IMAP - can not chose compatible AUTH backend

Start at the user_external IMAP backend and its CURL authentication setup, using the reported IMAP server capability response and shell commands as the reproduction case. The work is done when a compatible non-GSSAPI authentication backend can be selected or used successfully without disabling GSSAPI on the server.

Steps to reproduce

1. Setup IMAP auth with TLS enabled to server with AUTH=GSSAPI CAPABLITY

Expected behaviour

You can login to NC

Actual behaviour

No login - error message in log:
[user_external][3] ERROR: Could not connect to imap server via curl: Operation timed out after 10001 milliseconds with 0 out of 0 bytes received

test from Linux shell

# curl --basic -v imaps://email.example.com --user '[email protected]'
Enter host password for user '[email protected]':
*   Trying xx8.yy5.zz4.aa6:993...
* TCP_NODELAY set
* Connected to email.example.com (xx8.yy5.zz4.aa6) port 993 (#0)
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* 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 handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* Server certificate:
*  subject: CN=email.example.com
*  start date: Jun 17 02:22:59 2020 GMT
*  expire date: Sep 18 02:22:59 2020 GMT
*  subjectAltName: host "email.example.com" matched cert's "email.example.com"
*  issuer: C=US; O=Let's Encrypt; CN=Let's Encrypt Authority X3
*  SSL certificate verify ok.
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* old SSL session ID is stale, removing
< * OK Server 1 IMAP4rev1 Thu, 09 Jul 2020 11:08:40 +0200
> A001 CAPABILITY
< * CAPABILITY IMAP4rev1 AUTH=PLAIN AUTH=LOGIN AUTH=DIGEST-MD5 AUTH=CRAM-MD5 AUTH=NTLM AUTH=GSSAPI SORT THREAD=ORDEREDSUBJECT UIDPLUS QUOTA ACL NAMESPACE CHILDREN IDLE ID UNSELECT METADATA MULTISEARCH ESEARCH XLIST CREATE-SPECIAL-USE 
< A001 OK CAPABILITY Completed
> A002 AUTHENTICATE GSSAPI
< +
* gss_init_sec_context() failed: No Kerberos credentials available (default cache: FILE:/tmp/krb5cc_0). 
* Closing connection 0
* TLSv1.3 (OUT), TLS alert, close notify (256):
curl: (94) An authentication function returned an error

Problem is that this Linux server is (and will never be) used in Kerberos environment ...

I had to switch off GSSAPI auth backend on IMAP server, because there is not any way how to tell
user_external/CURL not to use it.
After it it works like charm ...*
BTW in owncloud with the old way to specify user_external config it still works

# curl --basic -v imaps://email.example.com --login-options "AUTH=PLAIN" --user '[email protected]'
Enter host password for user '[email protected]':
*   Trying 192.0.2.0:993...
* TCP_NODELAY set
* Connected to email.example.com (192.0.2.0) port 993 (#0)
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* 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 handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* Server certificate:
*  subject: CN=email.example.com
*  start date: Mar 16 07:11:09 2022 GMT
*  expire date: Jun 14 07:11:08 2022 GMT
*  subjectAltName: host "email.example.com" matched cert's "email.example.com"
*  issuer: C=US; O=Let's Encrypt; CN=R3
*  SSL certificate verify ok.
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* old SSL session ID is stale, removing
< * OK IMAP Server
> A001 CAPABILITY
< * CAPABILITY IMAP4rev1 AUTH=PLAIN AUTH=LOGIN AUTH=NTLM AUTH=GSSAPI UIDPLUS QUOTA ACL NAMESPACE CHILDREN IDLE ID UNSELECT METADATA MULTISEARCH ESEARCH XLIST CREATE-SPECIAL-USE 
< A001 OK CAPABILITY Completed
> A002 AUTHENTICATE PLAIN
< + 
> 123456789qwertzuiopasdfghjklyxcvbnm
< A002 OK AUTHENTICATE Completed
> A003 LIST "" *
.
.
.
< A003 OK LIST Completed
* Connection #0 to host email.example.com left intact

Possible solutions:

• enable choosing auth backend in user_external (CURLOPT_HTTPAUTH The HTTP authentication method(s) to use. The options are: CURLAUTH_BASIC, CURLAUTH_DIGEST, CURLAUTH_GSSNEGOTIATE, CURLAUTH_NTLM, CURLAUTH_ANY, and CURLAUTH_ANYSAFE. )
• try to auth with other auth backends too ...

Affected Authentication backend

IMAP and server with GSSAPI

Server configuration

User External App version: 2.1.0

Operating system: Ubuntu 20.04 LTS

Web server: Apache2 2.4.41

Database: Postgresql 12.9

PHP version: 7.4.3

Nextcloud version: 23.0.3

Updated from an older Nextcloud/ownCloud or fresh install: fresh

Where did you install Nextcloud from: nextcloud tar file

Signing status:

NO integrit URL ...

List of activated apps:

Enabled:
  - admin_audit: 1.13.0
  - bruteforcesettings: 2.4.0
  - cloud_federation_api: 1.6.0
  - comments: 1.13.0
  - dav: 1.21.0
  - federatedfilesharing: 1.13.0
  - files: 1.18.0
  - files_rightclick: 1.2.0
  - files_sharing: 1.15.0
  - files_trashbin: 1.13.0
  - files_versions: 1.16.0
  - files_videoplayer: 1.12.0
  - limit_login_to_ip: 3.1.0
  - logreader: 2.8.0
  - lookup_server_connector: 1.11.0
  - notifications: 2.11.1
  - oauth2: 1.11.0
  - provisioning_api: 1.13.0
  - serverinfo: 1.13.0
  - settings: 1.5.0
  - sharebymail: 1.13.0
  - twofactor_backupcodes: 1.12.0
  - updatenotification: 1.13.0
  - user_external: 2.1.0
  - viewer: 1.7.0
  - workflowengine: 2.5.0
Disabled:
  - accessibility: 1.5.0
  - activity: 2.12.0
  - circles: 23.1.0
  - contactsinteraction: 1.0.0
  - dashboard: 7.0.0
  - encryption
  - federation: 1.9.0
  - files_external
  - files_pdfviewer: 1.8.0
  - firstrunwizard: 2.8.0
  - nextcloud_announcements: 1.8.0
  - password_policy: 1.9.1
  - photos: 1.1.0
  - privacy: 1.3.0
  - recommendations: 0.7.0
  - support: 1.2.1
  - survey_client: 1.7.0
  - systemtags: 1.9.0
  - text: 3.0.1
  - theming: 1.10.0
  - user_ldap
  - user_status: 1.0.1
  - weather_status: 1.0.0

Nextcloud configuration:

# sudo -u www-data php occ config:list system
{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "cloud.example.com"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "version": "19.0.0.12",
        "overwrite.cli.url": "https:\/\/cloud.example.com",
        "htaccess.RewriteBase": "\/",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "mail_smtpmode": "smtp",
        "mail_smtpsecure": "tls",
        "mail_sendmailmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "user_backends": [
            {
                "class": "OC_User_IMAP",
                "arguments": [
                    "email.example.com",
                    993,
                    true
                ]
            }
        ],
        "loglevel": 3,
        "logfile": "\/var\/log\/nextcloud.log",
        "default_language": "cs_CZ",
        "updatechecker": true,
        "filelocking.enabled": true,
        "log_type": "errorlog",
        "trashbin_retention_obligation": "30, auto"
    }
}