Port 513 (login, who)

IANA assigns port 513 to login, who: remote login a la telnet; automatic authentication performed based on priviledged port numbers and distributed data bases which identify "authentication domains".

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

IANA assignment

Service nameProtoDescriptionAssigneeReference

login TCP remote login a la telnet; automatic authentication performed based on priviledged port numbers and distributed data bases which identify "authentication domains" - -

who UDP maintains data bases showing who's logged in to machines on a local net and the load average of the machine - -

Check what is listening on port 513

Linux: is anything listening on TCP 513?

sudo ss -tlnp 'sport = :513'

Linux: UDP 513

sudo ss -ulnp 'sport = :513'

Linux / macOS: which process owns the port

sudo lsof -nP -i :513

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':513 '

Windows (Command Prompt)

netstat -ano | findstr :513

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 513 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 513

From Windows

Test-NetConnection HOST -Port 513

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

nc -vzu -w 5 HOST 513

Block port 513 in the firewall

ufw

sudo ufw deny 513/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 513 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 513 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 513' -Direction Inbound -Protocol TCP -LocalPort 513 -Action Block

ufw

sudo ufw deny 513/udp