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Clustering Guide

Overview

Starting an etcd cluster statically requires that each member knows another in the cluster. In a number of cases, the IPs of the cluster members may be unknown ahead of time. In these cases, the etcd cluster can be bootstrapped with the help of a discovery service.

Once an etcd cluster is up and running, adding or removing members is done via runtime reconfiguration. To better understand the design behind runtime reconfiguration, we suggest reading the runtime configuration design document.

This guide will cover the following mechanisms for bootstrapping an etcd cluster:

Static
etcd Discovery
DNS Discovery

Each of the bootstrapping mechanisms will be used to create a three machine etcd cluster with the following details:

Name	Address	Hostname
infra0	10.0.1.10	infra0.example.com
infra1	10.0.1.11	infra1.example.com
infra2	10.0.1.12	infra2.example.com

Static

As we know the cluster members, their addresses and the size of the cluster before starting, we can use an offline bootstrap configuration by setting the initial-cluster flag. Each machine will get either the following environment variables or command line:

ETCD_INITIAL_CLUSTER="infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380"
ETCD_INITIAL_CLUSTER_STATE=new

--initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380 \
--initial-cluster-state new

Note that the URLs specified in initial-cluster are the advertised peer URLs, i.e. they should match the value of initial-advertise-peer-urls on the respective nodes.

If spinning up multiple clusters (or creating and destroying a single cluster) with same configuration for testing purpose, it is highly recommended that each cluster is given a unique initial-cluster-token. By doing this, etcd can generate unique cluster IDs and member IDs for the clusters even if they otherwise have the exact same configuration. This can protect etcd from cross-cluster-interaction, which might corrupt the clusters.

etcd listens on listen-client-urls to accept client traffic. etcd member advertises the URLs specified in advertise-client-urls to other members, proxies, clients. Please make sure the advertise-client-urls are reachable from intended clients. A common mistake is setting advertise-client-urls to localhost or leave it as default if the remote clients should reach etcd.

On each machine, start etcd with these flags:

$ etcd --name infra0 --initial-advertise-peer-urls http://10.0.1.10:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380 \
  --initial-cluster-state new

$ etcd --name infra1 --initial-advertise-peer-urls http://10.0.1.11:2380 \
  --listen-peer-urls http://10.0.1.11:2380 \
  --listen-client-urls http://10.0.1.11:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.11:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380 \
  --initial-cluster-state new

$ etcd --name infra2 --initial-advertise-peer-urls http://10.0.1.12:2380 \
  --listen-peer-urls http://10.0.1.12:2380 \
  --listen-client-urls http://10.0.1.12:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.12:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380 \
  --initial-cluster-state new

The command line parameters starting with --initial-cluster will be ignored on subsequent runs of etcd. Feel free to remove the environment variables or command line flags after the initial bootstrap process. If the configuration needs changes later (for example, adding or removing members to/from the cluster), see the runtime configuration guide.

TLS

etcd supports encrypted communication through the TLS protocol. TLS channels can be used for encrypted internal cluster communication between peers as well as encrypted client traffic. This section provides examples for setting up a cluster with peer and client TLS. Additional information detailing etcd’s TLS support can be found in the security guide.

Self-signed certificates

A cluster using self-signed certificates both encrypts traffic and authenticates its connections. To start a cluster with self-signed certificates, each cluster member should have a unique key pair (member.crt, member.key) signed by a shared cluster CA certificate (ca.crt) for both peer connections and client connections. Certificates may be generated by following the etcd TLS setup example.

On each machine, etcd would be started with these flags:

$ etcd --name infra0 --initial-advertise-peer-urls https://10.0.1.10:2380 \
  --listen-peer-urls https://10.0.1.10:2380 \
  --listen-client-urls https://10.0.1.10:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.10:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --client-cert-auth --trusted-ca-file=/path/to/ca-client.crt \
  --cert-file=/path/to/infra0-client.crt --key-file=/path/to/infra0-client.key \
  --peer-client-cert-auth --peer-trusted-ca-file=ca-peer.crt \
  --peer-cert-file=/path/to/infra0-peer.crt --peer-key-file=/path/to/infra0-peer.key

$ etcd --name infra1 --initial-advertise-peer-urls https://10.0.1.11:2380 \
  --listen-peer-urls https://10.0.1.11:2380 \
  --listen-client-urls https://10.0.1.11:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.11:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --client-cert-auth --trusted-ca-file=/path/to/ca-client.crt \
  --cert-file=/path/to/infra1-client.crt --key-file=/path/to/infra1-client.key \
  --peer-client-cert-auth --peer-trusted-ca-file=ca-peer.crt \
  --peer-cert-file=/path/to/infra1-peer.crt --peer-key-file=/path/to/infra1-peer.key

$ etcd --name infra2 --initial-advertise-peer-urls https://10.0.1.12:2380 \
  --listen-peer-urls https://10.0.1.12:2380 \
  --listen-client-urls https://10.0.1.12:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.12:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --client-cert-auth --trusted-ca-file=/path/to/ca-client.crt \
  --cert-file=/path/to/infra2-client.crt --key-file=/path/to/infra2-client.key \
  --peer-client-cert-auth --peer-trusted-ca-file=ca-peer.crt \
  --peer-cert-file=/path/to/infra2-peer.crt --peer-key-file=/path/to/infra2-peer.key

Automatic certificates

If the cluster needs encrypted communication but does not require authenticated connections, etcd can be configured to automatically generate its keys. On initialization, each member creates its own set of keys based on its advertised IP addresses and hosts.

On each machine, etcd would be started with these flags:

$ etcd --name infra0 --initial-advertise-peer-urls https://10.0.1.10:2380 \
  --listen-peer-urls https://10.0.1.10:2380 \
  --listen-client-urls https://10.0.1.10:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.10:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --auto-tls \
  --peer-auto-tls

$ etcd --name infra1 --initial-advertise-peer-urls https://10.0.1.11:2380 \
  --listen-peer-urls https://10.0.1.11:2380 \
  --listen-client-urls https://10.0.1.11:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.11:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --auto-tls \
  --peer-auto-tls

$ etcd --name infra2 --initial-advertise-peer-urls https://10.0.1.12:2380 \
  --listen-peer-urls https://10.0.1.12:2380 \
  --listen-client-urls https://10.0.1.12:2379,https://127.0.0.1:2379 \
  --advertise-client-urls https://10.0.1.12:2379 \
  --initial-cluster-token etcd-cluster-1 \
  --initial-cluster infra0=https://10.0.1.10:2380,infra1=https://10.0.1.11:2380,infra2=https://10.0.1.12:2380 \
  --initial-cluster-state new \
  --auto-tls \
  --peer-auto-tls

Error cases

In the following example, we have not included our new host in the list of enumerated nodes. If this is a new cluster, the node must be added to the list of initial cluster members.

$ etcd --name infra1 --initial-advertise-peer-urls http://10.0.1.11:2380 \
  --listen-peer-urls https://10.0.1.11:2380 \
  --listen-client-urls http://10.0.1.11:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.11:2379 \
  --initial-cluster infra0=http://10.0.1.10:2380 \
  --initial-cluster-state new
etcd: infra1 not listed in the initial cluster config
exit 1

In this example, we are attempting to map a node (infra0) on a different address (127.0.0.1:2380) than its enumerated address in the cluster list (10.0.1.10:2380). If this node is to listen on multiple addresses, all addresses must be reflected in the “initial-cluster” configuration directive.

$ etcd --name infra0 --initial-advertise-peer-urls http://127.0.0.1:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380 \
  --initial-cluster-state=new
etcd: error setting up initial cluster: infra0 has different advertised URLs in the cluster and advertised peer URLs list
exit 1

If a peer is configured with a different set of configuration arguments and attempts to join this cluster, etcd will report a cluster ID mismatch will exit.

$ etcd --name infra3 --initial-advertise-peer-urls http://10.0.1.13:2380 \
  --listen-peer-urls http://10.0.1.13:2380 \
  --listen-client-urls http://10.0.1.13:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.13:2379 \
  --initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra3=http://10.0.1.13:2380 \
  --initial-cluster-state=new
etcd: conflicting cluster ID to the target cluster (c6ab534d07e8fcc4 != bc25ea2a74fb18b0). Exiting.
exit 1

Discovery

In a number of cases, the IPs of the cluster peers may not be known ahead of time. This is common when utilizing cloud providers or when the network uses DHCP. In these cases, rather than specifying a static configuration, use an existing etcd cluster to bootstrap a new one. This process is called “discovery”.

There two methods that can be used for discovery:

etcd discovery service
DNS SRV records

etcd discovery

To better understand the design of the discovery service protocol, we suggest reading the discovery service protocol documentation.

Lifetime of a discovery URL

A discovery URL identifies a unique etcd cluster. Instead of reusing an existing discovery URL, each etcd instance shares a new discovery URL to bootstrap the new cluster.

Moreover, discovery URLs should ONLY be used for the initial bootstrapping of a cluster. To change cluster membership after the cluster is already running, see the runtime reconfiguration guide.

Custom etcd discovery service

Discovery uses an existing cluster to bootstrap itself. If using a private etcd cluster, create a URL like so:

$ curl -X PUT https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83/_config/size -d value=3

By setting the size key to the URL, a discovery URL is created with an expected cluster size of 3.

The URL to use in this case will be https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83 and the etcd members will use the https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83 directory for registration as they start.

Each member must have a different name flag specified. Hostname or machine-id can be a good choice. Or discovery will fail due to duplicated name.

Now we start etcd with those relevant flags for each member:

$ etcd --name infra0 --initial-advertise-peer-urls http://10.0.1.10:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --discovery https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83

$ etcd --name infra1 --initial-advertise-peer-urls http://10.0.1.11:2380 \
  --listen-peer-urls http://10.0.1.11:2380 \
  --listen-client-urls http://10.0.1.11:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.11:2379 \
  --discovery https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83

$ etcd --name infra2 --initial-advertise-peer-urls http://10.0.1.12:2380 \
  --listen-peer-urls http://10.0.1.12:2380 \
  --listen-client-urls http://10.0.1.12:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.12:2379 \
  --discovery https://myetcd.local/v2/keys/discovery/6c007a14875d53d9bf0ef5a6fc0257c817f0fb83

This will cause each member to register itself with the custom etcd discovery service and begin the cluster once all machines have been registered.

Public etcd discovery service

If no exiting cluster is available, use the public discovery service hosted at discovery.etcd.io. To create a private discovery URL using the “new” endpoint, use the command:

$ curl https://discovery.etcd.io/new?size=3
https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

This will create the cluster with an initial size of 3 members. If no size is specified, a default of 3 is used.

ETCD_DISCOVERY=https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

--discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

Each member must have a different name flag specified or else discovery will fail due to duplicated names. Hostname or machine-id can be a good choice.

Now we start etcd with those relevant flags for each member:

$ etcd --name infra0 --initial-advertise-peer-urls http://10.0.1.10:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

$ etcd --name infra1 --initial-advertise-peer-urls http://10.0.1.11:2380 \
  --listen-peer-urls http://10.0.1.11:2380 \
  --listen-client-urls http://10.0.1.11:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.11:2379 \
  --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

$ etcd --name infra2 --initial-advertise-peer-urls http://10.0.1.12:2380 \
  --listen-peer-urls http://10.0.1.12:2380 \
  --listen-client-urls http://10.0.1.12:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.12:2379 \
  --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de

This will cause each member to register itself with the discovery service and begin the cluster once all members have been registered.

Use the environment variable ETCD_DISCOVERY_PROXY to cause etcd to use an HTTP proxy to connect to the discovery service.

Error and warning cases

Discovery server errors

$ etcd --name infra0 --initial-advertise-peer-urls http://10.0.1.10:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de
etcd: error: the cluster doesn’t have a size configuration value in https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de/_config
exit 1

Warnings

This is a harmless warning indicating the discovery URL will be ignored on this machine.

$ etcd --name infra0 --initial-advertise-peer-urls http://10.0.1.10:2380 \
  --listen-peer-urls http://10.0.1.10:2380 \
  --listen-client-urls http://10.0.1.10:2379,http://127.0.0.1:2379 \
  --advertise-client-urls http://10.0.1.10:2379 \
  --discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de
etcdserver: discovery token ignored since a cluster has already been initialized. Valid log found at /var/lib/etcd

DNS discovery

DNS SRV records can be used as a discovery mechanism. The --discovery-srv flag can be used to set the DNS domain name where the discovery SRV records can be found. Setting --discovery-srv example.com causes DNS SRV records to be looked up in the listed order:

_etcd-server-ssl._tcp.example.com
_etcd-server._tcp.example.com

If _etcd-server-ssl._tcp.example.com is found then etcd will attempt the bootstrapping process over TLS.

To help clients discover the etcd cluster, the following DNS SRV records are looked up in the listed order:

_etcd-client._tcp.example.com
_etcd-client-ssl._tcp.example.com

If _etcd-client-ssl._tcp.example.com is found, clients will attempt to communicate with the etcd cluster over SSL/TLS.

If etcd is using TLS, the discovery SRV record (e.g. example.com) must be included in the SSL certificate DNS SAN along with the hostname, or clustering will fail with log messages like the following:

[...] rejected connection from "10.0.1.11:53162" (error "remote error: tls: bad certificate", ServerName "example.com")

If etcd is using TLS without a custom certificate authority, the discovery domain (e.g., example.com) must match the SRV record domain (e.g., infra1.example.com). This is to mitigate attacks that forge SRV records to point to a different domain; the domain would have a valid certificate under PKI but be controlled by an unknown third party.

The -discovery-srv-name flag additionally configures a suffix to the SRV name that is queried during discovery. Use this flag to differentiate between multiple etcd clusters under the same domain. For example, if discovery-srv=example.com and -discovery-srv-name=foo are set, the following DNS SRV queries are made:

_etcd-server-ssl-foo._tcp.example.com
_etcd-server-foo._tcp.example.com

Create DNS SRV records

$ dig +noall +answer SRV _etcd-server._tcp.example.com
_etcd-server._tcp.example.com. 300 IN  SRV  0 0 2380 infra0.example.com.
_etcd-server._tcp.example.com. 300 IN  SRV  0 0 2380 infra1.example.com.
_etcd-server._tcp.example.com. 300 IN  SRV  0 0 2380 infra2.example.com.

$ dig +noall +answer SRV _etcd-client._tcp.example.com
_etcd-client._tcp.example.com. 300 IN SRV 0 0 2379 infra0.example.com.
_etcd-client._tcp.example.com. 300 IN SRV 0 0 2379 infra1.example.com.
_etcd-client._tcp.example.com. 300 IN SRV 0 0 2379 infra2.example.com.

$ dig +noall +answer infra0.example.com infra1.example.com infra2.example.com
infra0.example.com.  300  IN  A  10.0.1.10
infra1.example.com.  300  IN  A  10.0.1.11
infra2.example.com.  300  IN  A  10.0.1.12

Bootstrap the etcd cluster using DNS

etcd cluster members can advertise domain names or IP address, the bootstrap process will resolve DNS A records. Since 3.2 (3.1 prints warnings) --listen-peer-urls and --listen-client-urls will reject domain name for the network interface binding.

The resolved address in --initial-advertise-peer-urls must match one of the resolved addresses in the SRV targets. The etcd member reads the resolved address to find out if it belongs to the cluster defined in the SRV records.

$ etcd --name infra0 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://infra0.example.com:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://infra0.example.com:2379 \
--listen-client-urls http://0.0.0.0:2379 \
--listen-peer-urls http://0.0.0.0:2380

$ etcd --name infra1 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://infra1.example.com:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://infra1.example.com:2379 \
--listen-client-urls http://0.0.0.0:2379 \
--listen-peer-urls http://0.0.0.0:2380

$ etcd --name infra2 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://infra2.example.com:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://infra2.example.com:2379 \
--listen-client-urls http://0.0.0.0:2379 \
--listen-peer-urls http://0.0.0.0:2380

The cluster can also bootstrap using IP addresses instead of domain names:

$ etcd --name infra0 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://10.0.1.10:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://10.0.1.10:2379 \
--listen-client-urls http://10.0.1.10:2379 \
--listen-peer-urls http://10.0.1.10:2380

$ etcd --name infra1 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://10.0.1.11:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://10.0.1.11:2379 \
--listen-client-urls http://10.0.1.11:2379 \
--listen-peer-urls http://10.0.1.11:2380

$ etcd --name infra2 \
--discovery-srv example.com \
--initial-advertise-peer-urls http://10.0.1.12:2380 \
--initial-cluster-token etcd-cluster-1 \
--initial-cluster-state new \
--advertise-client-urls http://10.0.1.12:2379 \
--listen-client-urls http://10.0.1.12:2379 \
--listen-peer-urls http://10.0.1.12:2380

Since v3.1.0 (except v3.2.9), when etcd --discovery-srv=example.com is configured with TLS, server will only authenticate peers/clients when the provided certs have root domain example.com as an entry in Subject Alternative Name (SAN) field. See Notes for DNS SRV.

Gateway

etcd gateway is a simple TCP proxy that forwards network data to the etcd cluster. Please read gateway guide for more information.

Proxy

When the --proxy flag is set, etcd runs in proxy mode. This proxy mode only supports the etcd v2 API; there are no plans to support the v3 API. Instead, for v3 API support, there will be a new proxy with enhanced features following the etcd 3.0 release.

To setup an etcd cluster with proxies of v2 API, please read the the clustering doc in etcd 2.3 release.

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Last modified August 18, 2021: fix v3.1 links (#450) (5a80a26)