I would like to add an extra IPv6 address so I can run a service on it but I want to retain the existing IPv6 address as the address to use for outgoing connections.When I add an ipv4 address withip addr add <address> dev <device>It gets added as secondary and is not used by default for outgoing connections but when I do the same for ipv6 the address is just added as a normal address and (all else being equal) linux uses the most recently added address for outgoing connections.There appears to be a deprecated flag which can mark an IPv6 address as non-default but there doesn't seem to be any obvious way to set it. Show us an output from ip and tell us how and where you see an address is primary and/or secondary.6: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP    link/ether 68:b5:99:78:63:d4 brd ff:ff:ff:ff:ff:ff    inet 5.153.225.206/26 brd 5.153.225.255 scope global bond0    inet 5.153.225.207/26 scope global secondary bond0    inet6 2001:41c9:1:3ce::11/64 scope global       valid_lft forever preferred_lft forever    inet6 2001:41c9:1:3ce::10/64 scope global       valid_lft forever preferred_lft forever    inet6 fe80::6ab5:99ff:fe78:63d4/64 scope link       valid_lft forever preferred_lft foreverThe extra IPv4 address is shown as secondary and is not used by default for outgoing connections but the extra IPv6 address is just listed the same as the first address and is chosen by default for outgoing connections. 
how do I add a second ipv6 address on Linux without making it default
linux;ipv6
The deprecated flag is set when the preffered lifetime of the address drops to zero. You can set the preffered lifetime flag when adding an address by doing.ip addr add <address> dev <device> preferred_lft <lifetime>you can also set it on an existing address ip addr change <address> dev <device> preferred_lft <lifetime>Setting it to zero will immediately mark the address as deprecated. I'm not sure what the units are if you set it to a value other than zero.Note the preffered_lft option and the ip addr change command do not seem to be documented in the manpage.