From: weidong <redacted>
Date: Wed, 14 Feb 2007 10:03:49 -0500

Hi, All
  When I tested Linux-2.6.20 and found that counter "ipOutNoRoutes" can
not increase correctly.
  The criteria is RFC2011

ipOutNoRoutes OBJECT-TYPE
    SYNTAX      Counter32
    MAX-ACCESS  read-only
    STATUS      current
    DESCRIPTION
            "The number of IP datagrams discarded because no route could
            be found to transmit them to their destination.  Note that
            this counter includes any packets counted in ipForwDatagrams
            which meet this `no-route' criterion.  Note that this
            includes any datagrams which a host cannot route because all
            of its default routers are down."
    ::= { ip 12 }

In current Linux TCP/IP stack, maybe we should not increase this counter
in "input path", but only increase it in "output path" due to the TCP/IP
stack performance.

Now in "output path", when TCP client tries to connect to an unreachable
server(net unreachable, so no route can be found), this counter has no
increment. When we use UDP sending UDP datagram to an net unreachable
address, this counter also has no increment.

Function need to fix:
tcp_v4_connect(); ip4_datagram_connect(); udp_sendmsg();

The following patch can fix the problems mentioned above

Thank you for this patch.

I think we need to make these checks more carefully.

Route lookup can fail for several reasons other than
no route being available.  Two examples are:

1) Out of memory error while creating route
2) IPSEC disallows communication to that flow ID

As a result, we'll probably best limiting the counter
increment when the error is either -EHOSTUNREACH or
-ENETUNREACH.

Probably, since this logic will be duplicated to several
locations, you'll want to implement the test inside of
a helper inline function.