How to stream large binary data from RPC/CGI to browser in LuCI with JSapi?

Zerogiven · February 7, 2026, 10:46pm

Hello there,

I'm developing a LuCI app that needs to export Podman volumes as .tar.gz files for browser download. Currently my RPC method runs podman volume export | gzip, writes the result to a temp file in /tmp, base64-encodes it, and returns it as JSON. This works for small volumes but fails for large ones because /tmp is tmpfs (RAM-backed) and fills up quickly.

What's the recommended way to stream large binary data directly to the browser in LuCI without buffering the entire file in /tmp? I've seen cgi-io/cgi-download can serve files, but those also require the file to exist on disk first. Ideally I'd pipe podman volume export | gzip straight to the HTTP response.

cheers

anon63541380 · February 7, 2026, 11:19pm

redirect to a static file

Zerogiven · February 8, 2026, 9:25am

Not sure if i understand it correctly, but i have a static file in the temp folder. Problem is for larger volumes for example with 3gb. To export them the only way, i think, is to stream the output because podman directly returns the file in the api.

However, a short (link) example or a few words more would help a lot

frollic · February 8, 2026, 9:41am

ln -s /temp/static.file /www/same.static.file

point to http://router.IP/same.static.file

Zerogiven · February 8, 2026, 10:44am

Yeah, nearly what i am doing for small files. /temp or /tmp can save a lot but not always 3gb. So create/save a 3gb(or more) file at the router is not acceptable - or if there is for example only 500mb space left and i want get a 1gb file, how should i statically link if i can not create it? The podman api returns the binary data so i could stream it directly to the browser instead of saving it at the router (to be honest if i have the file it does not matter where it is saved, with a rpc call i can get it from anywhere but that is not the problem itself )

I thought there must be a way because i also do not think that for example luci backup function is not possible if space is full.

Zerogiven · February 8, 2026, 10:56am

I think i have found it...

github.com/openwrt/luci

modules/luci-base/htdocs/luci-static/resources/fs.js

master


      
          	 * Execute the specified command, bypassing ubus.
          	 *
          	 * Note: The `command` must be either the path to an executable,
          	 * or a basename without arguments in which case it will be searched
          	 * in $PATH. If specified, the values given in `params` will be passed
          	 * as arguments to the command.
          	 *
          	 * This function will invoke the requested commands through the cgi-io
          	 * helper applet at `/cgi-bin/cgi-exec` which bypasses the ubus rpc
          	 * transport. This is useful to fetch large command outputs which might
          	 * exceed the ubus message size limits or which contain binary data.
          	 *
          	 * The cgi-io helper will enforce the same access permission rules as
          	 * the ubus based exec call.
          	 *
          	 * @param {string} command
          	 * The command to invoke.
          	 *
          	 * @param {string[]} [params]
          	 * The arguments to pass to the command.
          	 *

Zerogiven · February 8, 2026, 11:20am

Yes this works perfectly like i want, exported a 5gb volume without the need of saving the file before sending to the user

system · February 18, 2026, 11:20am

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