Don't skip DNAT for a routed network without userland-proxy#49577
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akerouanton merged 1 commit intomoby:masterfrom Mar 13, 2025
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Don't skip DNAT for a routed network without userland-proxy#49577akerouanton merged 1 commit intomoby:masterfrom
akerouanton merged 1 commit intomoby:masterfrom
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| for _, ipamCfg := range insp.IPAM.Config { | ||
| url := "http://" + net.JoinHostPort(ipamCfg.Gateway, "8080") | ||
| res := container.RunAttach(ctx, t, c, | ||
| container.WithNetworkMode(routedNetName), | ||
| container.WithCmd("wget", "-O-", "-T3", url), | ||
| ) | ||
| if tc.expResponse { | ||
| // 404 Not Found means the server responded, but it's got nothing to serve. | ||
| assert.Check(t, is.Contains(res.Stderr.String(), "404 Not Found"), "url: %s", url) | ||
| } else { | ||
| assert.Check(t, is.Contains(res.Stderr.String(), "download timed out"), "url: %s", url) | ||
| } | ||
| } |
If the userland-proxy is running, packets from one bridge network addressed to the host port are not DNAT'd - so that docker-proxy can pick them up, and therefore the packet bypasses the network isolation rules. Without the userland-proxy, there's no way for a packet from one bridge network to bypass the network isolation rules. So, in this case, DNAT is not skipped - and that at-least allows packets originating from the network that published the port to access the host port. Commit 0546d90 improved support for routed mode networks (allowing nat-mode networks access to containers in routed-mode networks, as well as just remote access). That commit changed the "SKIP DNAT" logic, making sure DNAT was skipped for a routed-mode network if the userland-proxy was enabled (so, containers in routed mode networks could access ports published by other networks). But, it still skipped DNAT for a routed mode network if the userland proxy was disabled - packets from the routed mode network aimed at any other network would be dropped by the network isolation rules anyway, and containers in a routed mode network don't need access to ports published from that network (because, by definition, there can't be any). However, network isolation rules can be worked-around with a rule in the DOCKER-USER chain, but the SKIP DNAT rule is harder to deal with. So, for routed-mode, only skip DNAT if the userland-proxy is enabled (just like nat-mode networks). Signed-off-by: Rob Murray <[email protected]>
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| // If the userland proxy is disabled, don't skip, so packets will be DNAT'd. That will | ||
| // enable access to ports published by containers in the same network. But, the INC rules | ||
| // will block access to that published port from containers in other networks. (However, | ||
| // users may add a rule to DOCKER-USER to work around the INC rules if needed.) |
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As shared privately, it'd be best to not require users to mess with their DOCKER-USER chain to enable that use-case. But, as you pointed out, it's too much risk for a patch release, and probably not worth implementing in iptables.
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- What I did
gateway_mode=routed#49509If the userland-proxy is running, packets from one bridge network addressed to the host port are not DNAT'd - so that docker-proxy can pick them up, and therefore the packet bypasses the network isolation rules.
Without the userland-proxy, there's no way for a packet from one bridge network to bypass the network isolation rules. So, in this case, DNAT is not skipped - and that at-least allows packets originating from the network that published the port to access the host port.
Commit 0546d90 improved support for routed mode networks (allowing nat-mode networks access to containers in routed-mode networks, as well as just remote access).
That commit changed the "SKIP DNAT" logic, making sure DNAT was skipped for a routed-mode network if the userland-proxy was enabled (so, containers in routed mode networks could access ports published by other networks).
But, it still skipped DNAT for a routed mode network if the userland proxy was disabled - packets from the routed mode network aimed at any other network would be dropped by the network isolation rules anyway, and containers in a routed mode network don't need access to ports published from that network (because, by definition, there can't be any).
However, network isolation rules can be worked-around with a rule in the DOCKER-USER chain, but the SKIP DNAT rule is harder to deal with.
- How I did it
For routed-mode, only skip DNAT if the userland-proxy is enabled (just like nat-mode networks).
- How to verify it
New regression test.
- Human readable description for the release notes