Re: pasta behaviour with multiple NICs

["Ben Woods" <pasta@ben.woods.am>]

Hi Stefano,

Thanks for the quick response.

I think my questions came from a misunderstanding of how pasta works. I was thinking about the container network namespace directly sending the traffic out the host physical interface based on the IP/gateway inside the netns.

Reading your answer, I think I understand now that in fact the network connection from inside the container netns is connected via a socket to pasta running on the host… and then pasta simply creates the TCP or UDP socket connection out the host physical interface using the host network stack. Is that correct?

That then explains why you’re saying that pasta itself is not choosing the egress interface, route or source IP… it’s the kernel that does that when pasta creates the TCP/UDP connection. Hence the traffic egress interface, source IP and next-hop should be the same as if it originated from a process on the host.

It does make we wonder what’s the purpose of assigning an IP/subnet/gateway inside the container netns at all - if all connections are sent via the socket and host pasta process then creates the actual connection?

Cheers,
Ben

On Fri, 25 Apr 2025, at 3:26 PM, Stefano Brivio wrote:
> Hi Ben,
>
> On Fri, 25 Apr 2025 14:54:18 +0800
> "Ben Woods" <pasta@ben.woods.am> wrote:
>
>> Hi everyone,
>> 
>> I'm struggling to understand how pasta will behave when the host has
>> multiple network interfaces. I can't see this mentioned in the
>> website or man page.
>
> Right, yeah, it's not really mentioned anywhere, sorry for that, and
> thanks for your question.
>
>> I'm using pasta with podman if that makes a difference.
>
> It shouldn't make a difference.
>
>> Example Scenario - 2 interfaces - eth0 (with default route) and eth1
>> in a different subnet.
>> 
>> When the podman container is created, inside the container there is a
>> single interface shown that mimics the eth0 interface name, IP,
>> gateway.
>> 
>> If traffic is initiated from the container to an IP within the eth1
>> subnet - how does pasta make it appear to come from the eth1 IP
>> address? Does it automatically apply NAT to achieve this?
>
> The operating system (unfortunately it's Linux only, so far) takes care
> of all that, pasta has no idea: it just opens a socket and connect()s
> it to the destination address (that might be bind() _and_ connect(), for
> UDP). The kernel then decides based on routing rules and tables.
>
> But yes, this typically results in NAT, at least with the default
> source address selection Linux does.
>
> In other words: it's as if your container and everything inside it
> behaved like a local process, network-wise, as seen from outside.
>
> Given that pasta isn't in charge of network (or even transport) headers
> "outside", it doesn't really "do NAT", but, with default options and a
> matching upstream interface, it avoids that NAT is done in the bigger
> picture.
>
>> If the host has a static route for a subnet not directly connected to
>> either eth0 or eth1, but the static route uses a next hop IP address
>> within the eth1 subnet - will pasta apply NAT to the eth1 IP address,
>> and the use the static route to send it via the next-hop router?
>
> This also reduces to a question about Linux, essentially. Yes, as far
> as I know, that would be the outcome: source NAT using a matching
> address assigned to eth1, if any (preferred source address).
>
> Does that answer your question?
>
> -- 
> Stefano

-- 
From: Ben Woods
ben@woods.am