this post was submitted on 27 Jun 2023
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Limit outbound SOCKS proxy requests with pf (self.openbsd)
submitted 2 years ago by wgs to c/openbsd
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Hi everyone,

For a very special use case of mine, I need to allow users to connect over SSH to my server to setup a SOCKS proxy:

ssh -D localhost:1080 -N my.server.domain.tld

However, I only want them to be able to reach 127.0.0.1 through this proxy (so, my server itself).

Is that even possible ?

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[–] otl 3 points 2 years ago (1 children)

You may be able to do this by specifying a rule with a user. For example, say I connect to puffy.example.org with user otl:

ssh -D 127.0.0.1:6969 otl@puffy.example.org

On the remote side, in pf.conf:

block out proto tcp all from self user otl

Untested. Curious to see what you come up with! See also pf.conf(5)

[–] wgs 5 points 2 years ago (1 children)

That's awesome, I didn't know you could filter by user ID ! I just tried and it works perfectly. I use the following:

block out proto { tcp udp } from self to port != domain user otl

This effectively blocks all outgoing connections for the given user (except DNS, as I want to allow forwarding DNS over SOCKS). Thanks a lot for the quick guidance !

[–] otl 1 points 2 years ago (1 children)

No worries! I assume you’re the same z3bra who posted on /r/unixporn? If so answering your question quickly is the least I could do; your posts got me back into Linux/Unix for fun during university (studying medical science) in 2014. That got me a job in IT straight out of uni then into software dev. It’s been a wild ride the past 9 years living in both Australia and the Netherlands. So big, big thanks to you!

[–] wgs 3 points 2 years ago

Yeah that's me, though that was a very long time ago haha. That's an awesome story, I could never guess simple posts like mines could be that much inspirational ^^ Thanks for sharing !