nginx服务器屏蔽上游错误码

平时的开发工作中，有时会遇到脚本权限不对导致403，文件被删除导致404，甚至后端业务异常导致5xx等情况，其实我们可以在服务器加上判断，检测当后端服务出现异常的时候前端返回一个指定的静态文件（也可以是一个动态资源）。

这样可以为一些关键业务（html或者动态资源，js等）配置此功能，当后端关键业务出现错误时，也会把一个指定的正确资源返回给用户。

想法自然是在nginx向客户端输出响应头时捕获信息，查看是不是异常，异常则跳转到指定文件

看了下nginx的第三方模块都没有实现这个功能，那还是自己来造轮子吧，最直接的开发方法是用openresty或者nginx+lua，nginx-lua提供了header_filter_by_lua挂载入口，可惜尝试下来却不行，仔细查看官方文档才发现这个入口里面禁用了一些关键IO操作的api。

Uses Lua code specified in <lua-script-str> to define an output header filter.

Note that the following API functions are currently disabled within this context:

Output API functions (e.g., ngx.say and ngx.send_headers)
Control API functions (e.g., ngx.exit and ngx.exec)
Subrequest API functions (e.g., ngx.location.capture and ngx.location.capture_multi)
Cosocket API functions (e.g., ngx.socket.tcp and ngx.req.socket).
Here is an example of overriding a response header (or adding one if absent) in

只能做nginx模块开发来实现了。折腾了3个晚上，艰苦的调试过程就不提了，直接贴代码吧。

nginx配置文件如下：

location ~ .php($|/) {
        fastcgi_pass   127.0.0.1:9000;
        fastcgi_index  index.php;
        fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
        include        fastcgi_params;
        up_error_code   403 404 500 503 504 505;
        up_error_go     /ok.html;

}

location / {
        root   html;
        index  index.html index.htm;
        up_error_code   502;
        up_error_go     /ok.html;
}

其中：

up_error_code是需要屏蔽的后段错误码类型，支持多个；

up_error_go是需要跳转到的资源位置

（502错误需要在location /里面才能捕获到，这个非常奇怪，原因还不清楚，所以除了要在proxy的location里面设置其他错误过滤外也要在根目录下过滤502错误才行。）

代码（ngx_http_upstream_error_go_module.c）实现较为简单，具体如下：

/*
 * Copyright (C) Ciaos
 */

#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>

typedef struct {
    ngx_array_t     *error_codes;
    ngx_str_t       file_path;
} ngx_http_upstream_error_go_conf_t;

static void *ngx_http_upstream_error_go_create_conf(ngx_conf_t *cf);
static char *ngx_http_upstream_error_go_merge_conf(ngx_conf_t *cf, void *parent,void *child);

static char *ngx_http_upstream_error_go_set_code(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);
static char *ngx_http_upstream_error_go_set_file_path(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);

static ngx_int_t ngx_http_upstream_error_go_init(ngx_conf_t *cf);

static ngx_command_t  ngx_http_upstream_error_go_commands[] = {
    { ngx_string("up_error_code"),
        NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_1MORE,
        ngx_http_upstream_error_go_set_code,
        NGX_HTTP_LOC_CONF_OFFSET,
        0,
        NULL },

    { ngx_string("up_error_go"),
        NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
        ngx_http_upstream_error_go_set_file_path,
        NGX_HTTP_LOC_CONF_OFFSET,
        0,
        NULL },

    ngx_null_command
};

static ngx_http_module_t  ngx_http_upstream_error_go_module_ctx = {
    NULL,                                  /* preconfiguration */
    ngx_http_upstream_error_go_init,              /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    ngx_http_upstream_error_go_create_conf,       /* create location configration */
    ngx_http_upstream_error_go_merge_conf         /* merge location configration */
};

ngx_module_t  ngx_http_upstream_error_go_module = {
    NGX_MODULE_V1,
    &ngx_http_upstream_error_go_module_ctx,       /* module context */
    ngx_http_upstream_error_go_commands,          /* module directives */
    NGX_HTTP_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};

static ngx_http_output_header_filter_pt  ngx_http_next_header_filter;
static ngx_http_output_body_filter_pt    ngx_http_next_body_filter;

static ngx_int_t
ngx_http_upstream_error_go_header_filter(ngx_http_request_t *r)
{
    ngx_uint_t      e;
    ngx_uint_t      *ecode;
    ngx_http_upstream_error_go_conf_t *clcf;

    clcf = ngx_http_get_module_loc_conf(r, ngx_http_upstream_error_go_module);
    if(clcf == NULL) {
        return NGX_ERROR;
    }

    if(clcf->error_codes){
        ecode = clcf->error_codes->elts;
        for(e = 0; e < clcf->error_codes->nelts; e ++){
            if(r->headers_out.status == *(ecode+e)){
                r->err_status = 0;
                ngx_str_set(&r->headers_out.status_line, "200 OK");
                ngx_http_internal_redirect(r, &clcf->file_path, &r->args);

                return NGX_ERROR;
            }
        }
    }

    return ngx_http_next_header_filter(r);
}

static ngx_int_t
ngx_http_upstream_error_go_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    return ngx_http_next_body_filter(r, in);
}

static char *
ngx_http_upstream_error_go_set_code(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_upstream_error_go_conf_t *clcf = conf;

    ngx_str_t       *value;
    ngx_uint_t      i;
    ngx_uint_t      *ecode;

    if(clcf->error_codes == NGX_CONF_UNSET_PTR) {
        clcf->error_codes = ngx_array_create(cf->pool, 4, sizeof(ngx_uint_t));
        if(clcf->error_codes == NULL)
        {
            return NGX_CONF_ERROR;
        }
    }
    value = cf->args->elts;
    for(i=1; i< cf->args->nelts; i++){
        ecode = ngx_array_push(clcf->error_codes);
        if(ecode == NULL) {
            return NGX_CONF_ERROR;
        }
        *ecode = ngx_atoi(value[i].data, value[i].len);
    }
    return NGX_CONF_OK;
}

static char *
ngx_http_upstream_error_go_set_file_path(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_upstream_error_go_conf_t *clcf = conf;
    ngx_str_t       *value;
    value = cf->args->elts;

    clcf->file_path = value[1];

    return NGX_CONF_OK;
}

static void *
ngx_http_upstream_error_go_create_conf(ngx_conf_t *cf)
{
    ngx_http_upstream_error_go_conf_t  *clcf;

    clcf = ngx_pcalloc(cf->pool, sizeof(ngx_http_upstream_error_go_conf_t));
    if (clcf == NULL) {
        return NULL;
    }

    clcf->error_codes = NGX_CONF_UNSET_PTR;

    return clcf;
}

static char *
ngx_http_upstream_error_go_merge_conf(ngx_conf_t *cf, void *parent, void *child)
{
    ngx_http_upstream_error_go_conf_t *prev = parent;
    ngx_http_upstream_error_go_conf_t *clcf = child;

    ngx_conf_merge_str_value(clcf->file_path, prev->file_path, "");
    ngx_conf_merge_ptr_value(clcf->error_codes, prev->error_codes, NULL);

    return NGX_CONF_OK;
}

static ngx_int_t
ngx_http_upstream_error_go_init(ngx_conf_t *cf)
{
    ngx_http_next_header_filter = ngx_http_top_header_filter;
    ngx_http_top_header_filter = ngx_http_upstream_error_go_header_filter;
    ngx_http_next_body_filter = ngx_http_top_body_filter;
    ngx_http_top_body_filter = ngx_http_upstream_error_go_body_filter;

    return NGX_OK;
}

 配置如下（config）：

ngx_addon_name=ngx_http_upstream_error_go_module
HTTP_AUX_FILTER_MODULES="$HTTP_AUX_FILTER_MODULES ngx_http_upstream_error_go_module"
NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/ngx_http_upstream_error_go_module.c"

将源代码和配置文件放到某个目录下，编译nginx（版本1.7.4测试通过）带上此模块即可，测试工具可用Test::Nginx（perl -t t/test.t或prove -r t），测试代码如下

use Test::Nginx::Socket;
repeat_each(1);
plan tests => 1 * repeat_each() * blocks();

our $config = <<"_EOC_";
        location /foo {
            fastcgi_pass   127.0.0.1;
            up_error_code   403 404 502 500 503 504 505;
            up_error_go     /index.html;
        }
_EOC_

run_tests();

__DATA__

=== TEST 1: upstream 403
--- http_config eval
"
    server{
        listen  127.0.0.1:80;
        location /foo {
            return 403;
        }
    }
"
--- config eval: $::config
--- request
GET /foo
--- error_code: 200

=== TEST 2: upstream 404
--- http_config eval
"
    server{
        listen  127.0.0.1:80;
        location /foo {
            return 404;
        }
    }
"
--- config eval: $::config
--- request
GET /foo
--- error_code: 200

=== TEST 3: upstream 502
--- http_config
--- config eval: $::config
--- request
GET /foo
--- error_code: 200