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Background
A private Docker registry such as Nexus Repository 3 will require authentication from your users in order to publish Docker images. In order to use authentication, the Docker Daemon implementation enforces that the registry connection uses HTTPS.
Nexus Repository 3 is not configured with HTTPS connectors by default, as configuring it requires an SSL certificate to be generated and configured manually.
For testing purposes, it is common that an administrator may want to use a self-signed TLS certificate for Nexus HTTPS connectors. Although self-signed certificates can encrypt HTTP communication and be generated quickly, they are generally considered untrustworthy because the certificate identity has not been signed/verified by a third party certificate authority (CA).
To work around the Docker Daemon considering the Nexus HTTPS as untrustworthy, the daemon has an option called --insecure-registry . By marking the Nexus hostname and port as insecure, the docker daemon does not validate the trustworthiness of the secure connection. The intent is to avoid errors that would normally occur when using an untrusted certificate.
However, the --insecure-registry flag has many unwanted side effects and can lead to obscure errors that would not be encountered otherwise.
Sonatype cannot recommend using the Docker --insecure-registry flag due to poorly defined semantics.
How To Generate a Self-Signed Certificate That Can Be Trusted By Docker Daemon
Rather than tell the Docker daemon to not validate a self-signed certificate by using --insecure-registry, the better practice is to tell it to trust the self-signed certificate explicitly.
Docker provides documentation which describes using openssl to generate a CA and server self-signed certificates. However those instructions can lead to error messages where Docker will still not trust your self-signed certificate. Also, Openssl is not implemented the same on all systems.
The steps listed below are an alternative, reliable way to generate a self-signed server certificate for Nexus in a platform neutral manner using Java keytool. The steps can be used to configure Docker to explicitly trust the connection to Nexus Repository.
Step 1: Generate a self-signed server certificate for Nexus using keytool
First determine the host name and externally accessible IP address of Nexus Repository that Docker will use. The host name and IP address will be embedded in the self-signed certificate so that host name certificate verification will not fail.
On the Nexus Repository host, go to <AppDir>/etc/ssl directory. Generate a public private key pair for Nexus to use for any HTTPS connector.
In the command below:
- Replace ${NEXUS_DOMAIN} with the DNS name of your Nexus host. The host name must be resolvable from the host running the Docker daemon.
- Replace ${NEXUS_IP_ADDRESS} with the IP address of your Nexus host. The IP address must be routable to Nexus from the host running Docker daemon.
keytool -genkeypair -keystore keystore.jks -storepass password -keypass password -alias jetty -keyalg RSA -keysize 2048 -validity 5000 -dname "CN=*.${NEXUS_DOMAIN}, OU=Example, O=Sonatype, L=Unspecified, ST=Unspecified, C=US" -ext "SAN=DNS:${NEXUS_DOMAIN},IP:${NEXUS_IP_ADDRESS}" -ext "BC=ca:true"
You should end up with a file named keystore.jks in the local directory. The CN value specifies what is known as a wildcard certificate which will match any sub-domain.
Step 2: Configure Nexus Repository to use the self-signed server certificate
For 3.1 or newer, see Inbound SSL - Configuring to Serve Content via HTTPS
In particular, make sure the SSLContextFactory is configured to point to the generated keystore file from step 1. Make sure these setting exist in jetty-https.xml:
Repository Manager 3.0.x Example: (<AppDir>/etc/jetty-https.xml)
<Set name="KeyStorePath"><Property name="karaf.etc"/>/ssl/keystore.jks</Set> <Set name="KeyStorePassword">changeit</Set> <Set name="KeyManagerPassword">changeit</Set>
<Set name="TrustStorePassword">changeit</Set>
Repository Manager 3.1.0 and Newer Example: (<AppDir>/etc/jetty/jetty-https.xml)
<Set name="KeyStorePath"><Property name="ssl.etc"/>/keystore.jks</Set> <Set name="KeyStorePassword">changeit</Set> <Set name="KeyManagerPassword">changeit</Set>
<Set name="TrustStorePassword">changeit</Set>
Restart Nexus to pick up any configuration changes.
Step 3: Configure Docker Daemon to trust the certificate
Use Oracle java `keytool` to retrieve and print the Nexus server certificate for the Nexus Repository instance running at ${NEXUS_DOMAIN}:${SSL_PORT} :
keytool -printcert -sslserver ${NEXUS_DOMAIN}:${SSL_PORT} -rfc
This command will print an encoded certificate surrounded by -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- lines.
If you have root access to the host running Docker daemon, the best option is to trust the Nexus server certificate at a system level. Copy the printed certificate and paste it into the file locations as described below.
Docker has general advice about how a unix based system can trust a certificate and we recommend to follow their advice.
A second option is to configure only Docker to trust the certificate on per host:port basis. Docker has specific advice on where certificates can be copied in order for them to be trusted automatically per host. The disadvantage is that for each Docker registry host:port accessed, a new certificate file must be added.