Port 9100: Node exporter / printing

What uses port 9100

Prometheus node_exporter exposes CPU, memory, disk and network metrics for a Linux host at http://host:9100/metrics. On printers, TCP 9100 is the raw printing port (HP JetDirect, AppSocket): anything sent to it gets printed.

Should port 9100 be open to the internet?

Keep private. Never expose this port to the internet. Keep it on localhost, a private network or behind a VPN or SSH tunnel.

Check what is listening on port 9100

sudo ss -ltnp 'sport = :9100'

sudo lsof -nP -iTCP:9100 -sTCP:LISTEN

Get-NetTCPConnection -LocalPort 9100 -State Listen
netstat -ano | findstr :9100

No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.

Fix “address already in use” on port 9100

Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:

sudo kill $(sudo lsof -t -iTCP:9100 -sTCP:LISTEN)

Stop-Process -Id (Get-NetTCPConnection -LocalPort 9100 -State Listen).OwningProcess

Allow port 9100 from trusted networks only

The examples allow 9100 only from 10.0.0.0/8; replace it with your own private network, office IP or VPN range.

sudo ufw allow from 10.0.0.0/8 to any port 9100 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 9100 proto tcp

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="9100" protocol="tcp" accept'
sudo firewall-cmd --reload

New-NetFirewallRule -DisplayName "Node exporter / printing 9100/TCP" -Direction Inbound -Protocol TCP -LocalPort 9100 -Action Allow -RemoteAddress 10.0.0.0/8

On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.

Port 9100 with Docker

# Publish on localhost only. Docker's port rules bypass ufw and firewalld,
# so -p 9100:9100 would expose the port to the whole network.
docker run -d -p 127.0.0.1:9100:9100 prom/node-exporter

The format is -p host_port:container_port. Containers on the same Docker network reach each other on the container port directly, without publishing it. Convert a long docker run command into a Compose file with docker run to compose.

Test port 9100 from another machine

nc -zv example.com 9100

Test-NetConnection example.com -Port 9100

Related ports

• Port 9090 (Prometheus): the default port of the Prometheus server.
• Port 9093 (Alertmanager / Kafka controller): used by Prometheus Alertmanager and often by Kafka KRaft controllers.

More monitoring and logging ports

161 (SNMP) , 162 (SNMP traps) , 514 (Syslog) , 4317 (OpenTelemetry (OTLP gRPC)) , 4318 (OpenTelemetry (OTLP HTTP)) , 5601 (Kibana) . See all ports in the port lookup.

Sources

• IANA port registry (not registered for this use)

Spotted a mistake? Email hello@devtoolsmith.com and we’ll fix it.

Frequently asked questions