Fix bugs in PickEphemeralPort for TCP.

Netstack always picks a random start point everytime PickEphemeralPort
is called. While this is required for UDP so that DNS requests go
out through a randomized set of ports it is not required for TCP. Infact
Linux explicitly hashes the (srcip, dstip, dstport) and a one time secret
initialized at start of the application to get a random offset. But to
ensure it doesn't start from the same point on every scan it uses a static
hint that is incremented by 2 in every call to pick ephemeral ports.

The reason for 2 is Linux seems to split the port ranges where active connects
seem to use even ones while odd ones are used by listening sockets.

This CL implements a similar strategy where we use a hash + hint to generate
the offset to start the search for a free Ephemeral port.

This ensures that we cycle through the available port space in order for
repeated connects to the same destination and significantly reduces the
chance of picking a recently released port.

PiperOrigin-RevId: 272058370

1 files changed, 1 insertions, 0 deletions

diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index 3842f1f7d..baf88bfab 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -36,6 +36,7 @@ go_library(
     ],
     deps = [
         "//pkg/ilist",
+        "//pkg/rand",
         "//pkg/sleep",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",