Port 24
Port 24 is a well-known port entry in the IANA registry, using TCP / UDP.
What is port 24 used for?
Port 24 is an IANA well-known port entry using TCP / UDP. The registry describes it as “any private mail system”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach this service connects to port 24. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 24 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.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Rick_Adams].
Check and block port 24
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 = :24'
sudo ss -ulpn 'sport = :24'Check on Windows
Get-NetTCPConnection -LocalPort 24 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 24 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 24/tcp
sudo ufw deny 24/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 24 -j DROP
sudo iptables -A INPUT -p udp --dport 24 -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 24 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 24 is usually used
Most traffic on port 24 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Mail clients sending messages through a relay or fetching them from a mailbox.
- Mail servers exchanging messages for a domain or syncing folders with clients.
- Deliverability, authentication and TLS tests that connect directly to the mail service.
Test the mail listener
Connect directly so the client configuration cannot hide a wrong hostname or blocked port.
nc -vz mail.example.com 24Common problems with port 24
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 24. 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 = :24' or sudo lsof -i :24. 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 24 at a glance
| Port number | 24 |
|---|---|
| Service name | Not named in the registry |
| Transport protocol | TCP / UDP |
| Category | |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | [Rick_Adams] |
| Registry description | any private mail system |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 24 used for?
Port 24 (Unnamed service) uses TCP / UDP. any private mail system.
How do I check if port 24 is open?
On Linux run `sudo ss -tlpn 'sport = :24'` and `sudo ss -ulpn 'sport = :24'`. On Windows run `Get-NetTCPConnection -LocalPort 24 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 24 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 24; empty output usually means nothing is bound to it.
How do I block port 24?
With UFW run `sudo ufw deny 24/tcp` and `sudo ufw deny 24/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 24 -j DROP` and `sudo iptables -A INPUT -p udp --dport 24 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 24 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy