[Bug 1910148] Re: Broadcast address detection is unreliable on Linux

[Andrew Johnson via Core-talk <core-talk at aps.anl.gov>]

** Changed in: epics-base
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1910148

Title:
  Broadcast address detection is unreliable on Linux

Status in EPICS Base:
  Fix Released

Bug description:
  I noticed that I have a Linux system where broadcasts are not sent to
  all configured broadcast addresses. The reason is easily identified:
  EPICS base still uses the SIOCGIFBRDADDR way to identify the addresses
  in osiSockDiscoverBroadcastAddresses. This fails in recent systems,
  when several IPs are added to one interface e.g. via "ip addr add" as
  NetworkManager likes to do:

  2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
      link/ether 44:39:d4:9d:25:b6 brd ff:ff:ff:ff:ff:ff
      inet 10.49.126.61/24 brd 10.49.126.255 scope global dynamic noprefixroute eno1
         valid_lft 834648sec preferred_lft 834648sec
      inet 10.49.125.2/24 brd 10.49.125.255 scope global eno1
         valid_lft forever preferred_lft forever

  In this case, only 10.49.126.255 is detected by EPICS (twice):

  ifreqNext() pifreq 0x55ccbc0cc5f0, size 0x28, ifr 0x0x55ccbc0cc618
  osiSockDiscoverBroadcastAddresses(): found IFACE: lo len: 0x10 current_ifreqsize: 0x28 
  osiSockDiscoverBroadcastAddresses(): net intf "lo" flags: 49
  osiSockDiscoverBroadcastAddresses(): ignoring loopback interface: "lo"
  ifreqNext() pifreq 0x55ccbc0cc618, size 0x28, ifr 0x0x55ccbc0cc640
  osiSockDiscoverBroadcastAddresses(): found IFACE: eno1 len: 0x10 current_ifreqsize: 0x28 
  osiSockDiscoverBroadcastAddresses(): net intf "eno1" flags: 1043
  found broadcast addr = a317eff
  osiSockDiscoverBroadcastAddresses(): net intf "eno1" found
  ifreqNext() pifreq 0x55ccbc0cc640, size 0x28, ifr 0x0x55ccbc0cc668
  osiSockDiscoverBroadcastAddresses(): found IFACE: eno1 len: 0x10 current_ifreqsize: 0x28 
  osiSockDiscoverBroadcastAddresses(): net intf "eno1" flags: 1043
  found broadcast addr = a317eff
  osiSockDiscoverBroadcastAddresses(): net intf "eno1" found

  The workaround here would be to put the second IP on another
  interface, i.e. eno1:1, but I believe the software shouldn't dictate
  that.

  The more modern way to identify all broadcast addresses is via
  AF_NETLINK, and the code is not overly complex. I actually do have a
  first patch ready, see the attachment. Obviously, this code should not
  be in the generic directory, and it should probably only be selected
  when #ifdef AF_NETLINK to also consider older versions of Linux (I
  think kernel ≥ 2.2 (from 1999) should work, but the oldest kernel I
  actually tested with the netlink code was 3.2.102). So this is just an
  RFC for now. Please excuse the bad formatting. I wasn't sure if there
  is a style file available for clang-format or the likes and didn't
  bother to do it by hand for this first demo.

  With the patch, the behavior is changed in one subtle way: Interfaces
  that are down are not ignored, because this information is not
  provided for RTM_GETADDR but for RTM_GETLINK. So I'd have to keep
  track of which interfaces are down in between these two calls. For
  other corner cases (loopback, PTP), the behavior should be identical
  as explained in the code.

  Obviously, osiLocalAddrOnce also still uses the old interface, but
  since it should only find any address, this should be OK.

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