# netclients: notifying myself about new devices at home

I occasionally look at my router's list of known devices and it always shows a long list. I know most devices but not all and the unknown devices make me a bit uncomfortable because I never know what is spying on me nowadays. If a new device appeared a few days ago, then I usually know what it was. If I see a device from 7 months ago, then I don't know what it was and that bothers my perfectionist self. It would be nice if I could get a notification when a new device is added and I could attach a comment to it. But alas my router doesn't provide such functionality.

But wait a moment, nowadays coding is very easy so maybe I could write my own little tool for this? So that's what I ended up doing. This post is just a slice-of-life look at how I approached it.

Figuring out how to read such a list programmatically from Go would be quite a long research undertaking for me. Not so in the AI age; it's very good at finding the details you need. I have a Fritz!Box router and it turns out it has a SOAP interface and people have written handy libraries for it. Listing the hosts it knows is exactly the example code at https://github.com/nitram509/gofritz. I created an API user for this task and copied that code from there:

  session := soap.NewSession(routerAddress, username, password)
  hosts, err := lan.XAvmGetHostList(session)
  if err != nil {
    return fmt.Errorf("netclients.GetNetclientList: %v", err)
  }
  for _, host := range hosts {
    fmt.Fprintf(report, "NetClient name=%s active=%t ip=%s mac=%s\n", host.HostName, host.Active, host.IPAddress, host.MACAddress)
  }

Then I created a simple config file with the auth data and the list of hosts I know about. The config file is one token per line with optional # comments (it's a simplified version of my @/pkgtrim format):

  # This file contains the list of clients on my network.

  # The first three tokens are the router/user/password combo.
  192.168.1.1
  someusernamehere
  somepasswordhere

  # Shared devices.
  fritzbox       # the fiber router's name
  rpi            # my raspberry pi 4
  hue            # Philips Hue Bridge for the lights, under the TV
  gamer          # gaming PC, next to the TV
  printer        # Epson ET-2720 in my office
  tvrouter       # a separate wifi router in access mode for a stronger wifi

  # Private devices.
  Phone1         # My phone
  Phone2         # wifey's phone
  Phone3         # kid's phone
  workpc         # my work PC
  laptop         # wifey's laptop
  DeLonghi       # wifey's coffee machine, sigh...

  # Guest devices.
  iPad           # kid's friend's tablet

I could add a lot more details (e.g. MAC addresses) but I like to start out simply and track only what I truly need. Parsing such a config into a slice of tokens is simple:

  var cfg []string
  for line := range strings.Lines(string(configData)) {
    fields := strings.Fields(line)
    if len(fields) == 0 || fields[0][0] == '#' {
      continue
    }
    cfg = append(cfg, fields[0])
  }
  if len(cfg) < 3 {
    return fmt.Errorf("netclients.KnownListTooSmall got=%d want=3+", len(cfg))
  }
  routerAddress, username, password = cfg[0], cfg[1], cfg[2]
  session := soap.NewSession(routerAddress, username, password)
  hosts, err := lan.XAvmGetHostList(session)
  ...

Now I just put the list into a map and print everything not found in the map:

  known := make(map[string]bool, len(cfg)-3)
  for _, c := range cfg[3:] {
    known[c] = true
  }
  report := &strings.Builder{}
  for _, host := range hosts {
    if !known[host.HostName] {
      fmt.Fprintf(report, "netclients.NewHost name=%s active=%t ip=%s mac=%s\n", host.HostName, host.Active, host.IPAddress, host.MACAddress)
    }
  }

And that's pretty much it. If a new host appears then I mourn the fact and then add it to my known hosts list along with a comment. The next re-run will not nag me about the new host. When the list of new hosts is non-empty, I also print the file I need to edit because otherwise I would forget where I keep this data.

I run it as part of my todo tool that I trigger several times during the day. In general that's where I put stuff I want to check occasionally (see @/task). I hate notifications with a passion but running a tool and having it report my outstanding items is fine. It's not an interrupt if I run the tool explicitly. It's also the tool where I check for new emails and where I check if my blog is still up or has new comments (see @/eventpolling) so this fits quite well there. This code runs in parallel with all the other random checks I have: https://github.com/ypsu/cfg/blob/main/todotool/todo.go.

Anyway, what I'm saying is that coding is easy nowadays so do go ahead and write the tools you need! Life feels better when we make our digital environment match our needs.

published on 2026-06-01

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