I'm having a similar problem with the integrated NIC (RealTek 8111GR) in my mobo (Asus Z87-A) and I'm a FreeNAS/FreeBSD newbie. (I'm coming from a Linux background.) How do I install this module in FreeNAS?
I hear ya bits. I'm not far off from you myself. Thankfully, with the kernel module posted by iveq, if you're running the 64bit build of FreeNAS 9.1.1, this should be pretty easy.
However, I should add, I have no idea if your 8111GR will be handled by this module. It sounds like a highly similar device, but it
could be significantly different, even if only by device ID or something, this driver
may not help you. For my 8111G, this did the trick:
Installing a pre-compiled FreeBSD Kernel Module: if_re on FreeNAS 9.1.1
Step 1: Download the file to a flash drive
http://forums.freenas.org/attachments/if_re-zip.2607/
Step 2: Mount the flash drive on your Freenas box
My flash drive was formatted with FAT32, so the mount command for me is "mount_msdosfs".
For people with FreeNAS running from a hard drive: The command you want to run is probably going to be
mount_msdosfs /dev/da1s1 /media
If you run FreeNAS from a USB drive like me, then you'll likely use
mount_msdosfs /dev/da2s1 /media
When you attach the USB drive, you should see a dmesg-like comment on the main console that'll tip you off to which /dev/daX device you're after. The "s1" postfix is pointing out what partition/slice on the device to mount.
Step 3: Remount root file system as read-write
You'll next need to re-mount your root file system as read-write so you can make changes.
mount -o rw /
Step 4: Copy the kernel module to the appropriate location
cp /media/if_re.ko /boot/kernel/
Step 5: Edit /boot/loader.conf to auto load the module
I used 'vi' to edit this file. You very likely want to edit it so that the module will automatically load on each boot. I added the following to the end of my
loader.conf:
## JTS: For Realtek Nic, not supported by default
## as of FreeNAS 9.1.1
if_re_load="YES"
Step 6: Reboot and verify
Reboot your system to check & confirm that
if_re is loading automatically.
Running
ifconfig is the acid test, if your nic is now configured, it should appear in the ifconfig output.
If it's not there, check if the kernel module is loaded. Run
kldstat and check for
if_re, if it's there, the module is loaded.
If it's not loaded, you can try manually loading the module to see if there are any error messages or something to tip you off.
kldload is the command to load the module. I think
kldload if_re would load it, but, as I said before I'm a Linux-type, I'm not totally sure if this will work on FreeBSD. I read somewhere it will, but I haven't tried since the above steps worked first try for me.
EDIT: In response to follow-up comments:
There was a ticket created for this issue about a year ago.
Indeed upgrades will nuke these changes, which is probably good since, as Cyberjock mentioned, future builds of the FreeBSD kernel used by FreeNAS will likely be different and this module (probably) won't work with those. Hopefully by the next release, the driver will have been added/activated in the FreeBSD build that is included with FreeNAS.
I wrote these instructions out like this in the event that some of us will need to refer to them on a regular basis.
EDIT 2: Maybe this is actually a terrible idea!
I've since noticed that while doing this did WORK at all.. I was having daily network problems. I don't know if it's the realtec hardware, driver or something else, but as soon as I switched NICs, my NAS performance got a lot more stable. So, it's probably worth your time to seriously consider not actually using this NIC. I'm now using an Intel Gigabit CT with vastly better stability.
Thanks to Cyberjock for pointing this out to me so I could test it, as it resolved the problem I was having. 
