netfilter: add iptables MASQUERADE target with strict hook validation and tests - #14606
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netfilter: add iptables MASQUERADE target with strict hook validation and tests#14606hugelgupf wants to merge 1 commit into
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… and tests
The `MASQUERADE` target (xt_nat) is a specialized Source NAT (SNAT)
target valid in the nat table's POSTROUTING chain. It dynamically
rewrites a packet's source IP address to the primary IP address of the
outgoing network interface and establishes a connection-tracking entry so
that return traffic is automatically de-masqueraded back to the original
client.
Kubernetes `kube-proxy` configures MASQUERADE in the KUBE-POSTROUTING
chain for traffic leaving the node or traversing virtual bridge hairpin
paths. Although gVisor's netstack already provided stack.MasqueradeTarget,
netfilter lacked a targetMaker for "MASQUERADE", causing `iptables-restore`
to fail ("MASQUERADE revision 0 not supported") and abort the entire
sync transaction.
### What is implemented:
- Protocol and layout validation: Translates iptables MASQUERADE target
(revisions 0, 1, 2) to stack.MasqueradeTarget for IPv4 and IPv6.
- Strict table and hook validation: Enforces that the MASQUERADE target
is only installed in the `nat` table on the `POSTROUTING` hook. Any
attempt to install MASQUERADE in other tables (filter/mangle/raw) or
other hooks (PREROUTING, INPUT, FORWARD, OUTPUT) is explicitly rejected
during ruleset loading.
- Dynamic egress source translation: stack.MasqueradeTarget queries the
outgoing AddressableEndpoint at action time and executes SNAT.
- Round-trip fidelity: Retains raw target bytes for faithful
iptables-save round-tripping.
- Unit and integration tests:
* Unit test in netfilter_test.go verifying checkLoopsAndChains rejects
MASQUERADE in non-POSTROUTING hooks.
* Integration tests in test/iptables/nat.go verifying UDP and TCP
masquerade rewrites as well as rejection of MASQUERADE in PREROUTING/filter.
### What is still missing / diverges vs. Linux:
- Interface DOWN flushes: Linux instantly deletes all conntrack entries
associated with an interface when the link drops its IP or goes DOWN;
gVisor relies on standard conntrack timeout reaping.
- Port range constraints: Specific port ranges passed via `--to-ports`
are preserved for iptables-save but port selection currently uses
netstack's standard ephemeral port allocator.
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Summary
masqueradeTargetMakerinpkg/sentry/socket/netfilter/masquerade.gofor IPv4 and IPv6, translating the iptables target tostack.MasqueradeTarget.MASQUERADEis only installed in thenattable on thePOSTROUTINGhook. Any rule placingMASQUERADEin other tables (filter/mangle/raw) or other hooks (PREROUTING, INPUT, FORWARD, OUTPUT) is rejected withEINVAL.pkg/sentry/socket/netfilter/netfilter_test.goand integration tests intest/iptables/nat.go(includingNATMasqueradeInvalidHookReject).Motivation & Context
Kubernetes
kube-proxyconfiguresMASQUERADEinKUBE-POSTROUTINGfor traffic leaving the node or traversing virtual bridge hairpin paths. Although netstack hadstack.MasqueradeTarget, netfilter had no targetMaker for"MASQUERADE", causingiptables-restoreto fail ("MASQUERADE revision 0 not supported") and aborting ruleset synchronization.Test Plan
bazel test //pkg/sentry/socket/netfilter:netfilter_test //pkg/tcpip/stack:stack_testtest/iptables:iptables_test(NATPostMasquerade*,NATMasqueradeInvalidHookReject)