Port 6121 (spdy)

IANA assigns port 6121 to spdy: SPDY for a faster web.

Registered (user) port, 1024-49151 TCP

IANA assignment

Service nameProtoDescriptionAssigneeReference

spdy TCP SPDY for a faster web Matthew Lloyd -

Registry dates for this port: registered 2010-04-26.

Linux: is anything listening on TCP 6121?

sudo ss -tlnp 'sport = :6121'

Linux / macOS: which process owns the port

sudo lsof -nP -i :6121

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':6121 '

Windows (Command Prompt)

netstat -ano | findstr :6121

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 6121 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 6121

From Windows

Test-NetConnection HOST -Port 6121

ufw

sudo ufw deny 6121/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 6121 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 6121 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 6121' -Direction Inbound -Protocol TCP -LocalPort 6121 -Action Block

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