network backend 'user' is not compiled into this binary

Question

I have build qemu-7.2.0 and then, when running it I am getting

qemu-system-x86_64: -nic user,model=virtio: network backend 'user' is not compiled into this binary

Apparently, I should enable this feature during build, but how to know which is it namely?

Answer 1

The 'user' networking backend is provided by the 'slirp' library; you get this message when the QEMU binary was built without slirp support compiled in.

As noted in the 7.2 changelog, QEMU no longer ships a copy of the slirp module with its sources. Instead you need to make sure you have installed your distro's libslirp development package (which is probably called libslirp-devel or libslirp-dev or something similar) before configuring and building QEMU. You need at least libslirp 4.7 or better.

QEMU's configure convention for optional features which require some build-time dependency is:

• by default, check for the dependency, and build in the feature if we find it
• if --enable-foo is passed, check for the dependency, and fail configure with an error if it is missing
• if --disable-foo is passed, don't check, and never build in the feature

So you can pass configure --enable-slirp to force it to give you an error if you haven't got the dependency, as a way of checking that you've installed the right system package for libslirp.

Answer 2

$ git clone https://gitlab.com/qemu-project/qemu.git

$ sudo apt-get install git libglib2.0-dev libfdt-dev libpixman-1-dev zlib1g-dev ninja-build meson

$ git clone https://gitlab.freedesktop.org/slirp/libslirp.git

$ cd libslirp

$ meson build

$ ninja -C build install

$ cd ..

$ cd qemu

$ mkdir -p bin/debug/native

$ cd bin/debug/native

$ ../../.././configure --enable-slirp --enable-debug

$ make -j$(nproc)

$ make install

$ cd ../../..