Can ipfw be used to limit concurrent requests from an IP?

Ian Smith smithi at
Sat May 28 14:28:12 UTC 2016

In freebsd-questions Digest, Vol 625, Issue 7, Message: 3
On Fri, 27 May 2016 20:34:56 +0100 Will Squire <will_squire at> wrote:

(please wrap lines < 80 columns if possible)

 > Can ipfw limit the number requests in a given amount of time from a 
 > specific IP?
 > To contextualise, if an IP sends requests in high concurrency (let's 
 > say 50 a second) can ipfw either block requests the exceed a 
 > threshold for that second (lets say the threshold is 20, 30 would be 
 > blocked), or ban/deny the given IP for exceeding a threshold?

Not as such.  If you know the specific IP address (or range, or subnet) 
you can use stateful rules with 'limit' instead of 'keep-state' to limit 
the maximum number of concurrent connections to the port/s configured in 
a given rule; see ipfw(8).  You cauld use a table of addresses to block
or limit rather than hard-coding them into rule/s.

While this is very useful for avoiding DoS of any particular service, it 
does not allow you to specify a rate, nor time limit, nor (directly) to 
block an IP address that's exceeding the given number of connections.

 > The aim is to lessen strain under DoS attacks, specifically for HTTP. 
 > The system is using Apache and mod_evasive has been added and tested, 
 > but it is not functioning correctly.

I haven't used (nor heard of) mod_evasive so can't comment on that, but 
limiting the total number of connections open to a given service can 
certainly mitigate the effect of such DoS attacks.

You could of course use /etc/inetd.conf (aka TCPwrappers) to limit 
connections in just the ways you want, though I'm not sure starting HTTP 
connections in that way is recommended these days.  I use if for FTP and 
POP3 connections, which works very well, thus:

sola# grep -v '#' /etc/inetd.conf
ftp     stream  tcp     nowait/7/3 root /usr/libexec/ftpd  ftpd -dll -S
pop3    stream  tcp     nowait/7/4 root /usr/local/libexec/qpopper qpopper -s -T 120

See inetd(1), particularly re the inetd.conf setting:

The above example limits pop3 connections to 7 children and 4 
connections per IP per minute.  Excess connections are logged to 
/var/log/messages (and console.log if enabled) thus:

May 21 12:31:59 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)
May 21 14:21:51 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)
May 21 14:21:52 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)
May 21 14:26:40 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)
May 21 15:34:53 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)
May 21 16:26:56 sola inetd[9671]: pop3 from exceeded counts/min (limit 4/min)

You could run a script to tail messages hunting for such lines, then add 
the IP to a table if you want; for example I run a script that instantly 
bans GET requests for certain strings to any of a number of webservers. 
I also tend to check logs and hand-add naughty nets such as the above to 
a block table, never to be seen again ..

I also use not dissimilar connection limits to sendmail's MTA, but 
that's done in sendmail's own configuration.

Others may know better ways to deal specifically with HTTP connections?

 > (P.S. The freebsd-ipfw list seems to be for development of the 
 > technology only, so asking this here. Please let me know if this 
 > isn?t the case)

It's usually fairly low volume and noone seems to mind usage questions, 
though the developers usually tend to let these go by.

cheers, Ian

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