Port 512 EXEC
Port 512 has a different IANA assignment for each transport protocol: TCP exec; UDP biff, comsat.
What is port 512 used for?
Port 512 carries a different IANA service on each transport protocol: TCP exec; UDP biff, comsat. 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 comsat, biff.
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 512 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 512
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 = :512'
sudo ss -ulpn 'sport = :512'Check on Windows
Get-NetTCPConnection -LocalPort 512 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 512 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 512/tcp
sudo ufw deny 512/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 512 -j DROP
sudo iptables -A INPUT -p udp --dport 512 -j DROPThese 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 512 safe to expose?
rexec is a legacy remote execution service with weak authentication. Disable it and use SSH instead. 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 512 is usually used
Most traffic on port 512 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Administrators connecting through a bastion host or VPN to manage a machine.
- Automation and CI jobs opening short-lived sessions to run commands.
- Port forwarding that keeps an internal service off the public network.
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 512Common problems with port 512
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 512. 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 = :512' or sudo lsof -i :512. 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 512 at a glance
| Port number | 512 |
|---|---|
| Service name | exec |
| Aliases | comsat, biff |
| Transport protocol | TCP / UDP |
| Assignments by protocol |
|
| Category | Remote access |
| Range | Well-known port (0–1023) |
| Risk level | High |
| Registered assignee | Not listed |
| Registry description | remote process execution; authentication performed using passwords and UNIX login names |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 512 used for?
Port 512 (EXEC). The IANA registry assigns a different service on each transport protocol: TCP exec; UDP biff, comsat.
How do I check if port 512 is open?
On Linux run `sudo ss -tlpn 'sport = :512'` and `sudo ss -ulpn 'sport = :512'`. On Windows run `Get-NetTCPConnection -LocalPort 512 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 512 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 512; empty output usually means nothing is bound to it.
How do I block port 512?
With UFW run `sudo ufw deny 512/tcp` and `sudo ufw deny 512/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 512 -j DROP` and `sudo iptables -A INPUT -p udp --dport 512 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 512 safe to expose?
rexec is a legacy remote execution service with weak authentication. Disable it and use SSH instead. 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.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy