Change TCP Window Size based on SrcPort, not DstPort.

When HTTPS fragment size was set, the program always used it for HTTP, even
if HTTP fragmentation was disabled. This is due to a bug which checked DstPort,
not SrcPort, and the packets passed DstPort != htons(80) HTTPS check.

README.md

@@ -65,6 +65,29 @@ And for x86_64:
 
 `make CPREFIX=x86_64-w64-mingw32 WINDIVERTHEADERS=/path/to/windivert/include WINDIVERTLIBS=/path/to/windivert/amd64`
 
+# How to install as Windows Service
+
+One way is using an [srvstart](http://www.rozanski.org.uk/software) program.  
+Unpack it to goodbyedpi directory and create 3 files:
+
+*goodbyedpi.ini*
+```INI
+[GoodByeDPI]
+startup=goodbyedpi.exe
+shutdown_method=winmessage
+auto_restart=n
+```
+*srvinstall.bat*
+```Batchfile
+srvstart install GoodByeDPI -c %CD%\goodbyedpi-svc.ini
+```
+*srvremove.bat*
+```Batchfile
+srvstart remove GoodByeDPI
+```
+Run these batch files as Administrator to install/remove service.  
+Open Windows Services panel to run service or make it start automaticaly.
+
 # Similar projects
 
 [zapret](https://github.com/bol-van/zapret) by @bol-van (for Linux).

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");
 
@@ -273,7 +280,8 @@ int main(int argc, char *argv[]) {
                 }
                 /* Handle OUTBOUND packet, search for Host header */
                 else if (addr.Direction == WINDIVERT_DIRECTION_OUTBOUND && 
-                         packet_dataLen > 16 && ppTcpHdr->DstPort == htons(80) &&
+                        packet_dataLen > 16 && ppTcpHdr->DstPort == htons(80) &&
+                        find_http_method_end(packet_data) &&
                         (do_host || do_host_removespace)) {
 
                     data_addr = find_host_header(packet_data, packet_dataLen);
@@ -297,21 +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");
+                                    }
                                 }
                             }
                         }
@@ -327,11 +355,11 @@ int main(int argc, char *argv[]) {
                 if (addr.Direction == WINDIVERT_DIRECTION_INBOUND && 
                     ppTcpHdr->Syn == 1) {
                     //printf("Changing Window Size!\n");
-                    if (do_fragment_http && ppTcpHdr->DstPort == htons(80)) {
+                    if (do_fragment_http && ppTcpHdr->SrcPort == htons(80)) {
                         change_window_size(packet, http_fragment_size);
                         WinDivertHelperCalcChecksums(packet, packetLen, 0);
                     }
-                    else if (do_fragment_https && ppTcpHdr->DstPort != htons(80)) {
+                    else if (do_fragment_https && ppTcpHdr->SrcPort != htons(80)) {
                         change_window_size(packet, https_fragment_size);
                         WinDivertHelperCalcChecksums(packet, packetLen, 0);
                     }