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// Network services

Port 853: DNS query-response protocol run over TLS

Port 853 is assigned by IANA to domain-s (DNS query-response protocol run over TLS).

853

How domain-s works

Network services
client:853#%&@!x9response? client:853#%&@!x9response?

The client sends a request to port 853 and the domain-s service answers. Everything is encrypted: someone on the path only sees unreadable bytes.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
EncryptedTLS or built-in crypto
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Network services
01 Security

Is port 853 dangerous?

We have no specific security note for port 853. Its risk depends on the program actually listening on it. General advice for network services services:

  • Answer only the networks that need the service (no open resolvers, relays or reflectors).
  • UDP services can be abused for DDoS amplification: rate-limit or block them at the edge.
  • Disable legacy test services that have no use in production.
02 Registry

IANA registry entries for port 853

ServicePortTransportDescriptionReferenceUpdated
domain-s 853 TCP, UDP DNS query-response protocol run over TLS RFC 7858 2022-04-11

// source: IANA service name and port number registry

03 Check it

Which program uses port 853, and is it reachable?

Linux

sudo ss -ltnup 'sport = :853'      # process listening on 853
sudo lsof -nP -i :853

macOS

sudo lsof -nP -i :853

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 853 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 853
netstat -ano | findstr :853

From another machine

nmap -p 853 -sV <host>
sudo nmap -sU -p 853 <host>
nc -zv <host> 853

Block it in the firewall

sudo ufw deny 853/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 853" -Direction Inbound -LocalPort 853 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 853 used for?

Port 853 is assigned by IANA to domain-s (DNS query-response protocol run over TLS).

Is port 853 TCP or UDP?

Port 853 is registered for TCP and UDP.

Is it safe to open port 853?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Answer only the networks that need the service (no open resolvers, relays or reflectors).

How do I check if port 853 is open?

On Linux run "ss -ltnup 'sport = :853'", on macOS "lsof -nP -i :853", and on Windows "netstat -ano | findstr :853" to see the local process. From another machine, "nmap -p 853 <host>" shows whether it is reachable.

05 Related
:53 DNS tcp · udp · moderate risk :135 MS RPC tcp · udp · high risk :123 NTP udp · tcp · moderate risk :67 DHCP server udp · tcp · moderate risk :111 rpcbind tcp · udp · high risk :9100 Raw printing (JetDirect) tcp · udp · high risk :631 IPP / CUPS tcp · udp · high risk :5353 mDNS udp · tcp · moderate risk

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