[Bug 224920] limited broadcast for Linux programs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224920

            Bug ID: 224920
           Summary: limited broadcast for Linux programs
           Product: Base System
           Version: 11.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: henry.hu.sh at gmail.com

Limited broadcast, or sending to 255.255.255.255, is the behavior specified in
RFC 947, which is deprecated (https://wiki.freebsd.org/NetworkRFCCompliance).
As a result, FreeBSD does not support it (sending to 255.255.255.255 results in
an ether address of the default gw), unless IP_ONESBCAST is specified.
However, for Linux programs, it seems to be still expected to work. Some
programs send UDP packets to 255.255.255.255 to discover devices on local
network. One such example is Canon's ScanGear MP program, which uses this way
to discover local scanners.

Sample trace:

21:06:37.887917 b0:6e:bf:2a:e7:23 > 04:a1:51:15:5e:11, ethertype IPv4 (0x0800),
length 166: 192.168.1.126.51201 > 255.255.255.255.161:  C="canon_admin"
GetRequest(104)  .1.3.6.1.4.1.1602.1.3.1.13.0 .1.3.6.1.4.1.1602.1.2.1.8.1.3.1.1
.1.3.6.1.4.1.1602.1.1.1.1.0 .1.3.6.1.4.1.1602.1.1.1.10.0
.1.3.6.1.4.1.1602.1.3.1.12.0

Expected:

21:05:46.430205 b0:6e:bf:2a:e7:23 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800),
length 166: 192.168.1.126.44951 > 255.255.255.255.161:  C="canon_admin"
GetRequest(104)  .1.3.6.1.4.1.1602.1.3.1.13.0 .1.3.6.1.4.1.1602.1.2.1.8.1.3.1.1
.1.3.6.1.4.1.1602.1.1.1.1.0 .1.3.6.1.4.1.1602.1.1.1.10.0
.1.3.6.1.4.1.1602.1.3.1.12.0

Since FreeBSD does not support this, this program cannot discover any devices.
The discovery code is in some binary blob, which is unable to be changed. For a
closer emulation of Linux behavior, I suggest that when running Linux programs,
we send a broadcast packet to the interface of the default route.

A workaround is letting ipfw do a forward:

fwd 192.168.1.255 udp from me to 255.255.255.255

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