Code:
┌─(ROOT@europa:pts/0)──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────(~)─┐
└─(2.06:22%:15:59:34%:#)── zcat /proc/config.gz | ag TUN ──(Thu, Oct 07)─┘
# CONFIG_SCHED_TUNE is not set
CONFIG_NET_IP_TUNNEL=y
CONFIG_NET_UDP_TUNNEL=y
CONFIG_NET_FOU_IP_TUNNELS=y
CONFIG_INET_XFRM_TUNNEL=y
CONFIG_INET_TUNNEL=y
CONFIG_INET_XFRM_MODE_TUNNEL=y
CONFIG_INET6_XFRM_TUNNEL=y
CONFIG_INET6_TUNNEL=y
CONFIG_INET6_XFRM_MODE_TUNNEL=y
CONFIG_IPV6_TUNNEL=y
CONFIG_IPV6_FOU_TUNNEL=y
# CONFIG_NET_ACT_TUNNEL_KEY is not set
CONFIG_LWTUNNEL=y
CONFIG_TUN=y
# CONFIG_TUN_VNET_CROSS_LE is not set
Likely to be exactly the same on the Sage.
As for the config entry, I *think* it's similar to the global Qt5 PROXY env var or something, but I don't remember the details (it might have been discussed here in the past, though).