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 destination 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 according 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 connections above to the given limit

The number of connections counted are system wide, not service specific. For example for saddr, you cannot 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:

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with daddr to limit the connections a server can accept

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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


              
    

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 connections to work. If knockd has not allowed new connections any traffic entering this chain will just return 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.

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 examples.

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 definition. 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 remote 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
    

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.

redirect; redirect to-port port¶

redirect is used internally by FireHOL helper commands.

Only FireHOL developers should need to use this action directly.