\fBhashlimit\fR uses hash buckets to express a rate limiting match (like the
\fBlimit\fR match) for a group of connections using a \fBsingle\fR iptables
rule. Grouping can be done per-hostgroup (source and/or destination address)
and/or per-port. It gives you the ability to express "\fIN\fR packets per time
quantum per group":
.TP
matching on source host
"1000 packets per second for every host in 192.168.0.0/16"
.TP
matching on source port
"100 packets per second for every service of 192.168.1.1"
.TP
matching on subnet
"10000 packets per minute for every /28 subnet in 10.0.0.0/8"
.PP
A hash limit option (\fB\-\-hashlimit\-upto\fP, \fB\-\-hashlimit\-above\fP) and
\fB\-\-hashlimit\-name\fP are required.
.TP
\fB\-\-hashlimit\-upto\fP \fIamount\fP[\fB/second\fP|\fB/minute\fP|\fB/hour\fP|\fB/day\fP]
Match if the rate is below or equal to \fIamount\fR/quantum. It is specified as
a number, with an optional time quantum suffix; the default is 3/hour.
.TP
\fB\-\-hashlimit\-above\fP \fIamount\fP[\fB/second\fP|\fB/minute\fP|\fB/hour\fP|\fB/day\fP]
Match if the rate is above \fIamount\fR/quantum.
.TP
\fB\-\-hashlimit\-burst\fP \fIamount\fP
Maximum initial number of packets to match: this number gets recharged by one
every time the limit specified above is not reached, up to this number; the
default is 5.
.TP
\fB\-\-hashlimit\-mode\fP {\fBsrcip\fP|\fBsrcport\fP|\fBdstip\fP|\fBdstport\fP}\fB,\fP...
A comma-separated list of objects to take into consideration. If no
\-\-hashlimit\-mode option is given, hashlimit acts like limit, but at the
expensive of doing the hash housekeeping.
.TP
\fB\-\-hashlimit\-srcmask\fP \fIprefix\fP
When \-\-hashlimit\-mode srcip is used, all source addresses encountered will be
grouped according to the given prefix length and the so-created subnet will be
subject to hashlimit. \fIprefix\fR must be between (inclusive) 0 and 32. Note
that \-\-hashlimit\-srcmask 0 is basically doing the same thing as not specifying
srcip for \-\-hashlimit\-mode, but is technically more expensive.
.TP
\fB\-\-hashlimit\-dstmask\fP \fIprefix\fP
Like \-\-hashlimit\-srcmask, but for destination addresses.
.TP
\fB\-\-hashlimit\-name\fP \fIfoo\fP
The name for the /proc/net/ipt_hashlimit/foo entry.
.TP
\fB\-\-hashlimit\-htable\-size\fP \fIbuckets\fP
The number of buckets of the hash table
.TP
\fB\-\-hashlimit\-htable\-max\fP \fIentries\fP
Maximum entries in the hash.
.TP
\fB\-\-hashlimit\-htable\-expire\fP \fImsec\fP
After how many milliseconds do hash entries expire.
.TP
\fB\-\-hashlimit\-htable\-gcinterval\fP \fImsec\fP
How many milliseconds between garbage collection intervals.