mail proxy: allow dynamically deciding whether to use SMTP authentication to the backend

Adds a `Auth-Method` as a valid response header for the HTTP
authentication server. When set to 'None', the proxy will not
attempt to perform SMTP authentication to the backend, even
when `proxy_smtp_auth` is `on`. This is useful when some backends
require authentication, while others don't allow it.

While this has been tested and works, the patch was created with an
incomplete understanding of the `proxy_smtp_auth` option, so it might
not be the cleanest implementation. Sharing as 'draft' for now until
I find the time to set up a better test environment and revise the
patch. Any feedback/pointers/etc welcome :)

Fixes #155

src/mail/ngx_mail.h

@@ -212,6 +212,7 @@ typedef struct {
     unsigned                starttls:1;
     unsigned                esmtp:1;
     unsigned                auth_method:3;
+    unsigned                proxy_auth_method:3;
     unsigned                auth_wait:1;
 
     ngx_str_t               login;

src/mail/ngx_mail_auth_http_module.c

@@ -677,6 +677,23 @@ ngx_mail_auth_http_process_headers(ngx_mail_session_t *s,
                 continue;
             }
 
+            if (len == sizeof("Auth-Method") - 1
+                && ngx_strncasecmp(ctx->header_name_start,
+                                   (u_char *) "Auth-Method",
+                                   sizeof("Auth-Method") - 1)
+                   == 0)
+            {
+                int value_len = ctx->header_end - ctx->header_start;
+                if (value_len == sizeof("none") - 1
+                    && ngx_strncasecmp(ctx->header_start,
+                                       (u_char *) "none",
+                                       sizeof("none") - 1)
+                       == 0)
+                {
+                    s->proxy_auth_method = NGX_MAIL_AUTH_NONE;
+                }
+            }
+
             /* ignore other headers */
 
             continue;
@@ -883,6 +900,7 @@ ngx_mail_auth_sleep_handler(ngx_event_t *rev)
 
         s->mail_state = 0;
         s->auth_method = NGX_MAIL_AUTH_PLAIN;
+        s->proxy_auth_method = NGX_MAIL_AUTH_PLAIN;
 
         c->log->action = "in auth state";
 

src/mail/ngx_mail_handler.c

@@ -714,6 +714,7 @@ ngx_mail_auth_cram_md5(ngx_mail_session_t *s, ngx_connection_t *c)
                    "mail auth cram-md5: \"%V\" \"%V\"", &s->login, &s->passwd);
 
     s->auth_method = NGX_MAIL_AUTH_CRAM_MD5;
+    s->proxy_auth_method = NGX_MAIL_AUTH_CRAM_MD5;
 
     return NGX_DONE;
 }
@@ -748,6 +749,7 @@ ngx_mail_auth_external(ngx_mail_session_t *s, ngx_connection_t *c,
                    "mail auth external: \"%V\"", &s->login);
 
     s->auth_method = NGX_MAIL_AUTH_EXTERNAL;
+    s->proxy_auth_method = NGX_MAIL_AUTH_EXTERNAL;
 
     return NGX_DONE;
 }

src/mail/ngx_mail_pop3_handler.c

@@ -464,6 +464,7 @@ ngx_mail_pop3_apop(ngx_mail_session_t *s, ngx_connection_t *c)
                    "pop3 apop: \"%V\" \"%V\"", &s->login, &s->passwd);
 
     s->auth_method = NGX_MAIL_AUTH_APOP;
+    s->proxy_auth_method = NGX_MAIL_AUTH_APOP;
 
     return NGX_DONE;
 }

src/mail/ngx_mail_proxy_module.c

@@ -605,8 +605,12 @@ ngx_mail_proxy_smtp_handler(ngx_event_t *rev)
         if (pcf->xclient) {
             s->mail_state = ngx_smtp_helo_xclient;
 
-        } else if (s->auth_method == NGX_MAIL_AUTH_NONE) {
+        } else if (s->proxy_auth_method == NGX_MAIL_AUTH_NONE) {
+            if (s->smtp_from.len) {
                 s->mail_state = ngx_smtp_helo_from;
+            } else {
+                s->mail_state = ngx_smtp_helo;
+            }
 
         } else if (pcf->smtp_auth) {
             s->mail_state = ngx_smtp_helo_auth;
@@ -667,8 +671,12 @@ ngx_mail_proxy_smtp_handler(ngx_event_t *rev)
         if (s->smtp_helo.len) {
             s->mail_state = ngx_smtp_xclient_helo;
 
-        } else if (s->auth_method == NGX_MAIL_AUTH_NONE) {
+        } else if (s->proxy_auth_method == NGX_MAIL_AUTH_NONE) {
+            if (s->smtp_from.len) {
                 s->mail_state = ngx_smtp_xclient_from;
+            } else {
+                s->mail_state = ngx_smtp_xclient_helo;
+            }
 
         } else if (pcf->smtp_auth) {
             s->mail_state = ngx_smtp_xclient_auth;
@@ -700,8 +708,12 @@ ngx_mail_proxy_smtp_handler(ngx_event_t *rev)
 
         pcf = ngx_mail_get_module_srv_conf(s, ngx_mail_proxy_module);
 
-        if (s->auth_method == NGX_MAIL_AUTH_NONE) {
+        if (s->proxy_auth_method == NGX_MAIL_AUTH_NONE) {
+            if (s->smtp_from.len) {
                 s->mail_state = ngx_smtp_helo_from;
+            } else {
+                s->mail_state = ngx_smtp_helo;
+            }
 
         } else if (pcf->smtp_auth) {
             s->mail_state = ngx_smtp_helo_auth;

src/mail/ngx_mail_smtp_handler.c

@@ -820,6 +820,7 @@ ngx_mail_smtp_rcpt(ngx_mail_session_t *s, ngx_connection_t *c)
                    "smtp rcpt to:\"%V\"", &s->smtp_to);
 
     s->auth_method = NGX_MAIL_AUTH_NONE;
+    s->proxy_auth_method = NGX_MAIL_AUTH_NONE;
 
     return NGX_DONE;
 }