CURL my api via .internal is way slower then via the address

Hello Fly community,

I hope this message finds you well. I am currently facing an issue with private networking in my Fly organization and would appreciate some guidance.

When I use the internal address (http://api-example.internal:8080) to curl my API instance from my front-end server, I notice a delay of approximately 5 seconds before receiving the response. However, when I use the address (, the response is instantaneous (from that same server).

I’m curious if anyone has encountered a similar situation or has insights into what might be causing this delay in private networking requests. Your expertise and suggestions would be immensely helpful.

curl -i http://api-example.internal:8080

If anyone could shed some light on this matter, I would be grateful.

Thank you in advance for your assistance!

Interesting! Try out this curl command from here, I’m interested to see if the pause is related to DNS resolution (time_namelookup).

Your case would be something like:

curl -s --connect-timeout 10 \
-w "time_namelookup: %{time_namelookup}, time_connect: %{time_connect}, time_appconnect: %{time_appconnect}, time_pretransfer: %{time_pretransfer}, time_redirect: %{time_redirect}, time_starttransfer: %{time_starttransfer}, time_total: %{time_total}\n" \
-o /dev/null \

And if you grab the private IPv6 of the api-example app, you can try to run a curl request directly to that to further see if DNS resolution might be the issue there.

1 Like

Hi, thanks for the answer, I’ve ran the command multiple times and I get the following outputs :

  • When I curl the instance through the private network, I get a result like this :
time_namelookup: 0.000636, time_connect: 0.202028, time_appconnect: 0.000000, time_pretransfer: 0.202072, time_redirect: 0.000000, time_starttransfer: 0.208683, time_total: 0.208750

However, I sometimes (approximately each 5 times), get a slower response like this :

time_namelookup: 0.000747, time_connect: 1.203455, time_appconnect: 0.000000, time_pretransfer: 1.203511, time_redirect: 0.000000, time_starttransfer: 1.213058, time_total: 1.213119
  • When I curl the instance throught the DNS, I get the following output :
time_namelookup: 0.000705, time_connect: 0.000860, time_appconnect: 0.022968, time_pretransfer: 0.023121, time_redirect: 0.000000, time_starttransfer: 0.031022, time_total: 0.031093

I guess the DNS resolution is not the issue here…

By the way, I think I had a configuration issue, so the 5 seconds delay is in reality less, but I still have a difference between the two methods, as seen in the time results.

Any chance this is happening because Fly shuts down VM’s due to inactivity? (Default configuration is to auto-shut off when idle).

The Fly Proxy controls that, however the fly-proxy is not used when connecting over private DNS like this. So, the VM may be shutting down since the proxy thinks it’s idle.

After leaving it for a few minutes, check fly status for the API app to see if it’s off - the delay could be it restarting. It might only restart then when using the way to access it, rather than over the private network.

You can configure the auto-shutdown behavior via fly.toml as per docs here: Automatically stop and start Machines · Fly Docs

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