We have a IOT device with several SIM cards aggregated to provided the communication for the upper layer applications.

During the test, we got several communications problems, after a short time debug and contacted with operators, we get to know that is the IOT DNS restrictions.

Say we have two SIM cards, and we want to access example.com.

We send DNS messages via the first one, and after that we can access the site via the first SIM card. If we access the site via the second card, the communication will be blocked. Because from the second card of view, the source is unknown.

To get around of it, the simple solution is to do name resolve periodically from all the SIM cards.

Ping is the first one came to mind, it has a -I option to specify the iterface/address.

-I interface
       interface  is  either an address, or an interface name.  If interface is an address, it sets source address to specified interface address.  If interface in an
       interface name, it sets source interface to specified interface.  For IPv6, when doing ping to a link-local scope address, link specification (by the '%'-nota‐
       tion in destination, or by this option) is required.

But it doesn't work, it only ensure ICMP message other than DNS message, even we use domain as the target.

The other one is dig, which has the similar option as ping:

-b address[#port]
    Set the source IP address of the query. The address must be a valid address on one of the host's network interfaces, or "0.0.0.0" or "::". An
    optional port may be specified by appending "#<port>"

It ensure DNS message via the specific address, and after periodicall dig the problem is solved.

#IOT #dig #DNS