I ran an “nmap -sUV -F” on my droplet, from the outside, and it returned the following:
Port state service version
53/udp open domain dnsmasq 2.90
Im running Debian 12. My firewall is solid. I first thought may be my outgoing rules to connect to the digitalocean dns servers was causing it. I removed the rules that allow for dns lookups and even tested it with an apt update and it failed like it should. So I did a “netstat -tulnp | grep 53” and got the following results:
udp 0 0 127.0.0.54:53 0.0.0.0:* 401/systemd-resolve
udp 0 0 127.0.0.53:53 0.0.0.0:* 401/systemd-resolve
udp 0 0 0.0.0.0:5355 0.0.0.0:* 401/systemd-resolve
My research indicates that the culprit is 127.0.0.53:53 through systemd-resolve process. But how is the outside world getting a response from a loopback address since the all addresses setting 0 0.0.0.0:5355 is port 5355 an not port 53 ? I don’t want to start messing with dnsmasq or editing the /etc/systemd/resolved.conf. Does any one know the answer?
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Add my own comment to my question: I should have check to see if I had dnsmasq 2.90 installed on my droplet. I do not. Even if I did debian 12 dnsmasq is 2.89. So I don’t think it is my droplet. But the question still stands then what is causing the nmap -sU result?
Hello, @paulsquid
You can run ss -tuln | grep :53 to see if any services are explicitly bound to all interfaces or the public interface.
Also confirm if dnsmasq is running and on which interfaces:
ps aux | grep dnsmasq
You might also want to check the configuration of dnsmasq to see which interfaces it’s bound to.
Given that systemd-resolved is configured to bind to the loopback address, it should not be exposed externally. The appearance of dnsmasq in the Nmap scan suggests that dnsmasq might be running and bound to an external interface.
Regards
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