Port 2181 (TCP) — ZooKeeper: what it is, how to check

Port 2181 ZooKeeper · TCP

Apache ZooKeeper client port: the coordination service used by older Kafka deployments, HBase, Solr and Hadoop.

Port	2181
Transport	TCP
Service	ZooKeeper — Client port
Category	Infrastructure
Range	registered (1024–49151)
Encrypted	No, plaintext
Exposure	Private network or VPN only

What runs on port 2181

Apache ZooKeeper client port: the coordination service used by older Kafka deployments, HBase, Solr and Hadoop. Unauthenticated by default and able to leak cluster topology and configuration, so it stays inside the cluster network. Newer Kafka (KRaft mode) does not need it.

Software that uses it: ZooKeeper · Kafka (pre-KRaft) · HBase · SolrCloud · ClickHouse Keeper (compatible)

Should port 2181 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

echo ruok | nc host 2181 answers imok if it is healthy (the four-letter words must be whitelisted).

Check port 2181

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :2181'
sudo lsof -i :2181

# from another machine: is it reachable?
nc -zv host 2181
nmap -p 2181 host
curl -v telnet://host:2181      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 2181

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 2181 proto tcp
sudo ufw deny 2181/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 2181 accept
nft add rule inet filter input tcp dport 2181 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 2181 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 2181 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=2181 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 2181" with no source is just "open".

Related ports

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions

Is port 2181 TCP or UDP?

TCP. ZooKeeper is a connection-oriented protocol; there is no UDP service on 2181 by default (a scanner reporting UDP 2181 open is usually seeing a filtered port).

Is port 2181 encrypted?

No, plaintext. Anything sent over it can be read by whoever is on the path, so it belongs on a trusted network or inside a VPN or SSH tunnel.

How do I check whether port 2181 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 2181 and which process. From another machine, nc -zv host 2181 or nmap -p 2181 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can ZooKeeper run on a different port?

Yes; the number is a convention, not a requirement. Changing it reduces log noise from scanners but does not add security, and every client, firewall rule and monitoring check then needs the new number. Keep the default unless you have a concrete reason.