By default Linux (randomly?) chooses “Client IPv6 Address” (given by HE.net) as source address for outbound connections. This can be inconvenient sometimes tough.

To override this, it’s possible to add a new address in “Routed IPv6 Prefixes” to eth0 in /etc/interfaces.d/eth0 …:

iface eth0 inet6 static
  address 2001:db8::1/64

… and abuse the definition of an IPv6 home address to override this with:

ip addr change 2001:db8::1 home dev eth0

To persist it, one can add a post-up command in /etc/network/interfaces.d/eth0:

# ...
iface eth0 inet6 static
  address 2001:db8::1/64
  post-up /sbin/ip addr change 2001:db8::1 home dev eth0

Edit: At first I tried to add a script to /etc/network/if-up.d with:

#!/bin/bash
set -e

/sbin/ip addr change 2001:db8::1 home dev eth0

It leads to failure in starting networking.service with RTNETLINK answers: File exists, though. I think the reason it failed was that I should have tested interface name before setting the address as home address. Using post-up command saves life here.