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  • Writer's pictureBrandon Hitzel

5 Easy Router Protection Techniques - includes Attack and Packet Analysis

Updated: Sep 26, 2019

Executing attacks against a router using Kali Linux along with Wireshark and the easy steps to protect against them.


I'm sure along your networking or security studies you've read about or came across articles about device hardening techniques or best practices. Doing things like disabling unused services and applying certain configurations in order to better protect your management/control/data planes etc. But how do the attacks look against the router or protocol you are protecting?

My main goal was to execute a few attacks and look at them in Wireshark to analyze, along with checking out what tools are easily available to accomplish this, as opposed to just showing some configuration and saying this is what you need to do.

One of the main objectives of a network and cyber security is availability, so you'll notice a theme of Denial of Service (DoS) attempts throughout the post. If a DoS condition is achieved against a router or switch then connection availability is affected. Thus, one of the main points of Network Defense shall be to harden your devices from such DoS attacks!

So for those that don't know you can run operating systems in Oracle Virtual Box like Kali Linux and then integrate that into your GNS3 topology which is what I did here. All the programs shown below were installed by default, except for Loki which is an older software that focuses on routing protocols. Loki was a challenge to install due to its dependencies and since most guides are from 2013 but if I can figure it out I'm sure others can as well. I actually went to one of my old virtual box VDI VHDs from 2017 which had Kali w/ Loki on it to refresh my memory. That is one reason its good to save items from your labs because if you ever have to go back it's serves as a good refresher.

Router Attack Lab Topology in GNS3
Router Attack Lab Topology in GNS3

Here is the GNS3 lab topology. I have 3 routers, 2x Cisco IOS and 1x Cisco IOS-XE and a virtual box with the newest Kali Linux 2019 distro. What I will probably do in the future is replace one of the virtual IOS routers with a Juniper router to perform some more testing with a different vendor. Likely I will have 1 or 2 more posts related to this. In this post I mainly focused on the local router (aka default gateway or first hop) for my target. The magnifying glass is where the packet captures were taken.

During my research and in the process I found most of the out-of-the-box tools and exploits specific to what I was looking to do lack luster and aged (i.e. pre-2010) and didn't experience stellar results, but that isn't to say there are limited options. Nevertheless, in this article I'll go through a few simple attacks and the easy steps you should take to protect your router or other networking devices against them.


1. Stop the enabled protocols you are using from broadcasting information

2. Secure the protocols you are using

3. Evaluate what services you are using and disable unused services

4. Secure everything with access control lists

5. Keep your software/firmware/OS up to date

Bonus: Be aware of what information your device is sharing



Your first step is to disable Cisco Discover Protocol (CDP) and Link Layer Discovery Protocol (LLDP) on unnecessary interfaces such as those of your guest networks/VLANs or on user facing subnets etc. These protocols are commonly enabled by default and send traffic out to find information about directly connected neighbors. Although very helpful, these two protocols can easy cause DoS conditions for your network devices.

This was the easiest and most successful attack of the few I looked at. You basically have a choice of to just recon the packets and obtain information, or gain the information and then perform a flooding disruption. Since these protocols are Layer 2, by just connecting to a LAN and running Wireshark you are able to see these packets without even obtaining and IP address (so the infiltrator can be STEALTH).

CDP Packet, Notice Make, Model, IP address, Software versio
CDP Packet, Notice Make, Model, IP address, Software version

The flood was easy to execute using yersinia. After about a minute or 2 the router was hosed at 100% CPU and then started throwing random logging messages (see below). Remember this is a virtual router under no load, so I'm sure one of your average production devices under some load would have faster results.

The defensive measure is to go under the interfaces and disable CDP or LLDP so its stops broadcasting and so the device stops processing the requests. Although I did see the overall CPU slightly elevated about 5-10% after disabling CDP both globally or on the interface while still running the attack, the CPU did not increment up to 100% which is good. However I did see the 5 second CPU spike a few times. I speculate this is due to the amount of broadcasts being seen on the interface since the flood attack has some volume.

CDP Layer 2 Flood Attack with Yersinia, also the random CDP messages being sent
CDP Layer 2 Flood Attack with Yersinia, also the random CDP messages being sent

Ping tests from one of the other routers showed responses from the victim at around 150+ milliseconds, and the router took a minute to recover even after I stopped sending the CDP messages and disabled CDP globally.

Notice the log entries in the left terminal screen after running the script for a few minutes.

CDP Layer 2 Flood Attack with Yersinia 2
CDP Layer 2 Flood Attack with Yersinia 2


Internal Routing Protocols

The next step is to stop sending IGP (EIGRP, RIP, OSPF) multicast messages on your guest or user facing LANs. The easiest way is to passive-interface default which is supported by most vendors in my research and then no-passive the interfaces you need to have a neighbor relationship on. This will prevent the messages from being sent out that link and also prevent neighbors from forming. Moreover, as you've probably read before use cryptographic authentication like MD5/SHA to minimize the chance of forming neighbors with rogue routers.

Beware, do not perform this mitigation on active production L3 devices unless you have a "commit confirm" type of device, can execute via a script, or have local console access etc. This is because your protocol adjacency will bounce when implemented which could kick you out :) - depending how you're are connecting.

First we look at the OSPF multicast Hello packet on the local LAN to begin our reconnaissance. You'll see common attributes within the packet shown.

OSPF Hello Packet, Notice the Area ID, DR priority, and Hello interval
OSPF Hello Packet, Notice the Area ID, DR priority, and Hello interval

I mentioned using the program Loki earlier and that it was old. I recall when I first used it a few years ago I was able to perform a malicious route injection pretty easily, but for some reason this time I could not get it to work. It could be because my GNS3/Virtual box has changed and is on a newer version or because my IOS is different (I think I had an older IOS last time). So your mileage may vary with this, plus I noticed the new 0.3.0 version did not have OSPF anymore like the older 0.2.4 version. Nevertheless, it's probably easier to just bridge your NIC adapter into GNS3 and have a virtual router perform this attack vs the showcased programs. Plus, you'd have more features at your disposal using that method.

Forming an OSPF Adjaceny to gather info and perform a routing attack
Forming an OSPF Adjaceny to gather info and perform a routing attack

We establish a neighbor-ship with the router first to learn the routing table. The concept would be to inject certain subnets to have them route to your location, or perform the holy grail of IGP Denial of Service attacks and inject a preferred default route into the routing table.

Moving on, a best practice with EIGRP for remote location routers (think hub and spoke) is to limit the query domain using the "stub" feature. This feature can also help prevent malicious route injection because even with a connection to a rogue router, the stub router will not forward the routes learned from it due to the behavior of the stub feature (you can configure it to forward, but by default only connected and summary routes are sent). Therefore, the upstream WAN or core will not learn of the bad path.

Similarly with OSPF a common practice is to use "stub areas" for remote locations or non-core routers which could help mitigate this malicious route injection. This is due to Type-5 LSAs not being generated inside stub areas, so the DoS would likely only effect the stub area (if at all). You'd probably have to generate a Type-3 LSA default route in order to propagate within the stub area, which would still not be preferred inside the backbone (Area 0).

OSPF Database Exchange
OSPF Database Exchange

Finally, you can see the OSPF database exchange after becoming neighbors. Notice the routes being exchanged and shown in the packets using wireshark. These routes were displayed in the routing table of our victim router in the second screen shot of this section. If one would have chosen to inject a bad route - as soon as this process was completed - if done correctly the bad route would have propagated immediately.

A raider could use this information to help minimize excessive discovery scanning on the target network, or use it to hone in on certain IPv4 subnets to selectively scan. However, if cryptographic authentication or the passive-interface command was implemented for this LAN, then the injection wouldn't have been possible.


Loki and yersinia both support HSRP hijacking which would cause black holing of LAN traffic or enable a man-in-the-middle. HSRP is a first-hop redundancy protocol commonly used in scenarios where there are two routers acting as default gateways. The protocol supports MD5 authentication which would help mitigate this vector. Yersinia seemed to have more options but I performed the testing with Loki as MITM attacks were beyond the scope of this lab. is Kali and 100.3 is the router. Note the virtual IP listed inside the packet of 100.1. I just wanted to mention this in order to further highlight the theme of knowing what can be performed when the protocols your router is utilizing are unsecured.

Yersinia's HSRP Hijack options
Yersinia's HSRP Hijack options


Access Control Lists and Services

Use Access Control Lists (ACLs) to protect router management access and access to services on your network devices. Use them for accessing SSH, FTP, SCP, HTTPS etc. Use the method of least privilege and only allow those certain management subnets that need access. Restricting access greatly reduces the attack surface and is very easy to implement from the basic home routers to the large service provider boxes (please tell me you have ACLs protecting management access!).

Below are 2 screenshots of a Nmap scan of a router with a before and after implementing an ACL to protect some of the services. Basically a scan was performed with the main services enabled and the second is with ACLs implemented on the VTY lines, NTP, SNMP, and HTTP services. I did perform many different scans to identify the open ports and enabled services which took minutes or less each to perform, so this illustrates how quick the attack can develop from recon to exploitation once an unprotected appliance is found.

Nmap Scan before implementing ACLs and disabling services
Nmap Scan before implementing ACLs and disabling services

Nmap results showing open ports on the Cisco router
Nmap results showing open ports on the Cisco router

Nmap scan showing 0 results after implementing ACLs and disabling services
Nmap scan showing 0 results after implementing ACLs and disabling services

Also consider reviewing your router configurations to ensure there are no unnecessary services enabled and that the services that are enabled are protected. Do you use the web GUI at all? or is there a need to have the HTTP server enabled? if not then consult your vendor's configuration guide to disable it, simple.

I noticed a few of the recent CVEs being released for Cisco IOS are HTTP based attacks, therefore having that service disabled mitigates the risk for those particular vulnerabilities - same thing applies for other exploits against their corresponding vulnerabilities. By having exposed services (i.e. open ports) it allows the attacker to pick and choose and attempt different methods of offense.

Consider which protocols are being used for your routers and switches as well - perhaps you need to change them. For instance, SCP and SFTP are the preferred secure protocols for downloading configurations while most people I see still use TFTP and FTP to perform these actions (even over the internet). TFTP and FTP are cleartext and are less secure than their SSH-based counterparts.

Likewise ensure for your RSA keys you are generating have strong key lengths like 2048. Its been well known keys under 1024 can easily be cracked for a while now. Enable SSH version 2 and set a max number of retries in addition to logging failed logins and setting a minimum accepted key size. This is an example of securing a service you use on your network device.

Here is a Nmap that was ran against a router with weak ciphers and SSH/HTTPS on defaults. I selected a bunch of scripts to check for vulnerable cryptography and such. Note: the router didn't have an ACL applied.

Nmap showing SSL vulnerability on a Cisco router
Nmap showing SSL vulnerability on a Cisco router

Some other services that are typically disabled are source routing, proxy arp, and IP redirects. Source routing would allow a end host to choose its routed path, proxy arp lets hosts communicate to other IP ranges without a default gateway, or if their subnet mask is configured in correctly. Moreover, redirects not only send ICMP packets but also notify the end host of a better path - which you should have already selected based on your topology.

A side note as well is to secure your Auxiliary/Modem or AUX ports (Cisco = "line aux 0") Because even with the VTY lines protected you need to apply a similar configuration like an ACL or "transport input none" in order to protect or disable the ports 6001 and 9001 etc. which I have seen still open on different models of Cisco devices. By having that service open it could provide telnet or other access leaving you vulnerable to potential offensive action. A good practice is to scan your router and see if there are any non-standard ports open.



One point I want to bring to your attention is the information Internet control message protocol (ICMP) can give away. Essentially depending on the situation the router will send ICMP replies letting the sending device know certain information. On some vendor's routers you have the opportunity to disable ip unreachables (or type 3 codes) on certain interfaces of your choosing so the device will not send such replies. Though, from my research you need to be tactical on where you disable these ICMP type 3 replies.

For example Path MTU discovery depends on an ICMP type 3 code 4 message to let the end host know fragmentation is needed. Furthermore, type 3 messages are used for traceroute - destination port unreachable (code 3). So by placing no ip unreachables on the end destination of the traceroute you will break it, however if you place it on the local LAN interface to protect the first hop device or on intermediate hops traceroute should still work in most cases. Same thing goes for disabling unreachables on an interface that has a lower MTU other than the default 1500 which would need to send the code 4 messages. Therefore, you need to think about which interfaces it would be best to disable this feature on.

As far as I know it is possible for an excessive amount of ICMP unreachables to cause a DoS condition due to excessive CPU usage. So consider rate-limiting them either with control plane policing, specific commands (e.g. "ip icmp rate-limit unreachable <#>"), or by disabling it all together on an interface. (check this, old but educational article on ICMP crafted messages)

Proceeding on, if you are using an ACL to restrict access or for traffic in/out of an interface, when a block entry is hit the device will respond with a type 3 code 13 " Communication administratively prohibited". This might be unwanted behavior as an attacker could utilize this to their advantage to find open ports or certain types of allowed flows. Here are the results of the first ping test which is dropped via an access-list.

ICMP code 13 packet after pinging traffic restricted by an Access Control List
ICMP code 13 packet after pinging traffic restricted by an Access Control List

As mentioned you can disable the ICMP type 3 replies on an interface, which might be a good consideration for a edge router that typically has an inbound access list screening some of the internet traffic as it reaches the network.

Below you can observe that after disabling unreachables there are no ping responses in the second ping test, so scanners would likely not even detect there is a device active if their traffic was dropped; however in the previous scenario the scanner would detect there was a device there even if it's traffic was dropped by the access list because of the ICMP reply.

Ping and Traceroute tests as viewed in wireshark before and after IP unreachables are disabled
Ping and Traceroute tests as viewed in wireshark before and after IP unreachables are disabled

Additionally, notice the difference between the first and second traceroutes where the second traceroute did not have a packet from the first hop router due to the ICMP type 3 replies being disabled. The final hop still had them enabled though.

Here is how the tests looked from the end host perspective. From the top down:

  1. Ping without ACL - successful

  2. Ping with ACL - Code 13 reply from first hop router

  3. Traceroute normal

  4. Traceroute with "no IP unreachables" on first hop interface

  5. Ping with ACL but with no ICMP replies - unsuccessful


Out of the Box Review

Kali linux has a variety of tools that are shipped with it that can literally exploit and devastate a variety of targets. However in my testing I found the tools had more exploits for Soho routers than for Cisco routers. The programs tested were cisco-torch, cisco global exploit (CGE), and metasploit framework. Moreover, a lot of the vulnerabilities seemed aged based on the listed date (if there was one). This is why its important to keep your software up to date, because as these issues with the code arise the vendors (hopefully) patch and protect against them. You wouldn't want to be taken down from an old vulnerability that was patched years ago because you neglected to update.

Back to the aged scripts, for example the cisco global exploit showed one option to be for catOS which is old and probably wouldn't be seen by most people and it had a few old HTTP CVEs from the early 2000s. I found one funny in that it sent a HTTP GET with a tons of AAAAAAAAAs which I assume did something once upon a time. The router responded with a 401 bad request and seemed unchanged after a few attempts (this could also be because it is a virtual device but I doubt it). I'm not really familiar with that CVE either.

HTTP exploit ran against a Cisco Router
HTTP exploit ran against a Cisco Router

Most the assaults I tried with cisco-torch and CGE were ineffective, except for the telnet buffer overflow when left unchecked (but like we discussed - disable unused services, Telnet should not be used because it is not secure). I even tried some NTP vulnerabilities which I expected my IOS to effected by like ntp_overflow and ntpd_reserved_dos but no dice.

This is how the Syslog #9 vulnerability looked from GCE.

Syslog DoS attempt from CGE
Syslog DoS attempt from CGE

You can see on metasploit there are a lot of options to choose from, but like with the previous examples the attacks I tried didn't do much against a more up to date (although still older IOS) protected device; although I only tested on virtual instances of Cisco routers. I'm sure some of those Soho scripts for D-links or the new Cisco RV small business devices are potent though.




I hope this post helps you in reviewing your routers or other Layer 3 network devices in order to better protect management access, along with your infrastructure's availability from Denial of Service attempts. Additionally, in the future I'm sure there will be more effective additions to the Kali distribution against enterprise routers vs. small business routers.

In this post we went through some packets and got down to the detailed level for a few of the offensive actions in order to better understand why we implement some of these security controls. Disabling unused services that broadcast out like CDP/LLDP and protecting your router using ACLs are simple yet effective means of network defense. If a protocol supports authentication then its wise to use it where possible. Also take into account the implications of leaving ICMP type 3 messages enabled on external facing interfaces.

The next post in this area I will probably do a write up about Control Plane Policing to protect the control/data planes, since that topic isn't covered a lot from what I've seen.

Finally, this should be a given but keep your devices patched and up to date!

Good luck out there.

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