Port 8123 (TCP) — ClickHouse HTTP: what it is, how to check

Port 8123 ClickHouse HTTP · TCP

ClickHouse's HTTP interface, used by its JDBC/ODBC drivers, Grafana and curl-style queries; the native TCP protocol is on 9000.

Port	8123
Transport	TCP
Service	ClickHouse HTTP
Category	Databases and caches
Range	registered (1024–49151)
Encrypted	No, plaintext
Exposure	Private network or VPN only

What runs on port 8123

ClickHouse's HTTP interface, used by its JDBC/ODBC drivers, Grafana and curl-style queries; the native TCP protocol is on 9000. Also Home Assistant's web UI, which is the first thing to check if 8123 is open on a home network. ClickHouse without a user password, reachable from outside, lets anyone run SELECT on every table.

Software that uses it: ClickHouse (HTTP) · Home Assistant · Polipo (historical proxy)

Should port 8123 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 "SELECT version()" | curl -s "http://host:8123/" --data-binary @- is the standard ClickHouse check.
Home Assistant: use its built-in TLS or a reverse proxy, and 2FA; many "smart home" compromises start at an exposed 8123.

Check port 8123

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :8123'
sudo lsof -i :8123

# from another machine: is it reachable?
nc -zv host 8123
nmap -p 8123 host
curl -v telnet://host:8123      # 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 8123

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 8123 proto tcp
sudo ufw deny 8123/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 8123 accept
nft add rule inet filter input tcp dport 8123 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 8123 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 8123 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=8123 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 8123" 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 8123 TCP or UDP?

TCP. ClickHouse HTTP is a connection-oriented protocol; there is no UDP service on 8123 by default (a scanner reporting UDP 8123 open is usually seeing a filtered port).

Is port 8123 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 8123 is open?

On the server, ss -ltnp (or -lunp for UDP) shows whether something listens on 8123 and which process. From another machine, nc -zv host 8123 or nmap -p 8123 host tests reachability. "Open" in a scan means something answered; a firewall usually makes the port filtered (no answer) rather than closed (RST).

Can ClickHouse HTTP 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.