Reimplement -s option to fix unACKed data

Old code used to reduce packet size by one byte to remove space in
HTTP Host header. This introduces one unACKed byte which OS later
tried to send to the host. This byte was \n (the last byte in original
packet) which broke POST requests.

New code in this commit moves "stolen" space in the end of User-Agent
header value and do not reduce packet size anymore.
User-Agent value is used because not all web servers are compatible
with additional space in the end of Host value.

Fix #3

goodbyedpi.c

@@ -27,6 +27,7 @@ static const char *http10_redirect_302 = "HTTP/1.0 302 ";
 static const char *http11_redirect_302 = "HTTP/1.1 302 ";
 static const char *http_host_find = "\r\nHost: ";
 static const char *http_host_replace = "\r\nhoSt: ";
+static const char *http_useragent_find = "\r\nUser-Agent: ";
 static const char *location_http = "\r\nLocation: http://";
 static const char *http_methods[] = {
     "GET ",
@@ -104,6 +105,12 @@ static PVOID find_host_header(const char *pktdata, int pktlen) {
                 http_host_find, strlen(http_host_find));
 }
 
+/* Finds User-Agent header with \r\n before it */
+static PVOID find_useragent_header(const char *pktdata, int pktlen) {
+    return dumb_memmem(pktdata, pktlen,
+                http_useragent_find, strlen(http_useragent_find));
+}
+
 static void change_window_size(const char *pkt, int size) {
     *(uint16_t*)(pkt + IPV4_HDR_LEN + TCP_WINDOWSIZE_OFFSET) = htons(size);
 }
@@ -138,8 +145,8 @@ int main(int argc, char *argv[]) {
         do_host_removespace = 0, do_additional_space = 0;
     int http_fragment_size = 2;
     int https_fragment_size = 2;
-    char *data_addr, *data_addr_rn, *host_addr, *method_addr;
-    int host_len, fromhost_uptoend_len;
+    char *data_addr, *data_addr_rn, *host_addr, *useragent_addr, *method_addr;
+    int data_len, host_len;
 
     printf("GoodbyeDPI: Passive DPI blocker and Active DPI circumvention utility\n");
 
@@ -298,21 +305,41 @@ int main(int argc, char *argv[]) {
                         else if (do_host_removespace) {
                             host_addr = data_addr + strlen(http_host_find);
 
-                            fromhost_uptoend_len = packet_dataLen - ((PVOID)host_addr - packet_data);
                             data_addr_rn = dumb_memmem(host_addr,
-                                                        fromhost_uptoend_len,
-                                                        "\r\n", 2);
+                                                       packet_dataLen - ((PVOID)host_addr - packet_data),
+                                                       "\r\n", 2);
                             if (data_addr_rn) {
+                                /* We move Host header value by one byte to the left and then
+                                 * "insert" stolen space to the end of User-Agent value because
+                                 * some web servers are not tolerant to additional space in the
+                                 * end of Host header.
+                                 *
+                                 * Nothing is done if User-Agent header is missing.
+                                 */
                                 host_len = data_addr_rn - host_addr;
-                                if (host_len <= 64) {
-                                    /* Move memory left by 1 byte and reduce packet size for 1 byte */
-                                    memmove(host_addr - 1, host_addr, fromhost_uptoend_len);
-                                    /* Reduce "Total Length" in IP header by 1 byte */
-                                    *(uint16_t*)(packet + IPV4_TOTALLEN_OFFSET) = ntohs(
-                                        htons(*(uint16_t*)(packet + IPV4_TOTALLEN_OFFSET)) - 1);
-                                    /* Reduce packetLen by 1 byte */
-                                    packetLen--;
-                                    //printf("Replaced Host header!\n");
+                                useragent_addr = find_useragent_header(packet_data, packet_dataLen);
+                                if (host_len <= 253 && useragent_addr && useragent_addr > host_addr) {
+                                    /* Performing action only if User-Agent header goes after Host */
+
+                                    useragent_addr += strlen(http_useragent_find);
+                                    /* useragent_addr is in the beginning of User-Agent value */
+
+                                    data_len = packet_dataLen - ((PVOID)useragent_addr - packet_data);
+                                    data_addr_rn = dumb_memmem(useragent_addr,
+                                                               data_len, "\r\n", 2);
+                                    /* data_addr_rn is in the end of User-Agent value */
+
+                                    if (data_addr_rn) {
+                                        data_len = (PVOID)data_addr_rn - (PVOID)host_addr;
+
+                                        /* Move one byte to the left from "Host:"
+                                         * to the end of User-Agen
+                                         */
+                                        memmove(host_addr - 1, host_addr, data_len);
+                                        /* Put space in the end of User-Agent header */
+                                        *(char*)(data_addr_rn - 1) = ' ';
+                                        //printf("Replaced Host header!\n");
+                                    }
                                 }
                             }
                         }