Provided by: firehol-doc_3.1.7+ds-3_all 
NAME
firehol-actions - actions for rules
SYNOPSIS
accept
accept with hashlimit name upto|above amount/period [burst amount] [mode {srcip|srcport|dstip|dst‐
port},...] [srcmask prefix] [dstmask prefix] [htable-size buckets] [htable-max entries] [htable-expire
msec] [htable-gcinterval msec]
accept with connlimit upto|above limit [mask mask] [saddr|daddr]
accept with limit requests/period burst [overflow action]
accept with recent name seconds hits
accept with knock name
reject [with message]
drop | deny
return
tarpit
DESCRIPTION
These actions are the actions to be taken on traffic that has been matched by a particular rule.
FireHOL will also pass through any actions that iptables(8) accepts, however these definitions provide
lowercase versions which accept arguments where appropriate and which could otherwise not be passed
through.
Note
The iptables(8) LOG action is best used through the optional rule parameter log since the latter
can be combined with one of these actions (FireHOL will generate multiple firewall rules to make
this happen). For more information see log and loglimit.
The following actions are defined:
accept
accept allows the traffic matching the rules to reach its destination.
For example, to allow SMTP requests and their replies to flow:
server smtp accept
accept with hashlimit name upto|above amount/period [burst amount] [mode {srcip|srcport|dstip|dstport},...]
[srcmask prefix] [dstmask prefix] [htable-size buckets] [htable-max entries] [htable-expire msec]
[htable-gcinterval msec]
hashlimit hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a group
of connections using a single iptables rule. Grouping can be done per-hostgroup (source and/or destina‐
tion address) and/or per-port.
name The name for the /proc/net/ipt_hashlimit/name entry.
upto amount[/second|/minute|/hour|/day] Match if the rate is below or equal to amount/quantum. It is
specified either as a number, with an optional time quantum suffix (the default is 3/hour).
above amount[/second|/minute|/hour|/day] Match if the rate is above amount/quantum.
burst amount 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. This option should be used
with caution - if the entry expires, the burst value is reset too.
mode {srcip|srcport|dstip|dstport},... A comma-separated list of objects to take into consideration. If
no mode option is given, srcip,dstport is assumed.
srcmask prefix When –hashlimit-mode srcip is used, all source addresses encountered will be grouped ac‐
cording to the given prefix length and the so-created subnet will be subject to hashlimit. prefix must
be between (inclusive) 0 and 32. Note that srcmask 0 is basically doing the same thing as not specifying
srcip for mode, but is technically more expensive.
dstmask prefix Like srcmask, but for destination addresses.
htable-size buckets The number of buckets of the hash table
htable-max entries Maximum entries in the hash.
htable-expire msec After how many milliseconds do hash entries expire.
htable-gcinterval msec How many milliseconds between garbage collection intervals.
Examples:
Allow up to 5 connections per second per client to SMTP server:
server smtp accept with hashlimit smtplimit upto 5/s
You can monitor it using the file /proc/net/ipt_hashlimit/smtplimit
accept with connlimit upto|above limit [mask mask] [saddr|daddr]
accept with connlimit matches on the number of connections per IP.
saddr matches on source IP. daddr matches on destination IP. mask groups IPs with the mask given upto
matches when the number of connections is up to the given limit above matches when the number of connec‐
tions above to the given limit
The number of connections counted are system wide, not service specific. For example for saddr, you can‐
not connlimit 2 connections for SSH and 4 for SMTP. If you connlimit 2 connections for SSH, then the
first 2 connections of a client can be SSH. If a client has already 2 connections to another service,
the client will not be able to connect to SSH.
So, connlimit can safely be used:
• with daddr to limit the connections a server can accept
• with saddr to limit the total connections per client to all services.
accept with limit requests/period burst [overflow action]
accept with limit allows the traffic, with new connections limited to requests/period with a maximum
burst. Run iptables -m limit --help for more information.
The default overflow action is to REJECT the excess connections (DROP would produce timeouts on otherwise
valid service clients).
Examples:
server smtp accept with limit 10/sec 100
server smtp accept with limit 10/sec 100 overflow drop
accept with recent name seconds hits
accept with recent allows the traffic matching the rules to reach its destination, limited per remote IP
to hits per seconds. Run iptables -m recent --help for more information.
The name parameter is used to allow multiple rules to share the same table of recent IPs.
For example, to allow only 2 connections every 60 seconds per remote IP, to the smtp server:
server smtp accept with recent mail 60 2
Note
When a new connection is not allowed, the traffic will continue to be matched by the rest of the
firewall. In other words, if the traffic is not allowed due to the limitations set here, it is
not dropped, it is just not matched by this rule.
accept with knock name
accept with knock allows easy integration with knockd (http://www.zeroflux.org/projects/knock/), a server
that allows you to control access to services by sending certain packets to “knock” on the door, before
the door is opened for service.
The name is used to build a special chain knock_<name> which contains rules to allow established connec‐
tions to work. If knockd has not allowed new connections any traffic entering this chain will just re‐
turn back and continue to match against the other rules until the end of the firewall.
For example, to allow HTTPS requests based on a knock write:
server https accept with knock hidden
then configure knockd to enable the HTTPS service with:
iptables -A knock_hidden -s %IP% -j ACCEPT
and disable it with:
iptables -D knock_hidden -s %IP% -j ACCEPT
You can use the same knock name in more than one FireHOL rule to enable/disable all the services based on
a single knockd configuration entry.
Note
There is no need to match anything other than the IP in knockd. FireHOL already matches every‐
thing else needed for its rules to work.
reject
reject discards the traffic matching the rules and sends a rejecting message back to the sender.
reject with message
When used with with the specific message to return can be specified. Run iptables -j REJECT --help for a
list of the --reject-with values which can be used for message. See REJECT WITH MESSAGES for some exam‐
ples.
The default (no message specified) is to send tcp-reset when dealing with TCP connections and icmp-port-
unreachable for all other protocols.
For example:
UNMATCHED_INPUT_POLICY="reject with host-prohib"
policy reject with host-unreach
server ident reject with tcp-reset
drop; deny
drop discards the traffic matching the rules. It does so silently and the sender will need to timeout to
conclude it cannot reach the service.
deny is a synonym for drop. For example, either of these would silently discard SMTP traffic:
server smtp drop
server smtp deny
return
return will return the flow of processing to the parent of the current command.
Currently, the only time return can be used meaningfully used is as a policy for an interface definition.
Unmatched traffic will continue being processed with the possibility of being matched by a later defini‐
tion. For example:
policy return
tarpit
tarpit captures and holds incoming TCP connections open.
Connections are accepted and immediately switched to the persist state (0 byte window), in which the re‐
mote side stops sending data and asks to continue every 60-240 seconds.
Attempts to close the connection are ignored, forcing the remote side to time out the connection after
12-24 minutes.
Example:
server smtp tarpit
Note
As the kernel conntrack modules are always loaded by FireHOL, some per-connection resources will
be consumed. See this bug report (http://bugs.sanewall.org/sanewall/issues/10) for details.
The following actions also exist but should not be used under normal circumstances:
mirror
mirror returns the traffic it receives by switching the source and destination fields. REJECT will be
used for traffic generated by the local host.
Warning
The MIRROR target was removed from the Linux kernel due to its security implications.
MIRROR is dangerous; use it with care and only if you understand what you are doing.
redirect; redirect to-port port
redirect is used internally by FireHOL helper commands.
Only FireHOL developers should need to use this action directly.
REJECT WITH MESSAGES
The following RFCs contain information relevant to these messages:
• RFC 1812 (http://www.ietf.org/rfc/rfc1812.txt)
• RFC 1122 (http://www.ietf.org/rfc/rfc1122.txt)
• RFC 792 (http://www.ietf.org/rfc/rfc0792.txt)
icmp-net-unreachable; net-unreach
ICMP network unreachable
Generated by a router if a forwarding path (route) to the destination network is not available.
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 792.
Note
Use with care. The sender and the routers between you and the sender may conclude that the
whole network your host resides in is unreachable, and prevent other traffic from reaching you.
icmp-host-unreachable; host-unreach
ICMP host unreachable
Generated by a router if a forwarding path (route) to the destination host on a directly connected net‐
work is not available (does not respond to ARP).
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 792.
Note
Use with care. The sender and the routers between you and the sender may conclude that your
host is entirely unreachable, and prevent other traffic from reaching you.
icmp-proto-unreachable; proto-unreach
ICMP protocol unreachable
Generated if the transport protocol designated in a datagram is not supported in the transport layer of
the final destination.
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 792.
icmp-port-unreachable; port-unreach
ICMP port unreachable
Generated if the designated transport protocol (e.g. TCP, UDP, etc.) is unable to demultiplex the
datagram in the transport layer of the final destination but has no protocol mechanism to inform the
sender.
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 792.
Generated by hosts to indicate that the required port is not active.
icmp-net-prohibited; net-prohib
ICMP communication with destination network administratively prohibited
This code was intended for use by end-to-end encryption devices used by U.S. military agencies.
Routers SHOULD use the newly defined Code 13 (Communication Administratively Prohibited) if they admin‐
istratively filter packets.
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 1122.
Note
This message may not be widely understood.
icmp-host-prohibited; host-prohib
ICMP communication with destination host administratively prohibited
This code was intended for use by end-to-end encryption devices used by U.S. military agencies.
Routers SHOULD use the newly defined Code 13 (Communication Administratively Prohibited) if they admin‐
istratively filter packets.
From RFC 1812, section 5.2.7.1. See RFC 1812 and RFC 1122.
Note
This message may not be widely understood.
tcp-reset
TCP RST
The port unreachable message of the TCP stack.
See RFC 1122.
Note
tcp-reset is useful when you want to prevent timeouts on rejected TCP services where the client
incorrectly ignores ICMP port unreachable messages.
SEE ALSO
• firehol(1) - FireHOL program
• firehol.conf(5) - FireHOL configuration
• firehol-interface(5) - interface definition
• firehol-router(5) - router definition
• firehol-params(5) - optional rule parameters
• FireHOL Website (http://firehol.org/)
• FireHOL Online PDF Manual (http://firehol.org/firehol-manual.pdf)
• FireHOL Online Documentation (http://firehol.org/documentation/)
AUTHORS
FireHOL Team.
FireHOL Reference Built 24 Jan 2024 firehol-actions(5)