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handle ACK in FIN, fix ACK to FIN + payload

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Sergio R. Caprile committed 2026-08-07 11:49:28 -03:00
1 parent 9f16806a22
commit 9c6b4edd37
3 files changed
+99 -30
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src/net_builtin.c
+19 -15
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@@ -1462,6 +1462,8 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
struct connstate *s = (struct connstate *) (c + 1);
struct mg_iobuf *io = c->is_tls ? &c->rtls : &c->recv;
uint32_t seq = mg_ntohl(pkt->tcp->seq);
// Evaluation order is important: 1) process FIN, 2) process keep-alive,
// 3) discard invalid SEG.SEQ, 4) process ACK and payload.
if (pkt->tcp->flags & TH_FIN) {
uint8_t flags = TH_ACK;
if (mg_ntohl(pkt->tcp->seq) != s->ack) {
@@ -1490,18 +1492,12 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
}
tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, flags,
mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
if (pkt->pay.len == 0) return; // if no data, we're done
} else if (pkt->pay.len <= 1 && mg_ntohl(pkt->tcp->seq) == s->ack - 1) {
// Keep-Alive (RFC-9293 3.8.4, allow erroneous implementations)
MG_VERBOSE(("%lu keepalive ACK", c->id));
tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
mg_htonl(s->seq), mg_htonl(s->ack), NULL, 0);
return; // no data to process
} else if (pkt->pay.len == 0) { // this is an ACK
if (pkt->tcp->flags & TH_ACK)
handle_ack(s, mg_ntohl(pkt->tcp->ack), mg_ntohs(pkt->tcp->win));
if (s->fin_rcvd && s->ttype == MIP_TTYPE_FIN) s->twclosure = true;
return; // no data to process
return; // RFC-9293 3.10.7.4 discard (incorrect) payload, ACK, window
} else if (seq != s->ack) {
uint32_t ack = (uint32_t) (mg_htonl(pkt->tcp->seq) + pkt->pay.len);
if (s->ack == ack) {
@@ -1511,14 +1507,20 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
tx_tcp(c->mgr->ifp, s->mac, &c->loc, &c->rem, c->dscp, TH_ACK,
mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
}
return; // drop it
} else if (io->size - io->len < pkt->pay.len &&
!mg_iobuf_resize(io, io->len + pkt->pay.len)) {
return; // drop it, RFC-9293 3.10.7.4: ignore ACKno
}
// Now process the segment for ACK and payload
if (pkt->tcp->flags & TH_ACK) {
handle_ack(s, mg_ntohl(pkt->tcp->ack), mg_ntohs(pkt->tcp->win));
if (pkt->pay.len == 0 && s->fin_rcvd && s->ttype == MIP_TTYPE_FIN)
s->twclosure = true;
}
if (pkt->pay.len == 0) return;
if (io->size - io->len < pkt->pay.len &&
!mg_iobuf_resize(io, io->len + pkt->pay.len)) {
mg_error(c, "oom");
return; // drop it
}
if (pkt->tcp->flags & TH_ACK)
handle_ack(s, mg_ntohl(pkt->tcp->ack), mg_ntohs(pkt->tcp->win));
// Copy TCP payload into the IO buffer. If the connection is plain text,
// we copy to c->recv. If the connection is TLS, this data is encrypted,
// therefore we copy that encrypted data to the c->rtls iobuffer instead,
@@ -1527,9 +1529,11 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
memcpy(&io->buf[io->len], pkt->pay.buf, pkt->pay.len);
io->len += pkt->pay.len;
MG_VERBOSE(("%lu SEQ %x -> %x", c->id, mg_htonl(pkt->tcp->seq), s->ack));
// Advance ACK counter
s->ack = (uint32_t) (mg_htonl(pkt->tcp->seq) + pkt->pay.len);
s->unacked += pkt->pay.len;
if (!(pkt->tcp->flags & TH_FIN)) { // FIN already advanced s->ack
// Advance ACK counter
s->ack = (uint32_t) (mg_htonl(pkt->tcp->seq) + pkt->pay.len);
s->unacked += pkt->pay.len;
}
// size_t diff = s->acked <= s->ack ? s->ack - s->acked : s->ack;
if (s->unacked > MG_TCPIP_WIN / 2 && s->acked != s->ack) {
// Send ACK immediately