Archive for April 11th, 2016
I had a working OpenVPN configuration. But it wasn’t the best it could be. The manpage for OpenVPN 2.3 (community.openvpn.net/openvpn/wiki/Openvpn23ManPage) was used to find particularly interesting options.
For most of the changes I had to find examples and more information through Googling, though blog.g3rt.nl/openvpn-security-tips.html is of particular note for popping up very often.
Improving TLS Security
auth SHA256so MACs on the individual packets are done with SHA256 instead of SHA1.
tls-version-min 1.2to drop SSL3 + TLS v1.0 support. This breaks older clients (2.3.2+), but those updated versions have been out for a while.
tls-ciphers allowed to a subset of Mozilla’s modern cipher list + DHE for older clients. ECDSA support is included for when ECDSA keys can be used. I’m uncertain of the usefulness of the ECDHE ciphers, as both my client and server support it, but the RSA cipher that’s 3rd in the list is still used. Continuing to investigate this.
The last 2 changes are gated by the
openvpn_use_modern_tls variable, which defaults to
- New keys are 2048 bit by default, downgraded from 4096 bit. This is based on Mozilla’s SSL guidance, combined with the expectation of being able to use ECDSA keys in a later revision of this playbook.
As part of the move to 2048 bit keys, the 4096 bit DH parameters are no longer distributed. It was originally distributed since generating it took ~75 minutes, but the new 2048 bit parameters take considerably less time.
Adding Cert Validations
OpenVPN has at least two kinds of certification validation available: (Extended) Key Usage checks, and certificate content validation.
Added the ability to verify the common name that is part of each certificate. This required changing the common names that each certificate is generated with, which means that the ability to wipe out the existing keys was added as well.
The server verifies client names by looking at the common name prefix using
verify-x509-name ... name-prefix, while the client checks the exact name provided by the server.
Again, both these changes are gated by a variable (
openvpn_verify_cn). Because this requires rather large client changes, it is off by default.
Wiping out & reinstalling
Added the ability to wipe out & reinstall OpenVPN. Currently it leaves firewall rules behind, but other than that everything is removed.
ansible-playbook -v openvpn.yml --extra-vars="openvpn_uninstall=true" --tags uninstall to just run the uninstall portion.
Connect over IPv6
Previously, you had to explicitly use
tcp6 to use IPv6. OpenVPN isn’t dual stacked if you use plain
tcp, which results in being unable to connect to the OpenVPN server if it has an AAAA record, on your device has a functional IPv6 connection, since the client will choose which stack to use if you just use plain
Since this playbook is only on Linux, which supports IPv4 connections on IPv6 sockets, the server config is now IPv6 by default (github.com/OpenVPN/openvpn/blob/master/README.IPv6#L50), by means of using
Hat tip to T-Mobile for revealing this problem with my setup.
- Add revoked cert check
Generate ellptic curve keys instead of RSA keys However, as noted above, ECDHE ciphers don’t appear to be supported, so I’m not sure of OpenVPN will support EC keys.
Add IPv6 within tunnel support (Possibly waiting for OpenVPN 2.4.0, since major changes are happening there)
This SO question seems to be my exact situation.
Tried splitting the assigned /64 subnet with:
ip -6 addr del 2607:5600:ae:ae::42/64 dev venet0 ip -6 addr add 2607:5600:ae:ae::42/65 dev venet0