ipfw limit src-addr woes

admin admin at azuni.net
Mon Feb 19 08:13:16 UTC 2007

Andre Santos wrote:
> On 2/18/07, admin <admin at azuni.net> wrote:
>> 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 local.ip.ad.dr,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
> The following command worked here (6.2-RC1). Only one connection was
> allowed to
> # ipfw add 1 allow tcp from any to 22 out via rl1 limit dst-addr 1
> Use the command "ipfw -d show" to see what connections are matching
> your dynamic rules.

# ipfw -d show | fgrep x.x.x.x | wc -l
$ netstat -na|fgrep x.x.x.x|fgrep ESTABLISHED|wc -l

Why is it that only 20 connections have been accounted for by ipfw's 
dynamic rules but there are actually 113 active connections from that IP 
at the moment? The limit src-addr is 75.

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