Port 873 rsync · TCP

What runs on port 873

rsync daemon mode: rsync talking its own protocol to an rsyncd that exports named modules (rsync host::module). Fast and great for mirrors, but unencrypted and authenticated only by a simple password. Most people run rsync over SSH (port 22) instead, which is encrypted and needs no daemon.

Software that uses it: rsync (rsyncd) · distribution mirrors · backup appliances

Should port 873 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• rsync host:: with no module name lists the modules, if "list = yes"; that is also what scanners do.
• For anything private, rsync -e ssh.

Check port 873

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :873'
sudo lsof -i :873

# from another machine: is it reachable?
nc -zv host 873
nmap -p 873 host
curl -v telnet://host:873      # 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 873

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 873 proto tcp
sudo ufw deny 873/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 873 accept
nft add rule inet filter input tcp dport 873 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 873 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 873 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=873 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 873" with no source is just "open".

Related ports

• 22 SSH — SSH, SCP, SFTP, git over ssh
• 21 FTP — FTP control; plaintext, replace with SFTP
• 2049 NFS — Network file system

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