PortAtlas.
Well-known port (0–1023) High risk

Port 513 LOGIN

Quick answer

Port 513 has a different IANA assignment for each transport protocol: TCP login; UDP who.

Check and block Is it safe? Troubleshooting FAQ
TransportTCP / UDP
RiskHigh
Typical useRemote access

What is port 513 used for?

Port 513 carries a different IANA service on each transport protocol: TCP login; UDP who. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default. The same port is also registered under who.

Each transport reaches a different service on this number, so match the protocol to the service you are diagnosing. A TCP check says nothing about a UDP listener on the same port, and the reverse is also true. Both transports still bind to the same number, which is why two processes can collide even when they use different protocols.

Use the commands below to confirm whether port 513 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.

Browse all Remote access ports

Source: IANA Service Name and Transport Protocol Port Number Registry.

Check and block port 513

Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.

Check on Linux

sudo ss -tlpn 'sport = :513' sudo ss -ulpn 'sport = :513'

Check on Windows

Get-NetTCPConnection -LocalPort 513 -State Listen -ErrorAction SilentlyContinue Get-NetUDPEndpoint -LocalPort 513 -ErrorAction SilentlyContinue

Block with UFW

sudo ufw deny 513/tcp sudo ufw deny 513/udp

Block with iptables

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

These commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.

Is port 513 safe to expose?

High

rlogin trusts host-based authentication and sends data in clear text. It should be disabled on any modern system. Treat this port as sensitive. Restrict it to trusted networks or a VPN, require authentication and logging, and never expose it directly to the public internet without a hardened gateway in front.

Where port 513 is usually used

Most traffic on port 513 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.

Demo

Test the listener

A TCP probe shows whether the remote-access service is reachable; the protocol client may still reject the session later.

nc -vz 127.0.0.1 513

Common problems with port 513

Connection refused or timed out

Refused usually means the host is reachable but nothing is listening on port 513. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.

Port already in use

Find the process holding the port with sudo ss -tlpn 'sport = :513' or sudo lsof -i :513. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.

Firewall rule did not survive a reboot

UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.

The port is blocked but still reachable

A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.

Port 513 at a glance

Port number513
Service namelogin
Aliaseswho
Transport protocolTCP / UDP
Assignments by protocol
  • TCP login — remote login a la telnet; automatic authentication performed based on priviledged port numbers and distributed data bases which identify "authentication domains"
  • UDP who — maintains data bases showing who's logged in to machines on a local net and the load average of the machine
CategoryRemote access
RangeWell-known port (0–1023)
Risk levelHigh
Registered assigneeNot listed
Registry descriptionremote login a la telnet; automatic authentication performed based on priviledged port numbers and distributed data bases which identify "authentication domains"
SourceIANA Service Name and Transport Protocol Port Number Registry

Frequently asked questions

What is port 513 used for?

Port 513 (LOGIN). The IANA registry assigns a different service on each transport protocol: TCP login; UDP who.

How do I check if port 513 is open?

On Linux run `sudo ss -tlpn 'sport = :513'` and `sudo ss -ulpn 'sport = :513'`. On Windows run `Get-NetTCPConnection -LocalPort 513 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 513 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 513; empty output usually means nothing is bound to it.

How do I block port 513?

With UFW run `sudo ufw deny 513/tcp` and `sudo ufw deny 513/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 513 -j DROP` and `sudo iptables -A INPUT -p udp --dport 513 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.

Is port 513 safe to expose?

rlogin trusts host-based authentication and sends data in clear text. It should be disabled on any modern system. Nevertheless, treat this port as sensitive. Restrict it to trusted networks or a VPN, require authentication and logging, and never expose it directly to the public internet without a hardened gateway in front.

See all Remote access ports

Primary data: IANA Service Name and Transport Protocol Port Number Registry.

Last reviewed 2026-10-30 · Methodology · About · Privacy