CyberHunter
Local dataset, no network
// File transfer & sharing

Port 873: rsync

rsync daemon.

873

How rsync works

File transfer & sharing
client:8731/32/33/3rsync client:8731/32/33/3rsync

Files are sent over port 873 in pieces and reassembled on the other side. It travels in clear text: anyone on the path can read it, passwords included.

Transport
TCP · UDP
Exposure risk
6/10 high if reachable from the Internet
Encryption
Clear textreadable on the network
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
File transfer & sharing
01 Security

Is port 873 dangerous?

Common attacks and weaknesses

  • Modules readable or writable without authentication, leaking backups.

How to secure it

  • Use rsync over SSH instead of the daemon, or set auth users and hosts allow.
02 Registry

IANA registry entries for port 873

ServicePortTransportDescriptionReferenceUpdated
rsync 873 TCP, UDP rsync - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 873, and is it reachable?

Linux

sudo ss -ltnup 'sport = :873'      # process listening on 873
sudo lsof -nP -i :873

macOS

sudo lsof -nP -i :873

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 873 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 873
netstat -ano | findstr :873

From another machine

nmap -p 873 -sV <host>
sudo nmap -sU -p 873 <host>
nc -zv <host> 873

Block it in the firewall

sudo ufw deny 873/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 873" -Direction Inbound -LocalPort 873 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 873 used for?

rsync daemon.

Is port 873 TCP or UDP?

Port 873 is registered for TCP and UDP. In practice the service mostly uses TCP.

Is it safe to open port 873?

It is rated 6/10 (high) when exposed to the Internet. Modules readable or writable without authentication, leaking backups. Use rsync over SSH instead of the daemon, or set auth users and hosts allow.

How do I check if port 873 is open?

On Linux run "ss -ltnup 'sport = :873'", on macOS "lsof -nP -i :873", and on Windows "netstat -ano | findstr :873" to see the local process. From another machine, "nmap -p 873 <host>" shows whether it is reachable.

05 Related
:445 SMB tcp · udp · critical risk :21 FTP tcp · udp · sctp · high risk :139 NetBIOS Session tcp · udp · high risk :69 TFTP udp · tcp · high risk :2049 NFS tcp · udp · sctp · high risk :20 File Transfer [Default Data] tcp · udp · sctp :115 Simple File Transfer Protocol tcp · udp :152 Background File Transfer Program tcp · udp

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