tcp: Derive MSS announced to guest/namespace from configured MTU if present

...and from the sending socket only if the MTU is not configured.

Otherwise, a connection to a host from a local guest, with a
non-loopback destination address, will get its MSS from the MTU of the
outbound interface with that address, which is unnecessary as we know
the guest can send us larger segments.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/tcp.c b/tcp.c
index 7de8fa9..d5e47c9 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1187,10 +1187,23 @@ static int tcp_send_to_tap(struct ctx *c, struct tcp_tap_conn *conn, int flags,
 	th->doff = sizeof(*th) / 4;
 
 	if (flags & SYN) {
+		uint16_t mss;
+
 		/* Options: MSS, NOP and window scale if allowed (4-8 bytes) */
 		*data++ = OPT_MSS;
 		*data++ = OPT_MSS_LEN;
-		*(uint16_t *)data = htons(info.tcpi_snd_mss);
+
+		if (c->mtu == -1) {
+			mss = info.tcpi_snd_mss;
+		} else {
+			mss = c->mtu - sizeof(sizeof *th);
+			if (IN6_IS_ADDR_V4MAPPED(&conn->a.a6))
+				mss -= sizeof(struct iphdr);
+			else
+				mss -= sizeof(struct ipv6hdr);
+		}
+		*(uint16_t *)data = htons(mss);
+
 		data += OPT_MSS_LEN - 2;
 		th->doff += OPT_MSS_LEN / 4;