ip-sentinel man page

ip-sentinel — keeps your IP space clean


ip-sentinel [--user|-u username] [--group|-g groupname] [--chroot|-r dir] [--ipfile|-i file] [--pidfile|-p file] [--logfile|-l file] [--errfile|-e file] [--mac mac] [--llmac mac] [--direction dir] [--action program] [--nofork|-n] [--help|-h] [--version] interface


ip-sentinel starts a daemon which tries to prevent unauthorized usage of IPs within an ethernet broadcastdomain by answering ARP requests. After receiving such a faked reply, the requesting party stores the MAC address in its ARP-table and will send future packets to this invalid MAC, rendering the IP unreachable.

After startup it opens its logfiles, goes into a chroot jail, drops privileges and forks itself into background by default. The parameters of these operations can by changed by the options described below.

Warning: with an uncareful configuration, ip-sentinel can bring down the entire network and your physical presence at the ip-sentinel machine and/or the switch will be required to fix it. Some switches can show an unexpected behavior are even crash when the special 802.* mac-addresses will be used.


--user|-u username and --group|-g groupname

After doing basical initializations, the program will drop its privilegies by changing its user and group ID. The username and groupname parameters determine these new IDs and must be an alphanumeric username/groupname or a numeric uid/gid.

Attention: When having a dietlibc-compiled version in a system which uses remote NSS (e.g. LDAP, NIS) to make passwd-lookups, you will have to use numeric IDs.

--chroot|-r dirname

Specifies the directory where the program shall go into and call chroot() after finishing its initialization. By default it is the homedirectory of the user specified with the --user option.

--ipfile|-i filename

Sets the name of the configuration file which contains the list of forbidden IPs. This name is relative to the chroot and is ips.cfg by default. This file will be reread automatically after changes.

--pidfile|-p filename

Sets the file where the pid of the daemon will be written into. This filename is relative to the root-filesystem and is /var/run/ip-sentinel.run by default.

--logfile|-l filename

Sets the file where normal operations will be logged. Normal operations are e.g. an ARP-reply for a forbidden IP. This filename is relative to the root-filesystem and /var/log/ip-sentinel.out by default.

--errfile|-e filename

Sets the file where error-conditions will be logged. Error-conditions are e.g. detected DOS attacks (bombing the ip-sentinel with ARP-requests) or errors in the configuration file. This filename is relative to the root-filesystem and /var/log/ip-sentinel.err by default.


Prevents daemon from going into background after initialization. This option may be useful when using advanced init-concepts like minit (http://www.fefe.de/minit/).

--direction dir

ip-sentinel can answer both requests asking for an intruder's address, and requests coming from an intruder asking for official addresses. This parameter specifies which requests shall be answered. Possible values are FROM (requests from intruders), TO (requests to intruders) and BOTH. The current default value is TO, but will probably change to BOTH in future versions.

--mac mac

Use mac as the default faked mac-address; see below for possible values. This parameter can be overriden in the ipfile for each questionable IP. The default value is RANDOM.

--llmac mac

Specifies the mac-address which will be used in the link-level headers when answering requests from intruders. Additionally to the values described below, ip-sentinel understands the SAME string, which means that the address from the ARP-header will be used. When not using the FROM or BOTH directions, the assigned value will not have any effect. The default is LOCAL.


When an intruder sends a request from a registered ip address and this option is set, ip-sentinel will send ARP replies which are poisoning this ip. When the @!srcmac syntax is used for the questionable ip, the srcmac is used as the is-at mac. Else, the mac specified by the --mac option is used.

This option takes an affect only when --destination BOTH or FROM is specified.

--action program

Execute program when a faked ARP reply will be generated. It will be called with 6 arguments:

  • type 0 when an intruder sent the request, 1 when a protected IP was requested and -1 else (should never happen)
  • spa the protocol address (IP) of the source
  • sha the hardware address (MAC) of the source
  • tpa the protocol address (IP) of the target
  • tha the hardware address (MAC) of the target
  • mac the generated MAC address

The program will be executed only without evaluating its return value or something else. Please note, that the program will be executed in the chroot with restricted rights.


Prints help message and exits.


Prints the version of the program and exits.


Specifies the network-interface where ip-sentinel listens for ARP-requests and sends ARP-replies. It is a name like "eth0"; aliases like "eth0:1" are not possible.

Configuration File

The location of the configfile is relatively to the choosen chroot-directory. Its default is determined at compilation time but it can be overriden by the --ipfile option.

The content of this file has the following syntax:

ip [mac]

Specifies an IP which shall be blocked. "Blocked" means that ARP-replies will be generated telling that the IP address ip has the MAC mac. When the MAC parameter is not given, a random one will be choosen.

Attention: Assigning a MAC in a switched environment will not have the desired effect, because the switch will be confused when receiving ARP-replies for the same IP on different ports (this one of the bad-host and this one of the ip-sentinel host).

Examples are: "  11:22:33:44:55:66" or "".

ip@[!]srcmac [mac]

Similarly to above, but when receiving ARP-requests, the entire sender-address (IP and MAC) will be compared against the ip and srcmac values. This setting affects packets from intruders only; ip-sentinel will not generate replies when ip is requested.

When the srcmac is negated with "!", only addresses from this ip which are not having this mac will be matching.

Examples are: "" or "!a:b:c:d:e:f".

*@srcmac [mac]

Every host using the srcmac will be blocked unless there is a matching ip@srcmac specification. This option does not support '!'  modifiers.

net/mask [mac]
net/mask@[!]srcmac [mac]

Specifies that IPs of an entiry network shall be blocked. See above for the meaning of "blocked". It is possible to specify a MAC address; else a random one will be generated.

Examples are: "", "" or " a:b:c:d:e:f".


The given IP address ip will be ignored. By default, any not specified address will be ignored but when having blocked netmasks it may be usefully to allow certain IPs.

An example is: "!"


Tells that IPs of the given network shall be ignored.

Examples are: "!" or "!" (the default).

# comment (and empty lines)

A comment; will be ignored

To be switch-friendly, there are only a few random MACs possible which are having the format "de:ad:be:ef:00:XX". Within a short timespan only 32 values are possible for XX.

When having overlapping networks and/or single IPs, this one with the most specified netmask (count of 1's) takes precedence. When netmasks are equal, networks which are using the "@srcmac" or "@!srcmac" syntax are taking precedence over those without source-macs. This "@..." rule does not apply to IPs. The behavior is unspecified when having overlapping networks with the same count of 1's and "@..."  specification, or when having duplicate IPs.


The lookup of single IPs has a complexity of O(log n) and this of netmasks a complexity of O(n).

Special MAC Addresses

Beside the usual hex-octets-delimited-by-colons mac addresses, ip-sentinel understands some special strings both on the commandline and in the configuration file:


expands to the mac-address of the used interface


means a random mac-address which is newly calculated on every usage


expands to 01:80:C2:00:00:00 which is the "Bridge Group Address".


expands to 01:80:C2:00:00:01 which is the "IEEE Std. 802.3x Full Duplex PAUSE operation". This MAC address will be blocked by a lot of switches and will probably become the default in future versions.

The 802.* addresses are having a special meaning for some switches and packets having them as destination-address will be dropped by the switch instead of flooding all ports. But it depends on the used switch how/if these macs are honored.

Some switches can show an unexpected behavior are even crash if the special 802.* mac-addresses will be used.


Except in comments, it is possible to specify ranges everywhere in the configuration file. These ranges are having the format "{from-to}" or "{item1,item2,...,itemN}". The first format includes any number beginning at "from" till "to" (inclusive), while the latter format expands to the listed items only. The expansion happens on a line-level and it is possible to use more than one range per line, so that

192.168.0.{1-3}     0:0:0:0:0:1
192.168.1.{1,3}     0:0:0:0:0:2

192.168.{2,4}.{1-3} 0:0:0:0:0:3

is the same like writing         0:0:0:0:0:1         0:0:0:0:0:1         0:0:0:0:0:1         0:0:0:0:0:1         0:0:0:0:0:1         0:0:0:0:0:3         0:0:0:0:0:3         0:0:0:0:0:3         0:0:0:0:0:3         0:0:0:0:0:3         0:0:0:0:0:3

Because there can be created very much entries with a single line (e.g. "{0-255}.{0-255}.{0-255}.{0-255}" would cover the entire IPv4 internet), ranges should be used sparely. When possible, large ranges should be expressed with netmasks.

Example                 ## Block anything
!           ## Allow IPs of the form 192.168.0.*               ## but block a:b:c:d:e:f   ## use a special mac for 802.1d        ## and 01:80:C2:00:00:00 for!1:1:1:1:1:1

*@b:b:b:b:b:b             ## block MAC b:b:b:b:b:b regardless of the IP

This setup will not send ARP-replies for the IPs 192.168.0.{3-255} but when a host tries to use e.g., ip-sentinel will tell that this IP has a MAC of "de:ad:be:ef:00:XX".

When an intruder is at "" and uses the mac "a:a:a:a:a:a:", a faked reply will be generated. Users at the same ip but another mac will be ignored.

In opposite, users with ip "" and mac "1:1:1:1:1:1" will be ignored but intruders with other macs (e.g. "2:2:2:2:2:2") are getting faked replies. When --poision is used, ip-sentinel will generate a " is at 1:1:1:1:1:1" arp-reply to a broad address.

See Also

RFC 826, IEEE Std 802.1D


Enrico Scholz <enrico.scholz@informatik.tu-chemnitz.de>


Dec 15 2004 ip-sentinel 0.11