# Getting the port number of serve/servlet when running on an ephemeral port
**URL:** https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130
**Category:** Questions & Answers
**Created:** [March 4, 2026, 12:22pm UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130 "2026-03-04T12:22:42Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@hnmdijkema](https://racket.discourse.group/u/hnmdijkema)
#### Post date: [March 4, 2026, 12:22pm UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130/1 "2026-03-04T12:22:42Z")
Dear all,
I've trouble finding this in the documentation.
Suppose I start the racket web framework as follows:
```scheme
(serve/servlet
(λ (req) (web-serve h req))
#:listen-ip "127.0.0.1"
#:port 0
#:command-line? #t
#:servlet-path ""
#:stateless? #t
#:servlet-regexp #rx"")
)))
```
In this case the server will listen on a port the OS chooses.
How do I get the port number it listens on?
---
### Author:  [@benknoble](https://racket.discourse.group/u/benknoble)
#### Post date: [March 4, 2026, 12:51pm UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130/2 "2026-03-04T12:51:48Z")
The [serve procedure](https://docs.racket-lang.org/web-server-internal/web-server.html#%28def._%28%28lib._web-server%2Fweb-server..rkt%29._serve%29%29) accepts a confirmation channel which can give you the port. I don’t think it’s available for some of the higher-level forms, though.
Here’s an [example use from my code](https://github.com/benknoble/frosthaven-manager/blob/ceab98845a4b96784022954d54bd91686496b4c3/server.rkt#L274). It’s relatively easy to combine with dispatch/servlet to do what some of the other interfaces do.
It might be nice if the other interfaces had this ability though.
---
### Author:  [@hnmdijkema](https://racket.discourse.group/u/hnmdijkema)
#### Post date: [March 7, 2026, 9:11am UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130/3 "2026-03-07T09:11:27Z")
Thanks Ben, I looked at your code and have managed to use the confirmation channel in my code. I just get the first value of it and that's the port number. Just a question:
```scheme
(match (async-channel-get port-ch)
[(? port-number? port) (values (~a "http://" (best-interface-ip-address) ":" port) stop)]
[(? exn? e) (raise e)]))
```
What are you doing here exactly?
---
### Author:  [@LiberalArtist](https://racket.discourse.group/u/LiberalArtist)
#### Post date: [March 7, 2026, 11:48pm UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130/4 "2026-03-07T23:48:36Z")
The second case exists because the server might fail to start, in which case the channel will get an exception value instead of a port number. Here's an example of a program that will produce such an exception (as long as you run it with normal privileges):
```scheme
#lang racket
(require web-server/web-server
racket/async-channel)
(define ach
(make-async-channel))
(define stop
(serve #:confirmation-channel ach
#:port 80 ;<-- needs privileges
#:dispatch void))
(begin0 (sync ach)
(sleep 1)
(stop))
```
---
### Author:  [@benknoble](https://racket.discourse.group/u/benknoble)
#### Post date: [March 8, 2026, 2:52pm UTC](https://racket.discourse.group/t/getting-the-port-number-of-serve-servlet-when-running-on-an-ephemeral-port/4130/5 "2026-03-08T14:52:49Z")
Philip covered the need to dispatch on whether we get a port or an exception. The only other thing I think you might be asking is "what is going on with that URL construction?"—the code that starts this server wants to tell the user 2 things:
- how to access the resulting website on a LAN (to play with friends); and
- how to stop the server.