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udp: degrade outer-ECN RX per-family on dual-stack sockets

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A failed IPV6_RECVTCLASS probe disabled ECN RX entirely, taking down
working IPv4 (v4-mapped) delivery with it, while the opposite failure
already only degraded. Treat both directions the same: each family
degrades to Not-ECT independently, and only a full-family failure
turns the cmsg parsing off.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp
This commit is contained in:
JackDoan
2026-07-29 17:17:56 -05:00
parent fbcdd83359
commit 8044b40c82
1 changed files with 12 additions and 12 deletions
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udp/udp_linux.go
+12 -12
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@@ -133,24 +133,24 @@ func (u *StdConn) prepareGRO() {
// field of each arriving datagram is delivered as ancillary data alongside
// the payload. ListenOut reads it via parseRecvCmsg and passes the codepoint
// through the EncReader for RFC 6040 combine on the decap side. Best-effort:
// we keep going on failure. Only the socket's own family gates support; on a
// dual-stack v6 socket a failed IPv4 probe just degrades v4 peers to Not-ECT
// (could be a v6-specific bind).
// we keep going on failure, and each family degrades independently — a peer
// whose family's probe failed just delivers no cmsg and lands as Not-ECT.
// Only a failure of every family the socket speaks turns the parsing off.
func (u *StdConn) prepareECNRecv() {
v4err := unix.SetsockoptInt(u.sysFd, unix.IPPROTO_IP, unix.IP_RECVTOS, 1)
err := v4err
var v6err error
if !u.isV4 {
err = unix.SetsockoptInt(u.sysFd, unix.IPPROTO_IPV6, unix.IPV6_RECVTCLASS, 1)
if err != nil {
err = errors.Join(v4err, err)
} else if v4err != nil {
u.l.Debug("udp: outer-ECN RX degraded", "reason", "kernel rejected probe on IPv4", "error", v4err)
}
v6err = unix.SetsockoptInt(u.sysFd, unix.IPPROTO_IPV6, unix.IPV6_RECVTCLASS, 1)
}
if err != nil {
u.l.Info("udp: outer-ECN RX disabled", "reason", "kernel rejected probe", "error", err)
switch {
case v4err != nil && (u.isV4 || v6err != nil):
u.l.Info("udp: outer-ECN RX disabled", "reason", "kernel rejected probe", "error", errors.Join(v4err, v6err))
recordCapability("udp.ecn_rx.enabled", false)
return
case v4err != nil:
u.l.Debug("udp: outer-ECN RX degraded", "reason", "kernel rejected probe on IPv4", "error", v4err)
case v6err != nil:
u.l.Debug("udp: outer-ECN RX degraded", "reason", "kernel rejected probe on IPv6", "error", v6err)
}
u.ecnRecvSupported = true
u.l.Info("udp: outer-ECN RX enabled")