On Mon, 23 Jan 2023 17:20:13 +1100 David Gibson wrote:

On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:

...

Hi Noah,

Sorry for the delay, I didn't check pending mailing list posts for a couple of days. Comments below:

On Tue, 17 Jan 2023 11:50:50 -0800 Noah Gold wrote:

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Hi folks,

libslirp and Passt have different approaches to sharing DNS resolvers with the guest system, each with their own benefits & drawbacks. On the libslirp project, we're discussing [1] how to support DNS failover. Passt already has support for this, but there is a drawback to its solution which prevents us from taking a similar approach: the resolvers are read exactly once, so if the host changes networks at runtime, the guest will not receive the updated resolvers and thus its connectivity will break.

So, passt/pasta kinda-sorta binds itself to a particular host interface, so DNS won't be the only issue if the host changes network. For one thing, at least by default the guest gets the same IP as the host, so if the host IP changes the guest will get out of sync. We'll mostly cope with that ok, but there will be some edge cases which will break (most obviously if after the network change the guest wants to talk to something at the host's old address / its current address).

Noah, by the way, if your usage for DNS failover is related to a virtual machine being migrated to another host with different addressing, mind that you could simply tell qemu to connect to a new instance of passt. That's something you can't do with libslirp. Would that solve your problem, or your issue is specifically related to DNS failover without any VM migration playing a role? -- Stefano

On Mon, 30 Jan 2023 16:11:38 -0800 Noah Gold wrote:

Sorry for the delay, I've been really busy this past week.

On Sun, Jan 22, 2023 at 10:26 PM David Gibson wrote:

...

On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:

...

Hi Noah,

Sorry for the delay, I didn't check pending mailing list posts for a couple of days. Comments below:

On Tue, 17 Jan 2023 11:50:50 -0800 Noah Gold wrote:

...

Hi folks,

libslirp and Passt have different approaches to sharing DNS resolvers with the guest system, each with their own benefits & drawbacks. On the libslirp project, we're discussing [1] how to support DNS failover. Passt already has support for this, but there is a drawback to its solution which prevents us from taking a similar approach: the resolvers are read exactly once, so if the host changes networks at runtime, the guest will not receive the updated resolvers and thus its connectivity will break.

So, passt/pasta kinda-sorta binds itself to a particular host interface, so DNS won't be the only issue if the host changes network. For one thing, at least by default the guest gets the same IP as the host, so if the host IP changes the guest will get out of sync. We'll mostly cope with that ok, but there will be some edge cases which will break (most obviously if after the network change the guest wants to talk to something at the host's old address / its current address).

...

Right -- the main motivation behind this (other than simplicity) is that we can close /etc/resolv.conf before sandboxing.

However, we could keep a handle on it, just like we do for PID and pcap files, while still unmounting the filesystem.

And we could also use inotify to detect changes I guess -- we do the same to monitor namespaces in pasta mode (see pasta_netns_quit_init()).

All true, but I'm not sure those are actually the most pressing issues we'll face with a host network change.

...

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libslirp's current approach is to DNAT a single address exposed to the guest to one of the resolvers configured on the host. The problem here is that if that one resolver goes down, the guest can't resolve DNS names. We're considering changing so that instead of a single address, we expose a set of MAXNS addresses, and DNAT those 1:1 to the DNS resolvers registered with the host. Because the DNAT table lives on the host side, we can refresh the guest's resolvers whenever the host's resolvers change, but without the need to expire a DHCP lease (even with short leases, the guest will still lose connectivity for a time).

Does this sound like an approach Passt would be open to adopting as well?

Yes, definitely, patches would be very welcome.

Hm, that's doesn't fit that easily into the passt model. For the most part we don't NAT at all, we only have a couple of special cases where we do. Because of that the problem with adding any extra NAT case is address allocation. Currently we use the host's gateway address, which mostly works but is a bit troublesome. I have some ideas I think will work better, but those don't necessarily get us more available addresses.

For libslirp we have the guest on a private subnet, so pulling addresses from that pool is pretty easy. For passt is the issue that there is no address range, or that the infrastructure to allocate from the range just doesn't exist yet?

[David is out this and next week] There's no address range because it's not designed with NAT in mind, even though it can do NAT. From what we discussed with David in the past, the idea, if I recall correctly, was that you could decide to, at least, remap a particular address instead of the gateway address (more on that below) -- and perhaps something more flexible with more addresses, but not an arbitrary number of them, as passt doesn't do dynamic memory allocation.

When you say "we use the host's gateway address", what is it used for exactly? (I didn't follow the loopback example below.)

The host's default gateway address (for both IPv4 and IPv6) is advertised, by default, as gateway address/next hop of default route, to the guest, via DHCP/NDP. Again by default (unless --no-map-gw is used), the guest can then use this address to refer to the host (and not its default gateway). See also the "Handling of traffic with local destination and source addressses" section in the NOTES of passt(1). However, this is, at the moment, unrelated to how DNS addresses are mapped: right now you can specify --dns-forward zero to two times (separately for IPv4 and IPv6) and that will forward DNS queries (with reverse mapping) to the first configured resolver. So, if you are happy with this kind of solution (with a NAT), you pick the addresses yourself, you don't need pools or ranges, and you would "just" need, on top of what's already available, to change, at runtime, the resolver passt forwards queries to (perhaps via inotify as I mentioned).

...

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Note that David (Cc'ed) is currently working on a generalised/flexible address mapping mechanism, some kind of (simple) NAT table as far as I understood it.

That's a bit overstating it. I'm making our current single NAT case (translating host side loopback to gateway address on the guest) more configurable. I have plans (or at least ideas) for a more generalized NAT mechanism, but I'm really not implementing that yet. What I'm doing now is kind of a soft prerequisite for that rework though (as well as useful in its own right).

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This might even address your DNS idea already, I'm not sure, I'd wait for him to comment.

Hadn't considered specifically that model, but it's a reasonbly natural extension of it (address allocation is still a complication). I'll certainly consider this case when I do more on this.

It sounds like there might be a path to using NAT, but it's not something that would be ready soon. Given that, would there be long term concerns with using NAT for DNS in the way proposed here? I understand we can't implement it now, but I'd like to understand if it's an approach we would still rather avoid, even long term.

I don't really see an issue with it, also because, actually, we already do it. :) ...even though it's for two address pairs only (internal/external IPv4/IPv6 addresses). If that's enough for your use case (more on that below), I think we can also implement a runtime change of resolvers now.

...

-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson

On Wed, Jan 25, 2023 at 9:55 AM Stefano Brivio wrote:

...

On Mon, 23 Jan 2023 17:20:13 +1100 David Gibson wrote:

...

On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:

...

Hi Noah,

Sorry for the delay, I didn't check pending mailing list posts for a couple of days. Comments below:

On Tue, 17 Jan 2023 11:50:50 -0800 Noah Gold wrote:

...

Hi folks,

libslirp and Passt have different approaches to sharing DNS resolvers with the guest system, each with their own benefits & drawbacks. On the libslirp project, we're discussing [1] how to support DNS failover. Passt already has support for this, but there is a drawback to its solution which prevents us from taking a similar approach: the resolvers are read exactly once, so if the host changes networks at runtime, the guest will not receive the updated resolvers and thus its connectivity will break.

So, passt/pasta kinda-sorta binds itself to a particular host interface, so DNS won't be the only issue if the host changes network. For one thing, at least by default the guest gets the same IP as the host, so if the host IP changes the guest will get out of sync. We'll mostly cope with that ok, but there will be some edge cases which will break (most obviously if after the network change the guest wants to talk to something at the host's old address / its current address).

Noah, by the way, if your usage for DNS failover is related to a virtual machine being migrated to another host with different addressing, mind that you could simply tell qemu to connect to a new instance of passt. That's something you can't do with libslirp.

It's not related to machine migration, though that's another interesting case with similar constraints. The use case I'm thinking about is for a mobile device that may experience network changes as part of its normal operation (e.g. changing wifi networks).

So... I admit I have no idea what happens exactly when you change parts of the host configuration, this kind of use case wasn't really a priority for passt in the... past. I expect it to mostly work. By default, we don't do NAT because (with default options) the address of the guest matches the address of the host. But once you change addresses and routes on the host, passt should just start doing NAT, it's implicit and not something you need to enable or disable. Would you have a chance to try it out in the use case you had in mind, so that we can go through any issue you might hit?

...

Would that solve your problem, or your issue is specifically related to DNS failover without any VM migration playing a role?

It's not related to migration, but I wonder whether there's an idea there which could be used. The approach I was taking was to make the network component resilient to network changes. But another option is to detect network changes and restart the network component. libslirp still needs a way to support exposing multiple servers though, and I wonder whether we would want to require library consumers to write network awareness into their applications as opposed to solving it for them.

Restarting the network component has a single, fundamental advantage, I think: it's a convenient way to reset a number of states and stored information in an implicit way. For example, it's better to reset TCP connections (stop the process, sockets close) than to let them hang. We could reset connections explicitly, of course, but this adds a bit of complexity. Still, with some effort we could make an attempt at actually keeping them alive. Maybe this even works with passt already. So I'm not really sure what would be the best approach. Making the network component resilient to network changes, in the long term, sounds more appropriate and elegant to me. I was just suggesting that, in the short term, restarting passt should cover whatever use case you might have. -- Stefano

On Mon, 13 Feb 2023 18:45:20 -0800 Noah Gold wrote:

On Thu, Feb 2, 2023 at 3:09 AM Stefano Brivio wrote:

...

On Mon, 30 Jan 2023 16:11:38 -0800 Noah Gold wrote:

...

Sorry for the delay, I've been really busy this past week.

On Sun, Jan 22, 2023 at 10:26 PM David Gibson wrote:

...

On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:

...

Hi Noah,

Sorry for the delay, I didn't check pending mailing list posts for a couple of days. Comments below:

On Tue, 17 Jan 2023 11:50:50 -0800 Noah Gold wrote:

...

Hi folks,

libslirp and Passt have different approaches to sharing DNS resolvers with the guest system, each with their own benefits & drawbacks. On the libslirp project, we're discussing [1] how to support DNS failover. Passt already has support for this, but there is a drawback to its solution which prevents us from taking a similar approach: the resolvers are read exactly once, so if the host changes networks at runtime, the guest will not receive the updated resolvers and thus its connectivity will break.

So, passt/pasta kinda-sorta binds itself to a particular host interface, so DNS won't be the only issue if the host changes network. For one thing, at least by default the guest gets the same IP as the host, so if the host IP changes the guest will get out of sync. We'll mostly cope with that ok, but there will be some edge cases which will break (most obviously if after the network change the guest wants to talk to something at the host's old address / its current address).

...

Right -- the main motivation behind this (other than simplicity) is that we can close /etc/resolv.conf before sandboxing.

However, we could keep a handle on it, just like we do for PID and pcap files, while still unmounting the filesystem.

And we could also use inotify to detect changes I guess -- we do the same to monitor namespaces in pasta mode (see pasta_netns_quit_init()).

All true, but I'm not sure those are actually the most pressing issues we'll face with a host network change.

...

...

libslirp's current approach is to DNAT a single address exposed to the guest to one of the resolvers configured on the host. The problem here is that if that one resolver goes down, the guest can't resolve DNS names. We're considering changing so that instead of a single address, we expose a set of MAXNS addresses, and DNAT those 1:1 to the DNS resolvers registered with the host. Because the DNAT table lives on the host side, we can refresh the guest's resolvers whenever the host's resolvers change, but without the need to expire a DHCP lease (even with short leases, the guest will still lose connectivity for a time).

Does this sound like an approach Passt would be open to adopting as well?

Yes, definitely, patches would be very welcome.

Hm, that's doesn't fit that easily into the passt model. For the most part we don't NAT at all, we only have a couple of special cases where we do. Because of that the problem with adding any extra NAT case is address allocation. Currently we use the host's gateway address, which mostly works but is a bit troublesome. I have some ideas I think will work better, but those don't necessarily get us more available addresses.

For libslirp we have the guest on a private subnet, so pulling addresses from that pool is pretty easy. For passt is the issue that there is no address range, or that the infrastructure to allocate from the range just doesn't exist yet?

[David is out this and next week]

There's no address range because it's not designed with NAT in mind, even though it can do NAT. From what we discussed with David in the past, the idea, if I recall correctly, was that you could decide to, at least, remap a particular address instead of the gateway address (more on that below) -- and perhaps something more flexible with more addresses, but not an arbitrary number of them, as passt doesn't do dynamic memory allocation.

Ah okay, it's sharing the host network by default? Or at least, doing its best to pretend that's the case?

Yes -- not sharing it, just pretending. Again, that's only by default.

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When you say "we use the host's gateway address", what is it used for exactly? (I didn't follow the loopback example below.)

The host's default gateway address (for both IPv4 and IPv6) is advertised, by default, as gateway address/next hop of default route, to the guest, via DHCP/NDP.

Again by default (unless --no-map-gw is used), the guest can then use this address to refer to the host (and not its default gateway). See also the "Handling of traffic with local destination and source addressses" section in the NOTES of passt(1).

However, this is, at the moment, unrelated to how DNS addresses are mapped: right now you can specify --dns-forward zero to two times (separately for IPv4 and IPv6) and that will forward DNS queries (with reverse mapping) to the first configured resolver.

So, if you are happy with this kind of solution (with a NAT), you pick the addresses yourself, you don't need pools or ranges, and you would "just" need, on top of what's already available, to change, at runtime, the resolver passt forwards queries to (perhaps via inotify as I mentioned).

Makes sense. The trouble is when N > 2, see below.

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...

Note that David (Cc'ed) is currently working on a generalised/flexible address mapping mechanism, some kind of (simple) NAT table as far as I understood it.

That's a bit overstating it. I'm making our current single NAT case (translating host side loopback to gateway address on the guest) more configurable. I have plans (or at least ideas) for a more generalized NAT mechanism, but I'm really not implementing that yet. What I'm doing now is kind of a soft prerequisite for that rework though (as well as useful in its own right).

...

This might even address your DNS idea already, I'm not sure, I'd wait for him to comment.

Hadn't considered specifically that model, but it's a reasonbly natural extension of it (address allocation is still a complication). I'll certainly consider this case when I do more on this.

It sounds like there might be a path to using NAT, but it's not something that would be ready soon. Given that, would there be long term concerns with using NAT for DNS in the way proposed here? I understand we can't implement it now, but I'd like to understand if it's an approach we would still rather avoid, even long term.

I don't really see an issue with it, also because, actually, we already do it. :) ...even though it's for two address pairs only (internal/external IPv4/IPv6 addresses). If that's enough for your use case (more on that below), I think we can also implement a runtime change of resolvers now.

Got it. The problem with just two pairs is when the host has N DNS resolvers, and N-1 of them are broken (N > 3 is unfortunately possible on the non unix systems (Windows) libslirp supports). It sounds like the *future* approach for passt might be tricky if dynamic allocation is completely off the table. Is some dynamic allocation permitted at initialization time? If so, we could detect the # of resolvers and perhaps take a start address as an argument?

If really necessary, I think we could consider to do some dynamic allocation before the seccomp profile is applied. However, how many addresses of resolvers could we possibly want? 16? 32? Reading around a bit, it looks like Windows DHCP servers generally support assigning 25 addresses. It's not the same thing, but it would suggest that 32 a reasonable choice -- and that's something we could merrily use to size static buffers.

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-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson

On Wed, Jan 25, 2023 at 9:55 AM Stefano Brivio wrote:

...

On Mon, 23 Jan 2023 17:20:13 +1100 David Gibson wrote:

...

On Sat, Jan 21, 2023 at 10:47:03AM +0100, Stefano Brivio wrote:

...

Hi Noah,

Sorry for the delay, I didn't check pending mailing list posts for a couple of days. Comments below:

On Tue, 17 Jan 2023 11:50:50 -0800 Noah Gold wrote:

> Hi folks, > > libslirp and Passt have different approaches to sharing DNS resolvers with > the guest system, each with their own benefits & drawbacks. On the libslirp > project, we're discussing [1] how to support DNS failover. Passt already has > support for this, but there is a drawback to its solution which prevents us > from taking a similar approach: the resolvers are read exactly once, so if the > host changes networks at runtime, the guest will not receive the updated > resolvers and thus its connectivity will break.

So, passt/pasta kinda-sorta binds itself to a particular host interface, so DNS won't be the only issue if the host changes network. For one thing, at least by default the guest gets the same IP as the host, so if the host IP changes the guest will get out of sync. We'll mostly cope with that ok, but there will be some edge cases which will break (most obviously if after the network change the guest wants to talk to something at the host's old address / its current address).

Noah, by the way, if your usage for DNS failover is related to a virtual machine being migrated to another host with different addressing, mind that you could simply tell qemu to connect to a new instance of passt. That's something you can't do with libslirp.

It's not related to machine migration, though that's another interesting case with similar constraints. The use case I'm thinking about is for a mobile device that may experience network changes as part of its normal operation (e.g. changing wifi networks).

So... I admit I have no idea what happens exactly when you change parts of the host configuration, this kind of use case wasn't really a priority for passt in the... past.

For the use case I'm looking at (present, not passt), it's probably fine for the typical thing to happen (all open sockets timeout or hit resets) since that's happening on the host anyways.

...

I expect it to mostly work. By default, we don't do NAT because (with default options) the address of the guest matches the address of the host. But once you change addresses and routes on the host, passt should just start doing NAT, it's implicit and not something you need to enable or disable.

Would you have a chance to try it out in the use case you had in mind, so that we can go through any issue you might hit?

I'm working exclusively with Windows at the moment, so presently this is more to make sure the adjustments we make in libslirp could be applied to passt... in the future. (Time travel aside, my vague understanding is that passt may be the successor for libslirp, at least based on the interest from the maintainers in keeping some compatibility in terms of features. I'd be very curious if someone could clarify how the two projects relate beyond solving very similar problems.)

I guess we should eventually add a FAQ section to the project website. Meanwhile, there's some kind of summary about that in slide 16 from a presentation at KVM Forum last year: https://static.sched.com/hosted_files/kvmforum2022/01/passt_kubevirt_kvm_for... (recording at: https://www.youtube.com/watch?v=U89bWP1HNgU). By the way, a Win2k port is up for grabs at https://bugs.passt.top/show_bug.cgi?id=8. I think the complexity might be similar to a FreeBSD/Darwin port (tracked at https://bugs.passt.top/show_bug.cgi?id=6).

Conceptually though, I'll definitely keep this thread updated if we run into issues implementing first in libslirp, as they may apply to passt as well.

Thanks!

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...

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Would that solve your problem, or your issue is specifically related to DNS failover without any VM migration playing a role?

It's not related to migration, but I wonder whether there's an idea there which could be used. The approach I was taking was to make the network component resilient to network changes. But another option is to detect network changes and restart the network component. libslirp still needs a way to support exposing multiple servers though, and I wonder whether we would want to require library consumers to write network awareness into their applications as opposed to solving it for them.

Restarting the network component has a single, fundamental advantage, I think: it's a convenient way to reset a number of states and stored information in an implicit way.

For example, it's better to reset TCP connections (stop the process, sockets close) than to let them hang. We could reset connections explicitly, of course, but this adds a bit of complexity.

Still, with some effort we could make an attempt at actually keeping them alive. Maybe this even works with passt already.

So I'm not really sure what would be the best approach. Making the network component resilient to network changes, in the long term, sounds more appropriate and elegant to me.

I was just suggesting that, in the short term, restarting passt should cover whatever use case you might have.

Makes sense. I agree, long term resiliency seems like the cleaner solution.

-- Stefano