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Port 520 (efs, router)

IANA assigns port 520 to efs, router: extended file name server.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
efs TCP extended file name server - -
router UDP local routing process (on site); uses variant of Xerox NS routing information protocol - RIP - -

Check what is listening on port 520

Linux: is anything listening on TCP 520?
sudo ss -tlnp 'sport = :520'
Linux: UDP 520
sudo ss -ulnp 'sport = :520'
Linux / macOS: which process owns the port
sudo lsof -nP -i :520
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':520 '
Windows (Command Prompt)
netstat -ano | findstr :520
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 520 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 520
From Windows
Test-NetConnection HOST -Port 520
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 520

Block port 520 in the firewall

ufw
sudo ufw deny 520/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 520 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 520 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 520' -Direction Inbound -Protocol TCP -LocalPort 520 -Action Block
ufw
sudo ufw deny 520/udp
iptables
sudo iptables -A INPUT -p udp --dport 520 -j DROP
nftables
sudo nft add rule inet filter input udp dport 520 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 520' -Direction Inbound -Protocol UDP -LocalPort 520 -Action Block

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