Mail: max_errors directive.

Similarly to smtpd_hard_error_limit in Postfix and smtp_max_unknown_commands
in Exim, specifies the number of errors after which the connection is closed.

src/mail/ngx_mail.h

diff --git a/src/mail/ngx_mail.h b/src/mail/ngx_mail.h
index 07104df..21178c3 100644
--- a/src/mail/ngx_mail.h
+++ b/src/mail/ngx_mail.h
@@ -115,6 +115,8 @@
     ngx_msec_t              timeout;
     ngx_msec_t              resolver_timeout;
 
+    ngx_uint_t              max_errors;
+
     ngx_str_t               server_name;
 
     u_char                 *file_name;
@@ -231,6 +233,7 @@
     ngx_uint_t              command;
     ngx_array_t             args;
 
+    ngx_uint_t              errors;
     ngx_uint_t              login_attempt;
 
     /* used to parse POP3/IMAP/SMTP command */

src/mail/ngx_mail_core_module.c

diff --git a/src/mail/ngx_mail_core_module.c b/src/mail/ngx_mail_core_module.c
index 4083124..115671c 100644
--- a/src/mail/ngx_mail_core_module.c
+++ b/src/mail/ngx_mail_core_module.c
@@ -85,6 +85,13 @@
       offsetof(ngx_mail_core_srv_conf_t, resolver_timeout),
       NULL },
 
+    { ngx_string("max_errors"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_num_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_core_srv_conf_t, max_errors),
+      NULL },
+
       ngx_null_command
 };
 
@@ -163,6 +170,8 @@
     cscf->timeout = NGX_CONF_UNSET_MSEC;
     cscf->resolver_timeout = NGX_CONF_UNSET_MSEC;
 
+    cscf->max_errors = NGX_CONF_UNSET_UINT;
+
     cscf->resolver = NGX_CONF_UNSET_PTR;
 
     cscf->file_name = cf->conf_file->file.name.data;
@@ -182,6 +191,7 @@
     ngx_conf_merge_msec_value(conf->resolver_timeout, prev->resolver_timeout,
                               30000);
 
+    ngx_conf_merge_uint_value(conf->max_errors, prev->max_errors, 5);
 
     ngx_conf_merge_str_value(conf->server_name, prev->server_name, "");
 

src/mail/ngx_mail_handler.c

diff --git a/src/mail/ngx_mail_handler.c b/src/mail/ngx_mail_handler.c
index 57503e9..246ba97 100644
--- a/src/mail/ngx_mail_handler.c
+++ b/src/mail/ngx_mail_handler.c
@@ -874,7 +874,20 @@
         return NGX_MAIL_PARSE_INVALID_COMMAND;
     }
 
-    if (rc == NGX_IMAP_NEXT || rc == NGX_MAIL_PARSE_INVALID_COMMAND) {
+    if (rc == NGX_MAIL_PARSE_INVALID_COMMAND) {
+
+        s->errors++;
+
+        if (s->errors >= cscf->max_errors) {
+            ngx_log_error(NGX_LOG_INFO, c->log, 0,
+                          "client sent too many invalid commands");
+            s->quit = 1;
+        }
+
+        return rc;
+    }
+
+    if (rc == NGX_IMAP_NEXT) {
         return rc;
     }