I use an iSavi IsatHub for Internet connectivity when I'm in the backcountry out of cellular service. This device is a BGAN terminal and it works by establishing a data link with an Inmarsat satellite and then provides a WiFi network to connect your devices.

Data usage is very expensive, so it offers a number of safeguards to prevent accidental usage. One of those safeguards is a firewall.

I only need SSH connectivity, so I blocked all outgoing ports with the exception of TCP port 22, UDP 53 and TCP 53. All inbound traffic is allowed.

This past week I was in the backcountry far from cell service. I fired up the IsatHub and connected my phone to the WiFi network. Much to my surprise, I started receiving text messages to my Verizon phone. More to my surprise, I was able to reply to those text messages and have a back and forth conversation. The satellite terminal has its own SIM card and SMS capabilities, but these messages were received over my Verizon SIM.

I've searched the net for any documentation on network ports used for WiFi texting and calling. I found this and it does include TCP53 and UDP53 (which I assume are for DNS lookups), but it also includes UDP500 and UDP4500 which are standard IPSEC VPN ports.

Does anyone have an idea of how this was possible? How the heck was I able to send text messages over WiFi with my Verizon phone when I've blocked all outgoing ports except those needed for SSH and DNS? Is it somehow related to accepting all inbound traffic? Could Verizon be re-appropriating port 53 for WiFi texting?

Edit: When I returned home, I connected my phone (airplane mode, wifi turned on) to my home WiFi network, sent a test text message and took a capture of the network ports in use. The only ports I saw in use between my phone and Verizon-owned IP addresses were ports UDP500, UDP4500 and TCP443 (500 and 4500 being used for the IPSEC VPN ports I mentioned earlier).

  • Unfortunately, questions about consumer-grade devices are explicitly off-topic here. You could try to ask this question on Android Enthusiasts or Ask Different, depending on your phone type.
    – Ron Maupin
    Jul 31 '17 at 22:10
  • @RonMaupin I disagree. This crux of this question involves the possible misappropriation of network ports or tunneling by a cellular carrier for WiFi texting. This is precisely the subject matter of this site. The Android phone is completely irrelevant.
    – Elliot B.
    Aug 1 '17 at 0:22
  • Why is it misappropriation? There is no law saying that any network device cannot use any open ports to perform the function for which it was created. If your phone wants to use any open ports, then that is up to the phone and its manufacturer. It is a consumer-grade device and a host, both of which are off-topic here. If you want to disagree with the closure, the box above explains that you can take it up on Network Engineering Meta.
    – Ron Maupin
    Aug 1 '17 at 0:30
  • @RonMaupin Did you even trouble yourself to read the scenario I've described? It would appear that Verizon runs their VPN for WiFi texting over port 53 (DNS) when the standard ports for an IPSEC VPN (UDP500 and UDP4500) are not available. That's what I call misappropriation. Call it whatever you want. That's not the point. The point is that I have a complicated networking question and this is the most appropriate Q&A site for the question. This is where the experts are on this subject matter. You believe my question is off-topic because my phone is an Android? Are you serious?
    – Elliot B.
    Aug 1 '17 at 0:39
  • What a host does, even by creating a VPN over non-standard ports is off-topic here. Again, for the third time, if you disagree with the closure, then take it up on Network Engineering Meta. You should really be taking it up with the phone manufacturer and/or Verizon, but we only deal with the network devices, not end-user devices, here, and only if the manufacturer offers optional, paid support for the network device (enterprise and some small-business devices). If it is Security you are worried about, then Information Security is the place to ask.
    – Ron Maupin
    Aug 1 '17 at 0:42

Could Verizon be re-appropriating port 53 for WiFi texting?

It's certainly plausible. Developers can get very creative in trying to get around network lockdown.

To find out for sure you would need to get a network sniffer on the connectionn

  • Do you recommend any tools for the job? Since I'm unable to check network ports on my phone nor on the BGAN terminal I'm assuming I'd have to bring in my Windows laptop and run some sort of WiFi sniffing program.
    – Elliot B.
    Jul 31 '17 at 21:38

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