FreeBSD socket bugs and peculiarities

Documented by Barath Raghavan, 11/2003 on FreeBSD 4.8-RELEASE

Writing to RAW sockets
----------------------
- ip_len and ip_off must be in host byte order


Reading from RAW sockets
------------------------
- ip_len does not include the IP header's length.  recvfrom() however
returns the packet's true length.  To get the true ip_len field do:
iphdr->ip_len += iphdr->ip_hl << 2;

- You may only read from RAW sockets bound with a protocol other than
IPPROTO_RAW

- ip_len is in host byte order

- You may only read packets for protocols or subprotocols that the kernel
does not process.  This includes things such as ICMP_ECHOREPLY and
ICMP_TIMESTAMP as well as nonstandard protocol numbers.


DIVERT sockets
--------------
- These differ in behavior from RAW sockets, but I haven't gotten a chance
to document their weirdness.


General Thoughts
----------------
- Linux RAW sockets are much better documented in modern Linux
distributions (Gentoo) and have no bugs that I've noticed.  Avoid FreeBSD
for raw sockets unless you have no choice.  If you need BSD, I've read
that OpenBSD has fixed several of these bugs and provides a raw socket
implementation similar to that of Linux.