buildroot/docs/manual/download-location.txt

Location of downloaded packages
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It might be useful to know that the various tarballs that are
downloaded by the Makefiles are all stored in the +DL_DIR+ which by
default is the +dl+ directory. It's useful, for example, if you want
to keep a complete version of Buildroot which is known to be working
with the associated tarballs. This will allow you to regenerate the
toolchain and the target filesystem with exactly the same versions.

If you maintain several Buildroot trees, it might be better to have a
shared download location. This can be accessed by creating a symbolic
link from the +dl+ directory to the shared download location:

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 $ ln -s <shared download location> dl
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Another way of accessing a shared download location is to create the
+BUILDROOT_DL_DIR+ environment variable.  If this is set, then the
value of DL_DIR in the project is overridden. The following line
should be added to +<~/.bashrc>+.

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 $ export BUILDROOT_DL_DIR <shared download location>
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