Background:
rl0 = very external (72.151.228.245)
xl0 = multihomed
10.4.0.1
10.2.4.1

10.2's router has a route that any packets going to 10.4 go through 10.2.4.1
This works.

This is the 10.4 router
Nothing flows from 10.4 to 10.2

DNS and another service are on 10.2

$ cat /etc/pf.conf
### macro name for external interface.
ext_if = "rl0"

### and the one for the internal interface
int_if = "xl0"

### We should not get external traffic from ANY of these addresses
martians = "{ 127.0.0.0/8, 192.168.0.0/16, 172.16.0.0/12, \
              10.0.0.0/8, 169.254.0.0/16, 192.0.2.0/24, \
              0.0.0.0/8, 240.0.0.0/4 }"

### all incoming traffic on external interface is normalized and fragmented
### packets are reassembled.
scrub in on $ext_if all fragment reassemble

### Allow anything from the LAN
pass out quick on $int_if proto { tcp, udp, icmp } from any to any modulate state
pass in quick on $int_if flags S/SA synproxy state

### set a default deny everything policy.
block all

### exercise antispoofing on the external interface, but add the local
### loopback interface as an exception, to prevent services utilizing the
### local loop from being blocked accidentally.
set skip on lo0
antispoof for $ext_if inet

### block anything coming from sources that we have no back routes for.
block in from no-route to any

### block packets that fail a reverse path check. we look up the routing
### table, check to make sure that the outbound is the same as the source
### it came in on. if not, it is probably source address spoofed.
block in from urpf-failed to any

### drop broadcast requests quietly.
block in quick on $ext_if from any to 255.255.255.255

### block packets claiming to come from reserved internal address blocks, as
### they are obviously forged and cannot be contacted from the outside world.
block in log quick on $ext_if from $martians to any
block out quick on $ext_if from any to $martians

### block probes that can possibly determine our operating system by disallowing
### certain combinations that are commonly used by nmap, queso and xprobe2, who
### are attempting to fingerprint the server.
### * F : FIN  - Finish; end of session
### * S : SYN  - Synchronize; indicates request to start session
### * R : RST  - Reset; drop a connection
### * P : PUSH - Push; packet is sent immediately
### * A : ACK  - Acknowledgement
### * U : URG  - Urgent
### * E : ECE  - Explicit Congestion Notification Echo
### * W : CWR  - Congestion Window Reduced
block in quick on $ext_if proto tcp flags FUP/WEUAPRSF
block in quick on $ext_if proto tcp flags WEUAPRSF/WEUAPRSF
block in quick on $ext_if proto tcp flags SRAFU/WEUAPRSF
block in quick on $ext_if proto tcp flags /WEUAPRSF
block in quick on $ext_if proto tcp flags SR/SR
block in quick on $ext_if proto tcp flags SF/SF

### keep state on any outbound tcp, udp or icmp traffic. modulate the isn of
### outgoing packets. (initial sequence number) broken operating systems
### sometimes don't randomize this number, making it guessable.
pass out on $ext_if proto { tcp, udp, icmp } from any to any modulate state

### Allow ICMP (the OS automatically throttles ICMP)
pass in on $ext_if proto icmp to any keep state

### normally, a client connects to the server and we handshake with them, then
### proceed to exchange data. by telling pf to handshake proxy between the client
### and our server, tcp syn flood attacts from ddos become uneffective because
### a spoofed client cannot complete a handshake.
### set a rule that allows inbound ssh traffic with synproxy handshaking.
pass in on $ext_if proto tcp from any to any port ssh flags S/SA synproxy state

### please don't let me break this... D=
pass in on $ext_if proto tcp from any to any port 8022 flags S/SA synproxy state

### set a rule that allows inbound www traffic with synproxy handshaking.
pass in on $ext_if proto tcp from any to any port www flags S/SA synproxy state

### Again for finger
pass in on $ext_if proto tcp from any to any port finger flags S/SA synproxy state

### identd
pass in on $ext_if proto tcp from any to any port ident flags S/SA synproxy state

### setup a table and ruleset that prevents excessive abuse by hosts
### that attempt to brute force the ssh daemon with repeated requests.
### any host that hammers more than 3 connections in 5 seconds gets
### all their packet states killed and dropped into a blackhole table.
table <ssh_abuse> persist file "/etc/abuse/ssh_abuse"
block in log quick from <ssh_abuse>
pass in on $ext_if proto tcp to any port ssh flags S/SA keep state (max-src-conn 5, max-src-conn-rate 3/5, overload <ssh_abuse> flush global)

### setup another table to block excessive http requests, but let's be nicer here.
table <http_abuse> persist file "/etc/abuse/http_abuse"
block in log quick from <http_abuse>
pass in on $ext_if proto tcp to any port www flags S/SA keep state (max-src-conn 24, max-src-conn-rate 32/5, overload <http_abuse> flush global)