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System Services
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14.8.2.1. RADIUS client settings
The system settings for a RADIUS client allow multiple different client settings to be created by name. L2TP
uses RADIUS by default, and if not set then the first settings found are used. However, you can set a named
RADIUS client setting to be used for each L2TP server setting. This then looks for the named client setting
for accounting and/or authentication.
The corresponding RADIUS servers are queried for the authentication or account messages. Each client setting
can list multiple servers. Normally the first matching setting is used, and all of the listed servers considered.
However, if all of the servers listed are currently blacklisted then the next matching named entry (i.e. with same
name) is considered, and its listed servers considered. You can see the status of each RADIUS server in the
Status/RADIUS menu. This includes the average response time, and the last 64 responses (good/bad).
The set of servers being considered are put in order based on their previous responses. The least recently
failed to respond are listed first and then the fastest responding servers listed first. Only the last 64 responses
being considered. The first 5 servers are then considered for answering the RADIUS query. If fewer than 5 are
available, then the list is repeated. This give 5 requests in a row to try, even if that is one server 5 times.
Each server is then given a timeout. The timeout is normally based on the scale-timeout multiplied by the
average response time of that server. If this is more than one fifth of the max-timeout then that is used instead.
The final (5th) server is given a timeout to extent to at least the min-timeout as total since the first request is
sent. This creates a sequence of requests to be sent to one or more RADIUS servers.
If, within the overall timeout, any of the servers respond then this is accepted. If none respond then all record
a timeout.
To allow servers to recognise duplicate requests, each request in the sequence that is to the same server has the
same content and ID. This allows the server to simply resend the previous reply if it was dropped.
In addition to these timeouts, it is also possible to set a maximum queue for the set of servers. This limits how
many concurrent requests can be waiting.
Tip
If your RADIUS servers are struggling, then set the queue lower, e.g. 8. If the response times have a lot
of jitter then consider setting the scale-timeout higher (the default is only 2, so try 3, 4, etc). For VoIP,
you will want a very fast server to respond to authentication used for call routing. For accounting you
may want to allow a longer scale and max timeout to ensure accounting requests are not lost.
14.8.2.2. Server blacklisting
For each request to a server, a log is made of whether there was a response or a timeout, and this is recored and
shown on the server status page. This logs the last 64 requests.
If all of the last 64 requests have failed then the server is blacklisted. This stops it being considered when there
are other servers to consider. If all are blacklisted then the blacklisted servers are used anyway.
However, it is quite possible for a server to go away when there are no current RADIUS requests, or even come
back when not being used for current requests. To allow for this the FireBrick sends status-server requests to
the server periodically, and records the responses in the 64 bit response queue. This means a blacklisted server
will be recorded as usable again once it starts answering such requests. It also means a server can become
blacklisted is a server stops responding to such requests without actually losing any real RADIUS requests.
If a server has never answered a status-server request, it is assumed not to be enabled. We strongly recommend
enabling this feature on your RADIUS servers. If not enabled then servers are provided with a dummy good
response periodically to take them out of blacklisted status and allow then to be tried occasionally in case they
are now working again.
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