Problem:

I’ve created a self-signed certificate for foo.localhost using a Let’s Encrypt recommendation using this Makefile:

include ../.env

configuration = csr.cnf
certificate = self-signed.crt
key = self-signed.key

.PHONY: all
all: $(certificate)

$(certificate): $(configuration)
    openssl req -x509 -out $@ -keyout $(key) -newkey rsa:2048 -nodes -sha256 -subj '/CN=$(HOSTNAME)' -extensions EXT -config $(configuration)

$(configuration):
    printf "[dn]\nCN=$(HOSTNAME)\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:$(HOSTNAME)\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth" > $@

.PHONY: clean
clean:
    $(RM) $(configuration)

I’ve then assigned that to a web server. I’ve verified that the server returns the relevant certificate:

$ openssl s_client -showcerts -connect foo.localhost:8443 < /dev/null
CONNECTED(00000003)
depth=0 CN = foo.localhost
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = foo.localhost
verify error:num=21:unable to verify the first certificate
verify return:1
---
Certificate chain
 0 s:/CN=foo.localhost
   i:/CN=foo.localhost
-----BEGIN CERTIFICATE-----
[…]
-----END CERTIFICATE-----
---
Server certificate
subject=/CN=foo.localhost
issuer=/CN=foo.localhost
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 1330 bytes and written 269 bytes
Verification error: unable to verify the first certificate
---
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256
    Session-ID: […]
    Session-ID-ctx: 
    Master-Key: […]
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket:
    […]

    Start Time: 1529622990
    Timeout   : 7200 (sec)
    Verify return code: 21 (unable to verify the first certificate)
    Extended master secret: no
---
DONE

How do I make cURL trust it without modifying anything in /etc? --cacert does not work, presumably because there is no CA:

$ curl --cacert tls/foo.localhost.crt 'https://foo.localhost:8443/'
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.haxx.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.

The goal is to enable HTTPS during development:

• I can’t have a completely production-like certificate without a lot of work to enable DNS verification in all development environments. Therefore I have to use a self-signed certificate.
• I still obviously want to make my development environment as similar as possible to production, so I can’t simply ignore any and all certificate issues. curl -k is like catch (Exception e) {}in this case – nothing at all like a browser talking to a web server.

In other words, when running curl [something] https://project.local/api/foo I want to be confident that

1. if TLS is configured properly except for having a self-signed certificate the command will succeed and
2. if I have any issues with my TLS configuration except for having a self-signed certificate the command will fail.

Using HTTP or --insecure fails the second criterion.

Solution:

Following these steps should solve your issue:

1. Download and save the self-signed certificate: echo quit | openssl s_client -showcerts -servername "${API_HOST}" -connect "${API_HOST}":443 > cacert.pem
2. Tell the curl client about it: curl --cacert cacert.pem --location --silent https://${API_HOST}

Also one could use wget and ignore certificates with: wget --no-check-certificate https://${API_HOST}