[GLLUG] Systemd on Debian

[Andy Smith]

Hello,

On Sat, May 23, 2020 at 09:07:01AM +0100, Chris Bell via GLLUG wrote:
> with the correct prefix used for sending and receiving.

In case it helps, I am often in the situation where a host has a
general purpose v6 address and several "service addresses" that are
only used as listening addresses for different services on the host.

I don't like outgoing packets to source from one of those service
addresses, but the default Linux behaviour is to use the last added
address on the interface as a source address.

There are two easy ways to influence this. One way is to add the
desired source address with a longer prefix, e.g. /128. If all
others are shorter prefix than this, e.g. /64s then the /128 will be
preferred.

The way I like better is to add the service addresses with a
preferred_lft of 0 like:

# ip addr add 2001:db8::1/64 dev $IFACE preferred_lft 0

That would then show up as "deprecated" in the "ip -6 addr" list,
which prevents it being used for any source address nut doesn't
interfere with it receiving packets addressed to it.

Cheers,
Andy

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