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Apply (D)DoS-Hardening - quark - quark web server
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---
commit 6d7c7b6ff701fafc2a649b21a66a92a9ab626221
parent 8aa213c123cbcfc2e50a77142c2206c202705e90
Author: Laslo Hunhold <[email protected]>
Date: Sat, 6 Feb 2021 01:15:53 +0100
Apply (D)DoS-Hardening
Until now, if quark found that in case of an incoming connection it
didn't have any vacant connection slots left, it would just not
accept() and thus block any further new connections until a slot was
free.
This may sound reasonable at first, but there are cases where the
connected clients are not benevolent and might firmly occupy all slots
with simple request flooding or more advanced attacks like slowloris,
R-U-Dead-Yet or Slow Read, which all boil down to sending or receiving
data really slowly. The latter attacks are very effective and require
very little resources on the attacker's side.
Thus, the only way to keep things going is to bite the bullet and always
accept a connection, even if it means dropping another connection for
it. In this case, an algorithm determines which client has the most
connections and drops the least-advanced connection of it (i.e. a
connection in the earliest possible stage).
Stress-tests with slowhttptest[0] and siege yielded excellent results,
where quark always remained responsive for the normal visitor despite
an active massive DoS-attack using the aforementioned methods.
Side-note: In the spirit of not having any memory allocations in quark
during "runtime", the constant-space algorithm used to determine the
client with the most connections is quadratic in time over the number
of slots. This, however, is not a big deal, given the number of slots
is always relatively small and such an out-of-connection-scenario is
an edge-case.
[0]:https://github.com/shekyan/slowhttptest
[1]:https://github.com/JoeDog/siege
Signed-off-by: Laslo Hunhold <[email protected]>
Diffstat:
M main.c | 134 +++++++++++++++++++++++++----…
M sock.c | 22 ++++++++++++++++++++++
M sock.h | 2 ++
3 files changed, 133 insertions(+), 25 deletions(-)
---
diff --git a/main.c b/main.c
@@ -45,15 +45,25 @@ logmsg(const struct connection *c)
inaddr_str[0] = '\0';
}
- printf("%s\t%s\t%d\t%s\t%s%s%s%s%s\n", tstmp, inaddr_str, c->res.statu…
- c->req.field[REQ_HOST], c->req.path, c->req.query[0] ? "?" : "",
- c->req.query, c->req.fragment[0] ? "#" : "", c->req.fragment);
+ printf("%s\t%s\t%s%.*d\t%s\t%s%s%s%s%s\n",
+ tstmp,
+ inaddr_str,
+ (c->res.status == 0) ? "dropped" : "",
+ (c->res.status == 0) ? 0 : 3,
+ c->res.status,
+ c->req.field[REQ_HOST][0] ? c->req.field[REQ_HOST] : "-",
+ c->req.path[0] ? c->req.path : "-",
+ c->req.query[0] ? "?" : "",
+ c->req.query,
+ c->req.fragment[0] ? "#" : "",
+ c->req.fragment);
}
static void
-close_connection(struct connection *c)
+reset_connection(struct connection *c)
{
if (c != NULL) {
+ shutdown(c->fd, SHUT_RDWR);
close(c->fd);
memset(c, 0, sizeof(*c));
}
@@ -152,7 +162,53 @@ response:
}
err:
logmsg(c);
- close_connection(c);
+ reset_connection(c);
+}
+
+static struct connection *
+get_connection_drop_candidate(struct connection *connection, size_t nslots)
+{
+ struct connection *c, *minc;
+ size_t i, j, maxcnt, cnt;
+
+ /*
+ * determine the most-unimportant connection 'minc' of the in-address
+ * with most connections; this algorithm has a complexity of O(n²)
+ * in time but is O(1) in space; there are algorithms with O(n) in
+ * time and space, but this would require memory allocation,
+ * which we avoid. Given the simplicity of the inner loop and
+ * relatively small number of slots per thread, this is fine.
+ */
+ for (i = 0, minc = NULL, maxcnt = 0; i < nslots; i++) {
+ /*
+ * the c is used to minimize in regard to importance
+ * within the same-address-group
+ */
+ c = &connection[i];
+
+ for (j = 0, cnt = 0; j < nslots; j++) {
+ if (!sock_same_addr(&connection[i].ia,
+ &connection[j].ia)) {
+ continue;
+ }
+ cnt++;
+
+ if (connection[j].state < c->state) {
+ /*
+ * we have found a connection with an
+ * even lower state and thus lower
+ * importance
+ */
+ c = &connection[j];
+ }
+ }
+ if (cnt > maxcnt) {
+ minc = c;
+ maxcnt = cnt;
+ }
+ }
+
+ return minc;
}
struct connection *
@@ -160,33 +216,61 @@ accept_connection(int insock, struct connection *connecti…
size_t nslots)
{
struct connection *c = NULL;
- size_t j;
+ size_t i;
/* find vacant connection (i.e. one with no fd assigned to it) */
- for (j = 0; j < nslots; j++) {
- if (connection[j].fd == 0) {
- c = &connection[j];
+ for (i = 0; i < nslots; i++) {
+ if (connection[i].fd == 0) {
+ c = &connection[i];
break;
}
}
- if (j == nslots) {
- /* nothing available right now, return without accepting */
-
- /*
- * NOTE: This is currently still not the best option, but
- * at least we now have control over it and can reap a
- * connection from our pool instead of previously when
- * we were forking and were more or less on our own in
- * each process
+ if (i == nslots) {
+ /*
+ * all our connection-slots are occupied and the only
+ * way out is to drop another connection, because not
+ * accepting this connection just kicks this can further
+ * down the road (to the next queue_wait()) without
+ * solving anything.
+ *
+ * This may sound bad, but this case can only be hit
+ * either when there's a (D)DoS-attack or a massive
+ * influx of requests. The latter is impossible to solve
+ * at this moment without expanding resources, but the
+ * former has certain characteristics allowing us to
+ * handle this gracefully.
+ *
+ * During an attack (e.g. Slowloris, R-U-Dead-Yet, Slow
+ * Read or just plain flooding) we can not see who is
+ * waiting to be accept()ed.
+ * However, an attacker usually already has many
+ * connections open (while well-behaved clients could
+ * do everything with just one connection using
+ * keep-alive). Inferring a likely attacker-connection
+ * is an educated guess based on which in-address is
+ * occupying the most connection slots. Among those,
+ * connections in early stages (receiving or sending
+ * headers) are preferred over connections in late
+ * stages (sending body).
+ *
+ * This quantitative approach effectively drops malicious
+ * connections while preserving even long-running
+ * benevolent connections like downloads.
*/
- return NULL;
+ c = get_connection_drop_candidate(connection, nslots);
+ c->res.status = 0;
+ logmsg(c);
+ reset_connection(c);
}
/* accept connection */
if ((c->fd = accept(insock, (struct sockaddr *)&c->ia,
&(socklen_t){sizeof(c->ia)})) < 0) {
if (errno != EAGAIN && errno != EWOULDBLOCK) {
- /* not much we can do here */
+ /*
+ * this should not happen, as we received the
+ * event that there are pending connections here
+ */
warn("accept:");
}
return NULL;
@@ -250,11 +334,11 @@ thread_method(void *data)
if (queue_event_is_error(&event[i])) {
if (c != NULL) {
queue_rem_fd(qfd, c->fd);
- close_connection(c);
+ c->res.status = 0;
+ logmsg(c);
+ reset_connection(c);
}
- printf("dropped a connection\n");
-
continue;
}
@@ -301,7 +385,7 @@ thread_method(void *data)
if (queue_mod_fd(qfd, c->fd,
QUEUE_EVENT_IN,
c) < 0) {
- close_connection(c);
+ reset_connection(c);
break;
}
break;
@@ -310,7 +394,7 @@ thread_method(void *data)
if (queue_mod_fd(qfd, c->fd,
QUEUE_EVENT_OUT,
c) < 0) {
- close_connection(c);
+ reset_connection(c);
break;
}
break;
diff --git a/sock.c b/sock.c
@@ -222,3 +222,25 @@ sock_get_inaddr_str(const struct sockaddr_storage *in_sa, …
return 0;
}
+
+int
+sock_same_addr(const struct sockaddr_storage *sa1, const struct sockaddr_stora…
+{
+ /* return early if address-families don't match */
+ if (sa1->ss_family != sa2->ss_family) {
+ return 0;
+ }
+
+ switch (sa1->ss_family) {
+ case AF_INET6:
+ return memcmp(((struct sockaddr_in6 *)sa1)->sin6_addr.s6_addr,
+ ((struct sockaddr_in6 *)sa2)->sin6_addr.s6_addr,
+ sizeof(((struct sockaddr_in6 *)sa1)->sin6_addr.s…
+ case AF_INET:
+ return ntohl(((struct sockaddr_in *)sa1)->sin_addr.s_addr) ==
+ ntohl(((struct sockaddr_in *)sa2)->sin_addr.s_addr);
+ default: /* AF_UNIX */
+ return strcmp(((struct sockaddr_un *)sa1)->sun_path,
+ ((struct sockaddr_un *)sa2)->sun_path) == 0;
+ }
+}
diff --git a/sock.h b/sock.h
@@ -12,5 +12,7 @@ int sock_get_uds_arr(const char *, uid_t, gid_t, int *, size_…
int sock_set_timeout(int, int);
int sock_set_nonblocking(int);
int sock_get_inaddr_str(const struct sockaddr_storage *, char *, size_t);
+int sock_same_addr(const struct sockaddr_storage *,
+ const struct sockaddr_storage *);
#endif /* SOCK_H */
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