XoT Testing: TLS peer certificate verification failed

[Klaus Darilion]

Hi! I want to test XoT between Bind9.20.6 primary and secondary.

On the primary I created a self-signed certificate with CN=xot-test-primary.ops.nic.at and configured bind:

# Create a 10years valid self-signed certificate:
#   openssl genpkey -algorithm RSA -out private.key -pkeyopt rsa_keygen_bits:2048
#   openssl req -new -key private.key -out request.csr -subj "/CN=xot-test-primary.ops.nic.at"
#   openssl x509 -req -days 3650 -in request.csr -signkey private.key -out certificate.crt
#   openssl x509 -text -noout -in certificate.crt
#   chmod g+r private.key
#
# Create DH-params file to enable Diffie-Hellman Perfect Forward Secrecy:
#   openssl dhparam -out dhparam.pem 4096
#
# https://bind9.readthedocs.io/en/v9.20.6/reference.html#namedconf-statement-tls
tls xot-test {
        cert-file "/etc/bind/certificate.crt";
        dhparam-file "/etc/bind/dhparam.pem";
        key-file  "/etc/bind/private.key";
};

options {
        listen-on                  { 193.46.106.51; };
        listen-on-v6               { 2a02:850:1:4::51; };
        listen-on    tls xot-test  { 193.46.106.51; };
        listen-on-v6 tls xot-test  { 2a02:850:1:4::51; };
};

That seems to work fine. Then I configured the secondary similar:
# Create a 10years valid self-signed certificate:
#   openssl genpkey -algorithm RSA -out private.key -pkeyopt rsa_keygen_bits:2048
#   openssl req -new -key private.key -out request.csr -subj "/CN=xot-test-secondary.ops.nic.at"
#   openssl x509 -req -days 3650 -in request.csr -signkey private.key -out certificate.crt
#   openssl x509 -text -noout -in certificate.crt
#   chmod g+r private.key
#
# Create DH-params file to enable Diffie-Hellman Perfect Forward Secrecy:
#   openssl dhparam -out dhparam.pem 4096
#
# https://bind9.readthedocs.io/en/v9.20.6/reference.html#namedconf-statement-tls
tls xot-test {
        #ca-file   "/etc/bind/ca.crt";          # Activating ca-file force client-certificates for incoming TLS connections
        cert-file "/etc/bind/certificate.crt";
        dhparam-file "/etc/bind/dhparam.pem";
        key-file  "/etc/bind/private.key";
        #remote-hostname "xot-test-primary.ops.nic.at";
}; // may occur multiple times

zone "test.klaus" {
    type secondary;
    file "/var/cache/bind/test.klaus";  // Path to your zone file

    primaries  {
      193.46.106.51    key "tsig-key" tls xot-test;
      2a02:850:1:4::51 key "tsig-key" tls xot-test;
    };

I copied the primary's certificate.crt to the secondary as ca.crt.

Using opportunistic TLS, zone transfer works fine.

But if I enable strict TLS, either by uncommenting 'ca-file' or 'remote-hostname' option, the TLS verification fails:

   transfer of 'test.klaus/IN' from 193.46.106.51#853: failed to connect: TLS peer certificate verification failed

But the setup on the primary looks fine. I can successfully open a TLS connection when using curl:
# curl -v https://xot-test-primary.ops.nic.at:853 --cacert ca.crt
* Host xot-test-primary.ops.nic.at:853 was resolved.
* IPv6: (none)
* IPv4: 193.46.106.51
*   Trying 193.46.106.51:853...
* Connected to xot-test-primary.ops.nic.at (193.46.106.51) port 853
* ALPN: curl offers h2,http/1.1
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
*  CAfile: ca.crt
*  CApath: /etc/ssl/certs
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384 / X25519 / RSASSA-PSS
* ALPN: server did not agree on a protocol. Uses default.
* Server certificate:
*  subject: CN=xot-test-primary.ops.nic.at
*  start date: Feb 27 14:02:56 2025 GMT
*  expire date: Feb 25 14:02:56 2035 GMT
*  common name: xot-test-primary.ops.nic.at (matched)
*  issuer: CN=xot-test-primary.ops.nic.at
*  SSL certificate verify ok.
*   Certificate level 0: Public key type RSA (2048/112 Bits/secBits), signed using sha256WithRSAEncryption


So, what am I doing wrong? Is Bind using a not-trivial TLS certificate verification? I also failed getting more verbose verification details. Any help is appreciated.

Thanks
Klaus


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