End-user troubleshooting of bad c-ares interaction with router

[Brad House]

What version of c-ares is installed?

What DNS servers are configured on your MacOS system when its not 
operating properly?  The output of "scutil --dns" would be helpful here.

On 1/17/24 12:09 PM, Nicholas Chammas via c-ares wrote:
> Hello,
>
> I am trying to troubleshoot a problem as an end-user of c-ares. I use 
> a library (Apache Spark Connect 
> <https://spark.apache.org/docs/latest/spark-connect-overview.html>) 
> that uses gRPC, which in turn uses c-ares. I am two levels removed 
> from c-ares itself and am a little out of my depth.
>
> I have a little Python script that connects to a remote Apache Spark 
> cluster via Spark Connect and runs a test query. When I run this 
> script on my home network, it takes over 20 seconds to run. When I 
> tether my workstation to my phone (which is connected via LTE), the 
> same script runs in a second or two. In both cases the script runs 
> successfully.
>
> I enabled some gRPC debug flags which print out a lot of information. 
> This led me to c-ares, as I believe the difference in runtime is 
> related somehow to DNS.
>
> I’ve extracted the log lines output by gRPC related to c-ares 
> <https://gist.github.com/nchammas/a4c9873d8158c323796e9b47c064e63a>. 
> (Be sure to scroll down to see both files; there is one for home and 
> one for LTE.) The gRPC codebase is hosted on GitHub, where you can 
> find the grpc_ares_wrapper.cc 
> <https://github.com/grpc/grpc/blob/v1.60.0/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.cc> file 
> mentioned in the log files.
>
> I tried changing the DNS servers configured in my home router but that 
> didn’t seem to help. Interestingly, however, if I set the same DNS 
> servers already configured in my home router directly on the network 
> interface I’m using, the 20 second delay disappears:
>
> ```
> networksetup -setdnsservers “My Network" 1.1.1.1 1.0.0.1
> ```
>
> But this setting doesn’t persist across restarts, and only Spark 
> Connect seems to have this problem. It seems there is some kind of bad 
> interaction between c-ares and my router.
>
> How can I dig deeper to understand what’s going wrong with my home 
> network? I checked the c-ares docs <https://c-ares.org/docs.html> but 
> I don’t see a way for an end-user to enable debug output from c-ares, 
> e.g. via an environment variable.
>
> Any suggestions? I’m running macOS 14.2.1. The router is an Apple AirPort.
>
> Nick
>
>
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