OpenSSH Patch Extends Tunneling Under OpenBSD

Jonatan Wallmander writes "We've written a small howto as well as produced a simple patch for OpenSSH that improves tunneling functionality in the ssh client on the OpenBSD platform (this should be OK on other platforms with some tweaking). It's a simple hack but works very good for us. We can have different IPs on the same BSD machine tunnel different hosts ... Without the patch you can only have one tunnel per BSD machine since it listens on INADDR_ANY.. Now all my computers on the LAN can access remote servers securely as if they were in the same room provided by a single BSD server. :)"

Comments

Balance of Perf and Ease?

[4of12]

I got educated on an earlier Slashdot story of how (a) how nice and easy it was to set up an encrypted tunnel using ssh instead of IPSec or a weird proprietary VPN product, (b) how TCP over TCP is a fundamentally bad idea and people were compensating by periodically restarting the tunnel service afresh to work around it.

How's the performance of this setup and does it address any of those problems?

Re:Balance of Perf and Ease?

[evilviper]

TCP over TCP a problem? I think not. You are probably thinking about OTHER things over TCP. UDP tunneling can have it's problems when there is a bandwidth crunch on the TCP connection.

TCP over TCP is just as good as regular TCP, even if it's a bit slower due to encryption (but compression makes it up to some extent).

Re:Balance of Perf and Ease?

[QuMa]

Depends on where the endpoints are. If you're generating the packets and stuffing them into a tcp tunnel on the same host you're fine. If your packets get out of order/dropped/whatever before they enter the tunnel you're still screwed though.

Re:Balance of Perf and Ease?

[Anonymous Coward]

ARG! All the replies to this post so far are so wrong that it's hurting my brain.

1. yes "TCP over TCP" is a bad effect. This is what happens when you tunnel an unreliable protocol (like IP encapsulated in PPP) over a reliable protocol (like TCP with something like SSH or SSL on top) The problem is that packet loss ends up causing retransmits both from the encapsulated TCP sessions and from the SSH/SSL layer on top. So you end up with a situation where it works perfectly when you're getting near zero packet loss, but as soon as there's even a moderate amount of packet loss the entire connection falls apart. Often the only way to recover from this is to restart the tunnel.

2. However, THIS ARTICLE ISN'T ABOUT THAT. It's using ssh'd builtin TCP forwarding to tunnel the data. So instead of "TCP-over-TCP" it's "TCP-to-TCP-to-TCP". At to point in the connection are there two TCP state machines both competing to retransmit. I'm sorry if this doesn't make much sense in text - I really need a whiteboard to explain it properly.

The short version is that the "TCP-over-TCP" problem is basically confined to schemes where they try to tunnel a PPP session over SSH (in order to get a full VPN). Just using SSH's port forwarding is _fine_.

Re:Balance of Perf and Ease?

[mindstrm]

No.. the reason tcp over tcp has issues is because you end up with both tcp layers trying to backoff and deal with congestion and retransmits rather than one, and things can happen such that the two layers start interfering with each other rather than helping. (one layer transmits, then another does, causing even more extra packets, etc)

Re:Balance of Perf and Ease?

[sshore]

TCP over TCP is a fundamentally bad

This isn't actually TCP over TCP. The connection is made to the local agent, and only the data, not the TCP headers, go over the tunnel.

No worries.

Re:Balance of Perf and Ease?

[halfelven]

TCP over TCP is a fundamentally bad idea

It is true on not-so-reliable networks.
It is false on the modern, low-loss Internet.

Excellent idea!

[Hanashi]

I read the paper, and found it to be a very elegant solution to the problem. I, too, have wished to have multiple tunnels on the same port. I only hope that this patch makes it into OpenSSH (and OpenSSH-portable, of course). I think it'll need some testing, but it seems like a good idea well executed.

Of VPN's and Legalities...

[bonsai_kitty]

This is an excelent idea and better yet sounds like it will work to bridge the OS gap. But is this considered a VPN? And if so should I be concerned about "The Long Arm" if I attempt it with a node in a state that prohibates such.

Re:Of VPN's and Legalities...

[Zebra_X]

No I don't think that you should be concerend about this. The law is vague about how what constitutes "hiding" the point of orgin of comunication.

A better way to look at it is this: you have a tunnel which has an orgin and an enpoint. Both of which are known physical locations, further more the computer that you are likely communicating with is also "known" becuase it's connected to a LAN which is in turn connected to a tunnel end point - which is a known location, not just to you, but at the very least your service provider. It's not as if you are trying to hide or even succeding in "hiding" the point of orgin for your communication.

I imagine that someone will go to court with this but it's not going to hold up - becuase communications networks require physical devices to do the communication and especially in the LAN->Internet scenario the origin of that communication is almost always known. Now bulding a tunnel from a cell phone, then from multiple servers and clearing out the log entries is something else...

Null encryption?

[Euphonious Coward]

This is slightly off-topic...

The OpenSSH sources list a "null" cipher, but I have had no success in establishing a connection using it. Is there a trick to it, or do I need to patch the sources to get a connection that sends in plaintext?

(Why? The traffic is already encrypted via IPSEC; I just want to use SSH's cool port-forwarding apparatus and its authentication, and don't want to pay for for encrypting twice.)

Re:Null encryption?

[sedawkgrep]

While I haven't done this in YEARS, I think you need to add a Ciphers line to /etc/ssh/sshd_config that contains 'none'. Be sure to include all the ciphers you may want to use because this list is exclusionary.

Otherwise, you can always use blowfish, rc4, or even AES (I think...) as they are all *MUCH* faster than 3des.

sedawkgrep

Re:Null encryption?

[Deagol]

Holy crap... I didn't realize that openssh defaulted to 3des (at least in Redhat RPMs). I forced both my server and clients to use blowfish and the interactive lag over my 33.6 modem connection dropped considderably.

Thanks for the tip. You learn something every day on this board!

Re:Null encryption?

[Euphonious Coward]

Thanks, I had tried that. I have a Ciphers line in my /etc/ssh/sshd_config already, so it will default to aes128-cbc, fall back to blowfish-cbc, and finally 3des-cbc. When I add "none" or "null", sshd complains when it reads the file: Bad SSH2 cipher spec.

I wonder if a plaintext cipher would compromise authentication. (Not that it matters in this case.)

Re:Null encryption?

[evilviper]

Bad SSH2 cipher spec.

That's the message you get when you've typed the name of the cipher incorrectly... Try the name followed by "-cbc", and I bet you'll have better luck.

I wonder if a plaintext cipher would compromise authentication. (Not that it matters in this case.)

Indeed it does, and more. With no encryption, there is no authentication, and no verification. So, not only can someone read your password, or intecept the challenge-response if you are using a cert or the ssh-agent, but someone could very easilly spoof the source address of a specially crafted packet, and, in turn, have that command executed as the logged-in user on the SSH server... This could work the other way around as well. Someone could make a packet that claims to be the server, and that data would be inserted into the data recieved by the client. Screen output, corrupting binary file transfers, whatever.

You didn't mention much about this, so I'm not sure if this is a concern for you. Also, I wonder why you would use SSH's port-forwarding... There are alternatives (depending on what you are doing with it) if you don't want the authentication/encryption.

Re:Null encryption?

[Euphonious Coward]

Adding ",none-cbc" to the ciphers list had the same result as just ",none" -- sshd rejected it.

It's not obvious that a null cipher must compromise authentication, and I can think of lots of uses for a channel that's authenticated but not bulk-encrypted, even without assuming that the channel is encrypted by IPSEC.

Re:Null encryption?

[evilviper]

Adding ",none-cbc" to the ciphers list had the same result as just ",none" -- sshd rejected it.

Hmm, I was just assuming OpenSSH imitated the commercial versions... After a quick search on google, I did find this patch against OpenSSH. Seems a rather simple change to get enencrypted connections working. I guess the OpenSSH guys just want to save users from themselves.

It's not obvious that a null cipher must compromise authentication,

Well, if you are using public-key auth (wouldn't recomend using ssh-agent in such a situation), you should be okay authentication-wise. But that's besides the point.

If the SSH connection is *entirely* going over an IPSec connection, there shouldn't be any problem. SSH won't be encrypting/authenticating the source, but IPSec should be able to do that part well enough.

Re:Null encryption?

[Euphonious Coward]

Thanks, I guess I should have googled for it myself.

Re:Null encryption?

[ViVeLaMe]

null encryption can be useful in the folllowing case:
you use public key auth and you like to use scp to automate some file transfers on your own network.
encryption can be quite a burden, and it's nice to be able to specify a null cipher on the command line when you know the files you transfer aren't especially sensitives.

Re:Null encryption?

[evilviper]

Possibly, but you'd better make a checksum on the source, and verify it at the destination. Encryption (even if you only use 40-bit rc4, or something equally weak) allows the client and server to verify the data is not corrupted, as well as preventing casual evesdroping...

I would sugest you look into using some weaker encryption modes, rather than none at all. If CPU power is an issue, crypto accelerators seriously improve performance, and can often be very cheap.

Re:Null encryption?

[giraffecock]

i googled your mother,cunt.

Re:Null encryption?

[Ed Avis]

The cipher 'none' does exist but normally it is specially disabled in the source code. I have a patch to OpenSSH to enable cipher 'none' but I haven't updated it in a while.

You beat me to it!

[BoomerSooner]

Ditto. Keep up the good work. I'm new to BSD (from Linux/Solaris) but I've been very impressed so far.

Small error in the examples...

[Gruturo]

I don't wish to be pedantic, and it doesn't compromise the understandability of the article, but in the drawing under chapter 3 (The basic solution), the "brown" OpenBSD file server is available to the Windows PC as .200, and NOT .5 as suggested.
The green OpenBSD box just does a simple port forwarding (from its own 139 to port 139 on 127.0.0.1 seen from the other endpoint's perspective) and makes it available non-loopback-only via the "-g" option (which btw won't work if you don't have "GatewayPorts yes" in your sshd_config file, and the last time I checked this was not exactly well documented). Therefore, 192.168.0.200:139 (actually 0.0.0.0:139, esp. without this patch :-) ) gets mapped to 127.0.0.1:139 (but on the OTHER end of the tunnel - thus the brown box).

The next example is correct (and shows the use of the patch).

Just my 0.02

Re:Small error in the examples...

[jonatanw]

thanks mate.. updated the picture to make things more clear. I also updated the title a bit..

Re:Small error in the examples...

[gskc]

Actually there is an bug in their patch too! Here is my patch to their patch for ssh.c :)

- options.gateway_addr = malloc(strlen(optarg));
+ options.gateway_addr = malloc(1 + strlen(optarg));
sprintf(options.gateway_addr,"%s",optarg);

You never know, this could lead to a buffer overrun and a possible exploit (options.gateway_addr is last seen going into getaddrinfo(...) ).

VPN's illegal?!?`

[Anonymous Coward]

I didn't know that there were any states that had outlawed VPN's. Do tell!

Re:VPN's illegal?!?`

[bonsai_kitty]

here is a previous Slashdot story on VPN paranoia...I just hope other states don follow suit.

Re:VPN's illegal?!?`

[Anonymous Coward]

It's called the Super DMCA. Search /. for it. There was a sfcronicle article on it a while back. It's scary.

TCP over TCP OK

[bill_mcgonigle]

TCP over TCP is a fundamentally bad idea

There may be some theoretical basis to this mantra, but in the real world it doesn't apply. I develop a product that uses TCP/TCP communications and it transfers hundreds of gigs a week from dozens of sites without any performance problems.

I think the way the real world works around the theoretical problems is that it's not really possible to maintain a TCP connection on the 'net for a long period of time unless you control the entire path. In my case, customer sites are always rebooting their routers, NAT boxes, etc. and connections rarely last longer than a day, often only several hours.

Fortunately, Unix has nice mechanisms for keeping things up, i.e. inittab.

Re:TCP over TCP OK

[sporty]

I wouldn't be surprised if you saw like, a 1% increase if you used straight tcp/ip.

tcp over tcp incures an x% size of packet increase. Reminds me of that stupid commerial. "We saved a nickel for a transaction" The go on and on about this stupid nickel and then they talk about how they do "20 million in transactions a day."

But hey, if you have the bandwidth, knock yourself out. :) If it works, it works.

Parent is good. Here it is with my bonus.

[Beryllium Sphere(tm)]

I'm reprinting something from an AC which got stuck at 0 despite being informative. I've dropped the first sentence.

cut here

1. yes "TCP over TCP" is a bad effect. This is what happens when you tunnel an unreliable protocol (like IP encapsulated in PPP) over a reliable protocol (like TCP with something like SSH or SSL on top) The problem is that packet loss ends up causing retransmits both from the encapsulated TCP sessions and from the SSH/SSL layer on top. So you end up with a situation where it works perfectly when you're getting near zero packet loss, but as soon as there's even a moderate amount of packet loss the entire connection falls apart. Often the only way to recover from this is to restart the tunnel.
2. However, THIS ARTICLE ISN'T ABOUT THAT. It's using ssh'd builtin TCP forwarding to tunnel the data. So instead of "TCP-over-TCP" it's "TCP-to-TCP-to-TCP". At to point in the connection are there two TCP state machines both competing to retransmit. I'm sorry if this doesn't make much sense in text - I really need a whiteboard to explain it properly.
The short version is that the "TCP-over-TCP" problem is basically confined to schemes where they try to tunnel a PPP session over SSH (in order to get a full VPN). Just using SSH's port forwarding is _fine_.

Obvious

[mindstrm]

Protocol overhead is obvious.. the real issue that people talk about is to do with how TCP deals with congestion... you end up with 2 layers of protocol trying to deal with a problem when only one needs to.. leading to some interesting issues... (things can theoretically get really slow)

Another approach for "transparent proxying"

[stefanb]

I just had to set up something similar, to be able to access a couple of machines at a client's site, and the client didn't want a real VPN.

The OpenSSH client has the nice feature of being able to act as a SOCKS4 proxy to dynamically build tunnels through a connection. What this means is that you don't have to specify every single forwarding when starting ssh, but you can use a socksified client to connect to ssh(1) and establish further tunnels on an as-needed basis.

Now, my (proprietary) software needed for this project is not socksified. What now? Ah! PFs rdr rules! (Also possible with IPFilter, IPFW (FreeBSD), IPChains...)

My client's private network address does not clash with the ones I use, so I set up my OpenBSD router to redirect TCP packets destined for the client's network to a local port. From there, transproxy (/usr/ports/www/transproxy) picks up the connection, and forwards it through the ssh(1) SOCKS4 proxy to the appropriate machine on the client's network.

The only thing left was to copy the client's private DNS zone over to my internal nameserver to be able to resolve names properly, and e voila!, I had my (almost) transparent ssh-based VPN.

The currect version of transproxy doesn't support SOCKS4, but I've submitted patches to the author. In the meantime, you can find my patches here.

Re:Another Small error in the examples...

[ScooterComputer]

Actually the second example shows an IP address of 192.168.1.46:139 for the friend's OBSD server in the diagram, but the affiliated command line (the second line) uses 192.168.0.10:139 for it.

What's wrong with port forwarding?

[CoolQ]

Can't you just tunnel to a different local port and use port forwarding or redir to map 139 on the proper interface to whatever you connected the tunnel to? It seems the patch is unnecessary...
--Quentin