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At my work there’s a HTTP/HTTPS proxy which requires authentication, I would like to access both ssh and a subversion repository through HTTP at my home server.
The easiest solution would be to use two IP addresses, but I don’t have an extra IP address available.
Ports other than 443 are blocked, and using port 80 won’t work because the HTTP proxy doesn’t support the HTTP WebDAV extensions required for subversion (and it’s also insecure).

The solution

Use pf’s overload feature to switch between services on a given port.

Using it is simple:
I can use svn whenever I want, if I would like to use ssh I open my browser, go to http://94.142.244.51 three times in 42 seconds and I can use ssh.
After closing ssh and waiting for a minute I can use svn again.

This solution will work for FreeBSD and OpenBSD. The concept can probably be implemented in most other stateful firewalls too.

/etc/pf.conf

Simplified ruleset for demonstration purposes

# Interface to filter
if="re0"

# My server's IP
ip="94.142.244.51"

# IP from my work
work={"X.X.X.X"}

# Table to keep track of who should be redirected from port 443 to port 22
table <rdr_ssh> persist

# Redirect everyone in the rdr_ssh table from port 443 to port 22
rdr on $if inet proto tcp from <rdr_ssh> to $ip port https -> $ip port ssh

# Default rules
block in log
pass out quick

# Allow traffic on port 443
pass in on $if proto tcp from any to $ip port https

# Allow traffic on port 80 for svn/ssh switching, allow it only from my work.
# Most (all?) browsers will try to load /favicon.ico so opening a page once
# are actually two TCP connections.
pass in log quick proto tcp from $work to $ip port http keep state
(max-src-conn-rate 6/42, overload <rdr_ssh>)

dummyserver.py

From pf.conf(5):

For stateful TCP connections, limits on established connections (connec-
tions which have completed the TCP 3-way handshake) can also be enforced
per source IP.

max-src-conn <number>
Limits the maximum number of simultaneous TCP connections which
have completed the 3-way handshake that a single host can make.

max-src-conn-rate <number> / <seconds>
Limit the rate of new connections over a time interval.  The con-
nection rate is an approximation calculated as a moving average.

It took me some time to realize that if no service is running on port 80 the connection attempt will just time out and the 3-way handshake is not completed.

You can use Python to start a simple webserver that does nothing.

#!/usr/bin/env python

from BaseHTTPServer import *

host = '94.142.244.51'
port = 80

HTTPServer((host, port), BaseHTTPRequestHandler).serve_forever()

Or you can use your webserver with a document root of /var/empty/ if you prefer.

/etc/crontab

It’s also useful to add a crontab entry to flush the table periodically to switch back to svn when you’re done with ssh.
To your /etc/crontab add:

* * * * * root /sbin/pfctl -t rdr_ssh -T expire 60 > /dev/null 2>&1

From pfctl(8):

-T expire number
			Delete addresses which had their statistics cleared
			more than number seconds ago.  For entries which
			have never had their statistics cleared, number
			refers to the time they were added to the table.

In other words: As long as a connection is open, the address won’t be removed, but if no connection has been open for 60 seconds the address is removed.

Or, if you prefer, you can manually clear the table with:

pfctl -t rdr_ssh -T del <ipaddress>

Further reading

Feedback

You can mail me at martin@arp242.net for feedback, questions, etc.

Copyright © 2010-2017 Martin Tournoij <martin@arp242.net>
This document is licensed under a cc-by 4.0 license.