How to know when a service goes away?

[probonopd]

https://github.com/grandcat/zeroconf#browse-for-services-in-your-local-network shows how to browse for services. But how can we catch the event when a service goes away?

avahi-browse -ar shows such events with

- enp0s25 IPv6 Go web server                                 Web Site             local

[frank-xlj]

I have the same question, and also how to unregister the service? Anybody knows if it's supported or not.
Hi @probonopd, have you found a way to get the notification when service goes away? I saw a PR is using TTL to distinguish the service is new added or removed. not sure if it makes sense.

[probonopd]

Unfortunately I have not found a way so far, but I did not search very deeply.

[betamos]

Second this, it's critical for keeping an up-to-date data structure of current clients. We'd need to:

1. Allow the user to know about expired service entries through the TTL values.
2. Handle "goodbye packets", which should expire a previously seen service entry immediately.

My understanding is that currently none of these "events" are available to the user, in that neither continuously refreshed entries, nor if they're expired through (1) or (2), will emit any new events after the initial discovery. Only the initial discovery is reliably observed.

Both are addressed in open PRs #93 and #60. However, they may conflict with each other as-is. For reference, in libp2p#31 both types of disconnects can be detected reliably and without duplicates.

Happy to help any way I can.

[probonopd]

Thanks @betamos, looking forward to seeing PRs #93 and #60 being integrated into main.

[jemoran42]

@grandcat Could we see the above mentioned PRs reviewed/merged soon? Thanks, everyone!

[betamos]

FYI I am actively maintaining a fork-of-a-fork at https://github.com/betamos/zeroconf

• It supports both explicit and timeout-based unannouncements
• It maintains state and only notifies when something changes
• However, it presents a different API so it's not a drop-in replacement

[KiddoV]

Any update or merge status on this?
In the meantime, I think we can do something like:

if resolver, err := zeroconf.NewResolver(nil); err != nil {
    log.Fatalln("Failed to initialize resolver:", err.Error())
} else {
    entries := make(chan *zeroconf.ServiceEntry)
    go func(results <-chan *zeroconf.ServiceEntry) {
        for entry := range results {
            peerAddr := fmt.Sprintf("%s:%d", entry.AddrIPv4[0].String(), entry.Port)
            log.Println("Discovered peer", peerAddr)  //<- Found the service
            //Track for peer disconnection
            go func() {
                if tcpConn, err := net.Dial("tcp", peerAddr); err != nil {
                    log.Println("Failed connecting to peer", peerAddr)
                } else {
                    defer tcpConn.Close()
                    for {
                        if _, err := tcpConn.Read(make([]byte, 1)); err != nil {
                            log.Println("Peer was disconnected", peerAddr) //<- Detect service goes away
                            return //Stop this routine
                        }
                    }
                }
            }
        }
    }(entries)}

Not sure if this is a good work-around?