Hide VPN users' passwords in virtual router (use PAP instead of CHAP)

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

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Remote Access VPN Services (L2TP-over-IPsec)
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##### CLOUDSTACK VERSION

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4.19, 4.20
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##### CONFIGURATION

- Remote Access VPN Service enabled on guest networks or VPCs
- Create VPN Users

##### SUMMARY

The users' credentials for the VPN remote access service are in plaintext in /etc/ppp/chap-secrets file. Even though the VRs are secured and only the platform administrators have access to them, it is not a good practice and and may even have security consequences to leave the passwords in plain text.
It would be a good improvement if it were possible to hashing this data (or something like that).

##### STEPS TO REPRODUCE

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- Enable VPN remote access service in guest network or VPC
- Create a VPN User
- Verify that the /etc/ppp/chap-secrets file on the virtual router contains the VPN user credentials in plaintext.
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##### EXPECTED RESULTS

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User credentials should not be in plaintext.
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##### ACTUAL RESULTS

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User credentials are in plaintext.
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Inizia dal percorso di provisioning di Remote Access VPN Service per i router virtuali e verifica come viene generato /etc/ppp/chap-secrets. Determina l’approccio di autenticazione supportato e le relative implicazioni di sicurezza prima di modificare il comportamento. L’attività è completata quando le credenziali VPN non vengono memorizzate in chiaro e la relativa copertura VPN verifica il risultato.

Scritto dal modello di indicizzazione a partire dal testo della issue.