Use Host header in pastebin sample rather than hardcoded constants

(This allows one to insert any string through the Host header in the
response, as there's no validation other than "there's no newline",
which is part of the HTTP parsing code.)

src/samples/pastebin/main.c

@@ -29,8 +29,6 @@
 #include "lwan-cache.h"
 #include "lwan-private.h"
 
-#define SERVER_NAME "paste.lwan.ws"
-#define SERVER_PORT 443
 #define CACHE_FOR_HOURS 2
 
 static struct cache *pastes;
@@ -123,9 +121,15 @@ static enum lwan_http_status post_paste(struct lwan_request *request,
             cache_coro_get_and_ref_entry(pastes, request->conn->coro, key);
 
         if (paste) {
+            const char *host_hdr = lwan_request_get_header(request, "Host");
+
+            if (!host_hdr)
+                return HTTP_BAD_REQUEST;
+
             response->mime_type = "text/plain";
             lwan_strbuf_printf(response->buffer, "https://%s/p/%zu\n\n",
-                               SERVER_NAME, (uint64_t)(uintptr_t)key);
+                               host_hdr, (uint64_t)(uintptr_t)key);
+
             return HTTP_OK;
         }
     }
@@ -136,6 +140,11 @@ static enum lwan_http_status post_paste(struct lwan_request *request,
 static enum lwan_http_status doc(struct lwan_request *request,
                                  struct lwan_response *response)
 {
+    const char *host_hdr = lwan_request_get_header(request, "Host");
+
+    if (!host_hdr)
+        return HTTP_BAD_REQUEST;
+
     response->mime_type = "text/plain";
 
     lwan_strbuf_printf(
@@ -152,7 +161,7 @@ static enum lwan_http_status doc(struct lwan_request *request,
         "response with different MIME-type.\n"
         "\n"
         "Items are cached for %d hours and are not stored on disk",
-        SERVER_NAME, SERVER_NAME, CACHE_FOR_HOURS);
+        host_hdr, host_hdr, CACHE_FOR_HOURS);
 
     return HTTP_OK;
 }