feature: clienthello.set_ciphers().

Sets the connection's TLS <= 1.2 cipher list and/or TLS 1.3 ciphersuites
from ssl_client_hello_by_lua*, e.g. keyed on the client hello server name.

On failure the connection's SSL object is left exactly as it was --
cipher list, ciphersuites, and error queue all untouched -- so the
handshake proceeds with the default list inherited from the server block
(ssl_ciphers/ssl_conf_command).

Requires ngx_http_lua_ffi_ssl_set_ciphers() from lua-nginx-module.

Author: climagabriel

lib/ngx/ssl/clienthello.lua

@@ -32,6 +32,7 @@ local ngx_lua_ffi_ssl_get_client_hello_ext
 local ngx_lua_ffi_ssl_set_protocols
 local ngx_lua_ffi_ssl_get_client_hello_ext_present
 local ngx_lua_ffi_ssl_get_client_hello_ciphers
+local ngx_lua_ffi_ssl_set_ciphers
 
 
 if subsystem == 'http' then
@@ -52,6 +53,9 @@ if subsystem == 'http' then
     int ngx_http_lua_ffi_ssl_get_client_hello_ciphers(ngx_http_request_t *r,
         unsigned short *ciphers,  size_t ciphers_len, char **err);
         /* Undefined for the stream subsystem */
+    int ngx_http_lua_ffi_ssl_set_ciphers(ngx_http_request_t *r,
+        const char *ciphers, const char *ciphersuites, char **err);
+        /* Undefined for the stream subsystem */
     ]]
 
     ngx_lua_ffi_ssl_get_client_hello_server_name =
@@ -63,6 +67,7 @@ if subsystem == 'http' then
         C.ngx_http_lua_ffi_ssl_get_client_hello_ext_present
     ngx_lua_ffi_ssl_get_client_hello_ciphers =
         C.ngx_http_lua_ffi_ssl_get_client_hello_ciphers
+    ngx_lua_ffi_ssl_set_ciphers = C.ngx_http_lua_ffi_ssl_set_ciphers
 
 
 
@@ -338,4 +343,34 @@ function _M.set_protocols(protocols)
     return nil, ffi_str(errmsg[0])
 end
 
+
+-- return ok, err
+-- on failure the connection's cipher list, ciphersuites and error queue
+-- are left untouched, so the handshake proceeds on the server defaults
+function _M.set_ciphers(ciphers, ciphersuites)
+    local r = get_request()
+    if not r then
+        error("no request found")
+    end
+
+    if ngx_phase() ~= "ssl_client_hello" then
+        error("API disabled in the current context")
+    end
+
+    if ciphers ~= nil and type(ciphers) ~= "string" then
+        error("ciphers must be a string")
+    end
+
+    if ciphersuites ~= nil and type(ciphersuites) ~= "string" then
+        error("ciphersuites must be a string")
+    end
+
+    local rc = ngx_lua_ffi_ssl_set_ciphers(r, ciphers, ciphersuites, errmsg)
+    if rc == FFI_OK then
+        return true
+    end
+
+    return nil, ffi_str(errmsg[0])
+end
+
 return _M

lib/ngx/ssl/clienthello.md

@@ -17,6 +17,7 @@ Table of Contents
     * [get_client_hello_ext_present](#get_client_hello_ext_present)
     * [get_client_hello_ext](#get_client_hello_ext)
     * [set_protocols](#set_protocols)
+    * [set_ciphers](#set_ciphers)
 * [Community](#community)
     * [English Mailing List](#english-mailing-list)
     * [Chinese Mailing List](#chinese-mailing-list)
@@ -294,6 +295,50 @@ Example: `ssl_clt.set_protocols({"TLSv1.1", "TLSv1.2", "TLSv1.3"})`
 
 [Back to TOC](#table-of-contents)
 
+set_ciphers
+----------------------
+**syntax:** *ok, err = ssl_clt.set_ciphers(ciphers, ciphersuites?)*
+
+**context:** *ssl_client_hello_by_lua**
+
+Sets the cipher list used by the current downstream SSL connection, e.g. keyed on the client hello server name.
+
+The `ciphers` string configures the TLS <= 1.2 cipher list in the OpenSSL cipher list format (see [ciphers(1)](https://docs.openssl.org/master/man1/openssl-ciphers/)), like the `ssl_ciphers` nginx directive. The optional `ciphersuites` string configures the TLS 1.3 ciphersuites, a colon-separated list like `TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256`, like the `ssl_conf_command Ciphersuites` nginx directive. Either argument may be `nil` to leave that part of the configuration unchanged.
+
+Returns `true` on success, or a `nil` value and a string describing the error otherwise.
+
+On failure the connection's SSL object is left exactly as it was -- cipher list, ciphersuites, and error queue all untouched -- so the handshake proceeds with whatever the connection already had, which is the default list inherited from the server block (`ssl_ciphers`/`ssl_conf_command`). Both strings are validated before either is applied, because a failed `SSL_set_cipher_list()` would otherwise still install the parsed list on the connection and leave the error queue dirty, aborting the handshake.
+
+Considering it is meaningless to set the cipher list after the cipher is negotiated,
+so this function may only be called in the context of [ssl_client_hello_by_lua*](https://github.com/openresty/lua-nginx-module/#ssl_client_hello_by_lua_block).
+
+This function requires OpenSSL 1.1.1 or later.
+
+Example:
+
+```nginx
+# nginx.conf
+server {
+    listen 443 ssl;
+    server_name   test.com;
+    ssl_client_hello_by_lua_block {
+        local ssl_clt = require "ngx.ssl.clienthello"
+        local name = ssl_clt.get_client_hello_server_name()
+        if name == "legacy.test.com" then
+            local ok, err = ssl_clt.set_ciphers("HIGH:!aNULL:!MD5")
+            if not ok then
+                ngx.log(ngx.ERR, "failed to set ciphers: ", err)
+                -- the handshake continues on the default ssl_ciphers
+            end
+        end
+    }
+    ssl_certificate test.crt;
+    ssl_certificate_key test.key;
+}
+```
+
+[Back to TOC](#table-of-contents)
+
 Community
 =========