CyberHunter
Local dataset, no network
// Other services

Port 101: NIC Host Name Server

Port 101 is assigned by IANA to hostname (NIC Host Name Server).

101

How hostname works

Other services
client:101hostnameresponse client:101hostnameresponse

The client sends a request to port 101 and the hostname service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Other services
01 Security

Is port 101 dangerous?

We have no specific security note for port 101. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 101

ServicePortTransportDescriptionReferenceUpdated
hostname 101 TCP, UDP NIC Host Name Server - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 101, and is it reachable?

Linux

sudo ss -ltnup 'sport = :101'      # process listening on 101
sudo lsof -nP -i :101

macOS

sudo lsof -nP -i :101

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 101 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 101
netstat -ano | findstr :101

From another machine

nmap -p 101 -sV <host>
sudo nmap -sU -p 101 <host>
nc -zv <host> 101

Block it in the firewall

sudo ufw deny 101/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 101" -Direction Inbound -LocalPort 101 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 101 used for?

Port 101 is assigned by IANA to hostname (NIC Host Name Server).

Is port 101 TCP or UDP?

Port 101 is registered for TCP and UDP.

Is it safe to open port 101?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Close the port if nothing on the host needs it.

How do I check if port 101 is open?

On Linux run "ss -ltnup 'sport = :101'", on macOS "lsof -nP -i :101", and on Windows "netstat -ano | findstr :101" to see the local process. From another machine, "nmap -p 101 <host>" shows whether it is reachable.

05 Related
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