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A Practical Guide to SSH Tunnels: Local and Remote Port Forwarding

181 points by signa11 5 days ago | 42 comments

My favorite use of this is peer-to-peer transfer of Docker images. The Docker CLI only allows you to use registries authenticated with HTTPS but there's an exception where it allows HTTP transfers over localhost.

So, if you use SSH tunneling to forward a port from localhost to a remote, then Docker unwittingly pushes to a remote. This is super useful "off the grid" with robotics/embedded applications where you don't want to bother with a registry and a good Internet connection.

Example, docker pussh: https://github.com/psviderski/unregistry

mmh0000 49 minutes ago

That's not quite true, you just need to add the `insecure-registries`[1] option with a list of either IP (or ip ranges) or hostnames that you want to allow without TLS.

```/etc/docker/daemon.json

  {
    "insecure-registries": ["10.100.0.0/24", "registry.yourmom.example.com:5000"]
  }

```

[1] https://docs.docker.com/reference/cli/dockerd/#insecure-regi...

QGQBGdeZREunxLe 2 hours ago

This is really useful as you don't have to add an entry under insecure-registries for local registries that don't have valid certificates.

bitlad 2 hours ago

You might as well handover the images to hackers.

Kampfschnitzel 3 hours ago

iirc there's a setting to allow docker to trust and use http registries

i set it up a few years ago for my homelab

afiori 30 minutes ago

Which makes me think that I have never heard of signed images/artefacts

buredoranna 5 hours ago

I'll mention it here, because I learned about it here.

"~C" will drop you into the SSH command line, allowing you to, among other things, effect port forwarding

  -L8080:localhost:443

Learning that "~C" exists, and what you can do with it, has supercharged my use of SSH tunnels, which were already awesome on their own.

But for some reason this has been disabled by default in more recent ssh configurations... to ensure its available

  -o EnableEscapeCommandline=yes

or, in your ~/.ssh/config

  EnableEscapeCommandline yes

(edit: formatting)

telotortium 5 hours ago

Important to note that `~` SSH commands work only right after you press Enter - it doesn’t trigger everywhere you press `~`.

Also EnableEscapeCommandline fortunately only affects `~C` - the all-important `~.` to kill a hung SSH session still works with it disabled.

ptaffs 3 hours ago

so many time i have inadvertently ended a session with a fat fingered ~.

wbadart 4 hours ago

I never pass up an opportunity to recommend the Cyber Plumber's Handbook: https://github.com/opsdisk/the_cyber_plumbers_handbook

Goes over similar content as TFA, in perhaps a little more depth. Indispensable sysadmin knowledge.

opsdisk 4 hours ago

Appreciate the mention wbadart!

ranger_danger 3 hours ago

Not mentioning the ssh -w option in that book should be a crime.

susu1111 4 hours ago

so good, I learn new things.

smw 4 hours ago

Need to mention sshuttle [0] here, as it magically solves a bunch of these problems without constant reconfiguration

[0] https://github.com/sshuttle/sshuttle

mystifyingpoi 2 minutes ago

sshuttle is amazing. I've used it extensively on stupidly configured networks, super useful tool.

phbeks 19 minutes ago

Thanks! I will keep this for reference. I use ssh alot but thie reminds me that I can learn new ways of tunneling :-)

chasil 6 hours ago

The article mentions bastions, but no jumphosting?

  ssh -J user1@bastion1,user2@bastion2 targetuser@targethost

Edit: Jumphosting was introduced in OpenSSH 7.3 2016-08-01.

https://www.openssh.org/releasenotes.html

saltcured 18 minutes ago

For me, this is always used via ProxyJump rules in my ~/.ssh/config

It is also nice that it works recursively, so I can logically structure my rules so that the one for my regular targets say to use bastion1, then the rule for bastion1 says to go via bastion 2, etc.

I find this easier to reason about and maintain rather than juggling a bunch of these multi-step rules.

dspillett 5 hours ago

It is surprising how many times I see this content (this version might be marked “Published: Jun 19, 2026” but I've definitely seen those exact diagrams before, starting at least a few years ago, and the same content around them in many tutorials before that) without it being updated to mention jump-hosts.

Support was added to OpenSSH about a decade ago? Even on a low moving Linux distro like Debian/LTS everyone should have support by now.

m348e912 2 hours ago

>ssh -J user1@bastion1,user2@bastion2 targetuser@targethost

Are you using SSH key auth or password authenticating three times when you do this?

chasil 24 minutes ago

If you don't have an agent running with an accessible key, then you will get three password prompts, with suggestions for any default keys.

The final target is a pre-elliptic curve OpenSSH server, so legacy is enabled. I could probably have removed that for clarity.

  C:\Users\me\>ssh -J me@bhost1,me@bhost2 -o KexAlgorithms=diffie-hellman-group14-sha1 -o HostKeyAlgorithms=ssh-rsa -o MACs=hmac-sha1 oracle@target
  Enter passphrase for key 'C:\Users\me/.ssh/id_ed25519':
  me@host1's password:
  Enter passphrase for key 'C:\Users\me/.ssh/id_ed25519':
  me@host2's password:
  oracle@target's password:
  Last login: Wed Jun 24 13:29:55 2026 from bhost2

That client is Microsoft's port of OpenSSH.

saltcured 16 minutes ago

I always use keys in my SSH agent.

Because the jump mechanism works via use of TCP forwarding, each host authn step is talking "directly" to your client. Importantly, this means it still works without requiring "agent forwarding" for the connection you are making.

hylaride 5 hours ago

Learning how SSH port forwarding is great as a pseudo-vpn for everything from GUI-client database access to (in physical infra) access to web-admin tools for appliances.

The socks proxy support can also deal with bad web filtering and privacy issues on public wifi networks (though nowadays if you're ssh'ing to a cloud IP, you'll get lots of "bot" restrictions).

saltcured 7 minutes ago

Yeah, I get use out of the SOCKS proxy mode in combination with a "split VPN" at work.

I need VPN to get into some internal resources via SSH, but there are lots of external/public/AWS resources I also need to access, and the full VPN adds too much overhead and fragility for those.

Using the available split VPN, I can point a browser instance at a localhost SOCKS proxy port to relay over SSH + VPN for other web resources I need to access internally.

Unfortunately, Firefox proxy config rules are sort of backwards for my needs. I want to say "only use proxy for these 3 domains" whereas it wants to use the proxy by default and only allow me to bypass specific domains.

bheadmaster 4 hours ago

If you have many different remote devices behind NATs or firewalls, a cool trick to access them all via EC2 server (or such) is to setup Remote Forwarding via UNIX socket on the server side, to devices' port 22. Preferably, UNIX socket filenames should start with a common prefix, so an SSH config can be written that will use ssh+socat in a ProxyCommand to establish the connection.

It's amazing how lightweight this method actually is. I have managed to connect hundreds of devices using a single EC2 nano instance.

saltcured 14 minutes ago

I think the more modern ProxyJump rule is superior for this. Just let it manage the actual TCP forwarding for you automatically. It's just the normal "bastion host" concept.

Particularly, you can use name patterns to apply the same rule broadly, assuming you have some systematic naming scheme for your eventual target devices.

ranger_danger 3 hours ago

Do you have more info on this method? How is the remote forwarding actually done?

mldbk 2 hours ago

[dead]

segphault 5 hours ago

Or you could just install something like Tailscale and never have to think about it again.

teddyh 6 hours ago

It’s amazing what you can learn by reading the manual.

felooboolooomba 5 hours ago

It is, because manuals are often not the best way to learn things. Most software manuals are reference manuals. SSH man page isn't too bad. I learned most of my SSH knowledge from it, but I'm not sure it's the best way to do it.

matltc 5 hours ago

For me, the best way to learn a tool is for a quick example or two showing its utility, then practicing with those, reading the man as needed on specific flags. Google or bot ”how do x" ? Repeat : done

Some pages have a nice up-front synopsis of flags, others put them in a wall of text. Browsing the former can supplant Google, /\b-x while paging is helpful for the latter.

felooboolooomba 5 hours ago

As a sysadmin, one of your biggest ROI is learning the ins and outs of SSH.

riobard 6 hours ago

There's a asymmetry here that "-R" works both for reverse static and dynamic (using SOCKS protocol) forwarding, but "-D" is required for dynamic forwarding which "-L" cannot do.

Why is that?

hylaride 4 hours ago

It's historical. Some older flags could be easily extended for dynamic port support and others could not.

trollbridge 6 hours ago

When I see one of these with obvious AI tells at the top (sentences lacking a subject or verb), I ask myself:

Can’t I just open up a harness and prompt “Teach me how to do X?”

lfx 5 hours ago

I do this all the time, I have a skill/gem with instructions on how I want to receive info, how to format and so on. Really helps to go fast to get the point.

Oxodao 5 hours ago

Could you share it? I'd be interested to get idea to make my own

lfx 3 hours ago

It goes like this: --- As an expert tutorial creator for experienced engineers, you take the input the user request and make interactive tutorial. Default style is technology, tech is mac and linux. Default style is 20mins, but you ask for the timeline. Also do not forget to provide the cost of technologies used. ---

GL26 6 hours ago

I personally do this, ask claude code to teach me about concepts I don't know about when it codes something, and only then I accept what it suggests to me

Bender 4 hours ago

Should add how to bypass MFA using phishing and SSH Multiplexing to the article.

matltc 5 hours ago

Very refreshing to see a utilitarian series such as this. Disappointing that the latest ai drama gets 20x more discussion and visibility on this site

ranger_danger 3 hours ago

Another option that I never see mentioned anywhere is -w which allows you to create either layer2 or layer3 tunnels via a tun(4) interface.