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Port 3304 (opsession-srvr)

IANA assigns port 3304 to opsession-srvr: OP Session Server.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
opsession-srvr TCP OP Session Server Amir Blich -
opsession-srvr UDP OP Session Server Amir Blich -

Check what is listening on port 3304

Linux: is anything listening on TCP 3304?
sudo ss -tlnp 'sport = :3304'
Linux: UDP 3304
sudo ss -ulnp 'sport = :3304'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3304
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3304 '
Windows (Command Prompt)
netstat -ano | findstr :3304
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3304 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3304
From Windows
Test-NetConnection HOST -Port 3304
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3304

Block port 3304 in the firewall

ufw
sudo ufw deny 3304/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3304 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3304 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3304' -Direction Inbound -Protocol TCP -LocalPort 3304 -Action Block
ufw
sudo ufw deny 3304/udp
iptables
sudo iptables -A INPUT -p udp --dport 3304 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3304 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3304' -Direction Inbound -Protocol UDP -LocalPort 3304 -Action Block

Related ports

3302mcs-fastmail3303opsession-clnt3305odette-ftp3306MySQL / MariaDB

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.