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Port 991 (nas)

IANA assigns port 991 to nas: Netnews Administration System.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
nas TCP Netnews Administration System Vera Heinau, Heiko Schlichting RFC 4707
nas UDP Netnews Administration System Vera Heinau, Heiko Schlichting RFC 4707

Registry dates for this port: last modified 2017-02-10.

Check what is listening on port 991

Linux: is anything listening on TCP 991?
sudo ss -tlnp 'sport = :991'
Linux: UDP 991
sudo ss -ulnp 'sport = :991'
Linux / macOS: which process owns the port
sudo lsof -nP -i :991
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':991 '
Windows (Command Prompt)
netstat -ano | findstr :991
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 991 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 991
From Windows
Test-NetConnection HOST -Port 991
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 991

Block port 991 in the firewall

ufw
sudo ufw deny 991/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 991 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 991 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 991' -Direction Inbound -Protocol TCP -LocalPort 991 -Action Block
ufw
sudo ufw deny 991/udp
iptables
sudo iptables -A INPUT -p udp --dport 991 -j DROP
nftables
sudo nft add rule inet filter input udp dport 991 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 991' -Direction Inbound -Protocol UDP -LocalPort 991 -Action Block

Related ports

989FTPS (implicit, data)990FTPS (implicit, control)992Telnet over TLS993IMAPS

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