Multihomed server issue

[Wang Jian]

2017-08-01 19:28 GMT+08:00 Wang Jian <larkwang at gmail.com>:
> 2017-08-01 11:06 GMT+08:00 Jason A. Donenfeld <Jason at zx2c4.com>:
>> On Tue, Aug 1, 2017 at 4:01 AM, Wang Jian <larkwang at gmail.com> wrote:
>>> 2017-07-31 23:34 GMT+08:00 Jason A. Donenfeld <Jason at zx2c4.com>:
>>>> On Fri, Jul 28, 2017 at 2:51 AM, Wang Jian <larkwang at gmail.com> wrote:
>>>>> The solution can be one of:
>>>>>
>>>>> 1. server can RTS (response to source), or can bind to arbitary
>>>>> address for outgoing
>>>>
>>>> The server does already respond to source.
>>>
>>> Sorry, I didn't make it clear. By saying RTS, I mean response to
>>> source link, that is,
>>> using called address and incoming link.
>>
>> You're still unclear to me. What?
>
> Let's say server has multiple interfaces, eth0, eth1, ... ethN, with
> IP0, IP1, ... IPn,
> If an incoming request is to eth1, to IP1, then the server's response
> packet will go out from eth1, and source is IP1.
>
> In some cases, it can be done using policy routing, but other cases not. I know
> a FreeBSD based VPN implements so called RTS.
>
> In my case, the server looks like
>
> eth0 = 10.1.1.2/24                    (default route, via 10.1.1.1/24)
> eth1.100 = 172.16.1.2/30         (policy routing: when source address
> is 111.111.1.0/24,  route via 172.16.1.1/30)
> eth1.200 = 172.16.2.2/30         (policy routing: when source address
> is 111.111.2.0/24,  route via 172.16.2.1/30)
> dummy0 = 111.111.1.2/24
> dummy1 = 111.111.2.2/24
>
> When a wireguard client contacts 111.111.1.2, the server responses UDP packet
> with source address 10.1.1.2 but not the desired 111.111.1.2, because
> of default route.
>
> I have mailed you my network setup privately. Sorry for inconvenience.

I managed to handle this issue for one public address

------
[Interface]
FwMark = 600
PostUp = ip rule add pref 20000 fwmark 600 table 600; ip route add
table 600 default via 172.16.1.1 src 111.111.1.2
------

With this workaround, for a wireguard interface, only one public address can
be used. We do need multiple public addresses for a reason: some clients
connect to one public address while other clients connect to another public
address.

We can use multple wg interfaces to address the limitation. But I do wish that
server can deduce public address which the client connects to, and use the
public address to response to the client, then the configuration will be
simple and straightforward.