admin wrote in msgid:

Hi, I'm trying to use ipfw's limit clause to limit the number of
connections a single IP can have at the same time in a transparent
web-proxy environment:

00350 skipto 401 tcp from x.x.x.x/x,y.y.y.y/y,z.z.z.z/z to any dst-port
80 in via if0 setup limit src-addr 10
00401 fwd,8080 tcp from x.x.x.x/x to any dst-port 80
... the rest fwd...

as I understand the manpage, when the current number of connectiions is
below 10, the action "skipto" is performed, else, the packet is dropped
and the search terminates. But...

the problem is that the src-addr limit is not enforced as some clients
somehow open a huge number (3-5 times the prescribed value) of
www-connections to some single address Out There, forcing you to bump up
certain sysctl variables (such as kern.ipc.nmbclusters,
kern.ipc.maxsockets, etc.) to mitigate the DOS effects. What might be
going on? Is ipfw broken, or am I misusing it?

OS: FreeBSD 6.2

I tested ipfw with the "limit" option and it works just fine.
I can open only one http connection from "" and hangs on
opening a second one with an error in the logfile.

# add 03000 allow log logamount 50 tcp from any to any dst-port 80 in limit 
dst-addr 1

My logfile:
Feb 18 16:16:57 jeremino kernel: ipfw: 3000 Accept TCP in via dc1
Feb 18 16:16:58 jeremino kernel: drop session, too many entries

You get the point. I know, indeed it works just great for many clients, including myself, but *some* clients manage to ignore the firewall rule and open many more connections in the ESTABLISHED state than allowed and eat up lots of memory with their send/recv queues... Instead of knocking my head on the wall I opted for posting here for help ;-)

I've decided to prove that I'm not crazy. This little code utilizes the BSD sockets API trying to open many connections to some outside web-site but just halts after crossing the limit (assuming the connections get transparently proxied by the problem firewalled-FreeBSD-proxy box on its path).

The question remains: why could some clients be immune to the limit?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>


int main(void)
        struct sockaddr_in sock_addr;
        struct in_addr in_addr;
        int i;

        if (inet_aton("", &in_addr) == 0) {
                return EXIT_FAILURE;

        sock_addr.sin_family = AF_INET;
        sock_addr.sin_addr = in_addr;
        sock_addr.sin_port = htons(80);
        for (i = 0; i < NUM_CONNS_TO_TRY_TO_OPEN; i++) {
                int s;

if ((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1) {
                        return EXIT_FAILURE;
if (connect(s, (struct sockaddr *) &sock_addr, sizeof sock_addr) != -1) {
                        fprintf(stderr, "%d ", i);
                } else {
                        return EXIT_FAILURE;

        return 0;

_______________________________________________ mailing list
To unsubscribe, send any mail to "[EMAIL PROTECTED]"

Reply via email to