The state of TACACS+

Colton Conor colton.conor at
Mon Dec 29 15:15:25 UTC 2014

We are able to implement TACAS+. It is my understanding this a fairly old
protocol, so are you saying there are numerous bugs that still need to be

A question I have is TACAS+ is usually hosted on a server, and networking
devices are configured to reach out to the server for authentication. My
question is what happens if the device can't reach the server if the
devices network connection is offline? Our goal with TACAS+ is to not have
any default/saved passwords. Every employee will have their own username
and password. That way if an employee gets hired/fired, we can enable or
disable their account. We are trying to avoid having any organization wide
or network wide default username or password. Is this possible? Do the
devices keep of log of the last successful username/password combinations
that worked incase the device goes offline?

On Sun, Dec 28, 2014 at 5:02 PM, Robert Drake <rdrake at> wrote:

> Picking back up where this left off last year, because I apparently only
> work on TACACS during the holidays :)
> On 12/30/2013 7:28 PM, Jimmy Hess wrote:
>> Even 5 seconds extra for each command may hinder operators, to the extent
>> it would be intolerable;     shell commands should run almost
>> instantaneously....  this is not a GUI, with an hourglass.   Real-time
>> responsiveness in a shell is crucial --- which remote auth should not
>> change.   Sometimes operators paste a  buffer with a fair number of
>> commands,  not expecting a second delay between each command ---  a
>> repeated delay, may also break a pasted sequence.
>> It is very possible for two of three auth servers to be unreachable,  in
>> case of a network break, but that isn't necessary.      The "response
>> timeout"  might be 5 seconds,  but in reality, there are cases where you
>> would wait  longer,  and that is tragic,   since there are some obvious
>> alternative approaches that would have had results  that would be more
>> 'friendly'  to the interactive user.
>> (Like remembering which server is working for a while,   or remembering
>> that all servers are down -- for a while,  and having a  50ms  timeout,
>>   with all servers queried in parallel,  instead of a 5 seconds timeout)
> I think this needs to be part of the specification.
> I'm sure the reason they didn't do parallel queries was because of both
> network and CPU load back when the protocol was drafted.  But it might be
> good to have local caching of authentication so that can happen even when
> servers are down or slow.  Authorization could be updated to send the
> permissions to the router for local handling. Then if the server dies while
> a session is open only accounting would be affected.
> That does increase the vendors/implementors work but it might be doable in
> phases and with partial support with the clients and servers negotiating
> what is possible.  The biggest drawback to making things like this better
> is you don't gain much except during outages and if you increase complexity
> too much you make it wide open for bugs.
> Maybe there is a simpler solution that keeps you happy about redundancy
> but doesn't increase complexity that much (possibly anycast tacacs, but the
> session basis of the protocol has always made that not feasible).  It's
> possible that one of the L4 protocols Saku Ytti mentioned, QUIC or MinimaLT
> would address these problems too.  It's possible that if we did the
> transport with BEEP it would also provide this, but I'm reading the docs
> and I don't think it goes that far in terms of connection assurance.
>> --
>> -JH
> So, here is my TACACS RFC christmas list:
> 1.  underlying crypto
> 2.  ssh host key authentication - having the router ask tacacs for an
> authorized_keys list for rdrake.  I'm willing to let this go because many
> vendors are finding ways to do key distribution, but I'd still like to have
> a standard ( for how to do this
> over LDAP in UNIX)
> 3.  authentication and authorization caching and/or something else

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