Don't attempt to use link-local adress for pool to pool copies

[maswan]

trafficstars

On some of our pools, we have a routing table like this:

::1 dev lo proto kernel metric 256 pref medium 2001:948:62:1::1234 dev eth0 proto kernel metric 100 pref medium fe80::/64 dev eth0 proto kernel metric 1024 pref medium default via fe80::123:44ff:4992:3111 dev eth0 proto ra metric 100 pref medium

Which confuses dCache's pp transfers since it attempts to use the link-local fe80 address instead of the globally routable one. The link-local address is only link-local though, it can not reach further than the local router, and not into the link-local parts of other pools, even though they too have the prefix fe80::/64.

That they attempted to use this was verified by ss showing: [fe80::12ac:a441:3117:2b21]:22605 with the other pool's link-local IP in there. Hardcoding the IP to the global one with pp interface also fixed the transfer issue.

My suggestion is to simply discard the fe80:: prefix from the logic selecting interface, much like we don't attempt to listen to 127.0.0.1 despite both the source and destination having that.

[kofemann]

Makes sense. We had other changes to prevent link-locals addresses from being published (https://github.com/dCache/dcache/commit/a09884f88581a1c27b451343f612f5cf852a9e20), but obviously, this was not sufficient (and even not backported to 8.2).

[maswan]

Ah, I see that I failed to find the previous issues by searching for "link-local" instead of "linklocal". And yes, this is impacting us in 8.2 today.

[ZNikke]

Depending on how the filtering is done, you might also see/utilize the scope flag on the interface, if that feels better than hard-coding exceptions. Redacted ip -6 addr show output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2001:aa:bb:cc::dd/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::aaaa:bbbb:cccc:ddd/64 scope link 
       valid_lft forever preferred_lft forever