FreeBSD project use SVN as default SCM. Source could be download with svnlite
software.
cd /usr/src svnlite checkout https://svn.freebsd.org/base/head .
cd /usr/src svnlite checkout https://web.freebsd.org/base/release/11.0.0 .
You can also get source from frozen tarball with fetch
command
cd /tmp fetch http://ftp.freebsd.org/pub/FreeBSD/releases/amd64/11.0-RELEASE/src.txz cd /usr/src tar xJvf /tmp/src.txz
cd /tmp fetch ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/11.0-RELEASE/src.txz cd /usr/src tar xJvf /tmp/src.txz
git clone https://github.com/freebsd/freebsd freebsdsrc