Secret discovery service (SDS)
TLS certificates, the secrets, can be specified in the bootstrap.static_resource secrets. But they can also be fetched remotely by secret discovery service (SDS).
The most important benefit of SDS is to simplify the certificate management. Without this feature, in k8s deployment, certificates must be created as secrets and mounted into the proxy containers. If certificates are expired, the secrets need to be updated and the proxy containers need to be re-deployed. With SDS, a central SDS server will push certificates to all Envoy instances. If certificates are expired, the server just pushes new certificates to Envoy instances, Envoy will use the new ones right away without re-deployment.
If a listener server certificate needs to be fetched by SDS remotely, it will NOT be marked as active, its port will not be opened before the certificates are fetched. If Envoy fails to fetch the certificates due to connection failures, or bad response data, the listener will be marked as active, and the port will be open, but the connection to the port will be reset.
Upstream clusters are handled in a similar way, if a cluster client certificate needs to be fetched by SDS remotely, it will NOT be marked as active and it will not be used before the certificates are fetched. If Envoy fails to fetch the certificates due to connection failures, or bad response data, the cluster will be marked as active, it can be used to handle the requests, but the requests routed to that cluster will be rejected.
If a static cluster is using SDS, and it needs to define a SDS cluster (unless Google gRPC is used which doesn’t need a cluster), the SDS cluster has to be defined before the static clusters using it.
The connection between Envoy proxy and SDS server has to be secure. One option is to run the SDS server on the same host and use Unix Domain Socket for the connection. Otherwise the connection requires TLS with authentication between the proxy and SDS server. Credential types in use today for authentication are:
mTLS – In this case, the client certificates for the SDS connection must be statically configured.
AWS IAM SigV4
SDS server
A SDS server needs to implement the gRPC service SecretDiscoveryService. It follows the same protocol as other xDS.
Note
SPIRE, an open-source reference implementation of the SPIFFE specification for production identity, can act as an SDS server for Envoy. See the SPIRE documentation for how to configure Envoy with SPIFFE and SPIRE.
SDS Configuration
SdsSecretConfig is used to specify the secret. Its field name is a required field. If its sds_config field is empty, the name field specifies the secret in the bootstrap static_resource secrets. Otherwise, it specifies the SDS server as ConfigSource. Only gRPC is supported for the SDS service so its api_config_source must specify a grpc_service.
SdsSecretConfig is used in two fields in CommonTlsContext. The first field is tls_certificate_sds_secret_configs to use SDS to get TlsCertificate. The second field is validation_context_sds_secret_config to use SDS to get CertificateValidationContext.
Key rotation
It’s usually preferrable to perform key rotation via gRPC SDS, but when this is not possible or desired (e.g. during bootstrap of SDS credentials), SDS allows for filesystem rotation when secrets refer to filesystem paths. This currently is supported for the following secret types:
By default, directories containing secrets are watched for filesystem move events. For example, a
key or trusted CA certificates at /foo/bar/baz/cert.pem will be watched at /foo/bar/baz.
Explicit control over the watched directory is possible by specifying a watched_directory path in
TlsCertificate and
CertificateValidationContext.
This allows watches to be established at path predecessors, e.g. /foo/bar; this capability is
useful when implementing common key rotation schemes.
An example of key rotation is provided below.
Example one: static_resource
This example show how to configure secrets in the static_resource:
static_resources:
secrets:
- name: server_cert
tls_certificate:
certificate_chain:
filename: certs/servercert.pem
private_key:
filename: certs/serverkey.pem
- name: client_cert
tls_certificate:
certificate_chain:
filename: certs/clientcert.pem
private_key:
filename: certs/clientkey.pem
- name: validation_context
validation_context:
trusted_ca:
filename: certs/cacert.pem
verify_certificate_hash:
E0:F3:C8:CE:5E:2E:A3:05:F0:70:1F:F5:12:E3:6E:2E:97:92:82:84:A2:28:BC:F7:73:32:D3:39:30:A1:B6:FD
clusters:
- connect_timeout: 0.25s
load_assignment:
cluster_name: local_service_tls
...
transport_socket:
name: envoy.transport_sockets.tls
typed_config:
"@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext
common_tls_context:
tls_certificate_sds_secret_configs:
- name: client_cert
listeners:
....
filter_chains:
transport_socket:
name: envoy.transport_sockets.tls
typed_config:
"@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.DownstreamTlsContext
common_tls_context:
tls_certificate_sds_secret_configs:
- name: server_cert
validation_context_sds_secret_config:
name: validation_context
In this example, certificates are specified in the bootstrap static_resource, they are not fetched remotely. In the config, secrets static resource has 3 secrets: client_cert, server_cert and validation_context. In the cluster config, one of hosts uses client_cert in its tls_certificate_sds_secret_configs. In the listeners section, one of them uses server_cert in its tls_certificate_sds_secret_configs and validation_context for its validation_context_sds_secret_config.
Example two: SDS server
This example shows how to configure secrets fetched from remote SDS servers:
1node:
2 cluster: envoy_cluster
3 id: envoy_node
4
5static_resources:
6 listeners:
7 - name: listener_0
8 address:
9 socket_address:
10 address: 0.0.0.0
11 port_value: 8000
12 filter_chains:
13 - transport_socket:
14 name: envoy.transport_sockets.tls
15 typed_config:
16 "@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.DownstreamTlsContext
17 common_tls_context:
18 tls_certificate_sds_secret_configs:
19 - name: server_cert
20 sds_config:
21 resource_api_version: V3
22 api_config_source:
23 api_type: GRPC
24 transport_api_version: V3
25 grpc_services:
26 - envoy_grpc:
27 cluster_name: sds_server_mtls
28 validation_context_sds_secret_config:
29 name: validation_context
30 sds_config:
31 resource_api_version: V3
32 api_config_source:
33 api_type: GRPC
34 transport_api_version: V3
35 grpc_services:
36 - envoy_grpc:
37 cluster_name: sds_server_uds
38 clusters:
39 - name: sds_server_mtls
40 typed_extension_protocol_options:
41 envoy.extensions.upstreams.http.v3.HttpProtocolOptions:
42 "@type": type.googleapis.com/envoy.extensions.upstreams.http.v3.HttpProtocolOptions
43 explicit_http_config:
44 http2_protocol_options: {}
45 load_assignment:
46 cluster_name: sds_server_mtls
47 endpoints:
48 - lb_endpoints:
49 - endpoint:
50 address:
51 socket_address:
52 address: 127.0.0.1
53 port_value: 8234
54 transport_socket:
55 name: envoy.transport_sockets.tls
56 typed_config:
57 "@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext
58 common_tls_context:
59 tls_certificates:
60 - certificate_chain:
61 filename: certs/servercert.pem
62 private_key:
63 filename: certs/serverkey.pem
64 - name: sds_server_uds
65 typed_extension_protocol_options:
66 envoy.extensions.upstreams.http.v3.HttpProtocolOptions:
67 "@type": type.googleapis.com/envoy.extensions.upstreams.http.v3.HttpProtocolOptions
68 explicit_http_config:
69 http2_protocol_options: {}
70 load_assignment:
71 cluster_name: sds_server_uds
72 endpoints:
73 - lb_endpoints:
74 - endpoint:
75 address:
76 pipe:
77 path: /tmp/uds_path
78 - name: example_cluster
79 load_assignment:
80 cluster_name: local_service_tls
81 endpoints:
82 - lb_endpoints:
83 - endpoint:
84 address:
85 socket_address:
86 address: 127.0.0.1
87 port_value: 8443
88 transport_socket:
89 name: envoy.transport_sockets.tls
90 typed_config:
91 "@type": type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext
92 common_tls_context:
93 tls_certificate_sds_secret_configs:
94 - name: client_cert
95 sds_config:
96 resource_api_version: V3
97 api_config_source:
98 api_type: GRPC
99 transport_api_version: V3
100 grpc_services:
101 - google_grpc:
102 target_uri: unix:/tmp/uds_path
103 stat_prefix: sds_uds_server
For illustration, above example uses three methods to access the SDS server. A gRPC SDS server can be reached by Unix Domain Socket path /tmp/uds_path and 127.0.0.1:8234 by mTLS. It provides three secrets, client_cert, server_cert and validation_context. In the config, cluster example_cluster certificate client_cert is configured to use Google gRPC with UDS to talk to the SDS server. The Listener needs to fetch server_cert and validation_context from the SDS server. The server_cert is using Envoy gRPC with cluster sds_server_mtls configured with client certificate to use mTLS to talk to SDS server. The validate_context is using Envoy gRPC with cluster sds_server_uds configured with UDS path to talk to the SDS server.
Example three: certificate rotation for xDS gRPC connection
Managing certificates for xDS gRPC connection between Envoy and xDS server introduces a bootstrapping problem: SDS server cannot manage certificates that are required to connect to the server.
This example shows how to set up xDS connection by sourcing SDS configuration from the filesystem. The certificate and key files are watched with inotify and reloaded automatically without restart. In contrast, Example two: SDS server requires a restart to reload xDS certificates and key after update.
clusters:
- name: control_plane
type: LOGICAL_DNS
connect_timeout: 1s
load_assignment:
cluster_name: control_plane
endpoints:
- lb_endpoints:
- endpoint:
address:
socket_address:
address: controlplane
port_value: 8443
typed_extension_protocol_options:
envoy.extensions.upstreams.http.v3.HttpProtocolOptions:
"@type": type.googleapis.com/envoy.extensions.upstreams.http.v3.HttpProtocolOptions
explicit_http_config:
http2_protocol_options: {}
transport_socket:
name: "envoy.transport_sockets.tls"
typed_config:
"@type": "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext"
common_tls_context:
tls_certificate_sds_secret_configs:
name: tls_sds
sds_config:
path: /etc/envoy/tls_certificate_sds_secret.yaml
validation_context_sds_secret_config:
name: validation_context_sds
sds_config:
path: /etc/envoy/validation_context_sds_secret.yaml
Paths to client certificate, including client’s certificate chain and private key are given in SDS config file /etc/envoy/tls_certificate_sds_secret.yaml:
resources:
- "@type": "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.Secret"
name: tls_sds
tls_certificate:
certificate_chain:
filename: /certs/sds_cert.pem
private_key:
filename: /certs/sds_key.pem
Path to CA certificate bundle for validating the xDS server certificate is given in SDS config file /etc/envoy/validation_context_sds_secret.yaml:
resources:
- "@type": "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.Secret"
name: validation_context_sds
validation_context:
trusted_ca:
filename: /certs/cacert.pem
In the above example, a watch will be established on /certs. File movement in this directory
will trigger an update. An alternative common key rotation scheme that provides improved atomicity
is to establish an active symlink /certs/current and use an atomic move operation to replace the
symlink. The watch in this case needs to be on the certificate’s grandparent directory. Envoy
supports this scheme via the use of watched_directory. Continuing the above examples:
resources:
- "@type": "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.Secret"
name: tls_sds
tls_certificate:
certificate_chain:
filename: /certs/current/sds_cert.pem
private_key:
filename: /certs/current/sds_key.pem
watched_directory:
path: /certs
resources:
- "@type": "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.Secret"
name: validation_context_sds
validation_context:
trusted_ca:
filename: /certs/current/cacert.pem
watched_directory:
path: /certs
Secret rotation can be performed with:
ln -s <path to new secrets> /certs/new && mv -Tf /certs/new /certs/current
Statistics
SSL socket factory outputs following SDS related statistics. They are all counter type.
For downstream listeners, they are in the listener.<LISTENER_IP>.server_ssl_socket_factory. namespace.
Name |
Description |
|---|---|
ssl_context_update_by_sds |
Total number of ssl context has been updated. |
downstream_context_secrets_not_ready |
Total number of downstream connections reset due to empty ssl certificate. |
For upstream clusters, they are in the cluster.<CLUSTER_NAME>.client_ssl_socket_factory. namespace.
Name |
Description |
|---|---|
ssl_context_update_by_sds |
Total number of ssl context has been updated. |
upstream_context_secrets_not_ready |
Total number of upstream connections reset due to empty ssl certificate. |
SDS has a statistics tree rooted in the sds.<SECRET_NAME>. namespace. In addition, the following statistics are tracked in this namespace:
Name |
Description |
|---|---|
key_rotation_failed |
Total number of filesystem key rotations that failed outside of an SDS update. |