You may need to up the kern.ipc.nmbclusters sysctl. If you are getting "out of mbuf" errors, this will fix that.
From 'man tuning':
kern.ipc.nmbclusters may be adjusted to increase the number of network
mbufs the system is willing to allocate. Each cluster represents approx-
imately 2K of memory, so a value of 1024 represents 2M of kernel memory
reserved for network buffers. You can do a simple calculation to figure
out how many you need. If you have a web server which maxes out at 1000
simultaneous connections, and each connection eats a 16K receive and 16K
send buffer, you need approximately 32MB worth of network buffers to deal
with it. A good rule of thumb is to multiply by 2, so 32MBx2 = 64MB/2K =
32768. So for this case you would want to set kern.ipc.nmbclusters to
32768. We recommend values between 1024 and 4096 for machines with mod-
erates amount of memory, and between 4096 and 32768 for machines with
greater amounts of memory. Under no circumstances should you specify an
arbitrarily high value for this parameter, it could lead to a boot-time
crash. The -m option to netstat(1) may be used to observe network clus-
ter use. Older versions of FreeBSD do not have this tunable and require
that the kernel config(8) option NMBCLUSTERS be set instead.
Add a line like the following to the /boot/loader.rc on the image.
set kern.ipc.nmbclusters=32768
That would take 64 MB RAM. With 128+ MB RAM and SmallWall, you could set it to that or higher, but setting it arbitrarily high may cause problems as stated above.
The default on FreeBSD and SmallWall is 1024, which is fine unless you require a huge number of connections. It's set to 1024 by default to limit memory consumption, and 1024 is more than enough for the vast majority of SmallWall installations.