Port 406 (imsp)

IANA assigns port 406 to imsp: Interactive Mail Support Protocol.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference

imsp TCP Interactive Mail Support Protocol John Myers -

imsp UDP Interactive Mail Support Protocol John Myers -

Linux: is anything listening on TCP 406?

sudo ss -tlnp 'sport = :406'

Linux: UDP 406

sudo ss -ulnp 'sport = :406'

Linux / macOS: which process owns the port

sudo lsof -nP -i :406

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':406 '

Windows (Command Prompt)

netstat -ano | findstr :406

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 406 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 406

From Windows

Test-NetConnection HOST -Port 406

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

nc -vzu -w 5 HOST 406

ufw

sudo ufw deny 406/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 406 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 406 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 406' -Direction Inbound -Protocol TCP -LocalPort 406 -Action Block

ufw

sudo ufw deny 406/udp

iptables

sudo iptables -A INPUT -p udp --dport 406 -j DROP

nftables

sudo nft add rule inet filter input udp dport 406 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 406' -Direction Inbound -Protocol UDP -LocalPort 406 -Action Block

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