[COMDEV-371] Apache httpd Module: How to retain original POST "response body" when using SetHandler?

• Type: Question
• Status: Closed
• Priority: Major
• Resolution: Information Provided
• Component/s: Comdev, Help Wanted
• Labels: Apache Proxy apache c module
• Environment: DEV
• Flags: Important
• Assignee: Unassigned
• Reporter: Harshil Fadia
• Created: 08/May/20 17:55
• Updated: 09/May/20 10:08
• Resolved: 09/May/20 10:08

Description

QUESTION: How can I trigger my C code and set variable in response header BUT still retain original Response body which server is generating for client.

Requirement: Write custom Apache module to read POST request body >> Take out data from request body >> Set particular response header attribute using request body >> Send response back to client

Details: I am using Apache as proxy for my ElasticSearch. I have successfully developed Apache Module in C and added it to httpd.conf. I have used "SetHandler" directive to trigger my custom C code.

It is all working fine. Request generated from client browser is going through my custom C Apache Module and in my C code, I am able to read POST request body from request_rec. I am also able to set response header with that value I am readying from request body.

Issue: If I invoke my module/code using SetHandler, when it comes out it does not have any response body and blank response body is sent to client browser. Where as when I disable my module/code, response is well generated a sent to client. So looks like by using SetHandler, response is lost.

 

My config/code are below:

1. httpd.conf

LoadModule example_module    /usr/lib64/httpd/modules/mycustommodule.so

<VirtualHost HOSTNAME:80>
  <Location /elasticsearch/_msearch>
    #SetHandler readbody-handler #calling my custom module
  </Location>
  #My KIBANA application is running on 5601 on same machine
  ProxyPass        / http://localhost:5601/ 
  ProxyPassReverse / http://localhost:5601/
</VirtualHost>

2. My custom C module

module AP_MODULE_DECLARE_DATA readbody_module = { 
    STANDARD20_MODULE_STUFF,
    NULL, /*Per-directory configuration handler */
    NULL, /*Merge handler for per-directory configurations */
    NULL, /*Per-server configuration handler */
    NULL, /*Merge handler for per-server configurations */
    NULL,
    regiter_hooks /*Our hook registering function */
};

static void register_hooks(apr_pool_t *pool)
{
    ap_hook_handler(readbody_handler, NULL, NULL, -10);
}

static int readbody_handler(request_rec *r)
{
    const char *buffer;

    if (!r->handler || strcmp(r->handler, "readbody-handler")) return (DECLINED);

    if (util_read(r, &buffer) == OK) //reading the body and assigning it into buffer
    {
        char s[2] = ":";
        char s2[2] = "\"";
        char *indexname;
        indexname = strtok(buffer, s);
        indexname = strtok(NULL, s);
        indexname = strtok(indexname, s2);
        indexname = strtok(NULL, s2);
        apr_table_setn(r->headers_out, "IndexPattern", indexname); //setting up response header
    }

    return OK;
}

static int util_read(request_rec *r, const char **rbuf)
{
    int rc;
    if ((rc = ap_setup_client_block(r, REQUEST_CHUNKED_ERROR)) != OK)
    {
        return rc;
    }
    if (ap_should_client_block(r))
    {
        char argsbuffer[HUGE_STRING_LEN];
        int rsize, len_read, rpos = 0;
        long length = r->remaining;
        *rbuf = apr_pcalloc(r->pool, length + 1);
        while ((len_read = ap_get_client_block(r, argsbuffer, sizeof(argsbuffer))) > 0)
        {
            if ((rpos + len_read) > length)
            {
                rsize = length - rpos;
            }
            else
            {
                rsize = len_read;
            }
            memcpy((char*) *rbuf + rpos, argsbuffer, rsize);
            rpos += rsize;
        }
    }
}
    return rc;