guessnet(8) guess which LAN a network interface is connected to


guessnet [options] [network_interface]


Guessnet guesses which LAN a network interface is connected to. Given a list of candidate profiles each of which includes a test description, guessnet runs all the tests in parallel and prints the name of the profile whose test was the first one to succeed. If no test succeeds within a certain timeout period then a default profile name is printed. After printing a profile name, guessnet immediately kills any tests that are still running and exits.

Candidate profiles are read either from a test description file or, in ifupdown mode, from /etc/network/interfaces.


Options follow the usual GNU conventions. In ifupdown mode, options can also be specified on the standard input in the form "<long-option-name>: <value>".
-C, --config-file=filename
Name of the configuration file to use if not specified on command line. Default: standard input or /etc/network/interfaces in ifupdown mode.
Only useful when operating in ifupdown mode (see below). Instructs guessnet to only consider logical interface names that start with physical interface name being mapped. (ie: eth0-home only matches when mapping eth0) Default: false.
Print debugging messages.
-d, --default=string
Interface name to print if no known networks are found. Default: none.
Show a brief summary of command line options.
-i, --ifupdown-mode
Operate in ifupdown mode: parse the input as if it is in the format of /etc/network/interfaces and read from /etc/network/interfaces instead of the standard input if the configuration filename is not specified. See the ifupdown mode section below for details.
Time in seconds to wait for the interface to initialize when it is not found already up at program startup. Default: 3 seconds.
Sleep a given number of seconds before starting operations. May be useful in case interface driver needs a little time to settle before reacting to commands. Default: 0 seconds.
Retry wireless network scanning a given amount. Useful if your driver needs some attempts to return a network list. Default: 1.
Send messages to syslog facility DAEMON, in addition to stderr.
-t, --timeout=int
Timeout in seconds used to wait for tests to terminate. Default: 5 seconds.
-v, --verbose
Operate verbosely.
Show the version number of the program.


guessnet takes as input a description of the tests it should perform. The test description file looks like this:

  # Empty lines and lines starting with '#' are ignored.
  # Other lines contain:
  #   <profile-name> <test-method> <parameters>
  # At home, look for a host with the given IP and MAC address
  home peer 00:01:02:03:04:05
  # At the university, check for the presence of at least one
  # of the following hosts
  university peer 05:06:03:02:01:0A
  university peer 15:13:B3:A2:2F:CD
  # If the peer doesn't reply to ARP packets coming from
  # then you can additionally specify a source address to use
  university peer 15:13:B3:A2:2F:CD
  # For the work network use a custom script
  work command /usr/local/bin/check_work
  # Commands are executed by "sh -c" so shell syntax can be used
  john-irda command grep -q `cat ~enrico/john-irda-id` /proc/net/irda/discovery
  # Location name and interface name are exported in NAME and IFACE
  weirdnet command /usr/local/bin/weirddetect "$NAME" "$IFACE"
  # Profile "none" is selected if no network signal is detected
  # (i.e. there is no cable plugged into the socket)
  no-net missing-cable
  # Match a wireless network with the given essid
  home wireless essid Home
  # You can also match the mac address of the access point
  home wireless mac 01:02:03:04:0A:0B
  # Or both
  home wireless essid Home mac 01:02:03:04:0A:0B
  # You can also match any open network
  anyopen wireless open

Every non-comment line represents a test to perform.

The first word in the line is the name that will be printed if the test succeeds.

The second word is the test type.

The remainder of the line contains parameters for the selected test; these vary depending on the test type.


ifupdown, Debian's standard network configuration system, permits one to define different "logical interfaces" (ifupdown's name for configuration profiles) and to choose among them when one configures a network interface. The choice can be delegated to an external "mapping" program. guessnet can be used as such a program if it is run in "ifupdown mode". guessnet runs in ifupdown mode if it is invoked as guessnet-ifupdown or if it is given the --ifupdown-mode option.

In ifupdown mode guessnet reads test data directly from the logical interface definitions in /etc/network/interfaces rather than from a separate test description file.

In ifupdown mode if names are passed to guessnet on its standard input then guessnet considers only those logical interface definitions; otherwise it considers them all. You can have ifupdown deliver data to guessnet's standard input using the map directive. See interfaces(5) for more information. If names are preceded with "!" character then match is inverted, meaning that all logical interfaces will be processed except for the ones specified in standard input. You cannot mix normal and negated interface names in the same mapping directive. Note: when using autofilter option (see above) you can broaden or tighten the automatic matching by specifying interface names as descripted.

Please note that you have to specify the fully qualified path to guessnet (/usr/sbin/guessnet-ifupdown), as otherwise it won't be run at system boot, as /usr/sbin is not on PATH of networking init script any more. Also, you need to ensure /usr is actually mounted at that moment.

In ifupdown mode options are selected by passing "<long-option-name>: <value>" on guessnet's standard input. This feature is provided because ifupdown cannot pass command line arguments to mapping scripts.

If you prefer you can precede the test keyword in /etc/network/interfaces with the word guessnet.

ifupdown does not allow two option lines in /etc/network/interfaces to start with the same word. To work around this limitation, multiple test (or guessnet) lines can have different numerals suffixed to their initial keywords (test1, test2, or guessnet1, guessnet2, and so on).

Here's an example of an /etc/network/interfaces file that has been set up for guessnet:

auto lo eth0
iface lo inet loopback
mapping eth0
        script /usr/sbin/guessnet-ifupdown
        # Scan all logical interfaces
        # More options can be given here, such as:
        # map timeout: 10
        # map verbose: true
        # map debug: true
        # map iwscan-tries: 23
        map default: none
mapping eth1
        script /usr/sbin/guessnet-ifupdown
        # Disable open net checking, just comment out if you are
        # desperate enough :) (see relative stanza below)
        map !eth1-anyopen
        # Scan only logical interfaces named eth1-*
        map autofilter: true
iface home inet static
        # Lines for resolvconf (if you use it: see apt-cache show resolvconf)
        # dns-search casa
        # dns-nameservers
        # Two tests, in case one of the two machines is down when we test
        test1 peer address mac 00:01:02:03:04:05
        test2 peer address mac 00:01:02:03:04:06
iface work inet static
        test command /usr/local/bin/check_work
iface work2 inet static
        # A source address has to be specified in case the peer
        # doesn't reply to ARP packets coming from
        test peer address mac 00:01:02:03:04:05 source
iface eth1-home inet static
        wireless-essid Home
        wireless-key s:myverysecret
        # Match a wireless network with the given essid
        test wireless essid Home
        # You can also match the mac address of the access point
        #test wireless mac 01:02:03:04:0A:0B
        # Or both
        #test wireless essid Home mac 01:02:03:04:0A:0B
iface eth1-work inet dhcp
        wireless-essid Work
        wireless-key s:myverysecretkey
        # Match a wireless network with the given essid
        # If you have spaces in the essid, use double quotes
        test wireless essid "Work place"
iface eth1-anyopen inet dhcp
        # You can also match any open network, if you are desperate :)
        wireless-essid any
        wireless-mode auto
        test wireless open
# If nothing else is found, try DHCP
iface none inet dhcp

Supported tests


Test description file syntax:
profile peer IP-address [MAC-address] [IP-address]
Ifupdown mode syntax:
test peer address IP-address [mac MAC-address] [source IP-address]
Look for peer using ARP. The test will succeed if a network interface with the specified IP address (and MAC address if specified) is connected to the local network.

One can omit the MAC address, in which case guessnet only tests for the presence of a host with the specified IP address.

If the peer whose presence you want to test for refuses to reply to ARP packets coming from then specify some source IP address from which the peer will accept requests.

Multiple peers can be specified (on multiple lines) but each peer must have a different IP address. This restriction may be eliminated in the future.

You can also omit the IP address and only use the MAC: that is useful to test for the existance of physical interfaces with changing IP addresses. This kind of scan uses an ICMP ping packet requires a source address in most cases, as hosts tend not to reply to pings coming from nowhere.


Test description file syntax:
profile wireless [essid essid] [mac MAC-address] [open|closed]
Ifupdown mode syntax:
test wireless [essid essid] [mac MAC-address] [open|closed]
Perform a wireless scan like iwlist scan does, and match the results.

The test succeeds if the scan reports at least one network for which all the tests (essid, mac of the access point, network is open or closed) match.

In case more than one profile matches a network, only the first one, as found in the configuration file, will succeed. This allows prioritising profiles: for example, you can prefer your home access point to an open network by listing it first in the configuration file.


Test description file syntax:
profile missing-cable
Ifupdown mode syntax:
test missing-cable
Check for link beat. The test is successful if link beat is not detected.

This feature allows guessnet to detect the case where there is no cable plugged into a network socket; in this case it makes no sense to go through other detection phases.

This test can be used in ifupdown mode too if a dummy logical interface is defined that includes the test missing-cable option. Bear in mind that when the cable is unplugged, ifupdown will consider the interface to be configured as this dummy logical interface. That is somewhat counterintuitive; one might prefer the interface to be deconfigured in that case. Unfortunately, guessnet is not currently able to tell ifup to refrain from configuring an interface. The problem can be solved, however, by means of the ifplugd(8) program.

Link beat detection is not supported on all network hardware. If the interface or its driver does not support link beat detection then this test does not succeed.


Test description file syntax:
profile command command
Ifupdown mode syntax:
test command command
Test using an arbitrary command. The test is considered successful if the command terminates with exit status 0.

Location name and interface name are exported to the script via the NAME and IFACE environment variables.

For backward compatibility, script can be used instead of command.

Experimental tests


Test description file syntax:
profile pppoe
Ifupdown mode syntax:
test pppoe
Use the pppoe program to send PADI packets in order to look for access concentrators. The test should succeed if a PPPOE modem is present on the given interface.

Using this test requires that pppoe be installed on the system.


Test description file syntax:
profile wireless [mac MAC-address] [essid ESSID]
Ifupdown mode syntax:
test wireless [mac MAC-address] [essid ESSID]
Test certain properties of the wireless interface. More specifically, test the MAC address and/or the ESSID of the associated access point. If both are given then MAC-address must precede ESSID.

Blanks may be included in the ESSID. For example,

    prof1 wireless essid My LAN
tests for an ESSID of "My LAN".

Note that the wireless test does not attempt to change these properties; it only examines them. This test is designed to work with programs such as waproamd which independently and dynamically manage the wireless network adapter to keep it associated to an access point.

Note that the wireless test is not yet implemented cleanly.

Note that if one of several tests terminates successfully then any other tests still running will be terminated with the KILL signal. Therefore, test programs should not need to do any special cleanup on exit.


Getting remote host MAC addresses

When you prepare the test data for guessnet you may need to know the MAC address of a remote interface in the local network. There are various ways to obtain this. The easiest is to use the arping utility by doing "arping [hostname]". If you don't have arping installed on your system then try the command "arp -a [hostname]" which will display the MAC address if it is in the ARP cache of your machine. You might want to ping the remote interface first to make sure that you have the information in the cache. You can also take a look at the /usr/share/doc/guessnet/examples/getmac script.

Multiple tests

Currently guessnet only supports specifying one kind of test per profile.


Guessnet was written by Enrico Zini <[email protected]> with contributions from Thomas Hood. The ARP network detection code was taken from laptop-netconf by Matt Kern <[email protected]>, which in turn in based on divine by Felix von Leitner <[email protected]>.

The Guessnet webpage is at .