Mail: client SSL certificates support.

The "ssl_verify_client", "ssl_verify_depth", "ssl_client_certificate",
"ssl_trusted_certificate", and "ssl_crl" directives introduced to control
SSL client certificate verification in mail proxy module.

If there is a certificate, detail of the certificate are passed to
the auth_http script configured via Auth-SSL-Verify, Auth-SSL-Subject,
Auth-SSL-Issuer, Auth-SSL-Serial, Auth-SSL-Fingerprint headers.  If
the auth_http_pass_client_cert directive is set, client certificate
in PEM format will be passed in the Auth-SSL-Cert header (urlencoded).

If there is no required certificate provided during an SSL handshake
or certificate verification fails then a protocol-specific error is
returned after the SSL handshake and the connection is closed.

Based on previous work by Sven Peter, Franck Levionnois and Filipe Da Silva.

src/mail/ngx_mail_ssl_module.c

diff --git a/src/mail/ngx_mail_ssl_module.c b/src/mail/ngx_mail_ssl_module.c
index f864d99..7dc642a 100644
--- a/src/mail/ngx_mail_ssl_module.c
+++ b/src/mail/ngx_mail_ssl_module.c
@@ -46,6 +46,15 @@
 };
 
 
+static ngx_conf_enum_t  ngx_mail_ssl_verify[] = {
+    { ngx_string("off"), 0 },
+    { ngx_string("on"), 1 },
+    { ngx_string("optional"), 2 },
+    { ngx_string("optional_no_ca"), 3 },
+    { ngx_null_string, 0 }
+};
+
+
 static ngx_command_t  ngx_mail_ssl_commands[] = {
 
     { ngx_string("ssl"),
@@ -146,6 +155,41 @@
       offsetof(ngx_mail_ssl_conf_t, session_timeout),
       NULL },
 
+    { ngx_string("ssl_verify_client"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_enum_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_ssl_conf_t, verify),
+      &ngx_mail_ssl_verify },
+
+    { ngx_string("ssl_verify_depth"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_num_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_ssl_conf_t, verify_depth),
+      NULL },
+
+    { ngx_string("ssl_client_certificate"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_str_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_ssl_conf_t, client_certificate),
+      NULL },
+
+    { ngx_string("ssl_trusted_certificate"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_str_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_ssl_conf_t, trusted_certificate),
+      NULL },
+
+    { ngx_string("ssl_crl"),
+      NGX_MAIL_MAIN_CONF|NGX_MAIL_SRV_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_str_slot,
+      NGX_MAIL_SRV_CONF_OFFSET,
+      offsetof(ngx_mail_ssl_conf_t, crl),
+      NULL },
+
       ngx_null_command
 };
 
@@ -198,6 +242,9 @@
      *     scf->certificate_key = { 0, NULL };
      *     scf->dhparam = { 0, NULL };
      *     scf->ecdh_curve = { 0, NULL };
+     *     scf->client_certificate = { 0, NULL };
+     *     scf->trusted_certificate = { 0, NULL };
+     *     scf->crl = { 0, NULL };
      *     scf->ciphers = { 0, NULL };
      *     scf->shm_zone = NULL;
      */
@@ -206,6 +253,8 @@
     scf->starttls = NGX_CONF_UNSET_UINT;
     scf->passwords = NGX_CONF_UNSET_PTR;
     scf->prefer_server_ciphers = NGX_CONF_UNSET;
+    scf->verify = NGX_CONF_UNSET_UINT;
+    scf->verify_depth = NGX_CONF_UNSET_UINT;
     scf->builtin_session_cache = NGX_CONF_UNSET;
     scf->session_timeout = NGX_CONF_UNSET;
     scf->session_tickets = NGX_CONF_UNSET;
@@ -238,6 +287,9 @@
                          (NGX_CONF_BITMASK_SET|NGX_SSL_SSLv3|NGX_SSL_TLSv1
                           |NGX_SSL_TLSv1_1|NGX_SSL_TLSv1_2));
 
+    ngx_conf_merge_uint_value(conf->verify, prev->verify, 0);
+    ngx_conf_merge_uint_value(conf->verify_depth, prev->verify_depth, 1);
+
     ngx_conf_merge_str_value(conf->certificate, prev->certificate, "");
     ngx_conf_merge_str_value(conf->certificate_key, prev->certificate_key, "");
 
@@ -248,6 +300,12 @@
     ngx_conf_merge_str_value(conf->ecdh_curve, prev->ecdh_curve,
                          NGX_DEFAULT_ECDH_CURVE);
 
+    ngx_conf_merge_str_value(conf->client_certificate,
+                         prev->client_certificate, "");
+    ngx_conf_merge_str_value(conf->trusted_certificate,
+                         prev->trusted_certificate, "");
+    ngx_conf_merge_str_value(conf->crl, prev->crl, "");
+
     ngx_conf_merge_str_value(conf->ciphers, prev->ciphers, NGX_DEFAULT_CIPHERS);
 
 
@@ -320,6 +378,35 @@
         return NGX_CONF_ERROR;
     }
 
+    if (conf->verify) {
+
+        if (conf->client_certificate.len == 0 && conf->verify != 3) {
+            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
+                          "no ssl_client_certificate for ssl_client_verify");
+            return NGX_CONF_ERROR;
+        }
+
+        if (ngx_ssl_client_certificate(cf, &conf->ssl,
+                                       &conf->client_certificate,
+                                       conf->verify_depth)
+            != NGX_OK)
+        {
+            return NGX_CONF_ERROR;
+        }
+
+        if (ngx_ssl_trusted_certificate(cf, &conf->ssl,
+                                        &conf->trusted_certificate,
+                                        conf->verify_depth)
+            != NGX_OK)
+        {
+            return NGX_CONF_ERROR;
+        }
+
+        if (ngx_ssl_crl(cf, &conf->ssl, &conf->crl) != NGX_OK) {
+            return NGX_CONF_ERROR;
+        }
+    }
+
     if (SSL_CTX_set_cipher_list(conf->ssl.ctx,
                                 (const char *) conf->ciphers.data)
         == 0)