When running vpnc in docker, get 'Cannot open "/proc/sys/net/ipv4/route/flush": Read-only file system'

Solution 1:

I am one of the core developers of OpenConnect and maintainers of the vpnc-script — which is used by both vpnc and OpenConnect for routing and DNS configuration.

This error message actually comes from the vpnc-script, not from vpnc itself, and…

1. This error doesn't matter at all. It come from the command ip -4 route flush cache, which triggers IPv4 route flushing, which is an unnecessary, deprecated, no-op in modern Linux kernels.

   We retain it only for backwards-compatibility, in case someone somewhere is running vpnc/OpenConnect on an annnnnnnnnnnnnnnnncieeeeeeent Linux kernel.

2. We suppressed the error message in a 2019 change to the standard vpnc-script.

   If you simply replace your vpnc-script with the latest version, the error will go away.

Solution 2:

As mentioned in the comments, there is no granular capability to be used with --add-cap, so you will have to run the container in privileged mode using --privileged in order to get rid of that warning.