Re: Netfilter redirect does not work with wireguard

From: Nico Schottelius <nico.schottelius@ungleich.ch>
To: "Ivan Labáth" <labawi-wg@matrix-dream.net>
Cc: WireGuard mailing list <wireguard@lists.zx2c4.com>
Subject: Re: Netfilter redirect does not work with wireguard
Date: Fri, 08 Nov 2019 16:29:31 +0100	[thread overview]
Message-ID: <87mud61j50.fsf@ungleich.ch> (raw)
In-Reply-To: <20191107215054.GA23531@matrix-dream.net>

Hello,

there are actually no replies at all, which is why I *assumed* that the
packets do not actually arrive at the wireguard code.

How does wireguard integrate into the kernel / netfilter flow? Or in
other words, how is the expected behaviour of something like what I am
trying to do?

Best,

Nico

p.s.: I was also thinking about needing SNAT, but I don't see any
replies generated at the moment.

Ivan Labáth <labawi-wg@matrix-dream.net> writes:

> Hello,
>
> have you checked the source port of replies, or whether
> there are any replies?
>
> # tcpdump -nn
>
> Tcpdump should show pre-NAT addresses on incoming packets
> and AFAIK post-NAT on outgoing packets, i.e. what goes on
> the wire.
>
> You might need to apply SNAT on outgoing packets, which
> might happen automatically with TCP connection tracking,
> but I wouldn't really expect it on connection-less UDP.
>
> Regards,
> Ivan
>
> On Thu, Nov 07, 2019 at 05:38:42PM +0100, Nico Schottelius wrote:
>>
>> Hello,
>>
>> I am experimenting with nft / netfilter redirects to support wireguard
>> packets on *any* udp port. I tried using the following configuration for
>> nftables:
>>
>> [17:34:14] vpn-2a0ae5c1:~# cat /etc/nftables.conf
>> #!/usr/sbin/nft -f
>>
>> flush ruleset
>>
>> table ip nat {
>> 	chain prerouting {
>> 		type nat hook prerouting priority 0;
>>
>> 		# SSH works
>> 		tcp dport != 22 redirect to 22
>>
>> 		# wireguard doesn't
>> 		udp dport != 51820 redirect to 51820
>> 	}
>>
>> 	chain postrouting {
>> 		type nat hook postrouting priority 0;
>> 	}
>> }
>>
>> However as you can see in the comments, this does not work with
>> wireguard, however it does work with SSH.
>>
>> I can see that wireguard is kernel space, and ssh user space, but does
>> that cause the netfilter part to be skipped or am I doing some silly
>> mistake here?
>>
>> Best regards,
>>
>> Nico
>>
>>
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