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Got a query about self-signed certificates that after doing several searches I don't feel I've got a concrete answer for.

Say I have generated a self-signed server certificate with CN=localhost. Does this mean that I can use that certificate in a server and be able to run that server on any machine in a LAN, where any client on the network with the certificate public key can communicate with the server (i.e. the server listens to any IP)?

As an example, I used the following script to generate certificates for use in a mutual TLS scenario (based on this answer):

echo Generate CA key:
openssl genrsa -passout pass:1111 -aes256 -out ca.key 4096

echo Generate CA certificate:
openssl req -passin pass:1111 -new -x509 -days 36500 -key ca.key -out ca.crt -subj  "/C=UK/ST=UK/L=London/O=YourCompany/OU=YourApp/CN=MyRootCA"

echo Generate server key:
openssl genrsa -passout pass:1111 -aes256 -out server.key 4096

echo Generate server signing request:
openssl req -passin pass:1111 -new -key server.key -out server.csr -subj  "/C=UK/ST=UK/L=London/O=YourCompany/OU=YourApp/CN=localhost"

echo Self-sign server certificate:
openssl x509 -req -passin pass:1111 -days 36500 -in server.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out server.crt

echo Remove passphrase from server key:
openssl rsa -passin pass:1111 -in server.key -out server.key

echo Generate client key
openssl genrsa -passout pass:1111 -aes256 -out client.key 4096

echo Generate client signing request:
openssl req -passin pass:1111 -new -key client.key -out client.csr -subj  "/C=UK/ST=UK/L=London/O=YourCompany/OU=YourApp/CN=localhost"

echo Self-sign client certificate:
openssl x509 -passin pass:1111 -req -days 36500 -in client.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out client.crt

echo Remove passphrase from client key:
openssl rsa -passin pass:1111 -in client.key -out client.key

What I am finding is that the server loads fine on some machines, however on other machines it fails to start, reporting that it could not bind to the port. I have checked that the port is definitely not being used by anything. Also the server starts fine if I don't use any certificates.

Am I doing something specifically wrong in the script, or is it not possible to have a certificate with CN=localhost in a server that should be able to wrong on any machine in a local network, and accept connections from any client on the network that trusts the public key?

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  • The certificate and the error that it cannot bind to a port have nothing to do with each other. This error has nothing to security too. And nothing is known about how the bindings of the unknown server change if you don't use certificates. Feb 19, 2020 at 22:12
  • @SteffenUllrich I have read the linked question, however I still don't think it answers my specific scenario. Ignoring the error I mentioned, is it possible to generate a self signed certificate and use it on any machine in a local network environment, and be able to connect to the server from another machine on the network? I am only using IP address/machine names to connect to the server.
    – Kakalokia
    Feb 19, 2020 at 22:33
  • A certificate is not bound to a machine. It is bound to a specific domain (or multiple domains) using the subject. To cite from the answer in the question I've linked to: "The key thing is that the certificate can only be used for the sites that it identifies itself as being valid for.". So, if you use it on all machines only to access localhost then it will work. Note though that at least Chrome will no longer accept the subject in CN but will expect it in SAN, which is a different topic. Feb 19, 2020 at 22:42
  • @SteffenUllrich One thing I can see with the server library I am using (gRPC) is that I can override the SSL target name used for host verification. If I set the certificate CN to localhost, and on the client override the target name with localhost as well, does that allow the scenario I'm describing? I have tried it and it works, however I am trying to confirm my theory in case I am missing an important piece.
    – Kakalokia
    Feb 19, 2020 at 22:42
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    As long as the server provides a certificate with the subject the client expects it will work. If the client can be made to expect a different name then the one from the URL you can do the things you describe. But it is very likely a bad idea to use the same certificate everywhere since if the private key is compromised on one machine it will be implicitly compromised on all. Feb 19, 2020 at 22:45

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