How Firewall authenticate user?

Dear Mr/Ms
I read in Buiding a VPN of Dave Koisur, ISBN 0471265264, I know that Firewall have strong authentication, but I don't know how it authenticate user? Is it store user Database somewhere? What protocol does it use to authenticate user? PPP or something?
Thanks in advance
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Firewalls authenticate vpn connections using pptp, L2tp or more commonly these days IPSec.  You then setup usernames according to which authentication you deploy.
Pete LongTechnical ConsultantCommented:
Agree though Authentication can be carried out by RADIUS where usernames etc are stored in a directory (or database) elsewhere
zero_kidAuthor Commented:
PPTP and L2TP use PPP to authenticate user. Username and Password are encypted using CHAP, MS-CHAP, MS-CHAP v2, or send in cleartext with PAP. It happens in PPP transaction.
PAQed with points (50) refunded

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