| Port | 853 |
|---|---|
| Transport | TCP |
| Service | DNS over TLS |
| Category | DNS |
| Range | well-known (0–1023, needs root to bind on Unix) |
| Encrypted | Yes, always (TLS or equivalent) |
| Exposure | Designed to face the internet |
Port 853 DNS over TLS · TCP
DNS over TLS: ordinary DNS inside a TLS connection, so the resolver on the path cannot read or alter queries.
What runs on port 853
DNS over TLS: ordinary DNS inside a TLS connection, so the resolver on the path cannot read or alter queries. Android's Private DNS, systemd-resolved and most routers can use it as clients; public resolvers (1.1.1.1, 9.9.9.9, 8.8.8.8) and Unbound, Knot Resolver and AdGuard Home serve it. DNS over QUIC also uses 853 on UDP.
Software that uses it: Unbound · Knot Resolver · AdGuard Home · stubby · systemd-resolved (client) · Cloudflare/Quad9/Google resolvers
Should port 853 be open?
Designed to face the internet. This port exists to be reachable from anywhere, so the usual question is not whether to open it but how the service behind it is configured and patched.
- kdig +tls @1.1.1.1 example.com or getdns_query are good test clients.
- Blocking 853 outbound is how some corporate networks force DNS through their own resolver; DoH on 443 then becomes the workaround, which is why they block that too.
Check port 853
# on the host: is anything listening, and which process? ss -ltnp 'sport = :853' sudo lsof -i :853 # from another machine: is it reachable? nc -zv host 853 nmap -p 853 host curl -v telnet://host:853 # raw TCP connect test, Ctrl-C to leave
A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.
Allow or block port 853
# ufw (Ubuntu/Debian) sudo ufw allow from 10.0.0.0/8 to any port 853 proto tcp sudo ufw deny 853/tcp # nftables nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 853 accept nft add rule inet filter input tcp dport 853 drop # iptables (legacy) iptables -A INPUT -p tcp --dport 853 -s 10.0.0.0/8 -j ACCEPT iptables -A INPUT -p tcp --dport 853 -j DROP # firewalld sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=853 protocol=tcp accept'
Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 853" with no source is just "open".
Related ports
Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.
Questions
Is port 853 TCP or UDP?
TCP. DNS over TLS is a connection-oriented protocol; there is no UDP service on 853 by default (a scanner reporting UDP 853 open is usually seeing a filtered port).
Is port 853 encrypted?
Yes, always (TLS or equivalent). The port number does not make it safe by itself: certificate validation, authentication and patching still decide.
How do I check whether port 853 is open?
On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 853 and which process. From another machine, nc -zv host 853 or nmap -p 853 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).
Can DNS over TLS run on a different port?
Yes; the number is a convention, not a requirement. Changing it reduces log noise from scanners but does not add security, and every client, firewall rule and monitoring check then needs the new number. Keep the default unless you have a concrete reason.