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Default Postfix master.cf

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prevent spam and brute force

Based on the search results provided, here are the steps you can take to block an IP address from accessing your Docker-based mail server:

• Use the iptables command to add a rule to the DOCKER-USER chain to block the IP address:

iptables -I DOCKER-USER -s <IP_ADDRESS> -j DROP

Replace <IP_ADDRESS> with the IP address you want to block. This will add a rule to the DOCKER-USER chain to drop all traffic from the specified IP address.

• To verify the rule has been added, run:

iptables -L DOCKER-USER

• If you want to log the dropped traffic from the DOCKER-USER chain, you can add an additional rule:

# log all the dropped traffic from the `DOCKER-USER` chain
iptables -A DOCKER-USER -j LOG

Keep in mind that blocking an IP address may not be a long-term solution, as the offender's IP address could change. It's better to focus on blocking repeat offenders and periodically checking the RBLs (Real-time Blackhole Lists) to see if the IP has been removed, indicating the owner has fixed the issue.

Another approach is to use the fail2ban tool, which can automatically detect and block IP addresses that are attempting to brute-force your mail server. The search results mention a potential issue with fail2ban when running behind a proxy, but there are workarounds available.

In summary, the key steps are to use iptables to block the specific IP address, and consider using a tool like fail2ban to automatically manage the blocking of malicious IP addresses.