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This creates a role for RFC3549 Netlink monitoring. If the OS supports it (currently, linux) then each pt creates a wsi with the netlink role and dumps the current routing table at pt init. It then maintains a cache of the routing table in each pt. Upon routing table changes an SMD message is issued as an event, and Captive Portal Detection is triggered. All of the pt's current connections are reassessed for routability under the changed routing table, those that no longer have a valid route or gateway are closed.
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Lws routing
lws is mainly built around POSIX sockets and operates from the information available from those. But in some cases, it needs to go a step further and monitor and understand the device routing table.
Recognizing loss of routability
On mobile devices, switching between interfaces and losing / regaining connections quickly is a given. But POSIX sockets do not act like that, the socket remains connected until something times it out if it no longer has a route to its peer, and the tcp timeouts can be in the order of minutes.
In order to do better, lws must monitor and understand how the routing table relates to existing connections, dynamically.
Linux: netlink
For linux-based devices you can build in netlink-based route monitoring
with -DLWS_WITH_NETLINK=1, lws aquires a copy of the routing table
when the context / pt starts up and modifies it according to netlink
messages from then on.
On Linux routing table events do not take much care about backing out changes made on interface up by, eg, NetworkManager. So lws also monitors for link / interface down to remove the related routes.
Actions in lws based on routing table
Both server and client connections now store their peer sockaddr in the wsi, and when the routing table changes, all active wsi on a pt are checked against the routing table to confirm the peer is still routable.
For example if there is no net route matching the peer and no gateway, the connection is invalidated and closed. Similarly, if we are removing the highest priority gateway route, all connections to a peer without a net route match are invalidated. However connections with an unaffected matching net route like 127.0.0.0/8 are left alone.
Intergration to other subsystems
If SMD is built in, on any route change a NETWORK message
{"rt":"add|del"} is issued.
If SMD is built in, on any route change involving a gateway, a NETWORK
message {"trigger":"cpdcheck", "src":"gw-change"} is issued. If
Captive Portal Detection is built in, this will cause a new captive
portal detection sequence.