Port 2181 (Apache ZooKeeper)

ZooKeeper client port, historically required by Kafka, HBase, Solr and Hadoop.

Registered (user) port, 1024-49151 TCP Message queues Do not expose to the internet

What port 2181 is used for

ZooKeeper client port, historically required by Kafka, HBase, Solr and Hadoop.

Security: should port 2181 be open?

Do not expose to the internet. Never expose: by default ZooKeeper allows unauthenticated reads and writes of cluster metadata, and the four-letter commands leak details.

IANA assignment

Service nameProtoDescriptionAssigneeReference

eforward TCP eforward Greg Pringle -

eforward UDP eforward Greg Pringle -

Check what is listening on port 2181

Linux: is anything listening on TCP 2181?

sudo ss -tlnp 'sport = :2181'

Linux / macOS: which process owns the port

sudo lsof -nP -i :2181

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':2181 '

Windows (Command Prompt)

netstat -ano | findstr :2181

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 2181 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 2181

From Windows

Test-NetConnection HOST -Port 2181

Block port 2181 in the firewall

ufw

sudo ufw deny 2181/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 2181 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 2181 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 2181' -Direction Inbound -Protocol TCP -LocalPort 2181 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.