HTTPfy

Port 403 (decap)

IANA assigns port 403 to decap.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
decap TCP decap - -
decap UDP decap - -

Check what is listening on port 403

Linux: is anything listening on TCP 403?
sudo ss -tlnp 'sport = :403'
Linux: UDP 403
sudo ss -ulnp 'sport = :403'
Linux / macOS: which process owns the port
sudo lsof -nP -i :403
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':403 '
Windows (Command Prompt)
netstat -ano | findstr :403
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 403 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 403
From Windows
Test-NetConnection HOST -Port 403
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 403

Block port 403 in the firewall

ufw
sudo ufw deny 403/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 403 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 403 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 403' -Direction Inbound -Protocol TCP -LocalPort 403 -Action Block
ufw
sudo ufw deny 403/udp
iptables
sudo iptables -A INPUT -p udp --dport 403 -j DROP
nftables
sudo nft add rule inet filter input udp dport 403 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 403' -Direction Inbound -Protocol UDP -LocalPort 403 -Action Block

Related ports

401ups402genie404nced405ncld

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