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Port 1243 (serialgateway)

IANA assigns port 1243 to serialgateway: SerialGateway.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
serialgateway TCP SerialGateway Stephen LaValley -
serialgateway UDP SerialGateway Stephen LaValley -

Check what is listening on port 1243

Linux: is anything listening on TCP 1243?
sudo ss -tlnp 'sport = :1243'
Linux: UDP 1243
sudo ss -ulnp 'sport = :1243'
Linux / macOS: which process owns the port
sudo lsof -nP -i :1243
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':1243 '
Windows (Command Prompt)
netstat -ano | findstr :1243
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 1243 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 1243
From Windows
Test-NetConnection HOST -Port 1243
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 1243

Block port 1243 in the firewall

ufw
sudo ufw deny 1243/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 1243 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 1243 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 1243' -Direction Inbound -Protocol TCP -LocalPort 1243 -Action Block
ufw
sudo ufw deny 1243/udp
iptables
sudo iptables -A INPUT -p udp --dport 1243 -j DROP
nftables
sudo nft add rule inet filter input udp dport 1243 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 1243' -Direction Inbound -Protocol UDP -LocalPort 1243 -Action Block

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