Synology DSM 7 and broken FTP support in curl

I recently updated my DS1517 to DSM 7 and noticed that FTP support has been left out in curl/libcurl they included. This is how I compiled the latest version of curl, including support for all omitted protocols. It still needs more fixing, since I was not able to compile it with SSL support (so no https, which is included in curl in DSM 7).

My guide is for the Synology DS1517 (ARM). You have to download the correct files for your NAS and set the correct options (paths and names) for the compile tools if you have another model.

The problem

For some unknown reason, Synology decided to drop support for all protocols except http and https in the included curl binary with DSM7:

root@DS1517:~# curl --version
curl 7.75.0 (arm-unknown-linux-gnueabi) libcurl/7.75.0 OpenSSL/1.1.1k zlib/1.2.11 c-ares/1.14.0 nghttp2/1.41.0
Release-Date: 2021-02-03
Protocols: http https
Features: alt-svc AsynchDNS Debug HTTP2 HTTPS-proxy IPv6 Largefile libz NTLM NTLM_WB SSL TrackMemory UnixSockets
root@DS1517:~#

The outcome of following this guide:

curl.ftp --version
curl 7.79.1 (arm-unknown-linux-gnueabihf) libcurl/7.79.1
Release-Date: 2021-09-22
Protocols: dict file ftp gopher http imap mqtt pop3 rtsp smtp telnet tftp
Features: alt-svc AsynchDNS IPv6 Largefile UnixSockets

As seen and mentioned above, I was not able to enable SSL in my compiled version, so this will not replace curl included in DSM7, but could be installed in /bin under another name as it has the libcurl statically linked in the binary.

What you need to compile for the Synology

The first thing you need is a Linux installation as a development system containing the Synology toolkit for cross-compiling.
A fairly standard installation will do, at least mine did (but that also includes PHP, MySQL, Apache and other useful stuff). This is preferably done on a virtual machine, but you can of course use a physical computer for it.

You also need the Synology DSM toolchain for the CPU in the NAS you want to compile for. I found the links in the Synology Developer Guide (beta).
There is also supposed to be a online version of the guide, but at least for me, all the links within it were not working.

Get the toolchain
To find out which toolchain you need, run the command ‘uname -a’:

root@DS1517:~# uname -a
Linux DS1517 3.10.108 #41890 SMP Thu Jul 15 03:42:22 CST 2021 armv7l GNU/Linux synology_alpine_ds1517

As seen above, the DS1517 reports “synology_alpine_ds1517”, so you should look for the “alpine” versions of downloads for this NAS.
Get the correct toolchain for your NAS from Synology toolkit downloads. For the DS1517, I downloaded the file “alpine-gcc472_glibc215_alpine-GPL.txz”:
Download and unpack on the development system:

wget "https://global.download.synology.com/download/ToolChain/toolchain/7.0-41890/Annapurna%20Alpine%20Linux%203.10.108/alpine-gcc472_glibc215_alpine-GPL.txz"
tar xJf alpine-gcc472_glibc215_alpine-GPL.txz -C /usr/local/

The above will download and unpack the toolchain to the /usr/local/arm-linux-gnueabihf folder. This contains Linux executables for the GNU compilers (gcc, g++ etc).

arm-linux-gnueabihf-gcc: No such file or directory
Now, whenever you try to execute any of the commands extracted to the bin directory, you will probably get the “No such file or directory” error (even with the correct path and filename and the file is executable).
If you examine the executable files using the ‘file’ command you will discover that these are 32-bit executables:

root@ubu-01:~# file /usr/local/arm-linux-gnueabihf/bin/arm-linux-gnueabihf-gcc-4.7.2
/usr/local/arm-linux-gnueabihf/bin/arm-linux-gnueabihf-gcc-4.7.2: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.15, stripped

I found the solution to the problem here:
arm-linux-gnueabihf-gcc: No such file or directory
In short:

dpkg --add-architecture i386
apt-get update
apt-get install git build-essential fakeroot
apt-get install gcc-multilib
apt-get install zlib1g:i386

Now that we have the cross-compiling toolkit working, let’s continue with curl.

Cross-compile curl for Synology NAS

The current version at the time I wrote the guide was 7.79.1, so I download the source and then uncompress it:

wget https://curl.se/download/curl-7.79.1.tar.gz
tar xfz curl-7.79.1.tar.gz
cd curl-7.79.1

Set some variables and GCC options

export TC="arm-linux-gnueabihf"
export PATH=$PATH:/usr/local/${TC}/bin
export CPPFLAGS="-I/usr/local/${TC}/${TC}/include"
export AR=${TC}-ar
export AS=${TC}-as
export LD=${TC}-ld
export RANLIB=${TC}-ranlib
export CC=${TC}-gcc
export NM=${TC}-nm

Build and install into installdir

./configure --disable-shared --enable-static --without-ssl --host=${TC} --prefix=/usr/local/${TC}/${TC}
make
make install

The above will build a statically linked curl binary for the Synology, and put the binary in the ‘bin’ folder indicated by the path specified with –prefix.

The final step is to copy the ‘curl’ binary over to the Synology (not to /bin yet) and test it, use “–version” to check that the binary supports FTP and the other by Synology omitted protocols:

./curl --version
curl 7.79.1 (arm-unknown-linux-gnueabihf) libcurl/7.79.1
Release-Date: 2021-09-22
Protocols: dict file ftp gopher http imap mqtt pop3 rtsp smtp telnet tftp
Features: alt-svc AsynchDNS IPv6 Largefile UnixSockets

If everything seems ok, copy the file to /bin and give it another name:

cp -p curl /bin/curl.ftp

If it complains about different version of curl and libcurl, you failed somewhere when trying to link the correct libcurl statically.

Most useful sources for this article:

https://global.download.synology.com/download/Document/Software/DeveloperGuide/Firmware/DSM/7.0/enu/DSM_Developer_Guide_7_0_Beta.pdf
https://thalib.github.io/2017/02/17/32bit-no-such-a-file-or-directory/
https://curl.se/docs/install.html