Port 2376 Docker API (TLS) · TCP

What runs on port 2376

Docker Engine API over TLS with mutual authentication: the client certificate is the password. Used by Docker Machine, some CI systems and Portainer. Still equivalent to root on the host for anyone holding a valid client certificate, so protect the keys and restrict by source.

Software that uses it: Docker Engine (--tlsverify) · Portainer · CI runners

Should port 2376 be open?

Internet-facing only with authentication and care. It can be exposed when the service behind it is authenticated, rate-limited and kept up to date; most deployments are better off behind a VPN, an allow-list or a reverse proxy.

• docker --tlsverify -H tcp://host:2376 version is the test; a bare TLS error means the CA does not match.

Check port 2376

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :2376'
sudo lsof -i :2376

# from another machine: is it reachable?
nc -zv host 2376
nmap -p 2376 host
curl -v telnet://host:2376      # 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 2376

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 2376 proto tcp
sudo ufw deny 2376/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 2376 accept
nft add rule inet filter input tcp dport 2376 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 2376 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 2376 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=2376 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 2376" with no source is just "open".

Related ports

• 2375 Docker API — Unencrypted; never expose
• 22 SSH — SSH, SCP, SFTP, git over ssh
• 6443 Kubernetes API

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