Circumventing SMTP Blocking by ISP using Digital Ocean

I want to setup a mail server. Easy! (some restrictions apply). However, my silly ISP is blocking connections on port 25 (the port for SMTP), incoming and outgoing. Harsh… Given that I don’t want to contact them and go through the trouble of trying to convince them to open it for special ol’ me, I decided that I would simply use my Digital Ocean droplet to route all traffic on port 25 for my mail server.

The tricky part of all of this is that I don’t want to simply forward all traffic from my mail server through the droplet like some kind of heathen. I want only SMTP traffic to be routed through this machine. Network bandwidth is expensive and I’d rather not route the rest of the services running on the mail server to New York and back! Talk about slow…

Network Topology

My problematic network setup looks something like this:

mail (mail server) <-> router <-> ISP

The problem is that all traffic going out of the router to the ISP will be filtered:

mail -(smtp.gmail.com:25)-> router -(smtp.gmail.com:25)-> ISP -(BLOCKED)->

client -(mail.ranvier.net:25)-> ISP -(BLOCKED)-> router

Ideally, we would like something like this:

mail -(smtp.gmail.com:25)-> router -(droplet:51820(smtp.gmail.com:25))-> ISP -(droplet:51820(smtp.gmail.com:25))-> droplet -(smtp.gmail.com:25)-> Digital Ocean

Where outgoing connections to port 25 from mail are forwarded by my router to a Digital Ocean droplet, which can then perform SNAT for mail and pass the connection along to where it ought to go.

The same would work in reverse to allow connections to the mail server on port 25 but just using a simple DNAT rule on the droplet for that. I’ll leave the ugly text diagram as an exercise for the reader if they feel up to it.

iptables Magic?

After some quick google searching, I found that other people had the same idea. So just some copy pasting and we’re good to go? Nah. First of all, I wanted to understand what was going on here. Secondly, I tried copy pasting ( :^) ) and it didn’t work because I have a somewhat different environment and the rules in that serverfault post weren’t entirely applicable.

The following iptables rule accomplishes a part of what we want:

router

# Mark these packets so they can be routed through droplet
iptables -A PREROUTING -t mangle -s {mail server internal IP} -p tcp --dport 25 -j MARK --set-mark 0x1000

This marks all packets originating from mail destined for port 25 with 0x1000. This, by itself, doesn’t do anything. However, we can make ip rules that use this mark for routing decisions.

ip rules

We first have to make the configuration for our alternate routing table:

router

# Add entry to routing tables
cat /etc/iproute2/rt_tables | grep -v '200 smtproute' > /etc/iproute2/rt_tables
echo '200 smtproute' >> /etc/iproute2/rt_tables

Then, we add routes and a rule for this table:

router

# Setup secondary routing table
ip route add default dev wg0 table smtproute
ip rule add fwmark 0x1000 table smtproute priority 32765

This will apply the routing table smtproute to any packet that is marked with 0x1000 and send said traffic out on the wireguard interface that connects router and droplet.

SNAT and DNAT on droplet

We’ve gotten the traffic from the mail server to the router and now finally to the droplet. But we need to make sure that the traffic is NAT’d b/c the IP of the mail server is on an internal network:

droplet

# Enable IP forwarding
sysctl -w net.ipv4.ip_forward=1  # Can be made persistent by editing in /etc/sysctl.conf
# NAT for traffic outgoing on internet interface
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

For traffic going into the mail server, we use a DNAT rule:

droplet

# Traffic incoming on port 25 should be routed to the mail server
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25 -j DNAT --to-destination {mail server internal IP}:25

Conclusion

If you want a TL; DR, here are all of the rules that I used to get this working:

Router Configuration

# Add entry to routing tables
cat /etc/iproute2/rt_tables | grep -v '200 smtproute' > /etc/iproute2/rt_tables
echo '200 smtproute' >> /etc/iproute2/rt_tables

# Setup secondary routing table
ip route add default dev wg0 table smtproute
ip rule add fwmark 0x1000 table smtproute priority 32765

# Mark SMTP packets for alternate routing
iptables -A PREROUTING -t mangle --source {mail server internal IP} -p tcp --dport 25 -j MARK --set-mark 0x1000

Droplet Configuration

# Enable IP forwarding
sysctl -w net.ipv4.ip_forward=1  # Can be made persistent by editing in /etc/sysctl.conf
# NAT for traffic outgoing on internet interface
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# Traffic incoming on port 25 should be routed to the mail server
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25 -j DNAT --to-destination {mail server internal IP}:25

This Netfilter Diagram was also very helpful in seeing how iptables and ip rules interact in the kernel, as well as seeing how all of the other chains and tables interact with each other.