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Using COA, Change of Authorization for Access and BNG platforms



In access deployments using RADIUS, during the access-accept we can pass reply items back to the NAS which allows us to configure per user configuration to alter the base template config or to apply extra features. These features normally can't change since RADIUS on itself doesn't allow for reauthorization. For that purpose COA (Change of Authorization) was developed allowing an active session to have its configuration changed based on effectively a new set of reply items that are downloaded to the NAS.

** Download a COA client for windows, MAC or linux below **

Latest version is v3.1 January 2017

The words NAS (network access server), BRAS (Broadband Remote Access Server) and BNG (Broadband next generation) are used interchangeably, they all refer to the same concept of aggregating subscribers.

Typically NAS is used in modem access scenarios, BRAS for PPPoA and PPPoE termination whereas BNG involves the concept of subscriber policies along with IP session termination (including PPPoX).

Core Issue

RADIUS servers are available in open source format on the web, for instance Livingston Radius server or Free Radius server are very popular. Also vendors have provided their own RADIUS servers such as Cisco Secure ACS. However there is not a wide variaty of COA tools out there unless they come with a "portal" type implementation in which COA is generally leveraged a lot. In this article I am presenting a COA tool that can be used from a normal linux station allowing you to pass a COA request to a NAS of your choice. The usage of the tool is explained as well as key parameters that you need to be providing in order to make a successful COA request.

Feature changes support with COA

What features can be changed via COA is highly dependant on the platform and software release that is being run. The COA tool will encapsulate your attributes and send them to the NAS, but it is the NAS's responsibility to apply the features and provide a proper status back on the implementation of it.

Features support in COA tool

  • Up to 10 attributes to be included in the COA request
  • Change of Authorization and Packet of Disconnect support
  • Random source ports or manually configurable
  • Encoding of the cisco-avpair subscriber:password="password" for account logon in VSA 249
  • Extended debug capability
  • Configurable via CLI or Configuration file
  • Request timeout support
  • Multi thread support
  • Encoding of strings, ip addresses and integers
  • Currently support on Linux, and W32. Solaris (solaris no longer supported and maintained!)
  • IPv6 Encoding
  • Various binary ISG codes supported (0A, 0B, 04 etc)

NAS configuration

The minimum configuration required for IOS looks like this


aaa server radius dynamic-author
server-key cisco
auth-type any

client determines from which source ip addresses we can accept a COA request. Sources not in the list will get ignored.

server-key is the encryption key to use for the MD5 authenticator computation and must match what the COA client will be using

auth-type defines which attributes are to be used for session identification.

     For instance, if you provide the Accounting-Session-Id and Username the auth-type any means that the first session found that matches EITHER      one of these check items will be subject to modification.

     Auth-type ALL means that all check items much match

With 4.2.0 IOS-Xr for the ASR9000 will have BNG with COA support also. Here is the configuration required in IOS-XR:


aaa server radius dynamic-author
port 1700
server-key cisco
auth-type any

client vrf default server-key cisco

A global server key is possible as well as a per client type key is also configurable. The listen port is configurable (same in IOS config omitted, as port 1700 is default in IOS).

COA Check items

To target a specific session you can use various attributes such as Framed-IP-Address, User-Name or Accounting-Session-Id.

It is recommended to always specify the accounting-session-id (attribute 44), the reason for that is that this att references a single session on any BNG as this number must be unique. The internal code lookups are much faster with this attribute then using user-name or framed-ip-address as these result in a lineair walk. Also user-name and FIP (sessions with same ip addr in different vrf's) may not be unique on the device

To provide extra safety to make sure you are targetting the right session, you can configure the auth-type match-all and send Acct-Session-Id (44) as well as a username (1) to have a fast lookup AND the safety that this username is indeed the one that we had in mind altering.

How to find the Accounting-Session-Id

You can lookup the accounting session id in the radius accountign records, but also in IOS or XR you can find the ID rather easily.

Note that the Accounting-Session-Id is generally a string that is perceived to be an integer.

In IOS the radius-record may prefix the acct-session-id STRING with a nas-port identifier like this:

Accounting Record

Thu May 26 10:22:59 2011
        Acct-Session-Id = "1/0/0/100.1_000000BA"
        Cisco-avpair = "ip:sub-qos-policy-out=briana"
        Framed-Protocol = PPP

IOS will strip and only use the 8 right most digits as the accounting session ID. In COA requests you could omit all 0's and just use "BA" for the id, however at the time of writing ios-xr does a string match and wants to see the 8 digits all together.

in IOS

Step 1: Find the subscriber of interest

NPE-G1#show subscr ses
Current Subscriber Information: Total sessions 1

Uniq ID Interface  State         Service      Identifier           Up-time
44      IP         authen        Local Term   0017.0e43.a1ac       00:00:29
45      Traffic-Cl unauthen      Ltm Internal                      00:00:29
46      Traffic-Cl unauthen      Ltm Internal                      00:00:29


Step 2: Take the subscribers internal ID and locate its record ID in the AAA databasre

NPE-G1#show subscr ses uid 44 det | i AAA_id
AAA_id 0000001B: Flow_handle 0

Step 3: Look into the AAA database for the found record to see what the accounting session id is.

For ISG sessions look at the Parent-Session-Id, for regular subscribers, look at the "session-id"

NPE-G1#sh aaa user 0x1B | i session-id
65684778 0 00000001 session-id(353) 4 48(30)
656848B0 0 00000001 session-id(353) 4 49(31)
656848F0 0 00000009 parent-session-id(352) 8 00000034


Step 1: Find the subscriber of interest:

RP/0/RSP1/CPU0:A9K-BOTTOM#show subscr sess all
Thu May 26 10:37:17.115 EDT
Codes: IN - Initialize, CN - Connecting, CD - Connected, AC - Activated,
       ID - Idle, DN - Disconnecting, EN - End

Type         Interface                State     Subscriber-IP

                                                LNS Address

PPPoE:PTA    BE1001.100.pppoe4        AC <<<

PPPoE:PTA    BE1001.200.pppoe5        AC

IP:DHCP         BE1001.2.ip3                  AC

PPPoE:LAC    BE1001.300.pppoe6        AC

Step 2: Detail the subscriber interface

RP/0/RSP1/CPU0:A9K-BOTTOM#show subscriber session filter interface bundle-e1001.100.pppoe4 detail
Thu May 26 10:38:42.647 EDT
Interface:                Bundle-Ether1001.100.pppoe4
Circuit ID:               Unknown
Remote ID:                "XTH_TEST"
Type:                     PPPoE:PTA
IP Address:     , VRF: default
Mac Address:              000b.5f2c.ef01
Account-Session Id:       00000067
Nas-Port:                 Unknown
Username:                 test
Subscriber Label:         0x00000067
Created:                  Tue May 24 12:00:57 2011
State:                    Activated
Access-interface:         Bundle-Ether1001.100

<output omitted>

COA Tool Manual

The  COA tool requires you to have a little bit of attribute knowledge in  RADIUS, that is, the attributes are identified by their enummerated  numbers rather then their name. Although you can look at a dictionary  file (attached) to map them should you need that.

The options can be specified all via a CLI, or can be provided in flat config file for ease of use and easy scripting.

The tool supports POD (packet of disconnect) as well as COA requests.


-n <ip addr> The IP address of the NAS that you want to send this COA request to
-N <ipv6> The IPv6 address of the NAS to be targeted (v3.0 new feature) either provide -n or -N
-p <int> The destination port on the NAS that is listening to COA requests (normally this is 1700)
-k <string> The secret-key that is used for the MD5 HASH computation, this must match the definition on the BNG/NAS router.
-d No sub argument needed, designates the tool to send a POD (packet of  disconnect) request rather then a COA request. If the session is found  it will get terminated.
-t <integer> By default the tool waits indefinitely for a response from the NAS.  The timeout option allows you to wait a number of seconds before the  tool exists
-s <int> Normally a random source port is selected by the tool that is used  to originate the request and listen for a response. If you wish to  specify the source port manually you can use this option. If there is a  single COA request on station X already using source port Q and the tool  is waiting for a response, then a second request cannot use source port  Q if fired from the same station X. An error will be thrown (socket /  bind error).
-f <string> Configuration file that holds the paramters described in a config file

The tool has the option for 6 attributes to be specified. The format is attribute_number,value

The Value is always perceived to be a string value, that means if  there are spaces involved, you need to embrace the string with quotes,  eg 18,"this is a test string"

If you like a certain value to be sent as an integer, for instance  for the Session-Timeout (27), then prefix the value with the word INT

example: 27,INT100 to send an integer value of 100

In case you need to send an ip address such as for Framed-IP-Address then prefix the ip with IP

example: 8,IP255.255.255.254

You can use the sample dictionary file attached to lookup the Attribute name to number to type (int, ip, string)

If you have an IPv6 Address for encoding, you can use the prefix V6 followed by the ipv6 address.

example: 98,"V6fe80::260:1111:feff:ffff"

Framed-IPv6-Prefix is automatically encoded (attribute 97).

-e Decode the response from the NAS into an attribute (integer) and value (string).
-r [0-255] Provide a static requestID, if omitted or out of bounds a random value is generated.
-x Extended debug output, follow what the tool is doing

Note: The bold options must always be provided otherwise the tool can't continue.

Using the Config file

The Tool has the ability to read values from a config file for ease of use. Sample config files will be provided below.

The following is the format of a config file:




Config file Parameters

Keyword CLI


the nas-ip address, the destination ip.
ipv6-address -N the nas-ipv6 address, destination IP of the BNG
secret -k secret key for md5 hash computation
destport -p destination port to send the request to
attribute0 (to 9) -0 to -9 the attributes to be encapsulated
sourceport -s define the source port for the request (optional)
timeout -t To set the timeout waiting for response (optional)
END n/a To denote the END of the config file reading stops after seeing this keyword

Note that parameters provided by CLI are NOT overwritten by the  config file, so the config file has precedence, eg if secret is provided  by cli using the -k CLIKEY and in the config file with secret=CFGKEY then the key used to hash is CFGKEY.

Formatting VSA's

This section described how to format a VSA

The vendor-specific attribute nubmer is 26

Cisco's vendor ID is 9

Cisco has a few VSA's defined such as:

Cisco-avpair, which is vendor attribute 1

Cisco-nas-port, which is vendor attribute 2

A few SSG attributes:

ATTRIBUTE       SSG-Account-Info              250     string  Cisco
ATTRIBUTE       SSG-Service-Info                251     string  Cisco
ATTRIBUTE       SSG-Command-Code         252     string  Cisco
ATTRIBUTE       SSG-Control-Info                253     string  Cisco

Microsoft is vendor 311 and has 2 key attributes commonly used:

ATTRIBUTE       MS-1st-NBNS-Server              30      ipaddr  Microsoft
ATTRIBUTE       MS-2nd-NBNS-Server              31      ipaddr  Microsoft

To provide a vsa into the tool you use the following format:

-1 26,9,1,"ip:ip-unnumbered=Loopback 123"

to send cisco-avpair with the ip unnumbered info


Account-Logon (config file)


Parameterized QOS (config file)

Adding a parent shaper and a child class with a priority queue policed

attribute3=26,9,1,ip:qos-policy-out=add-class(sub, (class-default), shape(800))
attribute4=26,9,1,ip:qos-policy-out=add-class(sub,(class-default, 3play-voip), pri-level(1), police(256,8))

Account-Logoff (cli)

# ./coa_new -n -p 1700 -k cisco -1 44,34 -2 26,9,1,"subscriber:command=account-logoff" -3 1,"0017.0e43.a1ac"

Release Notes

* VERSION 1.0 - first offical RELEASE

* version 1.1 - added random source port and transaction ID generation

* version 1.2 - added POD capability via the -d option

* version 1.3 - added capability for ssg account info converting

*               serivce logoff 0C to binary 0x0C

* version 1.4 - added capability for ssg account info converting

*               service logoff 0B to binary 0x0B

* version 1.5 - fixed bug in length field of attribute size

* version 1.6 - added session query 0x04

* version 1.7 - added timeout receive option -t

* version 1.8 - added manual source port configuration

* version 1.9 - detect integer strings and send them as int rather then string

*               a string prefix of INT tells the program to treat value as int.

* version 1.10- detect ip prefixes and convert accordingly with IP1.2.3.4

* version 2.0 - ability to read config from file with -f

* version 2.1 - fixed subscriber:password length calculation in v2.0

* version 2.2 - improved hexdump, added code comments, cleaned up code

* version 2.3 - added ability to decode the COA/POD response attributes via -e

*               user configurable requestID

* version 2.4 - config file parse chokes on empty line, fixed that issue

* version 2.5 - Adds support for VSA36 with SALT encryption

* version 2.6 - Fixed bug in salt length character

* version 2.7 - Added expanded source port range (+retry), increased attributes

* version 2.8 - Added IPV6 encoding capabilities via V6 prefix keyword

* version 2.9 - Fixed prefix length corruption crash attr 97

* version 3.0 - Added IPv6 transport for sending COA requests to the BNG

* version 3.1 - Fixed integer encoding to proper int formatting (4 bytes)

Related Information

Disclaimer: this is not an official Cisco supported tool but merely provided to verify, demonstrate and integrate COA requests with.

Xander Thuijs, CCIE #6775

Principal Engineer ASR9000



Hi, Xander:

Quick followup on this issue. What happens to an IP Session on ASR BNG when the DHCP lease timers expires. Say DHCP lease given to CPE is 24 hours. When those 24 hours go by, does the IPsession get killed on the BNG so that the CPE when trying get another DHCP has to reauthenticate? (Also, I think CPE will always ask for a renew well before lease time expires, so that is another problem)

Is there a way to make sure (say on DHCP server) that it won't grant the same IP address and force BNG to kill the session, forcing reauthentication?

Thanks in advance.


Cisco Employee

Hi Carlos,

an ip session would renew every time on the half lease time, once the dhcp server acks, the lease time is renewed then also. During renew, the session would ask (dhcp request) the same address, and the server and ack or nack that.

if ACK, it retains the addr, if it naks, the subscriber will fall back to discover upon which you can assign a new addr, but that is really a server implementation.

If the server doesn't ack or nack and the client attempts to renew and doesn't get an answer, it will release the addr (or should) and the binding on the BNG will disappear which will instruct the subscriber session to get destroyed also.

when that happens, the subscr session removed, the subscr will no longer be able to foward traffic, or well better put, the subscr traffic will hit the access interface, so depending on the config of the access if we can basically stop fowarding for this user.



Hi, Xander:

When you say:

"if ACK, it retains the addr, if it naks, the subscriber will fall back to discover upon which you can assign a new addr, but that is really a server implementation."

Say the server NAKs, will the session be destroyed by BNG at that very moment, so that when you fallback to discover the session has to be recreated and reauthenticated/authorized?



Cisco Employee

Hey carlos, yes if the server naks the subscr will fall back to discover and likely obtain a new address, for that reason the subscr session and binding need to be released also.

I'll reconfirm with a quick test and let you know if this is inaccurate, but for now assume this model.



Thank you, Xander. I'd appreciate it if you could confirm. I won't be able to get my hands on an ASR to test it myself for the next couple of weeks.

Cisco Employee

hey carlos, confirmed, the way I wrote it above is the way it is supposed to be operating.



First of all, Xander, thank you very much for taking the time. I know I speak for a lot of people here.

I was wondering if you could share whatever you did to your dhcp server to nak the subscriber. Is that a line on a linux dhcp server perhaps?

Cisco Employee

hi carlos,

you're very welcome, and thanks for that note, thats nice to hear!

I took an IOS dhcp server and tried to clear the binding on teh dhcp server in IOS, that didnt work reliably, so made a special image to NAK requests when they would come in for renewal.

Then i figured a way that if you exclude an address from the pool it should NAK it also.

ip dhcp-server exclude address <addr>

For linux dhcp, I dont know of a trick unfortunately.

MS dhcp server has the option though.



Do you know what the ms dhcp option is called so that i can look it up?



Cisco Employee

If you take the address out of the scope or put it in reserved state, the dhcp server will send a NAK then.



OK, now I get it. So this is a trick you're using to 'insert' a NAK in the renewal process, but NOT a 'feature' on the dhcp server where you are configuring the server so that the client/server setup goes all the way to lease time expiration by NAKing every renewal request.

Cisco Employee

yeah yeah exactly! There may be some dhcp servers out there probably where you can mark a lease for nak or something, but that is so server specific. I would assume that any decent SP dhcp server can do that, as it is a very useful feature to prevent semi static adds (which are usually more expensive).

In order to test the nak func, this is the trick to use (reserve addr from lease)

cheers! xander


And after long conversation: we were told that as of 4.3.1 the 'disconnect' command does exist now (even though I cannot find it anywhere in the command reference). Can you confirm its existence and if it does the same as in the ASR1000, kill the session that is?

RP/0/RSP0/CPU0:BNG_Demo(config-pmap-c)#10 ?

  activate      Activate

  authenticate  Authentication related configuration

  authorize     Authorize

  deactivate    Deactivate

  disconnect    Disconnect session

  set-timer     Set a timer to execute a rule on its expiry

  stop-timer    Disable timer before it expires



Cisco Employee

hi carlos, yes this will destroy the subscriber session.

it is new in XR4.3.1

policy-map type control subscriber testme

event account-logoff match-first

  class type control subscriber DHCP do-until-failure

   10 disconnect





Thank you very much, Xander.

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