Monitor "aliveness" of default bridges in Tor Browser
Tor Browser ships with several dozen default bridges. We currently have no automated way of learning when one of these bridges disappear, e.g., because the owner cannot afford her VPS bill anymore. If this happens, Tor Browser would then ship with dead bridges, which don't help anyone. This has happened before. Also, note that this ticket is not about measuring censorship.
We should find out when any of our default bridges disappear. A simple way to do so would be to test its TCP reachability, i.e., see if it's possible to establish a TCP handshake with its bridge port. Ideally, we want a test like: "ping the bridge n times per week and mark it as "alive" if it responded to at least n - m SYN segments." We should also think about how to sync a list of default bridges to test with the actual list of default bridges in Tor Browser, so we are testing what needs testing.
Prometheus may be able to help here, and has an exporter that can measure a machine's TCP reachability. Here's what anarcat found:
$ curl 'http://localhost:9115/probe?target=google.com:80&module=tcp_connect' # HELP probe_dns_lookup_time_seconds Returns the time taken for probe dns lookup in seconds # TYPE probe_dns_lookup_time_seconds gauge probe_dns_lookup_time_seconds 0.046783699 # HELP probe_duration_seconds Returns how long the probe took to complete in seconds # TYPE probe_duration_seconds gauge probe_duration_seconds 0.060479041 # HELP probe_failed_due_to_regex Indicates if probe failed due to regex # TYPE probe_failed_due_to_regex gauge probe_failed_due_to_regex 0 # HELP probe_ip_protocol Specifies whether probe ip protocol is IP4 or IP6 # TYPE probe_ip_protocol gauge probe_ip_protocol 6 # HELP probe_success Displays whether or not the probe was a success # TYPE probe_success gauge probe_success 1