Since Cowboy 2.14.0 an experimental implementation of HTTP/3 (or QUIC or RFC9000) has been released. That's a pretty good news, and if stable enough, will change a lot on the web ecosystem. Unfortunately, this feature is not documented yet - because unstable - but it's not a good reason to play with it and try to understand how to configure a cowboy server with HTTP/3 and QUIC.
Cowboy does not support natively QUIC and depends on quicer application to work correctly. To be clear, this is a BIG NO-NO for a deployment in production environment. The list of dependencies directly required by quicer is insane, it fetch many python-like modules and recompile OpenSSL from scratch (OpenSSL 3.1.7+quic, from an unmaintained and archived repository, with all security issues that could implies).
Anyway, the COWBOY_QUICER Erlang macro must be set to 1 to compile cowboy with HTTP/3 and QUIC support. Then, the rebar.config file will need few modifications. The configuration for the cowboy dependency must be overridden and the previous macro must be activated. The quicer depency is also required, because this is an experimental feature, cowboy does not include it. To test that, let reuse the buckaroo project created in a previous publication.
{erl_opts, [
debug_info
]}.
{deps, [
cowboy,
quicer
]}.
{shell, [{apps, [buckaroo]}]}.
{overrides, [
{add, cowboy, [
{erl_opts, [
{d, 'COWBOY_QUICER', 1}
]}
]}
]}.
A cleanup will be also required there with a full recompilation from scratch of all the dependencies.
$ rm -rf _build
$ rebar3 compile
...
In theory, after this step, cowboy should now be able to start a QUIC server and use HTTP/3. At least, an Erlang shell can be started without problem, and it seems buckaroo is running correctly.
$ rebar3 shell
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling buckaroo
Erlang/OTP 29 [erts-17.0] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]
Eshell V17.0 (press Ctrl+G to abort, type help(). for help)
===> Booted cowlib
===> Booted ranch
===> Booted cowboy
===> Booted buckaroo
1>
A TLS certificate is required, we don't need to create a clean one, simply generating a self-signed with openssl req for now will do the job.
$ openssl req -x509 -newkey rsa:4096 \
-keyout key.pem \
-out cert.pem \
-sha256 \
-days 3650 \
-nodes -subj \
"/C=XX/ST=StateName/L=CityName/O=CompanyName/OU=CompanySectionName/CN=CommonNameOrHostname"
If the certificate has been correctly created, quicer can be started via cowboy with the help of cowboy:start_quic/3 function. This function is not exposed by default and to have access to it, one must compile cowboy with COWBOY_QUIC macro enabled (what we configured previously). Anyway , to add this step, we can modify the buckaroo_cowboy module by creating a new function called start_quic/3. Most of the parameters will be reused from the normal way to start cowboy.
start_quic() ->
application:ensure_all_started(quicer),
cowboy:start_quic(
name(),
#{
socket_opts => transport_options() ++ [
{certfile, "cert.pem"},
{keyfile, "key.pem"}
]
},
protocol_options()
).
Let start the application now.
$ rebar3 shell
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling buckaroo
Erlang/OTP 29 [erts-17.0] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]
Eshell V17.0 (press Ctrl+G to abort, type help(). for help)
===> Booted cowlib
===> Booted ranch
===> Booted cowboy
===> Booted buckaroo
1> buckaroo_cowboy:start_quic().
{ok,#Ref<0.2047991422.1293549576.157826>}
2>
The service started... Okay, that's a good step but we still need to be sure it is listening on the right port and see if it's working correctly. Let use ss (or netstat if you are using another OS).
$ ss -nlpu sport 8081
State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=86))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=90))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=94))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=98))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=102))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=106))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=110))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=114))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=118))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=122))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=126))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=130))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=134))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=138))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=142))
UNCONN 0 0 *:8081 *:* users:(("beam.smp",pid=2784595,fd=146))
A QUIC service must listen on UDP, and this is the case. The output of ss command returns the what we are looking for, an application (beam.smp) listening to all interfaces on UDP/8082.
$ curl -kv --http3-only https://localhost:8081/; echo
* Host localhost:8081 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
* HTTPS-RR: -
* Trying [::1]:8081...
* SSL Trust: peer verification disabled
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384 / x25519 / RSASSA-PSS
* Server certificate:
* subject: C=XX; ST=StateName; L=CityName; O=CompanyName; OU=CompanySectionName; CN=CommonNameOrHostname
* start date: May 27 04:15:27 2026 GMT
* expire date: May 24 04:15:27 2036 GMT
* issuer: C=XX; ST=StateName; L=CityName; O=CompanyName; OU=CompanySectionName; CN=CommonNameOrHostname
* Certificate level 0: Public key type RSA (4096/152 Bits/secBits), signed using sha256WithRSAEncryption
* OpenSSL verify result: 12
* SSL certificate verification failed, continuing anyway!
* Established connection to localhost (::1 port 8081) from ::1 port 33253
* using HTTP/3
* [HTTP/3] [0] OPENED stream for https://localhost:8081/
* [HTTP/3] [0] [:method: GET]
* [HTTP/3] [0] [:scheme: https]
* [HTTP/3] [0] [:authority: localhost:8081]
* [HTTP/3] [0] [:path: /]
* [HTTP/3] [0] [user-agent: curl/8.20.0]
* [HTTP/3] [0] [accept: */*]
> GET / HTTP/3
> Host: localhost:8081
> User-Agent: curl/8.20.0
> Accept: */*
>
* Request completely sent off
< HTTP/3 200
< content-length: 5
< content-type: text/plain
< date: Wed, 27 May 2026 12:49:14 GMT
< server: Cowboy
<
* Connection #0 to host localhost:8081 left intact
hello
It looks good, cowboy returns the famous hello message and curl is correctly using HTTP/3.
Conclusion
I would really like to use HTTP/3 and QUIC in production with Erlang and cowboy, but I don't really think this is still a good time to use it. Many things must be improved, and based on the amount of dependencies required, it's a bit scary.
This project example can still be seen on niamtokik/buckaroo repository at Github.
Have fun!
Cover Image by Chris Liverani on Unsplash
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