Deconstructing the RADIUS CoA process

If you need to brush up on the RADIUS process, please read my previous post:
Following the 802.1X AAA process with Packet Captures

Everyone talks about it, yet I rarely meet folks that really understand what CoA (Change of Authorization) means for RADIUS authentication and client access. I recently spent a few hours troubleshooting RADIUS CoA and figure since it is fresh in my mind maybe I can share and hopefully help others out in the field.


In Summary: RADIUS Change of Authorization (RFC 3576 & RFC 5176) Allows a RADIUS server to send unsolicited messages to the Network Access Server (aka Network Access Device/Authenticator in Cisco terminology e.g. AP/WLC/Switch/Firewall) to change the connected client’s authorized state. This could mean anything from disconnecting the client, to sending different attribute value pairs to the Authenticator to change the device’s VLAN/ACL and more. It is fairly robust in what it can do so I may not go too deep as I want this to be consumable.

What RADIUS CoA is NOT: Magic!

I will be walking through CoA Use Cases, what CoA looks like from a PCAP perspective,  and how to gather data for troubleshooting.

RADIUS CoA Typical Use Cases:

Central captive portal (Open SSID with MAC filtering) – Especially with Cisco ISE, RADIUS CoA is the core feature set required for the captive portal. In the example below, we are redirecting a client to a splash page for either Authentication or Acceptable Use Policy review. As you can see below we have a pretty simple process.

  1. The client connects to the network (wired/wireless)
  2. Client MAC address is sent to RADIUS server as a username and password (Access-Request)
  3. RADIUS server responds with an Access-Accept and a URL redirect. (could also include a VLAN assignment)
  4. The client is redirected to the splash portal
  5. User logs in using the credentials required
  6. RADIUS server then sends a CoA with a request to reauthenticate
  7. Authenticator (AP/Switch/WLC) sends a CoA-ACK
  8. Authenticator sends an Access_Request with existing Session-Id and authentication data.
  9. RADIUS server then responds back with Access-Accept and any extra functions e.g. a Filter-ID for group policy assignment in Meraki Wireless.

Wireless and Wired CoA-Reauthenticate Process

Screen Shot 2018-01-16 at 2.17.07 PM

The above process is also used for secure device registration and URL redirects for blacklisting etc. but would involve a complete client authentication/reauthentication via EAP instead of MAC authentication. For an example check the shared captures labeled 1-of-2 and 2-of-2. These contain the EAPoL side and the RADIUS side.

Client Posturing – In some cases you may want to perform posturing on the end client. This, more often than not, requires a client on the end machine, whether it is a dissolvable agent with java, or a thick client like Cisco AnyConnect. The whole goal of posturing is making sure the clients that have access to your internal resources are properly secured from threats. A common scenario is a user removing or disabling Anti-Virus. When this event occurs it may be desired to limit that client’s access to the network until AV is reinstalled or enabled. This could be done through an ACL or VLAN change.

One of the difficult situations that arises when changing VLANs is the client may not release their IP address. In 802.11 this is easily handled by sending a disconnect-request instead of reauthentication. In wired authentication scenarios this is not typically recommended as it requires a port bounce and can take some tweaking to make work well, if at all. Instead of a VLAN change it is recommended to perform ACL changes to wired clients. On a catalyst switch this could be a dACL (Downloadable ACL) for instance.

Dynamic Network Restrictions – Closely following the use case above, a client’s access may need to be dynamically changed if they are not adhering to the network policy. Using products such as Cisco’s Stealthwatch in tandem with Cisco ISE, we could monitor a client for data dumping thresholds and change the VLAN/ACL applied to them or shut down the port to minimize the impact. This is just one example of the many possibilities.

Wireless Disconnect-Request Flow

Screen Shot 2018-01-17 at 11.28.18 AM

Now on to the Fun stuff….

To capture CoA packets:

The CoA packets are only seen between the authenticator and the authentication server. Therefore we need to capture between the authenticator and the authentication server as depicted below.

Screen Shot 2018-01-16 at 11.15.39 AM

In most environments this consists of using a SPAN/RSPAN port to capture traffic. Some vendors do provide the ability to perform tcpdumps/pcaps which can be a little easier, especially if you are offsite. For capture applications I tend to lean towards using wireshark as it is free and powerful. To download please go to

CoA Messages are sent on two different udp ports depending on the platform. Cisco standardizes on UDP port 1700, while the actual RFC calls out using UDP port 3799. These messages are all included in the “radius” wireshark filter.

Just in case you don’t have a test network please feel free to use the pcaps in this share:

CoA PCAP Examples

RADIUS CoA Packet Types

There are two different RADIUS CoA packets that are sent from the RADIUS Server (Authentication Server):

  • Disconnect-Request – Requests to terminate the session of the client.
  • CoA-Request – Requests to do a number of things from reauthenticate to port-bounce, shutdown, and more.

And there are four that are sent from the NAS/NAD/Authenticator:

  • Disconnect-ACK – Acknowledgment of successful disconnect
  • Disconnect-NAK – Failed session disconnect
  • CoA-ACK – Acknowledgment of successful CoA action
  • CoA-NAK – Failed CoA action

RADIUS Server Sourced Packets

In this section we will review the two CoA messages that are sent from the RADIUS server and the useful material in the packet.

Disconnect-Request Message

Wireshark Filter: radius.code == 40

This packet is sent from the RADIUS server and is used to simply disconnect the client from the current session. This also typically involves an immediate re-authentication by the client. Disconnect-Requests can/should be used in 802.11 situations where a VLAN change needs to occur. If we simply used a CoA-Request (as we’ll see later), the client may be changed to a new VLAN while keeping the IP address it obtained from the former VLAN, clearly causing problems.

Screen Shot 2018-01-12 at 10.29.29 AM

A few useful attributes in this message are:


The account terminate cause will let you know the reason for the request. This can vary but typically is classified as an Admin-Reset from a Cisco ISE Perspective.

Screen Shot 2018-01-12 at 10.31.56 AM

Audit-Session-ID & Calling-Station-ID

These fields can be used to filter information from your RADIUS server regarding the client MAC address (Calling-Station-Id) and session ID. So when you need to hunt down a particular failure in a log, you can correlate the logs via these two attributes.

Screen Shot 2018-01-12 at 10.33.09 AM

NAS Response Link

Wireshark helpfully gives a link to the frame that is the NAS response to the RADIUS server. This Disconnect-Ack packet will be reviewed in the authenticator sourced packets section later in this post.

Screen Shot 2018-01-12 at 11.02.31 AM


CoA Request

Wireshark Filter: radius.code == 43

Unlike the Disconnect-Request above, a CoA-Request can contain a number of actions. This can include anything from reauthentication to bouncing or shutting down a port. A lot of these can be vendor specific responses. So in this instance I am going to use the Cisco ISE CoA Request info. One thing to note is useful attributes are also still the Audit-Session-Id, Calling-Station-ID, and the Response Link as well as the attributes below.

Screen Shot 2018-01-12 at 4.49.13 PM

Useful Info:

Cisco-AVPair: subscriber:command = XXXXXX

This is where we are able to request that the authenticator perform a function. With Cisco ISE it is rolled into a Cisco-AVPair: subscriber:command.

For instance:

  • subscriber:command=reauthenticate

This request will cause a reauthentication either for the client via EAP, or the authenticator may send the MAC address and session ID again in the event that it is a MAC authenticated session.

Screen Shot 2018-01-17 at 2.29.52 PM


  • subscriber:command=bounce-host-port

This is a wired only CoA Request. A request to bounce the host port will end up with a link-down link-up event on the switchport. This can be useful for trying to move a client to a new VLAN if possible. This is not something I recommend defaulting to for guest portals however as it can take some tweaking to the core CoA configurations. In ISE this would involve rewriting the Network Device Profile and CoA ReAuth requests to include a port-bounce, which I do not believe is a recommended practice.

Screen Shot 2018-01-17 at 2.37.52 PM


  • subscriber:command=disable-host-port

This is another wired only CoA request. This will disable the switchport if the switch supports it. I have seen cases where the end switch may not support a port shutdown and will bounce the port instead. This is not a recommended CoA request for most situations as it takes manual intervention to resolve. Instead a VLAN or ACL change is far more effective, even if the VLAN doesn’t exist (blackhole).

Screen Shot 2018-01-17 at 3.41.15 PM

Authenticator Sourced Packets

Now we will review the packets that are sent in response to the CoA or disconnect request from the server. These are fairly simple and usually only include an ACK for pass or NAK for failure.


Wireshark Filter: radius.code == 41

This is an acknowledgment of a successful disconnect-request instruction from the authenticator to the RADIUS server. This packet can contain attributes such as the session that was disconnected, calling-station-id, or just simply the Message-Authenticator.

Screen Shot 2018-01-17 at 4.18.15 PM


Wireshark Filter: radius.code == 42

This is an acknowledgement of a failed disconnect-request. This might happen if the client is already disconnected, or if the session has ended prior to the disconnect request. In the example screenshot we can see a bit of useful information in the error cause attribute.

Screen Shot 2018-01-17 at 4.15.00 PM


Wireshark Filter: radius.code == 44

As with the Disconnect-ACK, the CoA-ACK is just an acknowledgement of the success of the CoA requested action. This packet can contain attributes such as the session that was disconnected, calling-station-id, or just simply the Message-Authenticator.

Screen Shot 2018-01-17 at 4.17.05 PM


Wireshark Filter: radius.code == 45

Once again just like the Disconnect-NAK, the CoA-NAK is an acknowledgement of a failed CoA action. This could be due to lack of support or the session has ended prior to the CoA-Request. Just like the Disconnect-NAK we get a nice Error-Cause for further troubleshooting.

Screen Shot 2018-01-17 at 4.15.38 PM


In closing

One thing to remember is CoA can be used to create some very complex if-this-then-that type scenarios. In the end however it is not a complex feature and definitely not magic! I hope this post was informative for you. If you find anything incorrect please let me know. Thanks and good luck!

Following the 802.1X AAA process with Packet Captures

EDIT: After chatting with David Westcott (@davidwestcott) I have made a few additions to this post. He has graciously asked that I add a little more details including the packet captures so everyone can follow along. This was a great idea, so please enjoy!

802.1X is typically the first step in one of the more advanced security implementations you will have to dip your toes into when moving your network to a secure state. That being said a lot of times we as engineers get stuck in a state of understanding enough to be dangerous and not enough to be highly successful and more importantly, capable. I personally learned a lot about dot1x via trial and error through implementations in the past as well as lab time at home and during my CCIE studies.

So the question is, how do we become masters of a protocol that is literally quite capable of being the success or demise of a network’s security and operation? 

…By taking advantage of the resources that are out there as well as practical labs. One thing to keep in mind is that while every vendor of a dot1x solution (Cisco, Aruba, FreeRADIUS, Microsoft) has a certain way of going about authentication, they all fall back to the same protocol: 802.1X/RADIUS. As long as you can understand that protocol to a fairly deep level you can troubleshoot any RADIUS environment.

A few great resources I have used and/or reviewed are:

Lab Minutes

Cisco ISE Communities Aggregate Post

Meraki Common RADIUS Error Codes with Microsoft NPS

Packet6 FreeRADIUS Install Guide

Katherine McNamara’s Network Node Blog

Now that being said I also recommend learning about not only how to debug the RADIUS servers via logs and troubleshooting tools, but being able to understand what it looks like from a packet capture perspective both wired and wireless and all the information you can glean from a pcap.

Overview of RADIUS/Dot1x Process

EAP & Radius Process

The above diagram is utilizing EAP-PEAP, this may look a little different depending on the Auth method.

Getting Started with Captures:

There are technically 2 points that you can grab packet captures, both of which will provide relevant information for troubleshooting. If you capture at Point 1 in the diagram below, the frame exchanges will be using EAPoL as a protocol. If you capture at Point 2 we will be seeing the traffic as RADIUS messages. That being said lets dive in!

dot1x packet Captures

Example Environment Details:

In this write up I used a Nexus 6P, Cisco Meraki MR33, and a Cisco ISE server. Included below are links to download the .pcap files.

Wireless EAPoL and Wired RADIUS packet capture files

Please download the files in the link above if you would like to follow along and review.

Wireless EAPoL capture clients:

STA (Supplicant) = 00:9a:cd:b7:c9:f0

AP (Authenticator) = e2:55:2d:f2:d1:54

Wired Radius Capture Clients:

AP (Authenticator) =

RADIUS Server (Authentication Server) =

Point 1: Supplicant to Authenticator (EAPoL)

You can capture between the client device (supplicant) and the access device (authenticator). At this point in the network you are looking at EAPoL messages. This is the communication method utilized that provides the Authenticator and the Client a line of communication prior to network access.

This is what the capture will look like:

Screen Shot 2017-07-08 at 11.22.32 AM

To perform this capture from a wireless perspective you will need either an access point capable of dumping/monitoring traffic, or a client that is capable of turning its adapter into monitor mode. Most of the time for wireless captures I use a Macbook Pro and Adrian’s Airtool App. This way is fairly flexible and quick.

If you are capturing for a wired authentication, you will want to perform a port SPAN or mirror to receive the needed frames.

Once you have performed the frame capture, you will want to open it in Wireshark and apply a few filter/s to get only the most useful info.


Screen Shot 2017-07-08 at 11.24.08 AM

In this case we combined 3 filters by the use of the &&. First we want to make sure we only see the eapol messages. We then want to filter on the Access point or switch as well as the client. This can be done in the case of wireless with wlan.addr == {mac address}. Quite often you will see wlan.ta/ra/dst etc. however in this case we want to simply isolate the mac addresses, not which position they are in the frame.

For wired we would still use the filter “eapol” and instead of wlan.addr we would use eth.addr.

The EAPoL portion of communication will vary depending on the authentication type. In my examples, we are using EAP-PEAP w/EAP-MsCHAPv2. This is a fairly standard form of authentication, and from a .pcap would resemble closely to EAP-TTLS w/PAP.

The useful portions that can usually be derived from a pcap are:

EAP-Identity Response:

Screen Shot 2017-07-08 at 11.18.59 AM

In this frame (frame #29 in the .pcap) you can see the Client’s (Supplicant) Identity being used of “employeealex“. This can be extremely useful when trying to determine if the supplicant is going to authenticate as the user or the machine account as well as what the user could be typing into the username prompt.

EAP Auth Method Negotiation and Credential Exchange:

Screen Shot 2017-07-08 at 11.52.33 AM.png

The first message in this clip is the server’s proposal of EAP-TLS (frame #32 in the .pcap), then the client’s response with, “Hey what about EAP-PEAP?” In some situations, depending on the RADIUS server configuration, the client may try to propose a method that is not permitted or supported by the server. This is where you would see that negotiation fail, and ultimately an Access-Reject/EAP-Failure.

However in this capture you can see the client and server negotiate EAP-PEAP. Once that is completed, the Server will present the client with its certificate. If the client does not trust the certificate from the server, and the user does not accept the certificate, the exchange will fail after the first frame or two of the handshake.

In this situation however the client trusts the server certificate and the two endpoints secure the medium with a TLS tunnel. Once secured you should notice that the protocol becomes purely TLS and since the traffic is encrypted, we can only see that the frames are “Application Data”. This is the point at which the client and server are exchanging inner authentication data such as EAP-MsCHAPv2 or EAP-TLS.

EAP Success(wired and wireless) and 4 Way Handshake (when the client is wireless):

Screen Shot 2017-07-08 at 11.20.25 AM

Once the client has been successfully authenticated and authorized, there is an EAP Success message sent back (frame #78) to signify the end of the process. If this is a wired client the process is over and the client is able to start transmitting and receiving data frames. If this is a wireless client, the station will utilize a few EAP attributes and the AP will utilize two MPPE key attributes in the Radius Access-Accept response to perform the 4 way handshake and create the encryption keys for secure communication (Frames 80, 82, 85, and 87).

Point 2: Authenticator to Authentication Server (RADIUS)

In the previous section we were only able to see the authentication process and not the authorization or accounting as that communication is only between the authenticator and authentication server. Luckily you are also able to capture traffic from the Access/Authenticator Device (i.e. switch/WLC/AP) directly to the Authentication Server (RADIUS server). I HIGHLY recommend capturing this traffic at the same time as your client EAPoL capture so you can reference/correlate the same data. At this point in the process you will be looking at RADIUS messages. These messages are typically not encrypted unless using AES-KeyWrap or RADSEC which is not common today unless in a highly secured environment or public environment where traffic is inherently insecure.

This is what the capture will look like:

Screen Shot 2017-07-08 at 11.32.10 AM

To perform this capture, we would want to span the traffic from the Authenticator or from the RADIUS server. The choice is yours. Once we have spanned the port we would perform a capture as usual and open in wireshark.

There are a few very useful filters in this capture as well that we will want to use to weed out the undesirable packets.


Screen Shot 2017-07-08 at 11.33.12 AM

As you can see we are not getting too complicated with our filters. We just want to combine filters once again with the && delimiter. Then we also want to filter on radius as the protocol, and the ip addresses of the Authenticator and Authentication server with ip.addr == {ip address}.

As you can see from the capture example above, the communication between the access device and RADIUS server are comprised of a ton of Access-Requests with Access-Challenges as the response from the RADIUS server. One thing to note is that all of the communication between the client and server during authentication is encapsulated in a RADIUS request. This can be extremely helpful in following how the Authenticator translates the information to the RADIUS server, as well as all the extra information that is appended. Now on that note….

The useful portions of the RADIUS exchanges are:

The initial Access-Request packet:

Screen Shot 2017-07-08 at 11.36.56 AM

In this packet (in the capture #31) we can see a ton of useful information in the form of Attribute Value Pairs. Here we can see for instance the username that is being sent as well as the connectivity information. When troubleshooting RADIUS environments it is a good idea to verify the NAS-Port-TypeService-Type, and Called-Station-ID. If you have watched any of my videos that I have created on how to configure Cisco ISE you will know we use these three attributes quite frequently for granular authorization policies.

One other piece of information that is extremely useful for troubleshooting is the NAS-IP-Address. This information is typically what is used to differentiate RADIUS clients in the RADIUS server. In my video on Network Devices and Groups you can see where in Cisco ISE we configure the Authenticator devices via their IP address. This information does need to match the NAS-IP-Address. If it does not the radius server will most likely not respond as the RADIUS client cannot be found.

The Final Access-Accept Packet:

Screen Shot 2017-07-08 at 11.38.05 AM

This packet (#55) could also be the Access-Reject packet if the authentication failed. In this instance however we passed authentication. As part of the authorization policies I even returned a Filter-ID that specifies a Group Policy in the Meraki Dashboard only giving me BYOD-Access. There can be many AVPs included depending on the platform including Airespace ACLs, AVC Rulesets, QoS parameters, VLAN assignment, User Roles, and much much more.

For those that are not familiar, in a Cisco Meraki wireless environment we can use Group Policies to specify different access through L3-L7 firewall rules as well as perform traffic shaping, policing, and QoS remarking on specific application traffic. 

RADIUS Accounting Messages:

Screen Shot 2017-07-08 at 11.39.20 AM

These messages (#56 & 57) provide the RADIUS server with connectivity information for authenticated and authorized clients. Notice the UDP ports change from 1812 to 1813 when the communication changes to Accounting. These are not mandatory for 802.1X however are useful in providing current connectivity stats to the RADIUS server. These will also show when a client disconnects with an Accounting-Stop message. As you can see in the above screenshot the message also include Event-Timestamps, and Session-IDs on top of the other connectivity information. You can also find the client IP address information under the Framed-IP-Address field.

In closing…

I hope this breakdown of the process was informative. It took me a while to honestly get to the point of being able to grab captures and decipher what they mean. If all of this is above your head just remember practice makes perfect. I recommend if you would like to get deeper into RADIUS to install something like FreeRADIUS and play around. Hands on experience beats any other form of study.

Thank you for reading!