Debugging SMS Delivery

Occasionally SMS will fail for a range of reasons. The best way to start debugging SMS failing is with a PCAP trace. 

Tracing an SMS

The application used for making traces can vary between systems and application. Wireshark, Tcpdump, TShark, etc. These are different applications for capturing and displaying network packet captures. This will enable to look at what the call is doing on the wire. 

A command will often look like the following and can be started from most nix based systems command line. 

tcpdump -w trace.pcap -i any

This simple command will produce a trace of all packets for any port on any interface and put it in a file called trace.pcap for use in a program like Wireshark for decoding the information. 

SMS as HTTPs

SMS is setup over HTTPs, this will require that a certificate is in place for a domain. The domain will require configuring as the SMS endpoint and the application put in place will require the certificate for that domain to be available to it. 

It is however possible to ensure that messages are being attempted by opening a socket and attempting to send messaging. A TCP connection will be attempted from one of our advertised IP addresses. Opening a local socket to receive the initial handshake request can be done with the following. 

netcat -l <IP_ADDRESS> 443

This will ensure that a local socket is open and the TCP request for the initial handshake can be received. The command above for making a PCAP trace can be used to filter for TCP traffic from our IP to ensure that the messaging will arrive.

400 Bad Request

This will occur if you are running an HTTP server to listen for requests. HTTPs will require to be hosted to listen on a domain with an associated certificate. An HTTP listener will see traffic arrive, but will give a response reporting malformed content if it is encrypted. 

Certificates

Always ensure that you have a valid certificate applied to your SMS endpoint to ensure proper delivery. If SMS suddenly stops working check the expiry date for your certificate and renew if required. 

Raise an Issue with cloudnumbering

When raising an issue with cloudnumbering support, we require the following information to investigate issues with SMS delivery. 

To number

From number

UTC Timestamp of SMS

Error received

Preferably we require 3 examples of the failures in case we require to escalate with any of our providers.