tcpip: fix IP_MTU_DISCOVER for locally-originated packets - #14143
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Fixes google#12319 When IP_MTU_DISCOVER is set to IP_PMTUDISC_DO or IP_PMTUDISC_PROBE, locally-originated packets exceeding the MTU should return ErrMessageTooLong (EMSGSIZE), matching Linux kernel semantics. Changes: 1. pkg/tcpip/network/ipv4/ipv4.go: Remove the IsForwardedPacket guard from the DF+fragmentation check. When the Don't Fragment bit is set, packets exceeding network MTU return ErrMessageTooLong. Default UDP sockets maintain PMTUDiscoveryDont (DF=false), preventing regressions for large unfragmented UDP datagrams (google#12479). 2. pkg/tcpip/transport/udp/endpoint.go: When UDP WritePacket fails with ErrMessageTooLong, queue a local error on the socket error queue when IP_RECVERR is enabled, matching Linux net/ipv4/udp.c:udp_send_skb(). 3. test/packetimpact/tests/ipv4_id_uniqueness_test.go: Update comment now that IP_PMTUDISC_DONT is supported. Assisted-by: Gemini CLI
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Fixes #12319
Problem
tracepathfails withtracepath: IP_MTU_DISCOVER: Operation not supportedbecausesetsockopt(IP_MTU_DISCOVER, IP_PMTUDISC_DO)andIP_PMTUDISC_PROBEpreviously returnedENOTSUP.A previous attempt to support this in PR #12366 was rolled back because of a regression where large UDP datagrams failed with
EMSGSIZE(#12479).Solution
pkg/tcpip/network/ipv4/ipv4.go:Remove the
IsForwardedPacketguard from theDontFragmentcheck. When the Don't Fragment (DF) bit is set and the packet exceeds the MTU, returnErrMessageTooLong(EMSGSIZE), matching Linux kernel semantics.IP_PMTUDISC_DONT,0) haveDF=false, so large datagrams continue to be fragmented locally without error, preventing regressions for workloads such as JavaDatagramChannel(Java runtime tests fail on buildkite #12479).pkg/tcpip/transport/udp/endpoint.go:When UDP
WritePacketfails withErrMessageTooLong, queue a local error on the socket's error queue ifIP_RECVERR/IPV6_RECVERRis enabled. This mirrors Linuxnet/ipv4/udp.c:udp_send_skb()behaviour, allowingtracepathto receive the error viaMSG_ERRQUEUE.test/packetimpact/tests/ipv4_id_uniqueness_test.go:Clean up stale TODO comment as
IP_PMTUDISC_DONTis supported.Verification
All unit tests, integration tests, and static checks passed:
//pkg/tcpip/network/ipv4:ipv4_test(passes all fragmentation & forwarding test cases)//pkg/tcpip/network/ipv4:ipv4_unit_test//pkg/tcpip/transport/udp:udp_x_test//pkg/tcpip/transport/internal/network:network_test//pkg/tcpip/transport/tcp:tcp_test//pkg/tcpip/tests/integration:...nogostatic analysis for all modified packagesAssisted-by: Gemini CLI