IP reverse dns

Is there a way to set an IP reverse dns value ?

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Do you mean mapping IPs over the private network back to an app? Or public DNS?

I mean declaring a domain to the public ip4 allocated to an app.


Is there any updates about it?

You can point your DNS A/AAAA records at your Fly app’s IP4 and IP6 addresses or CNAME your sub.domain.tld to your app’s fly.dev name (ref).

For instance, for one of my Fly apps, midway, here’s the DNS records Fly set up (on their fly.dev domain) for me:

➜ dig midway.fly.dev A midway.fly.dev AAAA +short

And here are the A/AAAA records I setup with my DNS provider (AWS for nx and Cloudflare for n):

➜  dig nx.rethinkdns.com A n.rethinkdns.com AAAA +short

With that, I am able to access my Fly app with names: midway.fly.dev or nx.rethinkdns.com or n.rethinkdns.com or any-random-63-chars.nx.rethinkdns.com.

As noted above, in my case, zones are split between AWS and Cloudflare, but you can also use Fly to register and host your DNS which makes dealing with TLS offload on Fly that much easier.

I suspect the original ask was about setting a PTR record for a fly app’s ip. I’m only familiar with them in the context of outbound smtp, which I’d be surprised if fly allowed in the first place.


Any update on the ability to set a reverse DNS record on a fly app IP? I would like to send a low volume of email from my applications for email and password resets and whatnot.

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Unfortunately even if Fly offered (as Azure/AWS/etc allow for their customer’s IPs, by support request) PTR records for your app’s public IP I don’t believe the PTR requirement would satisfy a PTR-for-sending-SMTP-from-Fly use case.

Any outbound SMTP, or indeed any TCP, connections from your app(s) will not be sourced from the app’s Fly public IP, see also here. Nor can you currently define the originating (shared) IP. Even if Fly offered PTR records for Fly app IPs - the outbound-static-IP issue would likely be a problem (re: SMTP/PTRs), as they’re not going to allow a single customer to set the PTR records for their numerous IPs from which your app’s traffic could be originated from.


Ah yeah good point. I guess I’ll just run a small vps with stable ip and PTR support for this. Would be cool if this was achievable on fly somehow though.