WG default routing

[Samuel Holland]

On 1/5/21 3:13 PM, Chris Osicki wrote:
> On Wed, Jan 06, 2021 at 01:25:30AM +0500, Roman Mamedov wrote:
>> On Tue, 5 Jan 2021 21:12:12 +0100
>> Chris Osicki <wg at osk.ch> wrote:
>>
>>> As far as I can see after few tests, AllowedIPs config file option has nothing to do with routing and I hope 
>>> it will stay like this.
>>
>> wg-quick uses AllowedIPs to also set up matching entries in the system routing
>> table. This can be disabled in its config.
>>
>>> It is just a filter
>>
>> It is not only a filter on incoming packets, but also WG's internal routing
>> table for knowing which packets should be sent to which peer.
> 
> I'm sorry to contradict you but after some more readig I have to :-)
> WG has no "internal routing table", wg-quick (which, BTW, is not the subject of my query) uses it to modify

Did you read this part of the home page?

https://www.wireguard.com/#conceptual-overview

	At the heart of WireGuard is a concept called Cryptokey Routing,
	which works by associating public keys with a list of tunnel IP
	addresses that are allowed inside the tunnel.

	[...]

	In the server configuration, when the network interface wants to
	send a packet to a peer (a client), it looks at that packet's
	destination IP and compares it to each peer's list of allowed
	IPs to see which peer to send it to.

	[...]

	In other words, when sending packets, the list of allowed IPs
	behaves as a sort of routing table, and when receiving packets,
	the list of allowed IPs behaves as a sort of access control
	list.

WireGuard itself does indeed have an internal routing table. And you
should really read that whole section.

> kernel routing tables, from the wg-quick man page:
> 
>        It infers all routes from the list of peers' allowed IPs, and automatically adds them to  the  system  routing
>        table.  If  one  of  those  routes is the default route (0.0.0.0/0 or ::/0), then it uses ip-rule(8) to handle
>        overriding of the default gateway.
> 
> So, in my test config I have a server, 10.10.10.1 and two clients, 10.10.10.2/3
> If on the server I remove the AllowedIPs option, no one can connect.
> Giving AllowedIPs = 10.10.10.0/24 both clients can connect and routing in them stays as it was.
> The same for the clients, without AllowedIPs = 10.10.10.0/24 cannot connect.
> 
> Thus, my question still remains: why this filtering function?

Because, as the WireGuard website explains, a tight, static binding
between a peer's identity and its IP address range is an extremely
useful building block, both for security and for designing a network
topology.

Cheers,
Samuel