http 1.0 keep-alive server and curl

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April 2019

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I'm busy with my own http server implementation on an embedded platform. Technically the server is HTTP 1.0 compliant, and therefore it expects the the client to send the header "Connection: Keep-Alive" to keep the connection open.

The implementation looks like this. I removed the code that parses the HTTP header and performs the request, to keep the post as short as possible:

int Service_Request(int conn) {

    struct ReqInfo reqinfo;
    volatile int resource = 0;
    int retval = 0;
    Req_Result req_result = GOT_REQ;

    InitReqInfo(&reqinfo);

    /* while we still have HTTP requests to process */
    while (req_result == GOT_REQ)
    {

        /*  Get HTTP request, there are 3 different return values:
         * GOT_REQ: we got a valid HTTP request
         * TIMEOUT_REQ we timed out waiting for a request
         * ERROR_REQ there was some error receiving from the socket
         * usually because the connection was closed by the peer*/
        req_result = Get_Request(conn, &reqinfo);
        if ( req_result == TIMEOUT_REQ)
        {
            /* timed out waiting for the client, exit */
            retval = 0;
            break;
        }
        else if (req_result == ERROR_REQ)
        {
            /* some error, exit */
            retval = -1;
            break;
        }
        /* Process the request GET, PUT and POST is supported*/
        if (reqinfo.method == GET) 
        {
            /* code to handle GET*/
        } 
        /* PUT and POST are handled in the same way */
        else if ((reqinfo.method == PUT) || (reqinfo.method == POST) )
        {
            /* Code to handle PUT and POST*/        
        } 
        else 
        {
            /* not supported, code should never get here */
            reqinfo.status = 501;
            Return_Error_Msg(conn, &reqinfo);
        }
        /*Diag_Msg("Client Request: \r\n");
        Diag_Msg(reqinfo.clientRequest);*/

        /*
         * the reqinfo.keep_alive flag will be set to 1 if the 
         * "Connection: Keep-Alive" header was sent by the client
         */
        if(reqinfo.keep_alive == 0)
        {
            break;
        }

        reqinfo.keep_alive_max--;
        if(reqinfo.keep_alive_max <= 0 )
        {
            /*
             * the connection has been reused for the maxmum amount of times, stop
             */
            break;
        }
        /*
         * If we get here, we will clear the memory used for the client request
         * and go to the beginning of the while loop to receive another request
         */
        Writeline(conn,"\r\n",2);
        FreeReqInfo(&reqinfo);

    }
    FreeReqInfo(&reqinfo);
    return (retval);
}

The Get_Request function looks like this:

Req_Result Get_Request(int conn, struct ReqInfo * reqinfo) {

    char   buffer[MAX_REQ_LINE] = {0};
    int    rval;
    fd_set fds;
    struct timeval tv;


    /*  Set timeout to 5 seconds if this is the first request since the client connected, wait 5 seconds
     * Otherwise, wait 5ms */
    if(reqinfo->first_request == 1)
    {
        tv.tv_sec  = 5;
        tv.tv_usec = 0;
        reqinfo->first_request = 0;
    }
    else
    {
        tv.tv_sec  = reqinfo->keep_alive_timeout;
        tv.tv_usec = 0;
    }

    /*  Loop through request headers. If we have a simple request,
    then we will loop only once. Otherwise, we will loop until
    we receive a blank line which signifies the end of the headers,
    or until select() times out, whichever is sooner.                */
    do {

    /*  Reset file descriptor set  */

    FD_ZERO(&fds);
    FD_SET (conn, &fds);


    /*  Wait until the timeout to see if input is ready  */

    rval = select(conn + 1, &fds, NULL, NULL, &tv);


    /*  Take appropriate action based on return from select()  */

    if ( rval < 0 ) 
    {
        Diag_Msg("Error calling select() in get_request()");
        return (ERROR_REQ);
    }
    else if ( rval == 0 ) {

        /*  input not ready after timeout  */

        return (TIMEOUT_REQ);

    }
    else {

        /*  We have an input line waiting, so retrieve it  */
        memset(buffer,0,MAX_REQ_LINE - 1);
        if(Readline(conn, buffer, MAX_REQ_LINE - 1) == -1)
        {
            return (ERROR_REQ);
        }
        if(reqinfo->clientRequest == NULL)
        { 
            reqinfo->clientRequest = calloc(MAX_REQ_LINE - 1, sizeof(char));
            strncpy(reqinfo->clientRequest,buffer,MAX_REQ_LINE - 1);
        }
        else
        {
            strncat(reqinfo->clientRequest,buffer,MAX_REQ_LINE - 1);
        }
        Trim(buffer);

        if ( buffer[0] == '\0' )
        break;

        if ( Parse_HTTP_Header(buffer, reqinfo) )
        break;
    }
    } while ( reqinfo->type != SIMPLE );

    return (GOT_REQ);
}

To describe the workings of this server in English: The server receives the first request. It parses the headers, if it finds the "Connection: Keep-Alive" header, it sets a flag. The server proceeds to process this request. WHen it is done it checks the keep-alive flag. If it is cleared, the server closes the connection. If set the server performs a clean-up operation and the proceeds to wait for another request over the same connection. And so on.

I tested this with curl:

C:\curl>curl -v -H "Connection: Keep-Alive" --data-binary @vid1.bin 10.84.67.129/s1p0:1/vid[1-2].bin

[1/2]: 10.84.67.129/s1p0:1/vid1.bin --> <stdout>
--_curl_--10.84.67.129/s1p0:1/vid1.bin
* About to connect() to 10.84.67.129 port 80 (#0)
*   Trying 10.84.67.129...
* connected
* Connected to 10.84.67.129 (10.84.67.129) port 80 (#0)
> POST /s1p0:1/vid1.bin HTTP/1.1
> User-Agent: curl/7.28.1
> Host: 10.84.67.129
> Accept: */*
> Connection: Keep-Alive
> Content-Length: 51200
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
>
* HTTP 1.0, assume close after body
< HTTP/1.0 100 Continue
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Server: DTSVU v0.1
< Content-Type: text/html
* HTTP/1.0 connection set to keep alive!
< Connection: Keep-Alive
< Keep-Alive: timeout=5, max=10
<
* Connection #0 to host 10.84.67.129 left intact

[2/2]: 10.84.67.129/s1p0:1/vid2.bin --> <stdout>
--_curl_--10.84.67.129/s1p0:1/vid2.bin
* Connection #0 seems to be dead!
* Closing connection #0
* About to connect() to 10.84.67.129 port 80 (#0)
*   Trying 10.84.67.129...
* connected
* Connected to 10.84.67.129 (10.84.67.129) port 80 (#0)
> POST /s1p0:1/vid2.bin HTTP/1.1
> User-Agent: curl/7.28.1
> Host: 10.84.67.129
> Accept: */*
> Connection: Keep-Alive
> Content-Length: 51200
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
>
* HTTP 1.0, assume close after body
< HTTP/1.0 100 Continue
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Server: DTSVU v0.1
< Content-Type: text/html
* HTTP/1.0 connection set to keep alive!
< Connection: Keep-Alive
< Keep-Alive: timeout=5, max=10
<
* Connection #0 to host 10.84.67.129 left intact
* Closing connection #0

As you can see, curl says: Connection #0 seems to be dead! afterthefirstrequest is completed. It then proceeds to close the connection and opens a new one. I'm sure I implemented the HTTP 1.0 keep-alive functionality correctly. SO my question is: what does curl expect over the connection after the first request is completed? Why does it decide the connection is dead?

PS the above code was adapted from http://www.paulgriffiths.net/program/c/webserv.php

1 answers

4

I solved it. If the server replies with HTTP/1.1 and "Content-Length: 0", curl reuses the connection. My server reply lookes like this

< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
< Server: DTSVU v0.1
< Content-Type: text/html
< Connection: Keep-Alive
< Keep-Alive: timeout=1, max=95
< Content-Length: 0

after the 5th reuse of the connection.