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DSL and Cable Modem Security

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Although high speed internet access may not have reached you yet, it probably will soon. The advantages are obvious, but there’s a dark side: security. I’m not going to talk about the more general aspects of securing your system here (I’ve done that in General Security), but only specifically about the issue of clear text passwords with telnet, pop, and ftp.

Out with the old, in with the new

Just because you have high speed access doesn’t mean you necessarily want to give up your present ISP. You may not want to change your email address. You may have a web site that you need to maintain with ftp. That’s my situation: my ISP hosts my mail and my web site, and I have no interest in changing that.

By the way, a few things you need to think about if you are planning such a move:

  • Does the DSL/Cable ISP have an SMTP server you can use for outgoing mail? The answer is almost certainly yes, but it doesn’t hurt to ask. Ask if they have a back-up server in case that one has problems. That was a very nice feature I had with my dial-up ISP- it was rare for the primary to have any problem, but once every year or so it would, and I’ d just switch to the secondary.
  • Do they have an NNTP (news) server? This may or may not be important to you, but if it is important, you’ll want to ask questions about the groups they carry and the article retention policies.
  • Are you getting a fixed IP address or will it be DHCP? You may have reason to need a static address, or you may not.
  • Do they care if you are not running Windows? If so, find someone else.
  • Do they care if you use NAT or a Proxy to allow machines on your internal lan to access the net through this machine?
  • Does their router/modem provide any security? If so, it’s important to understand what, both for its limitations and in case it blocks something you need to provide. You can get some idea of your security status by using an external scanning tool like those available at http://www.grc.com/default.htm. When I used that on my DSL line, I got informative messages on my screen that someone might be trying to break in.
  • If you are keeping your current ISP for mail, how are you going to get it? Will your ISP allow remote POP/IMAP access or will you forward it?

In the clear

Pop, telnet, and ftp all use clear-text passwords- that is, the passwords would be totally visible to someone snooping network traffic. That’s not necessarily a problem with a dial-up line (see the General Security article) but it could be a problem with DSL and it is definitely a concern with cable modems.

See Where can I obtain ssh binaries? if you can’t or don’t want to compile ssh yourself. These binaries are the OpenSSH version from http://www.openssh.org

You may also want to read SSH Basics.

Ssh replaces telnet. Of course that’s not all that it does, but for a beginning, that’s how you’ll want to test it. Naturally, your ISP has to run the sshd daemon- fortunately mine does.

ssh -l username yourisp.com

will attempt to login to “yourisp.com” as “username”. It will ask you for your password, and from that point on it looks like a telnet session (well, not really: telnet escape keys don’t work, for example). The entire session is encrypted, from the entry of your password right up until you log out. Nothing you do while logged in can be seen by someone sniffing the network.

You’d think that wouldn’t help you with ftp, but ssh has more tricks up its sleeves.

ssh -l username -L 1800:yourisp.com:21 yourisp.com

logs you in, just as before, but it also sets up a trap on port 1800, and that trap will just take anything that tries to use port 1800 on your local machine and forward it to port 21 (ftp) on yourisp.com. You can see that it does that by opening a different window on your home machine and trying:

telnet localhost 1800

(don’t do this from the login you made using ssh above; that won’t get you anything useful)

Yes, that’s right: you telnetted to port 1800 on your local machine, but you end up connected to the ftp server at your ISP (type “quit” to exit). What’s the point? The point is that the connection is now encrypted because it goes through ssh. Note: this obviously isn’t usually important for anonymous ftp- this is for sites that you must log in to as a real user, such as your hosted web site, where you need to upload files. The point is that you do not want anyone to be able to see your login and password.

The choice of 1800 is arbitrary and might not work if by chance port 1800 is already in use. If that is the case, just try another port. You can see ports in use with “netstat -a”.

If you have root access, you can set this up on port 21. You’ll need to comment out “ftpd” in /etc/inetd.conf, and send a “kill -1″ to the inetd daemon before you can do that, but then there wouldn’t be any need to use 1800 as a port number,

Unfortunately, that’s not quite enough. You need an ftp client that can be told to connect to port 1800 instead of the 21 it would default to, and you also need an ftp client that can do “passive” transfers. Why? Because your firewall should be set not to accept incoming connections, and keep in mind that this is important for both anonymous and authenticated ftp.

Passive ftp works by the client (that’s you) telling the server to use Passive mode-the client opens it’s own data connection, and the server uses that. The server is being “passive”- it isn’t actively opening connections. For your typical firewall, that’s much easier- the connection originates inside the firewall, therefore it’s OK (though the firewall does usually have to be told that this is OK ahead of time).

Ftp is a little different than most protocols. When you connect to an ftp server, you connect on what’s called the “Control” port (that’s port 21). When you want to transfer a file, the ftp server opens a data connection back to you (using port 20). There’s two connections: one that you originated, and one that the server opened for data. And there’s the problem for most firewalls: they block that data connection because it comes from outside.

Ian Peattie noted in a comp.unix.sco.misc posting that there is another way to solve the FTP problem:

To get round this, you can use an FTP proxy. Luckily, IPNAT comes with an FTP proxy built in to it. If you change your IPNAT rules to:

   map net1 172.16.1.0/24 -> 0.0.0.0/32 proxy port ftp ftp/tcp   map net1 172.16.1.0/24 -> 0.0.0.0/32 

then IPNAT will watch for the connection coming back from the remote server and will forward it on to the PC.

An ftp client that answers both of these problems (and is more than slick in other respects also) is “ncftp”, which is freely available on the web and from Skunkware. One caveat, though, at least for the SCO Skunkware version I used while writing this article: don’t run ncftp as root. It’s not that it won’t work, or that there’s some awful security hole- the problem is just that it’s a little broken and doesn’t properly create the bookmark and preferences files for the root user. These are supposed to go into $HOME/.ncftp, which is supposed to be created on first use of ncftp, but it doesn’t work for root. You shouldn’t be using root for ordinary work anyway.

Jean-Pierre Radley tells me that ncftp’s “root” problem can be fixed by adding “NCFTPDIR=/.ncftp” to root’s environment.

This is what an ncftp “bookmark” looks like:

Bookmark name: yourisp.com
Hostname: localhost
User: username
Password: ********
Account: none
Directory: /
Transfer type: Binary
Port: 21
Has SIZE command: Yes
Has MDTM command: Yes
Can use passive FTP: Yes
Operating System: UNIX
Comment: (none)

To create a new bookmark, type “hosts” at the ncftp prompt and follow the directions. The only things you’ll need to change are the port (if you are not using port 21) and the “Can use Passive FTP” (it’s No by default). Make sure that the Hostname is “localhost”- you can name the bookmark anything, but it’s got to connect to “localhost”.

The other thing you need is to change your preferences so that it will use passive ftp. Type “prefs”

Preferences

Default open mode: anonymous
Anonymous password: tony@scobox.landc.com
Blank lines between cmds: Yes
Default FTP mode: Passive, but fall back to port if needed
(rest of preferences ommitted)

It’s the “Default FTP mode” that you have to change to use Passive. Once this is all configured as shown, an “ncftp yourisp.com” (note that’s the Bookmark name; it’s really connecting to localhost) will connect to your ISP and your control channel (which is where your password would appear) is secure. Your data channel is not, but that’s unlikely to be a problem, and you could always separately encrypt anything that was.

Pop

For pop, you probably want to use the actual pop port, because many pop clients just assume 110 and can’t be configured otherwise. I had a pop client that I could configure, but for some reason that still didn’t work for me, and as my primary need (Netscape) insists on using 110, that’s what I did. That means taking POP3 out of /etc/inetd.conf (and you wouldn’t want to be running it there anyway) and making a connection to your old ISP like this:

ssh -l username -L 110:yourisp.com:110 yourisp.com

Bruce Garlock pointed out that you can make Netscape use a different port simply by entering (for example) localhost:1810 in the Incoming Mail Server Name box. That does work, thanks Bruce!

Note that there’s no reason to make two connections if you also want to use ftp:

ssh -l username -L 21:yourisp.com:21 -L 110:yourisp.com:110 yourisp.com

will take care of both forwardings at the same time.

Now all you have to do is tell your pop client (Netscape, etc.) that you want it to use “localhost” instead of your ISP. That’s all it takes; the actual session will be forwarded to your ISP by ssh, and it will be secure. Don’t forget that you’ll need to point your SMTP at your new ISP’s mail server and also News if you read news groups.

SCP

You can also use “scp” to copy files. Let’s say I want to copy “index.html” from my current directory to another machine. I might say:

scp index.html tony@aplawrence.com:/www/newstuff/index.html

I’d be asked for my password, and then the file would be securely copied.

Note: if this looks like it’s going to work, but then says “scp not found”, it is the OTHER end that is missing (or lacks a PATH to) scp. The other end could have sshd and not have scp.

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Copyright and Reprint Info

Originally appeared at http://www.aplawrence.com

A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com

DSL and Cable Modem Security
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