Port 90 DNSIX
Port 90 is the IANA-assigned dnsix port for DNSIX Securit Attribute Token Map (TCP / UDP).
What is port 90 used for?
Port 90 is assigned to dnsix and uses TCP / UDP. The IANA registry describes the service as “DNSIX Securit Attribute Token Map”. 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 dnsix connects to port 90. 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 90 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 DNS and naming ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Charles_Watt].
Check and block port 90
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 = :90'
sudo ss -ulpn 'sport = :90'Check on Windows
Get-NetTCPConnection -LocalPort 90 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 90 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 90/tcp
sudo ufw deny 90/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 90 -j DROP
sudo iptables -A INPUT -p udp --dport 90 -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 90 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 90 is usually used
Most traffic on port 90 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Recursive or authoritative resolution for clients on a LAN or in a VPC.
- Forwarders passing queries between internal and public resolvers.
- Debugging name-resolution failures with dig or nslookup.
Query the resolver
Send one query straight to the server so the answer does not depend on the client's configured resolver.
dig @127.0.0.1 -p 90 example.comCommon problems with port 90
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 90. 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 = :90' or sudo lsof -i :90. 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 90 at a glance
| Port number | 90 |
|---|---|
| Service name | dnsix |
| Transport protocol | TCP / UDP |
| Category | DNS and naming |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | [Charles_Watt] |
| Registry description | DNSIX Securit Attribute Token Map |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 90 used for?
Port 90 (DNSIX) uses TCP / UDP. DNSIX Securit Attribute Token Map.
How do I check if port 90 is open?
On Linux run `sudo ss -tlpn 'sport = :90'` and `sudo ss -ulpn 'sport = :90'`. On Windows run `Get-NetTCPConnection -LocalPort 90 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 90 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 90; empty output usually means nothing is bound to it.
How do I block port 90?
With UFW run `sudo ufw deny 90/tcp` and `sudo ufw deny 90/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 90 -j DROP` and `sudo iptables -A INPUT -p udp --dport 90 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 90 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