DragonFly BSD


Building a filtering bridge with DragonFlyBSD

A filtering bridge transparently filters packets in a network. This has its advantages and disadvantages. Disadvantages include: getting the bridge up (the handbook is outdated on this), active FTP, directions.

I have spent many days getting my bridge up & running. Thought I would share the information with others in order to help avoiding the hassle I had to face

Materials and Methods

I used a dual PII/400 system with 2 Planet-ENW-9607 /sk(4)/ NICs and the machine was running DragonflyBSD 1.6.1.

Setting up bridging

To enable bridging and PF support on DragonFly you have two options: load the required kernel modules or compile them directly into the kernel. I chose the latter, because ALTQ support ha to be compiled into the kernel anyway in order to use traffic shaping (and I had to recompile for SMP as well :P). Here is my kernel config (only the parts that differ from GENERIC.):

machine         i386
cpu             I686_CPU
ident           SMP
# Bridging support
pseudo-device  bridge
options         ALTQ            #alternate queueing
options         ALTQ_CBQ        #class based queueing
options         ALTQ_RED        #random early detection
options         ALTQ_RIO        #triple red for diffserv (needs RED)
options         ALTQ_HFSC       #hierarchical fair service curve
options         ALTQ_PRIQ       #priority queue
options         ALTQ_NOPCC      #don't use processor cycle counter
options         ALTQ_DEBUG      #for debugging
# PF
device          pf
device          pfsync
device          pflog
# Symmetric Multiprocessing support
options        SMP                     # Symmetric MultiProcessor Kernel
options        APIC_IO                 # Symmetric (APIC) I/O

After compiling, installing and rebooting I got bridging support. If you don't need/want to compile a kernel you can simply open (or create) * /boot/loader.conf* and add these lines:


and if you don't want to reboot, you can load the modules realtime: # kldload pf.ko # kldload pflog.ko # kldload if_bridge.ko Well, we have bridging support now, we have to create an interface that represents the bridge and tell which NICs belong to it.

Edit /etc/rc.conf and include: # Replace x.x.x.x with the desired IP. ifconfig_sk0="inet x.x.x.x netmask" ifconfig_sk1="up" cloned_interfaces="bridge0" ifconfig_bridge0="addm sk0 addm sk1 up" pf_enable="YES" pflog_enable="YES" There is no need to have IPs in a bridge, but it is generally a good idea to have one interface with an IP address assigned, and if we would like to do filtering, it is necessary (well, for me at least. :P) It is also good because you can SSH into the machine or run it as a DHCP server, caching DNS server etc. as well (I run dhcpd on it, but won't cover that now, there are many great howto's out there). That's it, we have a running bridge with PF. Now let's go and configure filtering!

Configuring the firewall

I will not cover every PF option under the sun, go figure yourself, the PF FAQ is great, although it lacks some details that I will partially cover. I insist on readingg how lists, macros and filtering rules work before reading on. Be aware of DragonFly's PF version being the same as OpenBSD 3.6's one! First off, one needs to understand that a bridge is symmetrical from the aspect of traffic, hence it's a good idea to filter on only one interface. In my practice, the interface having the IP usually resides LAN-side, while the IPless one does on the WAN-side:


On other interfaces we just pass everything:

pass quick on { $int_if, lo0, bridge0} all

OK, we do the usual filtering, all good, then we get the good ol' active FTP problem. Well, since we have no new-wave ftp-proxy in the base system, nor does ftpsesame work on DragonFly, we'll have to stick with the ftp-proxy that comes with inetd. We enable it in /etc/inetd.conf (uncommenting the last line in the default config):

ftp-proxy       stream  tcp     nowait  root    /usr/libexec/ftp-proxy  ftp-proxy

and (re)start inetd:

# rcrestart inetd

It will listen on any interfaces by default, so you may want to limit access to port 8021 in PF. Now we set up PF to forward FTP traffic from the LAN to the ftp-proxy. On a router, this can be done with a simple traffic redirection - we redirect traffic from the external interface to the internal interface (even ftp-proxy needs an IP to listen on :-P):

# ftp-proxy S01E01
# Do you remember? ($int_if) always gives you the IP of that inerface, you can use it like (rl0) as well.

rdr on $ext_if inet proto tcp from to any port ftp -> ($int_if) port 8021 but given that the bridge operates in Layer 2, we just have rewritten this packet, nothing more, it will leave the bridge and disappear ( rdr * directives never work on a bridge!*). However, we can actually route it to another interface, and since the packet has just been changed to be destined to hit the IP of that interface, we have successfully done a forwarding, on a bridge it requires these two steps, though. We use the route-to keyword for this (and if you can NOT find anything about it in the PF FAQ, then you are right, there are a few words on it in the pf.conf(5) manpage, though):

# ftp-proxy S01E02
pass in log quick on $ext_if route-to ( lo0 ) proto tcp from <intnet> to ($int_if) port 8021

We actually route it to the loopback interface, I am unsure why it has to be done this way. (FIXME) Well, all we have to do now to allow FTPDATA connections from port 20 of the ftpserver to our machine's IP. This can easily be achieved as ftp-proxy has a dedicated user called proxy, and we have uid/gid filtering in pf (another poorly documented feature):

# ftp-proxy S01E03 a. k. a. season finale
pass in log quick on $ext_if inet proto tcp from port 20 to ($int_if) user proxy keep state

and we will have working active FTP behind our bridge.

References & acknowledgements

man pf.conf PF FAQ A mailing list entry that I found on Google :)

Thanks to: