\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 prot
"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\fR, \fB--hashlimit-above\fR) and
\fB--hashlimit-name\fR are required.
.TP
\fB--hashlimit-upto\fR \fIamount\fR[\fB/second\fR|\fB/minute\fR|\fB/hour\fR|\fB/day\fR]
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\fR \fIamount\fR[\fB/second\fR|\fB/minute\fR|\fB/hour\fR|\fB/day\fR]
Match if the rate is above \fIamount\fR/quantum.
.TP
\fB--hashlimit-burst\fR \fIamount\fR
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\fR [\fBsrcip\fR|\fBsrcport\fR|\fBdstip\fR|\fBdstport\fR[\fB,\fR...]]
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\fR \fIprefix\fR
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\fR \fIprefix\fR
Like --hashlimit-srcmask, but for destination addresses.
.TP
\fB--hashlimit-name\fR \fIfoo\fR
The name for the /proc/net/ipt_hashlimit/foo entry.
.TP
\fB--hashlimit-htable-size\fR \fIbuckets\fR
The number of buckets of the hash table
.TP
\fB--hashlimit-htable-max\fR \fIentries\fR
Maximum entries in the hash.
.TP
\fB--hashlimit-htable-expire\fR \fImsec\fR
After how many milliseconds do hash entries expire.
.TP
\fB--hashlimit-htable-gcinterval\fR \fImsec\fR
How many milliseconds between garbage collection intervals.