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Port 4728 (capmux)

IANA assigns port 4728 to capmux: CA Port Multiplexer.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
capmux TCP CA Port Multiplexer Nigel Groves -
capmux UDP CA Port Multiplexer Nigel Groves -

Registry dates for this port: registered 2006-06.

Check what is listening on port 4728

Linux: is anything listening on TCP 4728?
sudo ss -tlnp 'sport = :4728'
Linux: UDP 4728
sudo ss -ulnp 'sport = :4728'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4728
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':4728 '
Windows (Command Prompt)
netstat -ano | findstr :4728
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 4728 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 4728
From Windows
Test-NetConnection HOST -Port 4728
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4728

Block port 4728 in the firewall

ufw
sudo ufw deny 4728/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 4728 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 4728 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 4728' -Direction Inbound -Protocol TCP -LocalPort 4728 -Action Block
ufw
sudo ufw deny 4728/udp
iptables
sudo iptables -A INPUT -p udp --dport 4728 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4728 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4728' -Direction Inbound -Protocol UDP -LocalPort 4728 -Action Block

Related ports

4726a26-fap-fgw4727fcis, fcis-disc4729gsmtap4730gearman

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