Block passive DPI packets only with "Connection: close". Fixes #17.

Some servers set "don't fragment" flag and never increase TCP ID
field. If they send HTTP redirection to another website, it would
be blocked by the program.
This is a hack to block redirects only with "Connection: close"
header as presumably legal redirects are most likely would
use keep-alive.

README.md

@@ -38,7 +38,7 @@ Use `goodbyedpi.exe -4` if it works for your ISP's DPI. This is the fastest mode
 
 ### Passive DPI
 
-Most Passive DPI send HTTP 302 Redirect if you try to access blocked website over HTTP and TCP Reset in case of HTTPS, faster than destination website. Packets sent by DPI have always have IP Identification field equal to `0x0000` or `0x0001`, as seen with Russian providers. These packets, if they redirect you to another website (censorship page), are blocked by GoodbyeDPI.
+Most Passive DPI send HTTP 302 Redirect if you try to access blocked website over HTTP and TCP Reset in case of HTTPS, faster than destination website. Packets sent by DPI always have IP Identification field equal to `0x0000` or `0x0001`, as seen with Russian providers. These packets, if they redirect you to another website (censorship page), are blocked by GoodbyeDPI.
 
 ### Active DPI
 
@@ -49,7 +49,7 @@ Active DPI is more tricky to fool. Currently the software uses 4 methods to circ
 * Removing space between header name and value in `Host` header
 * Adding additional space between HTTP Method (GET, POST etc) and URI
 
-These methods should not break any website as are fully compatible with TCP and HTTP standards, yet it's sufficient to prevent DPI data classification and to circumvent censorship. Additional space may break some websites, although it's acceptable by HTTP/1.1 specification (see 19.3 Tolerant Applications).
+These methods should not break any website as they're fully compatible with TCP and HTTP standards, yet it's sufficient to prevent DPI data classification and to circumvent censorship. Additional space may break some websites, although it's acceptable by HTTP/1.1 specification (see 19.3 Tolerant Applications).
 
 The program loads WinDivert driver which uses Windows Filtering Platform to set filters and redirect packets to the userspace. It's running as long as console window is visible and terminates when you close the window.
 
@@ -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.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

@@ -27,7 +27,9 @@ 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 *connection_close = "\r\nConnection: close";
 static const char *http_methods[] = {
     "GET ",
     "HEAD ",
@@ -90,8 +92,9 @@ static int is_passivedpi_redirect(const char *pktdata, int pktlen) {
     if (memcmp(pktdata, http11_redirect_302, strlen(http11_redirect_302)) == 0 ||
         memcmp(pktdata, http10_redirect_302, strlen(http10_redirect_302)) == 0)
     {
-        /* Then check if this is a redirect to new http site */
-        if (dumb_memmem(pktdata, pktlen, location_http, strlen(location_http))) {
+        /* Then check if this is a redirect to new http site with Connection: close */
+        if (dumb_memmem(pktdata, pktlen, location_http, strlen(location_http)) &&
+            dumb_memmem(pktdata, pktlen, connection_close, strlen(connection_close))) {
             return 1;
         }
     }
@@ -104,6 +107,12 @@ 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 +147,8 @@ 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");
 
@@ -274,6 +283,7 @@ 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) &&
+                        find_http_method_end(packet_data) &&
                         (do_host || do_host_removespace)) {
 
                     data_addr = find_host_header(packet_data, packet_dataLen);
@@ -297,24 +307,44 @@ 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,
+                                                       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--;
+                                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");
                                     }
                                 }
                             }
+                        }
 
                         WinDivertHelperCalcChecksums(packet, packetLen, 0);
                     }
@@ -327,11 +357,11 @@ 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);
                     }