GUESSNET(8)

📦 version 0.58build2, in ubuntu jammy, ubuntu noble

💅 GUESSNET — the manual page, but make it make sense

section 8, System Manager's Manual — yeah bestie, this is a sysadmin page, not one of your little userland ones 🎀

✨ NAME ✨

guessnet — guess which LAN a network interface is connected to 🦋

🌸 SYNOPSIS 🌸

guessnet [options] [network_interface]

💖 DESCRIPTION 💖

Okay so, plot twist: guessnet guesses which LAN a network interface is connected to. Iconic behaviour honestly.

Here's the tea: you hand it a list of candidate profiles, and each profile has a test description attached. guessnet runs all the tests in parallel and prints the name of the profile whose test was the first one to succeed. If literally no test succeeds within a certain timeout period, then it prints a default profile name instead. And the second it prints a profile name, it immediately kills any tests that are still running and dips out. Bye 👋

Where do those candidate profiles come from? Either from a test description file, or — in ifupdown mode — from /etc/network/interfaces. Slay.

🎀 OPTIONS 🎀

Options follow the usual GNU conventions. And in ifupdown mode, options can also be specified on the standard input in the form "<long-option-name>: <value>". Cute right?

  • -C, --config-file=filename — the name of the configuration file to use, if it wasn't specified on the command line. Default: standard input, or /etc/network/interfaces in ifupdown mode.
  • --autofilter — only useful when you're operating in ifupdown mode (peep the section below 👇). Tells guessnet to only consider logical interface names that start with the physical interface name being mapped. Like, eth0-home only matches when mapping eth0. Default: false.
  • --debug — print debugging messages. ✨ detective mode ✨
  • -d, --default=string — the interface name to print if no known networks are found. Default: none.
  • --help — show a brief summary of command line options. (Bestie behaviour.)
  • -i, --ifupdown-mode — operate in ifupdown mode: parse the input as if it's in the format of /etc/network/interfaces, and read from /etc/network/interfaces instead of the standard input if the configuration filename isn't specified. See the ifupdown mode section below for the details.
  • --init-time=int — time in seconds to wait for the interface to initialize when it's not found already up at program startup. Default: 3 seconds.
  • --init-delay=int — sleep a given number of seconds before starting operations. Lowkey useful when the interface driver needs a little time to settle before it reacts to commands. Default: 0 seconds.
  • --iwscan-tries=int — retry wireless network scanning a given amount. Useful if your driver needs a few attempts to return a network list. Default: 1.
  • --syslog — 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. 🗣️
  • --version — show the version number of the program.

🦋 TEST DESCRIPTION FILE 🦋

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 192.168.1.1 00:01:02:03:04:05

# At the university, check for the presence of at least one
# of the following hosts
university peer 130.136.1.1 05:06:03:02:01:0A
university peer 130.136.1.2 15:13:B3:A2:2F:CD

# If the peer doesn't reply to ARP packets coming from 0.0.0.0
# then you can additionally specify a source address to use
university peer 130.136.1.2 15:13:B3:A2:2F:CD 130.136.1.250

# 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. One line = one test, no cap.

  • 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 — and these vary depending on the test type. iykyk.

🌈 IFUPDOWN MODE 🌈

ifupdown, Debian's standard network configuration system, lets you define different "logical interfaces" (that's ifupdown's name for configuration profiles) and choose among them when you configure a network interface. And here's the kicker: that choice can be delegated to an external "mapping" program. guessnet can be used as such a program — if you run it in "ifupdown mode". guessnet runs in ifupdown mode if it's invoked as guessnet-ifupdown, or if it's 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. Chef's kiss 👨🍳💋

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 info. If names are preceded with a "!" character then the match is inverted, meaning all logical interfaces will be processed <em>except</em> the ones specified in standard input. And heads up: you cannot mix normal and negated interface names in the same mapping directive. Note: when using the autofilter option (see above), you can broaden or tighten the automatic matching by specifying interface names as described.

⚠️ Bestie, listen. You have to specify the fully qualified path to guessnet (/usr/sbin/guessnet-ifupdown), because otherwise it won't be run at system boot — /usr/sbin is not on the PATH of the 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 exists because ifupdown cannot pass command line arguments to mapping scripts. Not ideal but here we are 💅

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

Also: 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. Slay, we adapt 🦋

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
     address 192.168.1.2
     netmask 255.255.255.0
     broadcast 192.168.1.255
     gateway 192.168.1.1

     # Lines for resolvconf (if you use it: see apt-cache show resolvconf)
     # dns-search casa
     # dns-nameservers 192.168.1.1 192.168.2.1

     # Two tests, in case one of the two machines is down when we test
     test1 peer address 192.168.1.1 mac 00:01:02:03:04:05
     test2 peer address 192.168.1.3 mac 00:01:02:03:04:06

iface work inet static
     address 10.1.1.42
     netmask 255.255.255.0
     broadcast 10.1.1.255
     gateway 10.1.1.1
     test command /usr/local/bin/check_work

iface work2 inet static
     address 192.168.2.23
     netmask 255.255.255.0
     broadcast 192.168.2.255
     gateway 192.168.2.1
     # A source address has to be specified in case the peer
     # doesn't reply to ARP packets coming from 0.0.0.0
     test peer address 192.168.2.1 mac 00:01:02:03:04:05 source 192.168.2.23

iface eth1-home inet static
     wireless-essid Home
     wireless-key s:myverysecret
     address 192.168.1.5
     netmask 255.255.255.0
     gateway 192.168.1.1
     dns-nameservers 192.168.1.1
     # 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

...and that's where part 1 dips, bestie. More coming, don't go anywhere 💖

✨ Supported tests ✨

💖 peer 💖

📝 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]

💬 Description:

Okay so bestie, this one is looking for a peer using ARP. The test is a total slay if there's a network interface with the specified IP address (and MAC address if you specified one) connected to the local network. 💅

You can totally omit the MAC address, and in that case guessnet only tests for the presence of a host with the specified IP address. Easy peasy. ✨

Now, if the peer whose presence you want to test for is being extra and refuses to reply to ARP packets coming from 0.0.0.0, then you gotta specify some source IP address from which the peer will accept requests. Like, give them a valid return address, bestie. 📬

You can specify multiple peers (on multiple lines) but each peer must have a different IP address. This restriction may be eliminated in the future, so keep an eye out. 👀

You can also omit the IP address and only use the MAC: that's super useful to test for the existence of physical interfaces with changing IP addresses. This kind of scan uses an ICMP ping packet and requires a source address in most cases, as hosts tend not to reply to pings coming from nowhere. Like, they're not gonna answer if they don't know who's calling. 🙈

🦋 wireless 🦋

📝 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]

💬 Description:

This one performs a wireless scan like iwlist scan does, and then matches the results. It's giving very much "let me see what's around me". 📡

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. So it's a vibe check for the network. ✨

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. So put your faves first, periodt. 👑

🌸 missing-cable 🌸

📝 Test description file syntax:

profile missing-cable

⚡ Ifupdown mode syntax:

test missing-cable

💬 Description:

This one checks for link beat. The test is successful if link beat is not detected. So if there's no cable, it's a win? Iconic. 🎀

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. Like, why bother if there's no physical connection? 💀

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. So if that's your vibe, check out ifplugd(8). 🫶

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. So if your hardware is not giving, this test won't slay. 😭

🎀 command 🎀

📝 Test description file syntax:

profile command command

⚡ Ifupdown mode syntax:

test command command

💬 Description:

Test using an arbitrary command. The test is considered successful if the command terminates with exit status 0. So if it exits clean, you're good. ✅

Location name and interface name are exported to the script via the NAME and IFACE environment variables. So your script can know where it's at. 🌍

For backward compatibility, script can be used instead of command. So you can still use the old school name, no cap. 🧢

🧪 Experimental tests 🧪

🧚 pppoe 🧚

📝 Test description file syntax:

profile pppoe

⚡ Ifupdown mode syntax:

test pppoe

💬 Description:

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. So it's like, hey, is there a modem here? 📞

Using this test requires that pppoe be installed on the system. So make sure you have it, bestie. 💾

🌈 wireless 🌈

📝 Test description file syntax:

profile wireless [mac MAC-address] [essid ESSID]

⚡ Ifupdown mode syntax:

test wireless [mac MAC-address] [essid ESSID]

💬 Description:

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. So order matters, okay? 💅

Blanks may be included in the ESSID. For example,

prof1 wireless essid My LAN

tests for an ESSID of "My LAN". So you can have spaces, it's fine. ✨

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. So it's just looking, not touching. 👀

Note that the wireless test is not yet implemented cleanly. So it's a work in progress, bestie. <orange>🚧</orange>

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. So like, don't stress about cleanup, the signal will handle it. 💀

📝 NOTES 📝

🕵️ 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. So many options, bestie! 🕵️‍♀️

🧩 Multiple tests

Currently guessnet only supports specifying one kind of test per profile. So like, one test per profile, that's the rule. 🚫

👀 SEE ALSO 👀

✍️ AUTHOR ✍️

Guessnet was written by Enrico Zini <enrico@debian.org> with contributions from Thomas Hood. The ARP network detection code was taken from laptop-netconf by Matt Kern <matt@debian.org>, which in turn in based on divine by Felix von Leitner <felix@fefe.de>. So much talent in this collab! 👑

The Guessnet webpage is at http://guessnet.alioth.debian.org. Go check it out! 🌐

That's the whole vibe, bestie! This manual page is for GUESSNET(8) and it's dated 4 November 2007. Stay slaying with your network detection! 💖✨

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