Let's now have a look at authentication servers. There are many possibilities and you may have a user database, probably using active directory or Free IPA LDAP, SQL and so on. Or you might have a radius server, for example for 802.1x wireless authentication. It could well be captive portal and lots of other possibilities. Users cannot access the network without them so having replicated instances, preferably running live-live is quite essential and we need to implement active monitoring for these, for example Nagios plugins.

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