Port 8121 (apollo-data)

IANA assigns port 8121 to apollo-data: Apollo Data Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference

apollo-data TCP Apollo Data Port Anthony Carrabino -

apollo-data UDP Apollo Data Port Anthony Carrabino -

Registry dates for this port: registered 2003-08.

Linux: is anything listening on TCP 8121?

sudo ss -tlnp 'sport = :8121'

Linux: UDP 8121

sudo ss -ulnp 'sport = :8121'

Linux / macOS: which process owns the port

sudo lsof -nP -i :8121

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':8121 '

Windows (Command Prompt)

netstat -ano | findstr :8121

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 8121 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 8121

From Windows

Test-NetConnection HOST -Port 8121

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 8121

ufw

sudo ufw deny 8121/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 8121 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 8121 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 8121' -Direction Inbound -Protocol TCP -LocalPort 8121 -Action Block

ufw

sudo ufw deny 8121/udp

iptables

sudo iptables -A INPUT -p udp --dport 8121 -j DROP

nftables

sudo nft add rule inet filter input udp dport 8121 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 8121' -Direction Inbound -Protocol UDP -LocalPort 8121 -Action Block

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