[CentOS] IP routing basics?

Mon Jun 12 19:42:23 UTC 2023
lejeczek <peljasz at yahoo.co.uk>

Hi guys.

I've always try to leave as mush as possible to the kernel 
but here I had to resort to source/rule based routing and I 
wonder why.
Any expert cared to share some light on what is 
(not)happening here

HOME -- tunnel --> public iface SHED gw  A iface; B iface 
<-- to respectively --> SOME box A iface; B iface

HOME can get to both A & B ifaces, (which are different 
subnets) all the way up to the SOME box

Now, a DIFFERENT box runs/is behind/on that SOME box, which 
connects to A iface & B iface respectively (which SOME's A & 
B ifaces are linux bridges) need - so it appears - manual 
rules in order to have HOME get to DIFFERENT's iface B

with IPs:

10.3.9.0/24 -> tunnel -> SHED's 10.1.1.254 & 10.3.1.254 ->  
SOME's 10.1.1.99 & 10.3.1.99 - all good!
---||--- -> DIFFERENT's 10.1.1.50 - works
---||--- -> DIFFERENT's 10.3.1.50 - for this one I need to 
set rules (on DIFFERENT only) so HOME can get to it

All boxes have METRICs set the same way:
iface on 10.1.1.0/24 runs with 111 & on 10.3.1.0/24 runs 
with 113 metrics.

  So I must(?) use these, so HOME can get to DIFFERENT's 
both ifaces:
-> $ ip ro ls table 113
default via 10.3.1.254 dev enp0s3
-> $ ip ro ls table 114
default via 10.1.1.254 dev enp0s10
& rules:
32764:    from 10.3.1.0/24 to 10.3.9.0/24 lookup 113
32765:    from 10.1.1.0/24 to 10.3.9.0/24 lookup 114

I'd prefer to not to use human-set rules but if not 
possible, I'll be happy with an explanation.

many tanks, L.