Thank you for your reply.
But I need more help.

How do I set request parameter of https://www.googleapis.com/oauth2/v2/userinfo？

Logs：

dovecot[30307]: lmtp(30320): Connect from 10.243.148.174
dovecot[30307]: lmtp(30320): Disconnect from 10.243.148.174: Remote closed connection (state=READY)
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: Host created
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: Host session created
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: Need to perform DNS lookup
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: Performing asynchronous DNS lookup
dovecot[30307]: auth: Debug: http-client[1]: request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]: Submitted (requests left=1)
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: DNS lookup successful; got 20 IPs
dovecot[30307]: auth: Debug: http-client: peer 172.217.31.170:443 (shared): Peer created
dovecot[30307]: auth: Debug: http-client: peer 172.217.31.170:443: Peer pool created
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Peer created
dovecot[30307]: auth: Debug: http-client[1]: queue https://www.googleapis.com:443: Setting up connection to 172.217.31.170:443 (SSL=www.googleapis.com) (1 requests pending)
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Linked queue https://www.googleapis.com:443 (1 queues linked)
dovecot[30307]: auth: Debug: http-client[1]: queue https://www.googleapis.com:443: Started new connection to 172.217.31.170:443 (SSL=www.googleapis.com)
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Creating 1 new connections to handle requests (already 0 usable, connecting to 0, closing 0)
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Making new connection 1 of 1 (0 connections exist, 0 pending)
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Connecting
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Waiting for connect (fd=22) to finish for max 0 msecs
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: HTTPS connection created (1 parallel connections exist)
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Client connected (fd=22)
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Connected
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Starting SSL handshake
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x10, ret=1: before/connect initialization
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: before/connect initialization
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv2/v3 write client hello A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv2/v3 read server hello A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv2/v3 read server hello A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv2/v3 read server hello A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 read server hello A
dovecot[30307]: auth: Received valid SSL certificate: /OU=GlobalSign Root CA - R2/O=GlobalSign/CN=GlobalSign
dovecot[30307]: auth: Received valid SSL certificate: /C=US/O=Google Trust Services/CN=GTS CA 1O1
dovecot[30307]: auth: Received valid SSL certificate: /C=US/ST=California/L=Mountain View/O=Google LLC/CN=upload.video.google.com
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 read server certificate A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 read server key exchange A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 read server done A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 write client key exchange A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 write change cipher spec A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 write finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 flush data
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv3 read finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv3 read finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv3 read finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=-1: SSLv3 read finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1001, ret=1: SSLv3 read finished A
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x20, ret=1: SSL negotiation finished successfully
dovecot[30307]: auth: Debug: www.googleapis.com: SSL: where=0x1002, ret=1: SSL negotiation finished successfully
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: SSL handshake successful
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Ready for requests
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Successfully connected (1 connections exist, 0 pending)
dovecot[30307]: auth: Debug: http-client: peer 172.217.31.170:443: Successfully connected (1 connections exist, 0 pending)
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: Using 1 idle connections to handle 1 requests (1 total connections ready)
dovecot[30307]: auth: Debug: http-client[1]: queue https://www.googleapis.com:443: Connection to peer 172.217.31.170:443 claimed request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Claimed request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]
dovecot[30307]: auth: Debug: http-client[1]: request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]: Sent header
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: No more requests to service for this peer (1 connections exist, 0 pending)
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Got 401 response for request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]: Unauthorized (took 46 ms + 59 ms in queue)
dovecot[30307]: auth: Error: oauth2(fukudata,118.103.29.199,<mgm9vz25BTZ2Zx3H>): oauth2 failed: No username returned
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: Response payload stream destroyed (0 ms after initial response)
dovecot[30307]: auth: Debug: http-client[1]: request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]: Finished
dovecot[30307]: auth: Debug: http-client[1]: queue https://www.googleapis.com:443: Dropping request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]
dovecot[30307]: auth: Debug: http-client: host www.googleapis.com: Host is idle (timeout = 1799906 msecs)
dovecot[30307]: auth: Debug: http-client[1]: request [Req1: GET https://www.googleapis.com/oauth2/v2/userinfo]: Free (requests left=1)
dovecot[30307]: auth: Debug: http-client[1]: peer 172.217.31.170:443: No requests to service for this peer (1 connections exist, 0 pending)
dovecot[30307]: auth: Debug: http-client: conn 172.217.31.170:443 [1]: No more requests queued; going idle (timeout = 60000 msecs)
dovecot[30307]: lmtp(30309): Connect from 10.243.148.174
dovecot[30307]: lmtp(30309): Disconnect from 10.243.148.174: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30320): Connect from 10.243.148.174
dovecot[30307]: lmtp(30320): Disconnect from 10.243.148.174: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30320): Connect from 10.243.148.174
dovecot[30307]: lmtp(30320): Disconnect from 10.243.148.174: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30309): Connect from 10.243.148.174
dovecot[30307]: lmtp(30309): Disconnect from 10.243.148.174: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30309): Connect from 10.243.148.110
dovecot[30307]: lmtp(30309): Disconnect from 10.243.148.110: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30309): Connect from 10.243.148.110
dovecot[30307]: lmtp(30309): Disconnect from 10.243.148.110: Remote closed connection (state=READY)
dovecot[30307]: lmtp(30309): Connect from 10.243.148.110
dovecot[30307]: lmtp(30309): Disconnect from 10.243.148.110: Remote closed connection (state=READY)
sshd[30475]: Connection closed by 10.243.150.20 port 48174 [preauth]
dovecot[30307]: imap-login: Disconnected (auth service reported temporary failure): user=<fukudata>, method=PLAIN, rip=118.103.29.199, lip=10.243.150.190, session=<mgm9vz25BTZ2Zx3H>
dovecot[30307]: lmtp(30317): Connect from 10.243.148.174
dovecot[30307]: lmtp(30317): Disconnect from 10.243.148.174: Remote closed connection (state=READY)

I would appreciate your reply.

Yours faithfully,

2021年1月19日(火) 15:34 Aki Tuomi <aki.tuomi@open-xchange.com>:

> On 19/01/2021 07:17 福田泰葵 <taiki.fukuda@justsystems.com> wrote:
>
>
> Dear Sir or Madam
> Unable to build OAuth2.0 authentication to Gmail using dovecot as proxy.
> I have a question about how to use dovecot as a proxy to perform OAuth 2.0 authentication to Gmail using a mail client.

Mail client is required, in this case, to provide valid oauth2 bearer token. I don't think google supports other ways.

>   1. Is the following all I need to do to authenticate to Gmail using dovecot as a proxy?
>   * passdb
>   passdb {
>   driver = oauth2
>   mechanisms = oauthbearer xoauth2
>   args = /etc/dovecot/dovecot-oauth2.token.conf.ext
>   } 
>   passdb {
>   driver = oauth2
>   mechanisms = plain login
>   args = /etc/dovecot/dovecot-oauth2.plain.conf.ext
>   }
>   

The plain config is a way to do 'password grant' authentication. This is when username and password is used to acquire a bearer token.

>   * create dovecot-oauth2.token.conf.ext and dovecot-oauth2.plain.conf.ext
>   * create gmail service account api
>   2. grant_url in dovecot-oauth2.token.conf.ext and dovecot-oauth2.plain.conf.ext is URL for obtaining a Google access token for a web server that I have built myself?
>   3. I use a Gmail service account, so I don’t need a client ID and secret ID, right?
>   4. Do I set introspection_url to the URL of my own web server with the access token used for authentication to Google as the response?

No. The introspection URL needs to point to a location where dovecot can figure out more information about the user with token. If I recall correctly, the token endpoint

For gmail, you need to use https://www.googleapis.com/oauth2/v2/userinfo

>   5. The documentation says “pass_attrs = host=127.0.0.1”, but if you are authenticating to Gmail, I should use
>   “pass_attrs = proxy=y host=%{if;%s;eq;imap;imap.gmail.com (http://imap.gmail.com);%{if;%s;eq;pop3;smtp .gmail.com (http://gmail.com);pop.gmail.com (http://pop.gmail.com)}} port=%{if;%s;eq;imap;993;%{if;%s;eq;pop3;587;465}} proxy_mech=xoauth2 pass=%{oauth2:access_token} user=%{oauth2:email oauth2:email}”?

I would use something more readable, like passwd-file driver with username_format=%s

The access token is also imported as %{token} in passdb.

>   6. What is the difference between dovecot-oauth2.token.conf.ext and dovecot-oauth2.plain.conf.ext ? Do I need to configure both?
> I used https://doc.dovecot.org/configuration_manual/authentication/oauth2/#proxy as a reference.
> I would appreciate your reply.
> Yours faithfully,
> ------------------------------
> e-mail: taiki.fukuda@justsystems.com
> TEL: 03-5324-7900
> mobile: 080-6198-7328
> ------------------------------

So this might work

/etc/dovecot/oauth2-token.conf.ext

introspection_url = https://www.googleapis.com/oauth2/v2/userinfo
introspection_mode = auth
username_attribute = email
pass_attrs = proxy=y proxy_mech=xoauth2

/etc/dovecot/dovecot.conf

auth_mechanisms = xoauth2 oauthbearer

passdb {
  driver = oauth2
  args = /etc/dovecot/oauth2-token.conf.ext
  result_success = continue-ok
}

passdb {
  driver = passwd-file
  args = username_format=%s /etc/dovecot/endpoints
  skip = unauthenticated
}

/etc/dovecot/endpoints

imap::::::: host=imap.gmail.com
pop3::::::: host=pop3.gmail.com
submission::::::: host=smtp.gmail.com

Aki