diff --git a/include/esockd.hrl b/include/esockd.hrl
index 8a4a2f6..7a65363 100644
--- a/include/esockd.hrl
+++ b/include/esockd.hrl
@@ -71,4 +71,24 @@
 -type ssl_option() :: ssl:ssl_option().
 -endif. % OTP_RELEASE
 
+
+-define(ERROR_MAXLIMIT, maxlimit).
+
+-define(ARG_ACCEPTED, accepted).
+-define(ARG_CLOSED_SYS_LIMIT, closed_sys_limit).
+-define(ARG_CLOSED_MAX_LIMIT, closed_max_limit).
+-define(ARG_CLOSED_OVERLOADED, closed_overloaded).
+-define(ARG_CLOSED_RATE_LIMITED, closed_rate_limited).
+-define(ARG_CLOSED_OTHER_REASONS, closed_other_reasons).
+
+-define(ACCEPT_RESULT_GROUPS,
+        [
+            ?ARG_ACCEPTED,
+            ?ARG_CLOSED_SYS_LIMIT,
+            ?ARG_CLOSED_MAX_LIMIT,
+            ?ARG_CLOSED_OVERLOADED,
+            ?ARG_CLOSED_RATE_LIMITED,
+            ?ARG_CLOSED_OTHER_REASONS
+        ]).
+
 -endif. % ESOCKD_HRL
diff --git a/src/esockd.erl b/src/esockd.erl
index 4a3e507..bafb581 100644
--- a/src/esockd.erl
+++ b/src/esockd.erl
@@ -411,7 +411,23 @@ parse_opt([_|Opts], Acc) ->
 %% @doc System 'ulimit -n'
 -spec(ulimit() -> pos_integer()).
 ulimit() ->
-    proplists:get_value(max_fds, hd(erlang:system_info(check_io))).
+    find_max_fd(erlang:system_info(check_io)).
+
+find_max_fd([]) ->
+    %% Magic!
+    %% According to Erlang/OTP doc, erlang:system_info(check_io)):
+    %% Returns a list containing miscellaneous information about the emulators
+    %% internal I/O checking. Notice that the content of the returned list can
+    %% vary between platforms and over time. It is only guaranteed that a list
+    %% is returned.
+    1023;
+find_max_fd([CheckIoResult | More]) ->
+    case lists:keyfind(max_fds, 1, CheckIoResult) of
+        {max_fds, N} when is_integer(N) andalso N > 0 ->
+            N;
+        _ ->
+            find_max_fd(More)
+    end.
 
 -spec(to_string(listen_on()) -> string()).
 to_string(Port) when is_integer(Port) ->
diff --git a/src/esockd_acceptor.erl b/src/esockd_acceptor.erl
index 7471953..a1749ae 100644
--- a/src/esockd_acceptor.erl
+++ b/src/esockd_acceptor.erl
@@ -320,12 +320,13 @@ inc_stats(#state{proto = Proto, listen_on = ListenOn}, Tag) ->
     _ = esockd_server:inc_stats({Proto, ListenOn}, Counter, 1),
     ok.
 
-counter(accepted) -> accepted;
-counter(emfile) -> closed_sys_limit;
-counter(enfile) -> closed_sys_limit;
-counter(overloaded) -> closed_overloaded;
-counter(rate_limited) -> closed_rate_limited;
-counter(_) -> closed_other_reasons.
+counter(accepted) -> ?ARG_ACCEPTED;
+counter(emfile) -> ?ARG_CLOSED_SYS_LIMIT;
+counter(enfile) -> ?ARG_CLOSED_SYS_LIMIT;
+counter(?ERROR_MAXLIMIT) -> ?ARG_CLOSED_MAX_LIMIT;
+counter(overloaded) -> ?ARG_CLOSED_OVERLOADED;
+counter(rate_limited) -> ?ARG_CLOSED_RATE_LIMITED;
+counter(_) -> ?ARG_CLOSED_OTHER_REASONS.
 
 start_connection(ConnSup, Sock, UpgradeFuns) when is_pid(ConnSup) ->
     esockd_connection_sup:start_connection(ConnSup, Sock, UpgradeFuns);
diff --git a/src/esockd_connection_sup.erl b/src/esockd_connection_sup.erl
index 395f3b1..ef4de1b 100644
--- a/src/esockd_connection_sup.erl
+++ b/src/esockd_connection_sup.erl
@@ -47,6 +47,8 @@
         , code_change/3
         ]).
 
+-include("esockd.hrl").
+
 -type(shutdown() :: brutal_kill | infinity | pos_integer()).
 
 -type option() :: {shutdown, shutdown()}
@@ -61,7 +63,6 @@
           mfargs :: esockd:mfargs()
          }).
 
--define(DEFAULT_MAX_CONNS, 1024).
 -define(TRANSPORT, esockd_transport).
 -define(ERROR_MSG(Format, Args),
         error_logger:error_msg("[~s] " ++ Format, [?MODULE | Args])).
@@ -149,7 +150,7 @@ call(Sup, Req) ->
 init(Opts) ->
     process_flag(trap_exit, true),
     Shutdown = get_value(shutdown, Opts, brutal_kill),
-    MaxConns = get_value(max_connections, Opts, ?DEFAULT_MAX_CONNS),
+    MaxConns = resolve_max_connections(get_value(max_connections, Opts)),
     RawRules = get_value(access_rules, Opts, [{allow, all}]),
     AccessRules = [esockd_access:compile(Rule) || Rule <- RawRules],
     MFA = get_value(connection_mfargs, Opts),
@@ -162,7 +163,7 @@ init(Opts) ->
 handle_call({start_connection, _Sock}, _From,
             State = #state{curr_connections = Conns, max_connections = MaxConns})
     when map_size(Conns) >= MaxConns ->
-    {reply, {error, maxlimit}, State};
+    {reply, {error, ?ERROR_MAXLIMIT}, State};
 
 handle_call({start_connection, Sock}, _From,
             State = #state{curr_connections = Conns, access_rules = Rules, mfargs = MFA}) ->
@@ -207,7 +208,10 @@ handle_call({add_rule, RawRule}, _From, State = #state{access_rules = Rules}) ->
             end
     catch
         error:Reason ->
-            error_logger:error_msg("Bad access rule: ~p, compile errro: ~p", [RawRule, Reason]),
+            logger:log(error, #{msg => "bad_access_rule",
+                                rule => RawRule,
+                                compile_error => Reason
+                               }),
             {reply, {error, bad_access_rule}, State}
     end;
 
@@ -283,7 +287,12 @@ get_state_option(connection_mfargs, #state{mfargs = MFA}) ->
     MFA.
 
 set_state_option({max_connections, MaxConns}, State) ->
-    State#state{max_connections = MaxConns};
+    case resolve_max_connections(MaxConns) of
+        MaxConns ->
+            State#state{max_connections = MaxConns};
+        _ ->
+            {error, bad_max_connections}
+    end;
 set_state_option({shutdown, Shutdown}, State) ->
     State#state{shutdown = Shutdown};
 set_state_option({access_rules, Rules}, State) ->
@@ -453,3 +462,27 @@ log(Level, Error, Reason, Pid, #state{mfargs = MFA}) ->
 get_module({M, _F, _A}) -> M;
 get_module({M, _F}) -> M;
 get_module(M) -> M.
+
+resolve_max_connections(Desired) ->
+    MaxFds = esockd:ulimit(),
+    MaxProcs = erlang:system_info(process_limit),
+    resolve_max_connections(Desired, MaxFds, MaxProcs).
+
+resolve_max_connections(undefined, MaxFds, MaxProcs) ->
+    %% not configured
+    min(MaxFds, MaxProcs);
+resolve_max_connections(Desired, MaxFds, MaxProcs) when is_integer(Desired) ->
+    Res = lists:min([Desired, MaxFds, MaxProcs]),
+    case Res < Desired of
+        true ->
+            logger:log(error,
+                       #{msg => "max_connections_config_ignored",
+                         max_fds => MaxFds,
+                         max_processes => MaxProcs,
+                         desired => Desired
+                        }
+                      );
+        false ->
+            ok
+    end,
+    Res.
diff --git a/src/esockd_server.erl b/src/esockd_server.erl
index 20572ba..21629e2 100644
--- a/src/esockd_server.erl
+++ b/src/esockd_server.erl
@@ -46,6 +46,8 @@
         , code_change/3
         ]).
 
+-include("esockd.hrl").
+
 -record(state, {
     listener_props :: #{esockd:listener_ref() => #{_Name => _Value}}
 }).
@@ -97,12 +99,7 @@ del_stats({Protocol, ListenOn}) ->
 
 -spec ensure_stats({atom(), esockd:listen_on()}) -> ok.
 ensure_stats(StatsKey) ->
-    Stats = [accepted,
-             closed_sys_limit,
-             closed_overloaded,
-             closed_rate_limited,
-             closed_other_reasons],
-    ok = ?MODULE:init_stats(StatsKey, Stats),
+    ok = ?MODULE:init_stats(StatsKey, ?ACCEPT_RESULT_GROUPS),
     ok.
 
 -spec get_listener_prop(esockd:listener_ref(), _Name) -> _Value | undefined.
diff --git a/test/esockd_SUITE.erl b/test/esockd_SUITE.erl
index d7746c3..5f2b63d 100644
--- a/test/esockd_SUITE.erl
+++ b/test/esockd_SUITE.erl
@@ -222,9 +222,22 @@ t_get_set_max_connections(_) ->
     ?assertEqual(16, esockd:get_max_connections({udp_echo, 7000})),
     ok = esockd:close(udp_echo, 7000).
 
+t_get_set_invalid_max_connections(_) ->
+    MaxFd = esockd:ulimit(),
+    MaxProcs = erlang:system_info(process_limit),
+    Invalid = max(MaxFd, MaxProcs) + 1,
+    {ok, _LSup} = esockd:open(echo, 7000, [{connection_mfargs, echo_server}]),
+    Expected = min(MaxFd, MaxProcs),
+    ?assertEqual(Expected, esockd:get_max_connections({echo, 7000})),
+    esockd:set_max_connections({echo, 7000}, 2),
+    ?assertEqual(2, esockd:get_max_connections({echo, 7000})),
+    esockd:set_max_connections({echo, 7000}, Invalid),
+    ?assertEqual(2, esockd:get_max_connections({echo, 7000})),
+    ok = esockd:close(echo, 7000).
+
 t_get_set_max_conn_rate(_) ->
     LimiterOpt = #{module => esockd_limiter, capacity => 100, interval => 1},
-    {ok, _LSup} = esockd:open(echo, 7000, 
+    {ok, _LSup} = esockd:open(echo, 7000,
                               [{limiter, LimiterOpt}, {connection_mfargs, echo_server}]),
     ?assertEqual({100, 1}, esockd:get_max_conn_rate({echo, 7000})),
     esockd:set_max_conn_rate({echo, 7000}, LimiterOpt#{capacity := 50, interval := 2}),
diff --git a/test/esockd_acceptor_SUITE.erl b/test/esockd_acceptor_SUITE.erl
index f76604c..275d4ee 100644
--- a/test/esockd_acceptor_SUITE.erl
+++ b/test/esockd_acceptor_SUITE.erl
@@ -28,11 +28,13 @@
 -define(COUNTER_OVERLOADED, 2).
 -define(COUNTER_RATE_LIMITED, 3).
 -define(COUNTER_SYS_LIMIT, 4).
--define(COUNTER_OTHER_REASONS, 5).
+-define(COUNTER_MAX_LIMIT, 5).
+-define(COUNTER_OTHER_REASONS, 6).
 -define(COUNTER_LAST, 10).
 
 counter_tag_to_index(accepted) -> ?COUNTER_ACCPETED;
 counter_tag_to_index(closed_sys_limit) -> ?COUNTER_SYS_LIMIT;
+counter_tag_to_index(closed_max_limit) -> ?COUNTER_MAX_LIMIT;
 counter_tag_to_index(closed_overloaded) -> ?COUNTER_OVERLOADED;
 counter_tag_to_index(closed_rate_limited) -> ?COUNTER_RATE_LIMITED;
 counter_tag_to_index(closed_other_reasons) -> ?COUNTER_OTHER_REASONS.
@@ -90,7 +92,7 @@ connect(Port, Timeout, Opts0) ->
 %% This is the very basic test, if this fails, nothing elese matters.
 t_normal(Config) ->
     Port = ?PORT,
-    Server = start(Port, no_limit()),
+    Server = start(Port, no_rate_limit()),
     {ok, ClientSock} = connect(Port),
     try
         ok = wait_for_counter(Config, ?COUNTER_ACCPETED, 1, 2000)
@@ -129,7 +131,7 @@ t_rate_limitted(Config) ->
 %% Failed to spawn new connection process
 t_error_when_spawn(Config) ->
     Port = ?PORT,
-    Server = start(Port, no_limit(), #{start_connection_result => {error, overloaded}}),
+    Server = start(Port, no_rate_limit(), #{start_connection_result => {error, overloaded}}),
     {ok, Sock1} = connect(Port),
     try
         ok = wait_for_counter(Config, ?COUNTER_OVERLOADED, 1, 2000),
@@ -141,7 +143,7 @@ t_error_when_spawn(Config) ->
 %% Failed to tune the socket opts
 t_einval(Config) ->
     Port = ?PORT,
-    Server = start(Port, no_limit(), #{tune_fun => {fun(_) -> {error, einval} end, []}}),
+    Server = start(Port, no_rate_limit(), #{tune_fun => {fun(_) -> {error, einval} end, []}}),
     {ok, Sock1} = connect(Port),
     try
         ok = wait_for_counter(Config, ?COUNTER_OTHER_REASONS, 1, 2000),
@@ -157,7 +159,7 @@ t_sys_limit(Config) ->
     meck:new(prim_inet, [passthrough, no_history, unstick]),
     meck:expect(prim_inet, async_accept, fun(_, _) -> {error, emfile} end),
     Port = ?PORT,
-    Server = start(Port, no_limit()),
+    Server = start(Port, no_rate_limit()),
     try
         %% acceptor to enter suspending state after started
         %% because async_accept always returns {error, emfile}
@@ -178,9 +180,21 @@ t_sys_limit(Config) ->
         stop(Server)
     end.
 
+%% Failed to spawn new connection process
+t_max_limit(Config) ->
+    Port = ?PORT,
+    Server = start(Port, no_rate_limit(), #{start_connection_result => {error, ?ERROR_MAXLIMIT}}),
+    {ok, Sock1} = connect(Port),
+    try
+        ok = wait_for_counter(Config, ?COUNTER_MAX_LIMIT, 1, 2000),
+        disconnect(Sock1)
+    after
+        stop(Server)
+    end.
+
 t_close_listener_socket_cause_acceptor_stop(_Config) ->
     Port = ?PORT,
-    #{acceptor := Acceptor, lsock := LSock} = start(Port, no_limit()),
+    #{acceptor := Acceptor, lsock := LSock} = start(Port, no_rate_limit()),
     Mref = monitor(process, Acceptor),
     unlink(Acceptor),
     unlink(LSock),
@@ -239,8 +253,8 @@ pause_then_allow(Pause) ->
      }.
 
 %% make a no-limit limiter
-no_limit() ->
-    #{module => ?MODULE, name => no_limit}.
+no_rate_limit() ->
+    #{module => ?MODULE, name => no_rate_limit}.
 
 %% limiter callback
 consume(_Token, #{name := pause_then_allow} = Limiter) ->
@@ -250,7 +264,7 @@ consume(_Token, #{name := pause_then_allow} = Limiter) ->
         #{current := allow} ->
             {ok, Limiter}
     end;
-consume(_Token, #{name := no_limit} = Limiter) ->
+consume(_Token, #{name := no_rate_limit} = Limiter) ->
     {ok, Limiter}.
 
 now_ts() -> erlang:system_time(millisecond).