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Port 3723 (sychrond)

IANA assigns port 3723 to sychrond: Sychron Service Daemon.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
sychrond TCP Sychron Service Daemon Robert Marinelli -
sychrond UDP Sychron Service Daemon Robert Marinelli -

Registry dates for this port: registered 2003-03.

Check what is listening on port 3723

Linux: is anything listening on TCP 3723?
sudo ss -tlnp 'sport = :3723'
Linux: UDP 3723
sudo ss -ulnp 'sport = :3723'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3723
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3723 '
Windows (Command Prompt)
netstat -ano | findstr :3723
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3723 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3723
From Windows
Test-NetConnection HOST -Port 3723
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3723

Block port 3723 in the firewall

ufw
sudo ufw deny 3723/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3723 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3723 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3723' -Direction Inbound -Protocol TCP -LocalPort 3723 -Action Block
ufw
sudo ufw deny 3723/udp
iptables
sudo iptables -A INPUT -p udp --dport 3723 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3723 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3723' -Direction Inbound -Protocol UDP -LocalPort 3723 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.