Error on installing `networkmanager-l2tp 1.2.10-3` in Arch Linux

References:

• https://gist.github.com/pastleo/aa3a9524664864c505d637b771d079c9
• https://askubuntu.com/questions/981165/l2tp-ipsec-failing-to-connect-kubuntu-17-10

When run the command to install the package `networkmanager-l2tp 1.2.10-3`, an error occurs like below:

$ yaourt -S networkmanager-l2tp

...

==> Validating source files with md5sums...
    network-manager-l2tp-1.2.10.tar.gz ... Passed
    network-manager-l2tp-1.2.10.tar.gz.asc ... Skipped
==> Verifying source file signatures with gpg...
    network-manager-l2tp-1.2.10.tar.gz ... FAILED (unknown public key 49A7787EF8D3C039)
==> ERROR: One or more PGP signatures could not be verified!
==> ERROR: Makepkg was unable to build networkmanager-l2tp.
==> Restart building networkmanager-l2tp ? [y/N]
==> --------------------------------------------
==>

I resolved by installing NetworkManager-l2tp manually.

$ sudo pacman -S strongswan xl2tpd
$ git clone https://github.com/nm-l2tp/network-manager-l2tp.git
$ cd network-manager-l2tp
$ ./autogen.sh
$ ./configure --prefix=/usr --sysconfdir=/etc \
    --localstatedir=/var --libexecdir=/usr/lib/NetworkManager \
    --with-pppd-plugin-dir=/usr/lib/pppd/2.4.7
$ sed -i -e 's/ -shared / -Wl,-O1,--as-needed\0/g' libtool
$ make
$ sudo make install
$ sudo systemctl restart NetworkManager

And then, add new VPN connection following steps:

• Go to Settings -> Network -> Add VPN -> Click Layer 2 Tunneling Protocol (L2TP).
• Enter Connection name
• Click VPN tab
• Enter Gateway, Username, Password
• Click IPsec Settings
• Check Enable IPsec tunnel to L2TP host
• Enter Pre-shared Key
• Click Save

After that, if VPN connection is failed after configured correctly, then it could be an issue with an issue with 3DES cipher setting. Try this step

• Go to Settings -> Network -> Add VPN -> Click Layer 2 Tunneling Protocol (L2TP).
• Edit VPN connection that you just created
• Click VPN tab
• Click IPSec Settings
• Enter Phase1 Algorithms as 3des-sha1-modp1024!
• Enter Phase2 Algorithms as 3des-sha1!
• Click OK
• Click Save

Try again.