net: support marking accepting TCP sockets

When using mark-based routing, sockets returned from accept()
may need to be marked differently depending on the incoming
connection request.

This is the case, for example, if different socket marks identify
different networks: a listening socket may want to accept
connections from all networks, but each connection should be
marked with the network that the request came in on, so that
subsequent packets are sent on the correct network.

This patch adds a sysctl to mark TCP sockets based on the fwmark
of the incoming SYN packet. If enabled, and an unmarked socket
receives a SYN, then the SYN packet's fwmark is written to the
connection's inet_request_sock, and later written back to the
accepted socket when the connection is established.  If the
socket already has a nonzero mark, then the behaviour is the same
as it is today, i.e., the listening socket's fwmark is used.

Black-box tested using user-mode linux:

- IPv4/IPv6 SYN+ACK, FIN, etc. packets are routed based on the
  mark of the incoming SYN packet.
- The socket returned by accept() is marked with the mark of the
  incoming SYN packet.
- Tested with syncookies=1 and syncookies=2.

Change-Id: I26bc1eceefd2c588d73b921865ab70e4645ade57
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 2ea4c45cf1c8736ccd577eff75058bb32ed0ca95..2042261408b9616712b56e0ed3db642b8221975f 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -584,6 +584,16 @@ tcp_fastopen - INTEGER
 
        See include/net/tcp.h and the code for more details.
 
+tcp_fwmark_accept - BOOLEAN
+       If set, incoming connections to listening sockets that do not have a
+       socket mark will set the mark of the accepting socket to the fwmark of
+       the incoming SYN packet. This will cause all packets on that connection
+       (starting from the first SYNACK) to be sent with that fwmark. The
+       listening socket's mark is unchanged. Listening sockets that already
+       have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
+       unaffected.
+       Default: 0
+
 tcp_syn_retries - INTEGER
        Number of times initial SYNs for an active TCP connection attempt
        will be retransmitted. Should not be higher than 255. Default value