[23.0 backport] Clear conntrack entries for published UDP ports#44752
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[23.0 backport] Clear conntrack entries for published UDP ports#44752
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Conntrack entries are created for UDP flows even if there's nowhere to route these packets (ie. no listening socket and no NAT rules to apply). Moreover, iptables NAT rules are evaluated by netfilter only when creating a new conntrack entry. When Docker adds NAT rules, netfilter will ignore them for any packet matching a pre-existing conntrack entry. In such case, when dockerd runs with userland proxy enabled, packets got routed to it and the main symptom will be bad source IP address (as shown by moby#44688). If the publishing container is run through Docker Swarm or in "standalone" Docker but with no userland proxy, affected packets will be dropped (eg. routed to nowhere). As such, Docker needs to flush all conntrack entries for published UDP ports to make sure NAT rules are correctly applied to all packets. - Fixes moby#44688 - Fixes moby#8795 - Fixes moby#16720 - Fixes moby#7540 - Fixes moby/libnetwork#2423 - and probably more. As a precautionary measure, those conntrack entries are also flushed when revoking external connectivity to avoid those entries to be reused when a new sandbox is created (although the kernel should already prevent such case). Signed-off-by: Albin Kerouanton <albinker@gmail.com> (cherry picked from commit b37d343) Signed-off-by: Cory Snider <csnider@mirantis.com>
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I was going to open a backport PR too. Thanks for doing it 😁
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This is not needed anymore since Docker Engine v23. This version is unmaintained since May 2025 (not that old). See: - moby/moby#44752 - moby/moby#44742 Fix #2153 (in a way)
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This is not needed anymore since Docker Engine v23. This version is unmaintained since May 2025 (not that old). See: - moby/moby#44752 - moby/moby#44742 Fix #2153 (in a way)
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Conntrack entries are created for UDP flows even if there's nowhere to route these packets (ie. no listening socket and no NAT rules to apply). Moreover, iptables NAT rules are evaluated by netfilter only when creating a new conntrack entry.
When Docker adds NAT rules, netfilter will ignore them for any packet matching a pre-existing conntrack entry. In such case, when dockerd runs with userland proxy enabled, packets got routed to it and the main symptom will be bad source IP address (as shown by #44688).
If the publishing container is run through Docker Swarm or in "standalone" Docker but with no userland proxy, affected packets will be dropped (eg. routed to nowhere).
As such, Docker needs to flush all conntrack entries for published UDP ports to make sure NAT rules are correctly applied to all packets.
As a precautionary measure, those conntrack entries are also flushed when revoking external connectivity to avoid those entries to be reused when a new sandbox is created (although the kernel should already prevent such case).
Signed-off-by: Albin Kerouanton albinker@gmail.com
(cherry picked from commit b37d343)
Signed-off-by: Cory Snider csnider@mirantis.com
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