Port 23 TELNET
Port 23 is the IANA-assigned telnet port for Telnet (TCP / UDP).
What is port 23 used for?
Port 23 is assigned to telnet and uses TCP / UDP. The IANA registry describes the service as “Telnet”. 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 telnet connects to port 23. 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 23 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. Registered assignee: [Jon_Postel]. Reference: [RFC 854].
Check and block port 23
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 = :23'
sudo ss -ulpn 'sport = :23'Check on Windows
Get-NetTCPConnection -LocalPort 23 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 23 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 23/tcp
sudo ufw deny 23/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 23 -j DROP
sudo iptables -A INPUT -p udp --dport 23 -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 23 safe to expose?
Telnet transmits everything, including passwords, in clear text and is a favourite target for botnets. Replace it with SSH. 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 23 is usually used
Most traffic on port 23 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 23Common problems with port 23
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 23. 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 = :23' or sudo lsof -i :23. 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 23 at a glance
| Port number | 23 |
|---|---|
| Service name | telnet |
| Transport protocol | TCP / UDP |
| Category | Remote access |
| Range | Well-known port (0–1023) |
| Risk level | High |
| Registered assignee | [Jon_Postel] |
| Reference | [RFC 854] |
| Registry description | Telnet |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 23 used for?
Port 23 (TELNET) uses TCP / UDP. Telnet.
How do I check if port 23 is open?
On Linux run `sudo ss -tlpn 'sport = :23'` and `sudo ss -ulpn 'sport = :23'`. On Windows run `Get-NetTCPConnection -LocalPort 23 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 23 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 23; empty output usually means nothing is bound to it.
How do I block port 23?
With UFW run `sudo ufw deny 23/tcp` and `sudo ufw deny 23/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 23 -j DROP` and `sudo iptables -A INPUT -p udp --dport 23 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 23 safe to expose?
Telnet transmits everything, including passwords, in clear text and is a favourite target for botnets. Replace it with SSH. 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