On Fri, Jan 06, 2023 at 03:38:41PM +0100, Arne Schwabe wrote:
> This limits the nubmer of packets OpenVPN will respond to. This avoid
> OpenVPN server being abused for refelection attacks in a large scale
> as we gotten a lot more efficient with the cookie approach in our
> initial connection approach.
>
> The defaults of 100 attempts per 10s should work for most people,
> esepcially since completed three way handshakes are not counted. So
> the default will throttle connection attempts on server with high packet
> loss or that are actually under a DOS.
>
> The 100 per 10s are similar in size to the old 2.5 and earlier behaviour
> where every initial connection attempt would take up a slot of the
> max-clients sessions and those would only expire after the TLS timeout.
> This roughly translates to 1024 connection attempts in 60s on an
> empty server.
>
[...]
> diff --git a/doc/man-sections/server-options.rst
> b/doc/man-sections/server-options.rst
> index 99263fff3..cbb2c3c92 100644
> --- a/doc/man-sections/server-options.rst
> +++ b/doc/man-sections/server-options.rst
> @@ -184,6 +184,25 @@ fast hardware. SSL/TLS authentication must be used in
> this mode.
> For the best protection against DoS attacks in server mode, use
> ``--proto udp`` and either ``--tls-auth`` or ``--tls-crypt``.
I think we should add a sentence about the relationship between connect-freq and
connect-freq-initial. Because users will wonder. Specifically are there attacks
that connect-freq protects against that connect-freq-initial does not?
>
> +--connect-freq-initial args
> + (UDP only) Allow a maximum of ``n`` initial connection packet responses
> + per ``sec`` seconds from to clients.
Remove "from"?
> +
> + Valid syntax:
> + ::
> +
> + connect-freq-initial n sec
> +
> + OpenVPN starting at 2.6 is very efficient in responding to initial
> + connection packets. When not limiting the initial responses
> + an OpenVPN daemon can be abused in reflection attacks.
> + This option is designed to limit the rate OpenVPN will respond to initial
> + attacks.
> +
> + Connection attempts that complete the initial three-way handshake
> + will not be counted against the limit. The default is to allow
> + 100 initial connection per 10s.
> +
> --duplicate-cn
> Allow multiple clients with the same common name to concurrently
> connect. In the absence of this option, OpenVPN will disconnect a client
[...]
> diff --git a/src/openvpn/mudp.c b/src/openvpn/mudp.c
> index c27c6da5b..4ef2f355b 100644
> --- a/src/openvpn/mudp.c
> +++ b/src/openvpn/mudp.c
> @@ -82,6 +82,16 @@ do_pre_decrypt_check(struct multi_context *m,
> struct openvpn_sockaddr *from = &m->top.c2.from.dest;
> int handwindow = m->top.options.handshake_window;
>
> + if (verdict == VERDICT_VALID_RESET_V3 || verdict ==
> VERDICT_VALID_RESET_V2)
> + {
> + /* Check if we are still below our limit for sending out
> + * responses */
> + if (!reflect_filter_rate_limit_check(m->initial_rate_limiter))
> + {
> + return false;
> + }
> + }
> +
> if (verdict == VERDICT_VALID_RESET_V3)
> {
> /* Extract the packet id to check if it has the special format that
> @@ -244,6 +254,10 @@ multi_get_create_instance_udp(struct multi_context *m,
> bool *floated)
>
> if (frequency_limit_event_allowed(m->new_connection_limiter))
> {
> + /* a successful three-way handshake only counts against
> + * connect-freq but not against connect-req-initial */
"connect-freq-initial"
> +
> reflect_filter_rate_limit_decrease(m->initial_rate_limiter);
> +
> mi = multi_create_instance(m, &real);
> if (mi)
> {
> diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
> index 0ba509fa0..1a7f278f7 100644
> --- a/src/openvpn/multi.c
> +++ b/src/openvpn/multi.c
> @@ -52,6 +52,7 @@
> #include "crypto_backend.h"
> #include "ssl_util.h"
> #include "dco.h"
> +#include "reflect_filter.h"
>
> /*#define MULTI_DEBUG_EVENT_LOOP*/
>
> @@ -368,6 +369,8 @@ multi_init(struct multi_context *m, struct context *t,
> bool tcp_mode)
> */
> m->new_connection_limiter = frequency_limit_init(t->options.cf_max,
> t->options.cf_per);
> + m->initial_rate_limiter =
> initial_rate_limit_init(t->options.cf_initial_max,
> +
> t->options.cf_initial_per);
>
> /*
> * Allocate broadcast/multicast buffer list
> @@ -729,6 +732,7 @@ multi_uninit(struct multi_context *m)
> mbuf_free(m->mbuf);
> ifconfig_pool_free(m->ifconfig_pool);
> frequency_limit_free(m->new_connection_limiter);
> + initial_rate_limit_free(m->initial_rate_limiter);
> multi_reap_free(m->reaper);
> mroute_helper_free(m->route_helper);
> multi_tcp_free(m->mtcp);
[...]
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index ee3783046..e756af948 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -480,6 +480,7 @@ static const char usage_message[] =
> " as well as pushes it to connecting clients.\n"
> "--learn-address cmd : Run command cmd to validate client virtual
> addresses.\n"
> "--connect-freq n s : Allow a maximum of n new connections per s
> seconds.\n"
> + "--connect-freq-initial n s : Allow a maximum of n replies for initial
> connections attempts per s seconds.\n"
> "--max-clients n : Allow a maximum of n simultaneously connected
> clients.\n"
> "--max-routes-per-client n : Allow a maximum of n internal routes per
> client.\n"
> "--stale-routes-check n [t] : Remove routes with a last activity
> timestamp\n"
> @@ -864,6 +865,8 @@ init_options(struct options *o, const bool init_gc)
> o->n_bcast_buf = 256;
> o->tcp_queue_limit = 64;
> o->max_clients = 1024;
> + o->cf_initial_per = 10;
> + o->cf_initial_max = 100;
> o->max_routes_per_client = 256;
> o->stale_routes_check_interval = 0;
> o->ifconfig_pool_persist_refresh_freq = 600;
> @@ -1555,6 +1558,8 @@ show_p2mp_parms(const struct options *o)
> SHOW_BOOL(duplicate_cn);
> SHOW_INT(cf_max);
> SHOW_INT(cf_per);
> + SHOW_INT(cf_initial_max);
> + SHOW_INT(cf_initial_per);
> SHOW_INT(max_clients);
> SHOW_INT(max_routes_per_client);
> SHOW_STR(auth_user_pass_verify_script);
> @@ -7452,6 +7457,22 @@ add_option(struct options *options,
> options->cf_max = cf_max;
> options->cf_per = cf_per;
> }
> + else if (streq(p[0], "connect-freq-initial") && p[1] && p[2] && !p[3])
> + {
> + long cf_max, cf_per;
> +
> + VERIFY_PERMISSION(OPT_P_GENERAL);
> + char *e1, *e2;
> + cf_max = strtol(p[1], &e1, 10);
> + cf_per = strtol(p[2], &e2, 10);
> + if (cf_max < 0 || cf_per < 0 || *e1 != '\0' || *e2 != '\0')
> + {
> + msg(msglevel, "--connect-freq-initial parameters must be
> integers and >= 0");
> + goto err;
> + }
> + options->cf_initial_max = cf_max;
> + options->cf_initial_per = cf_per;
> + }
> else if (streq(p[0], "max-clients") && p[1] && !p[2])
> {
> int max_clients;
Regards,
--
Frank Lichtenheld
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