests from the client and server simultaneously using -d
  • iperf2 uses a multi-threaded architecture, while iperf3 uses single-threaded. To be honest, I haven’t seen any way that this actually affects performance of the application. I’d be really curious if anyone has some input on this.

I hope this was helpful, and I hope I did both of these cool programs a small amount of justice. I’m really curious to see if anyone has any other input or differences they know about. Please fee free to comment or reach out directly.

How To Install Free Range Routing (FRR) on Ubuntu 20.04 and Rocky Linux 8.5

The latest version of my favorite routing protocol software, Free Range Routing 8.1 was recently released on November 9th.

Free Range Routing is a fork of the Quagga project that improves upon it and adds lots more features and new protocols. My favorite protocol that is added is EIGRP, which was originally a Cisco proprietary protocol until Cisco released a draft RFC in 2013. Free Range Routing makes it easy to spin up a Linux router and exchange routes via EIGRP. Since Cisco routers speak EIGRP, you can also exchange routes with them too! Today we’ll just exchange routes between Ubuntu 20.04 and Rocky Linux 8.5 via EIGRP.


Ubuntu and Rocky Linux in GNS3

We have a simple network here with an Ubuntu and Rocky Linux VM’s acting as IP routers. Without adding routes, Ubuntu does not know about, and Rocky does not know about EIGRP can educate them. I should mention – each of the Alpine nodes has a default route pointing to the .1 in their subnet (Ubuntu and Rocky), which is a typical setup in most networks.


In a previous post, I installed FRR on Ubuntu 18.04 via the snap store. You can still do that, but it looks like the snap version hasn’t been updated with 8.1. I’m sure it will be updated soon, but let’s install it via the binary packages that FRR provides just to do something different.

For Rocky Linux, you can find instructions here. They are RPM packages for CentOS, and in my testing I found them to work fine for Rocky Linux. Per their instructions, we’ll run these commands:

curl -O$FRRVER-repo-1-0.el8.noarch.rpm
sudo yum install ./$FRRVER*
sudo yum install frr frr-pythontools

We’ll need to modify /etc/frr/daemons and turn on the protocols we want, in this case EIGRP:

vi /etc/frr/daemons

eigrpd=yes #find this line and set to yes

Then you’ll need to restart frr:

systemctl restart frr

The process is similar on Ubuntu. The debian-based instructions are on this page. Following those, we’ll run these commands:

curl -s | sudo apt-key add -
echo deb $(lsb_release -s -c) $FRRVER | sudo tee -a /etc/apt/sources.list.d/frr.list
sudo apt update && sudo apt install frr frr-pythontools

We’ll need do modify the daemons file similar to above and run the exact same systemctl command to restart frr.

Installation is complete!

Configure FRR and EIGRP

To setup EIGRP routing, we’ll enter the FRR vtysh configuration tool that should be familiar if you’ve used either Quagga or Cisco IOS routers. On Ubuntu we’ll do this:


Hello, this is FRRouting (version 8.1).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

u20vm# conf t
u20vm(config)# router eigrp 10
u20vm(config-router)# network
u20vm(config-router)# network
u20vm(config-router)# ^Z
u20vm# exit

On Rocky Linux, it’s almost exactly the same but the second network to add is


Hello, this is FRRouting (version 8.1).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

rl85vm# conf t
rl85vm(config)# router eigrp 10
rl85vm(config-router)# network
rl85vm(config-router)# network
rl85vm(config-router)# ^Z
rl85vm# exit

Since Rocky is runnning firewalld by default, you’ll need to either stop it with systemctl stop firewalld or go through the process to allow EIGRP-related traffic through the firewall.

We should be able to see that each router has the other’s connected route now installed in its table. On Ubuntu we can see is installed from vtysh with show ip route (edited somewhat for brevity):

u20vm# show ip route

E [90/28160] is directly connected, ens3, weight 1, 00:41:19
C>* is directly connected, ens3, 00:41:57
E>* [90/30720] via, ens3, weight 1, 00:40:55
E [90/28160] is directly connected, ens4, weight 1, 00:40:43
C>* is directly connected, ens4, 00:41:57

And likewise on Rocky Linux we can see is installed:

rl85vm# show ip route

E [90/28160] is directly connected, ens3, weight 1, 00:43:07
C>* is directly connected, ens3, 00:43:45
E [90/28160] is directly connected, ens4, weight 1, 00:42:50
C>* is directly connected, ens4, 00:43:45
E>* [90/30720] via, ens3, weight 1, 00:42:38

A wireshark (if you’re running GNS3) will show the EIGRP messages flowing. If you catch it right at the start, you can see updates messages and not just hellos:

Wireshark capture of EIGRP traffic between Ubuntu and Rocky Linux


This should be easy, we’ll just ping between the Alpine Linux nodes. (make sure each has a default route pointing to .1)

/ # ping
PING ( 56 data bytes
64 bytes from seq=0 ttl=63 time=2.662 ms

It works!

Hope you liked it.

How To Test Network Bandwidth With iperf3 in Linux

Testing network bandwidth in multiple flavors in Linux is simple with a tool called iperf. There’s two main versions – iperf2 and iperf3. Project maintainers apparently completely rewrote iperf3 from scratch to make the the tool simpler and to support some new features.

Update 12/12/2021: One of the authors of iperf2 reached out to me. Iperf2 is currently very much actively developed. You can find the most recent code on its page. Iperf3 was indeed rewritten from scratch as the wikipedia page says, but mostly to meet the U.S. Department of Energy’s use cases. Iperf3’s github page clearly states the the DoE owns the project.

For testing bandwidth properly, you need to be running in server mode on one endpoint and client mode on the other. For this experiement, we will run the server on Rocky Linux 8.5 and the client on Ubuntu 20.04.


iperf3 test in GNS3

This is about as simple of a topology as I can think of. Two nodes on either end of a single link, Ubuntu at running iperf3 client and Rocky at running iperf3 server.

Iperf3 installation

On Ubuntu, iperf3 can be installed from distribution sources with apt-get:

apt-get install iperf3

Same on Rocky Linux but with yum:

yum install iperf3

Run iperf3 bandwidth test

First we need to start the server process on Rocky Linux with one command:

iperf3 -s

Then you should see the server listening for incoming tests:

iperf3 server listening on Rocky Linux 8.5

Then from the Ubuntu client, one command will run the test:

iperf3 -c

The output will give us our bandwith test results, which can be see on either the client or server:

Connecting to host, port 5201
[  5] local port 59628 connected to port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec   176 MBytes  1.48 Gbits/sec  685    230 KBytes       
[  5]   1.00-2.00   sec   173 MBytes  1.45 Gbits/sec  738    113 KBytes       
[  5]   2.00-3.00   sec   170 MBytes  1.42 Gbits/sec  1004    191 KBytes       
[  5]   3.00-4.00   sec   175 MBytes  1.47 Gbits/sec  714    123 KBytes       
[  5]   4.00-5.00   sec   182 MBytes  1.52 Gbits/sec  458    163 KBytes       
[  5]   5.00-6.00   sec   204 MBytes  1.71 Gbits/sec  443    314 KBytes       
[  5]   6.00-7.00   sec   180 MBytes  1.51 Gbits/sec  910    130 KBytes       
[  5]   7.00-8.00   sec   191 MBytes  1.60 Gbits/sec  849    123 KBytes       
[  5]   8.00-9.00   sec   172 MBytes  1.44 Gbits/sec  564    170 KBytes       
[  5]   9.00-10.00  sec   184 MBytes  1.54 Gbits/sec  412    225 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.76 GBytes  1.52 Gbits/sec  6777             sender
[  5]   0.00-10.04  sec  1.76 GBytes  1.51 Gbits/sec                  receiver

iperf Done.

A wireshark capture in GNS3 between the two hosts (or tcpdump on the links if you’re not in GNS3) will show the packets flying while the test is running:

Wireshark capture from GNS3 of iperf3 test

Hope you liked it!

How To Install (and lab) Keepalived on Ubuntu 20.04 and Rocky Linux 8.5

Keepalived is an open source software project that can do many things related to high availability. One of these many things is the Virtual Router Redundancy Protocol, which provides for high availability for IP networking. In other words, you can have two routers and if one goes down, the second one kicks in automatically.

The way this works is two or more routers exchange VRRP messages on a subnet and based on their configuration, decide who is the master and who is the backup. Once this is decided they will agree on a pre-configured “virtual” IP, or an IP that is not configured on an interface, but a floating one that either router can assume responsibility for should the other one fail for some reason and VRRP messages stop flowing.


VRRP lab in GNS3

The relevant network here is on the bottom half, where a subnet of is configured. The Ubuntu server has on its ens3 interface, while Rocky has on its ens3 interface. They will both have keepalived installed and through VRRP share virtual IP of The Alpine linux “PC” at the bottom which is acting as a workstation or desktop computer will have its default route configured to point to, the VRRP address.

Ubuntu 20.04 configuration

On the Ubuntu server we’ll install the keepalived available from the package manager with this:

apt-get install keepalived

Once that’s installed, we’ll write the configuration file which is in /etc/keepalived/keepalived.conf. You’ll need to create the keepalived.conf file:

vrrp_instance VI_1 {
	state MASTER
	interface ens3
	virtual_router_id 51
	priority 100
	advert_int 1
	authentication {
		auth_type PASS
		auth_pass james
	virtual_ipaddress {

I’ll go through the parameters here:

  • state MASTER: the state that the router will start in.
  • interface ens3: VRRP protocol messages should flow is ens3.
  • virtual_router_id: An integer that both routers should have configured to the same thing.
  • priority: who wins the master/backup election – higher numerical means higher priority.
  • advert_int: backup waits this long (multiplied by 3) after messages from master fail before becoming master
  • authentication: a clear text password authentication.
  • virtual_ipaddress: the agreed-upon virtual IP that the routers will share

Restart keepalived to load the config:

systemctl restart keepalived

Rock Linux configuration

Since Rocky uses yum for package management, we install keepalived like this:

yum install keepalived

And in the /etc/keepalived/keepalived.conf file we’ll write this:

vrrp_instance VI_1 {
	state BACKUP
	interface ens3
	virtual_router_id 51
	priority 99
	advert_int 1
	authentication {
		auth_type PASS
		auth_pass james
	virtual_ipaddress {

Make sure to restart keepalived to load the config:

systemctl restart keepalived

The only parameters that are different are the state and the priority.

Verify keepalived and VRRP

The first thing you can check on Ubuntu is the /var/log/syslog file to make sure keepalived started and is in the correct state:

tail /var/log/syslog
Dec  1 12:23:47 u20vm Keepalived[12349]: Opening file '/etc/keepalived/keepalived.conf'.
Dec  1 12:23:47 u20vm Keepalived[12349]: Starting VRRP child process, pid=12360
Dec  1 12:23:47 u20vm Keepalived_vrrp[12360]: Registering Kernel netlink reflector
Dec  1 12:23:47 u20vm Keepalived_vrrp[12360]: Registering Kernel netlink command channel
Dec  1 12:23:47 u20vm Keepalived_vrrp[12360]: Opening file '/etc/keepalived/keepalived.conf'.
Dec  1 12:23:47 u20vm Keepalived_vrrp[12360]: Registering gratuitous ARP shared channel
Dec  1 12:23:47 u20vm Keepalived_vrrp[12360]: (VI_1) Entering BACKUP STATE (init)
Dec  1 12:23:48 u20vm Keepalived_vrrp[12360]: (VI_1) received lower priority (99) advert from - discarding
Dec  1 12:23:51 u20vm Keepalived_vrrp[12360]: message repeated 3 times: [ (VI_1) received lower priority (99) advert from - discarding]
Dec  1 12:23:51 u20vm Keepalived_vrrp[12360]: (VI_1) Entering MASTER STATE

On Rocky the journalctl -e command showed me the keepalived logs.

Once you’ve confirmed that keepalived is in the right state, you can prove it further with a wireshark capture. If you’re doing this lab in GNS3 like I am, it’s easy, just right click the link (in this case, between the Ubuntu server and the switch) and capture on it. Otherwise you can use something like tcpdump on the Ubuntu/Rocky routers themselves. Right when you restart the keepalived process you will see packets going back and forth. That’s the master election process and exchange of parameters/neighbor establishment:

VRRP election in wireshark

But once the routers are in agreement and VRRP is working, packets will only flow from the master to the designated (in the RFC) VRRP multicast address at

VRRP keepalives in wireshark

We can also further prove that when we initiate a ping to from the Alpine “PC” (it’s a docker container), we can see that traffic is flowing through the Ubuntu router.

Simulate a failure

We’ll simulate a failure by shutting the ens3 interface on the Ubuntu router, like so:

ip link set ens3 down

The Rocky router will observe that VRRP “hello” messages are no longer going to, and quickly assume the role of master and take over for I did a continuous ping on the Alpine PC and it didn’t actually show any failed pings! We can see that traffic is now flowing through the Rocky router:

Now let’s “fail back” to the Ubuntu router by re-enabling the ens3 interface:

ip link set ens3 up
ip addr add dev ens3

And we should be able to see keepalived resuming master responsibilities in the Ubuntu /var/log/syslog:

Dec  1 12:44:53 u20vm Keepalived_vrrp[12379]: Netlink reports ens3 up
Dec  1 12:44:53 u20vm systemd-networkd[602]: ens3: Gained carrier
Dec  1 12:44:55 u20vm systemd-networkd[602]: ens3: Gained IPv6LL
Dec  1 12:45:04 u20vm Keepalived_vrrp[12379]: (VI_1) Entering BACKUP STATE
Dec  1 12:45:04 u20vm Keepalived_vrrp[12379]: (VI_1) received lower priority (99) advert from - discarding
Dec  1 12:45:07 u20vm Keepalived_vrrp[12379]: message repeated 3 times: [ (VI_1) received lower priority (99) advert from - discarding]
Dec  1 12:45:08 u20vm Keepalived_vrrp[12379]: (VI_1) Entering MASTER STATE

While the Rocky log will show a similar message about becoming backup. We can see that pings are once again flowing through the Ubuntu router:

Hope you liked it.

How To Check If A UDP Port Is Listening Using Netcat

Netcat is a super cool tool to perform a great number of different testing and troubleshooting tasks. On Ubuntu Linux, netcat comes built-in so you don’t even have to install it. A common question I get from people trying to troubleshoot a UDP-based application is how to check if a UDP port is “listening” or not. Most folks are familiar with the telnet-based method of checking if a TCP port is listening (e.g., telnet 80), but UDP remains elusive.

Today let’s take a look at why UDP is tricky to check and a method using the netcat tool to see if a server is listening on a particular UDP port.


We’re using a very simple topology here, my PC is at and the server we’re going to check for an open UDP port is at My PC runs an Ubuntu 20.04 Desktop OS, while the server is running the Ubuntu Server version.

TCP and UDP are different protocols

TCP is designed to establish “reliable” connections, meaning packets are either delivered on-time and in order, or the connection fails. To guarantee delivery, it must first establish a “socket”, which is where the TCP handshake comes in. All TCP sockets start like this:

Client –> [SYN] –> Server
Client <– [SYN/ACK] <– Server
Client –> [ACK] –> Server

Once this handshake is done, a socket and connection is established.

A telnet test will succeed if this TCP handshake completes properly, letting you know that the port you’re testing is open.

The problem with UDP is that no such handshake exists, because it was designed specifically to not have connections or reliability. Neither is “better”, they’re just used for different things.

So how do you test for an open UDP connection if there’s not a protocol handshake or guaranteed response? If an application is NOT listening on a particular UDP port, and a UDP packet arrives on that port, the OS will send back an ICMP message of type 3 and code 3, meaning “destination unreachable” and “port unreachable”, respectively. Because UDP lacks a protocol message for this purpose, the ICMP message serves to notify the sender that they’re barking up the wrong tree.

Set up a netcat UDP server

Setting up a server to listen on a UDP port of our choosing is easy using netcat. Just run this command on the Ubuntu server:

netcat -ul 2000 &

The -ul switches specify UDP and “listen” mode, respectively. The ampersand at the end puts the process in the background so we can get our shell back. After running this command, we can do a quick netstat to see that there is indeed a UDP server listening on port 2000:

netstat -an | grep 2000
udp        0      0  *

Use netcat on the client to test the UDP port

From “James’s PC” on the diagram, I’ll run the following command:

nc -vz -u 2000

The -v and -u options specify verbose and UDP mode, respectively. The -z option tells netcat not to send any data with the packet. At the end are the IP address of the server and to UDP port 2000. If the port is open, we should see output like this:

Connection to 2000 port [udp/*] succeeded!

The message is well and good, but well all know packets don’t lie. If we take a wireshark capture of the interaction, we can see why this success message is produced:

Five UDP packets were sent by netcat on James’s PC to make sure no response came back from the server. If there’s no ICMP response, we can infer that the port is indeed open.

The netcat UDP server will shut down after hitting it once with UDP packets, so it’s no longer listening on port 2000. If we run our command again, we’ll get no output from netcat and wireshark clearly shows the ICMP type 3 code 3 message being sent back from the server:

UDP port testing is not like TCP at all, since we can only infer the port’s open state, not guarantee it like TCP.

I hope this one was helpful!

3 Essential Linux Networking Commands

Here at Question Computer we do a fair amount of lab creation, especially with Linux and networking. I’d like to share my essential commands that I use for setting up a basic IP network since in most of my articles I assume that part is already set up.

Stand aside, ifconfig (it’s been deprecated). iproute2 is the official utility to interact with the Netlink Linux kernel interface, which provides a way for the Linux kernel’s networking stack to be configured. That’s a bit of a long-winded way of saying if you want an IP address, use iproute2. iproute2 is split up into a number of different child commands, each for a different part of the network stack, such as ip addresses, interfaces, routing, etc.

Keep in mind – iproute2 manages Linux network configurations on the fly. If you want your configurations to survive a reboot, you’ll either need to write a startup script or you can use the Ubuntu tool for managing network configs – netplan.


We’ll be working with a basic topology that will illustrate the different commands we’ll be using here. By the end of the lab, we should have IP connectivity between all nodes. Ubuntu20.04-2 and Ubuntu20.04-3 will be serving as routers by forwarding and have it enabled in their /etc/sysctl.conf files.

1. ip link

We’ll start with Ubuntu20.04-1 at the bottom left. Before we can even get to assigning IP addresses, we need to know what the interfaces are called, and to turn them on. The ip link command will show us what they are:

ip link

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 0c:96:94:ff:00:00 brd ff:ff:ff:ff:ff:ff

We can ignore the “lo” loopback interface, the one that is connected to the actual network is “ens3”. Let’s enable it:

ip link set ens3 up

Now that it’s enabled, we can assign an IP address to this interface. This command will need to be run on all interfaces on all nodes. I won’t show that, as it’s a bit repetitive. It can be run either before or after an IP address is assigned, but I like to do it before.

2. ip address

To assign an IP address to Ubuntu20.04-1’s ens3 interface, this command will do it:

ip address add dev ens3

Hopefully the arguments there are pretty self-explanatory. “add” means we’re adding an IP address, and “dev” just means we’re assigning the address to the interface that comes after it.

Now that we know how to assign an IP address, the rest of the nodes are easy.


ip address add dev ens3
ip address add dev ens4


ip address add dev ens3
ip address add dev ens4


ip address add dev ens3

3. ip route

The way to manage routes with iproute2 is with the ip route command.

First, on Ubuntu20.04-1, we’ll need to add a default route which is the most common basic configuration for an endpoint node. This command will add it:

ip route add default via

The via parameter allows you to specify where to send traffic that is being sent using the default route. In this case, it’s the Ubuntu router at the top left, Ubuntu20.04-2.

Ubuntu20.04-2 is directly connected to and with interfaces ens3 and ens4, but it does not know about Adding a static route for that subnet pointing to (the other Ubuntu router at the top right) will allow traffic to flow:

ip route add via

A similar but mirror image of that command can be run on Ubuntu20.04-3:

ip route add via

And finally we’ll add another default to Ubuntu20.04-4:

ip route add default via

All connectivity should be in place!


Let’s try to ping from Ubuntu20.04-1 to Ubuntu20.04-4, all the way across the environment.

From Ubuntu20.04-1:


PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=62 time=1.97 ms

It works! Ping is a simple but handy tool. Always remember that when you get a reply, it means not only did traffic make it to its destination, but it came back too.

Wireguard VPN on Ubuntu 20.04

Wireguard is an attempt to improve VPN tunnels in a number of ways – simpler code, less compute, easier configuration, the list goes on. If we’re comparing it to IPsec, I would say that yes, it’s a bit easier to configure. One of the main differences is that it does not rely on the classic two IPsec options for keys – PSK and X.509 certificates. Instead, it relies on public key/private key similar to SSH.

Today, we’re going to configure a very simple policy-based site-to-site Wireguard VPN. By “policy-based” I mean that tunneled traffic is determined by a pre-written configuration in the Wireguard configuration file, not by static or dynamic routes. By site-to-site, I mean this is not a remote-access (road warrior) VPN, it’s designed to connect the subnets that sit behind the two VPN peers.


The two Ubuntu20.04 machines are serving as routers and VPN peers. No routing is in place for PC1 and PC2 to talk to each other. While the two Ubuntu machines can connect to each other, there is no network-level encryption. That’s where Wireguard comes in.

Generate key pairs

If you haven’t installed it yet, wireguard can be installed with apt-get:

apt-get install wireguard

We’ll use the wg command to generate keys on Ubuntu20.04-1. These two commands will generate a private and public key pair:

wg genkey > private1.key
wg pubkey < private1.key > public1.key

We’ll do the same on Ubuntu20.04-2

wg genkey > private2.key
wg pubkey < private2.key > public2.key

Tunnel configurations

Then we’ll write a tunnel config file for Ubuntu20.04-1 in /etc/wireguard/wg0.conf:

PrivateKey = uOnrdfW/ANcU+fh+RUjlb3TQlFWIdwbOpDpAA+NkonY=
Address =
ListenPort = 8888

PublicKey = JjSDqdzjPOX+iIAaWCjxxg1yZ76jAd6jfSfv/1AlojI=
Endpoint =
AllowedIPs =,

You’ll notice I took the keys out of the key files and pasted them into the config file. Under “Interface” we have Ubuntu20.04-1’s own private key, while the public key of Ubuntu20.04-2 is under “Peer”. The “Address” parameter is the “glue network” of the tunnel. This will be the virtual subnet that exists inside the tunnel encryption. The “AllowedIPs” parameter is where tunneled traffic is specified (interesting traffic). You need to put the destination subnet here. Since this is Ubuntu20.04-1 and is the destination on the other side of the tunnel, we put as part of the “AllowedIPs” parameter. Also put the glue network in here as well.

We can write a similar one for Ubuntu20.04-2 in /etc/wireguard/wg0.conf. For Ubuntu20.04-2, is on the other side of the tunnel so will be in “AllowedIPs”. Under “Interface”, we have its own private key, while the public key of Ubuntu20.04-1 is pasted under “Peer”.

PrivateKey = KJgjkPQVhOX5CyYYWr7B6v1AbI7H2kEtBi4wdhAES2g=
Address =
ListenPort = 8888

PublicKey = +GOlIMgLAnLZujraI8m4F6JyWZOpxWGRAPSUqkwrZyg=
Endpoint =
AllowedIPs =,

To bring the tunnels up, on each Ubuntu machine run this command:

wg-quick up wg0

To verify the configuration is loaded, use “wg show”. I ran this one from Ubuntu20.04-2:

wg show
interface: wg0
  public key: JjSDqdzjPOX+iIAaWCjxxg1yZ76jAd6jfSfv/1AlojI=
  private key: (hidden)
  listening port: 8888

peer: +GOlIMgLAnLZujraI8m4F6JyWZOpxWGRAPSUqkwrZyg=
  allowed ips:,
  latest handshake: 28 minutes, 48 seconds ago
  transfer: 1.09 KiB received, 2.63 KiB sent

We should be able to ping the tunnel glue network IPs (here from Ubuntu20.04):

root@u20vm:/home/james# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=5.22 ms

And PC1 and PC2 can ping each other too:

PC1> ping

84 bytes from icmp_seq=1 ttl=62 time=4.942 ms

You’ll notice we did not have to add any routes for and to be able to reach each other. The Wireguard configuration added routing automatically, which is why I am calling this type of tunnel “policy-based”.

And the most fun part, Wireshark. We can see the traffic going back and forth, and the protocol is labeled “Wireguard”. Pretty cool, right?

Wireshark capture of Wireguard traffic

Hope you liked this one.