LPIC-2 DNS, Web and Mail Services Practice Question
An organization's Postfix mail server is being used as an open relay. The administrator wants to restrict relaying to only authenticated users from the internal network. Which combination of settings is correct?
⚠ Common exam trap
The trap here is that candidates often place reject_unauth_destination first to block open relays, forgetting that it must come after permit_mynetworks and permit_sasl_authenticated to avoid blocking legitimate internal or authenticated relays.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
It enforces relaying restrictions in the correct order: first permit mail from trusted internal networks (permit_mynetworks), then permit authenticated users (permit_sasl_authenticated), and finally reject all other unauthorized destinations (reject_unauth_destination). This ensures that only authenticated users or hosts in mynetworks can relay, while blocking open relay attempts from external unauthenticated sources.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
smtpd_helo_restrictions = permit_sasl_authenticated, reject_unauth_destination
Why it's wrong here
HELO restrictions are not appropriate for controlling relaying.
- ✗
smtpd_recipient_restrictions = reject_unauth_destination, permit_mynetworks, permit_sasl_authenticated
Why it's wrong here
Rejects unauthorized destinations first, which would block legitimate relay attempts.
- ✗
smtpd_client_restrictions = permit_mynetworks, reject_unauth_destination
Why it's wrong here
Client restrictions insufficient; recipient restrictions are needed for relay control.
- ✓
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
Why this is correct
Allows internal networks and authenticated users before rejecting unauthorized destinations.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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