Add max_window options

When enabled, TCP packets are forced to use maximum window size for debugging purposes or to avoid conntrack breaks (possibly due to dropped window change ACKs)

bpf/egress.c

@@ -192,7 +192,8 @@ int egress_handler(struct __sk_buff* skb) {
   __be32 csum_diff = 0;
   try_tc(mangle_data(skb, ip_end + sizeof(*udp), &csum_diff, padding));
   decl_shot(struct tcphdr, tcp, ip_end, skb);
-  update_tcp_header(tcp, payload_len, seq, ack_seq, conn_cwnd);
+  update_tcp_header(tcp, payload_len, seq, ack_seq,
+                    settings->max_window ? 0xffff << CWND_SCALE : conn_cwnd);
 
   __u32 csum_off = ip_end + offsetof(struct tcphdr, check);
   redecl_shot(struct tcphdr, tcp, ip_end, skb);

bpf/ingress.c

@@ -294,8 +294,10 @@ int ingress_handler(struct xdp_md* xdp) {
   bpf_spin_unlock(&conn->lock);
 
   if (flags & TCP_FLAG_SYN && flags & TCP_FLAG_ACK) log_conn(LOG_CONN_ACCEPT, &conn_key);
-  if (will_send_ctrl_packet)
-    send_ctrl_packet(&conn_key, flags, seq, ack_seq, flags & TCP_FLAG_RST ? 0 : cwnd);
+  if (will_send_ctrl_packet) {
+    __u32 out_cwnd = flags & TCP_FLAG_RST ? 0 : settings->max_window ? 0xffff << CWND_SCALE : cwnd;
+    send_ctrl_packet(&conn_key, flags, seq, ack_seq, out_cwnd);
+  }
   if (unlikely(flags & TCP_FLAG_RST)) {
     log_destroy(&conn_key, DESTROY_INVALID, cooldown);
     use_pktbuf(RB_ITEM_FREE_PKTBUF, pktbuf);

common/defs.h

@@ -221,8 +221,9 @@ struct filter_settings {
         };
       } keepalive, k; };
       __s16 padding;
+      __s16 max_window;
     };
-    __s16 array[7];
+    __s16 array[8];
   };
 };
 // clang-format on
@@ -238,10 +239,11 @@ static const struct filter_settings DEFAULT_SETTINGS = {
   .handshake.array = {2, 3},
   .keepalive.array = {180, 10, 3, 600},
   .padding = 0,
+  .max_window = true,
 };
 
 static const struct filter_settings FALLBACK_SETTINGS = {
-  .array = {-1, -1, -1, -1, -1, -1, -1},
+  .array = {-1, -1, -1, -1, -1, -1, -1, -1},
 };
 
 static inline void filter_settings_apply(struct filter_settings* local,

docs/mimic.1.md

@@ -36,6 +36,9 @@
 `-p, --padding`
 : Padding size appended to each packet. Pass 'random' to use random padding.
 
+`-W, --max-window`
+: Always use maximum window size in TCP packets.
+
 `-F, --file=PATH`
 : Load configuration from file
 
@@ -108,7 +111,7 @@ See **/usr/share/doc/mimic/eth0.conf.example** for detailed examples.
 `log.verbosity`
 : Controls how much information should be printed. Log level equal to or higher (in number) than log verbosity will be discarded. Both number and string matching log levels are accepted. Number must be greater than or equal to 0. Defaults to info (2). Available levels are: 0 - error (cannot be discarded), 1 - warn, 2 - info, 3 - debug, 4 - trace.
 
-`handshake`, `keepalive`, `padding`
+`handshake`, `keepalive`, `padding`, `max_window`
 : See [**OPTIONS**](#options).
 
 `filter`

src/config.c

@@ -116,11 +116,11 @@ static int parse_int_seq(char* str, int* nums, size_t len) {
   return nums_idx;
 }
 
-__attribute__((unused)) static int parse_bool(const char* str, bool* result) {
+static int parse_bool(const char* str, __s16* result) {
   if (strcmp("true", str) == 0 || strcmp("1", str) == 0)
-    *result = true;
+    *result = 1;
   else if (strcmp("false", str) == 0 || strcmp("0", str) == 0)
-    *result = false;
+    *result = 0;
   else
     ret(-EINVAL, _("invalid boolean value: '%s'"), str);
   return 0;
@@ -159,6 +159,8 @@ static int parse_setting(const char* k, char* v, struct filter_settings* setting
     try(parse_keepalive(v, &settings->keepalive));
   else if (strcmp("padding", k) == 0)
     try(parse_padding(v, &settings->padding));
+  else if (strcmp("max_window", k) == 0)
+    try(parse_bool(v, &settings->max_window));
   else
     return 0;
   return 1;

src/run.c

@@ -42,6 +42,7 @@ static const struct argp_option options[] = {
   {"keepalive", 'k', N_("t:i:r:s"), 0, N_("Controls keepalive mechanism"), 2},
   {"padding", 'p', N_("bytes"), 0,
    N_("Padding size appended to each packet. Pass 'random' to use random padding."), 2},
+  {"max-window", 'W', NULL, 0, N_("Always use maximum window size in TCP packets"), 2},
   {"file", 'F', N_("PATH"), 0, N_("Load configuration from file"), 3},
   {},
 };
@@ -77,6 +78,9 @@ static inline error_t args_parse_opt(int key, char* arg, struct argp_state* stat
     case 'p':
       try(parse_padding(arg, &args->gsettings.padding));
       break;
+    case 'W':
+      args->gsettings.max_window = true;
+      break;
     case 'F':
       args->file = arg;
       break;
@@ -356,21 +360,22 @@ static int do_routine(int conns_fd, const char* ifname) {
             time_diff_sec(tstamp, conn.stale_tstamp) >= conn.settings.keepalive.stale) {
           reset = remove = true;
         } else if (conn.settings.keepalive.time > 0 && retry_secs >= conn.settings.keepalive.time) {
+          __u32 cwnd = conn.settings.max_window ? 0xffff << CWND_SCALE : conn.cwnd;
           if (conn.settings.keepalive.interval <= 0) {
             reset = true;
           } else if (conn.retry_tstamp >= conn.reset_tstamp) {
             log_conn(LOG_DEBUG, &key, _("sending keepalive"));
             conn.reset_tstamp = tstamp;
             conn.keepalive_sent = true;
-            send_ctrl_packet(&key, TCP_FLAG_ACK, conn.seq - 1, conn.ack_seq, conn.cwnd, ifname);
+            send_ctrl_packet(&key, TCP_FLAG_ACK, conn.seq - 1, conn.ack_seq, cwnd, ifname);
             bpf_map_update_elem(conns_fd, &key, &conn, BPF_EXIST | BPF_F_LOCK);
           } else {
             int reset_secs = time_diff_sec(tstamp, conn.reset_tstamp);
             if (reset_secs >= conn.settings.keepalive.retry * conn.settings.keepalive.interval) {
               reset = true;
             } else if (reset_secs % conn.settings.keepalive.interval == 0) {
               log_conn(LOG_DEBUG, &key, _("sending keepalive"));
-              send_ctrl_packet(&key, TCP_FLAG_ACK, conn.seq - 1, conn.ack_seq, conn.cwnd, ifname);
+              send_ctrl_packet(&key, TCP_FLAG_ACK, conn.seq - 1, conn.ack_seq, cwnd, ifname);
             }
           }
         }

src/show.c

@@ -62,6 +62,7 @@ int show_overview(int whitelist_fd, struct filter_settings* gs, int log_verbosit
     fprintf(out, _(", padding random"));
   else if (gs->padding)
     fprintf(out, _(", padding %d"), gs->padding);
+  if (gs->max_window) fprintf(out, _(", max window"));
   fprintf(out, "\n");
 
   char buf[FILTER_FMT_MAX_LEN];
@@ -97,6 +98,9 @@ int show_overview(int whitelist_fd, struct filter_settings* gs, int log_verbosit
       else
         fprintf(out, ",padding=%d", info.settings.padding);
     }
+    if (a->max_window != b->max_window) {
+      fprintf(out, _(",max_window=%s"), info.settings.max_window ? "true" : "false");
+    }
     if (strlen(info.host) != 0) fprintf(out, _(" %s(resolved from %s)"), RESET GRAY, info.host);
     fprintf(out, RESET "\n");
   }