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<p>Hi all,</p>
<p>I was able to get to the clubroom today and hook up the 4G modem
with the new SIM. Here are the details so far:</p>
<ul>
<li>The 4G modem is 192.168.4.1/24 with DHCP off, plugged into an
access port of the switch Kerosene</li>
<li>Murasoi's spare RJ45 is 192.168.4.2, static, with that port on
Kerosene also set to an access port</li>
<li>Both those access ports have been added to VLAN 42, and are
separate from the rest of the network</li>
<li>The spare port on Murasoi is `eth0`, with `eth1` being the
regular uplink and `eth2` being the 10G fibre trunk to the rest
of the clubroom</li>
<li>I've firewalled off the 4G link from forwarding, so it can't
inadvertently leak traffic into or out of the network</li>
<li>I've confirmed the 4G link is functional, e.g. via `ping -I
eth0 8.8.8.8`<br>
</li>
<li>Murasoi can talk to the modem, but not yet to the wider
internet until policy-based routing is set up</li>
</ul>
<p>A bit more on that last point:</p>
<p>Right now, Murasoi knows to send any traffic destined for the
rest of 192.168.4.0/24 out `eth0` with a source address of
192.168.4.2. But if I want to send a packet to, say, Google's
8.8.8.8, it'll hit the default route and be sent via the UWA
uplink. Even if my software forces a source address of
192.168.4.2, that route will be hit and UWA will end up with a
packet it doesn't know how to deal with. That's because normal
routing only consideres the destination, not the source.</p>
<p>Some software, like `ping`, can force using a specific interface
as well/instead of a source address. That's how I could test the
link's functionality. But most software can't. What we need is to
make some rules about which source addresses go out which routes.
That's called _policy-based routing_, and it's the job of commands
like `ip rule`.</p>
<p>I'll be working on getting that started tomorrow.</p>
<p>Cheers,</p>
<p>James [MPT]</p>
<p>PS: PBR is a very flexible technique. UCC is lucky to have a
fast, low-latency and unmetered connection to the net, so we don't
really need anything more. This backup link is intended only in
the case of serious issue, and isn't meant to carry normal traffic
even then. But if we had multiple links with different pros and
cons, PBR lets you determine which traffic goes where, based on a
range of criteria.</p>
<p>PPS: If you're reading all this and wondering what the heck I'm
going on about, feel free to pester me on IRC/Discord or in reply.
I'll see if I can get it explained better.<br>
</p>
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