Port 113 AUTH
Port 113 is the IANA-assigned auth port for Authentication Service (TCP / UDP).
What is port 113 used for?
Port 113 is assigned to auth and uses TCP / UDP. The IANA registry describes the service as “Authentication Service”. 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 ident.
Ports are the transport-layer address a service listens on. A client that wants to reach auth connects to port 113. 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 113 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 Authentication and access ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Mike_St_Johns].
Check and block port 113
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 = :113'
sudo ss -ulpn 'sport = :113'Check on Windows
Get-NetTCPConnection -LocalPort 113 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 113 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 113/tcp
sudo ufw deny 113/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 113 -j DROP
sudo iptables -A INPUT -p udp --dport 113 -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 113 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 113 is usually used
Most traffic on port 113 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Network devices authenticating users through a central service.
- Wi-Fi, VPN and remote-access gateways enforcing access policy.
- Accounting and session logs used for audits.
Probe the auth service
Authentication protocols are sensitive to shared secrets and source addresses, so test from an authorised client.
nc -vz 127.0.0.1 113Common problems with port 113
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 113. 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 = :113' or sudo lsof -i :113. 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 113 at a glance
| Port number | 113 |
|---|---|
| Service name | auth |
| Aliases | ident |
| Transport protocol | TCP / UDP |
| Category | Authentication and access |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | [Mike_St_Johns] |
| Registry description | Authentication Service |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 113 used for?
Port 113 (AUTH) uses TCP / UDP. Authentication Service.
How do I check if port 113 is open?
On Linux run `sudo ss -tlpn 'sport = :113'` and `sudo ss -ulpn 'sport = :113'`. On Windows run `Get-NetTCPConnection -LocalPort 113 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 113 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 113; empty output usually means nothing is bound to it.
How do I block port 113?
With UFW run `sudo ufw deny 113/tcp` and `sudo ufw deny 113/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 113 -j DROP` and `sudo iptables -A INPUT -p udp --dport 113 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 113 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