iptables Questions

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1080p

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#84206 26-May-2011 20:18

Hey all,

I am messing about with iptables on a VPS and have some questions.

I have drawn myself a little mind map of what I want flowing in and out of the server and was hoping you'd all take a look and make sure I am not missing anything spectacular.

I haven't written any rules yet but here is what I want:

IPv6: Drop all in and outbound.

Allow all established and related connections.

Inbound:
SSH - Allow (custom port)
IRC - Allow (custom port)
HTTP - Allow (80)

Block everything else (drop silently)

Outbound:
DNS - Allow (53)
IRC - Allow (6697, 7000)
HTTP - Allow (80)

Block everything else (drop silently)

Have I forgotten anything that is essential to the server operating? As you can see this VPS will not be doing a lot (IRC bouncer, webserver) but I'd like to make sure it is as secure as I can.

Does anyone have a good tutorial they could link me to as well. I am getting there in terms of understanding but like to have things spelled out the first time I do 'em.

muppet

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#474543 26-May-2011 20:20

For your inbound rules don't forget to allow "established" sessions in. Otherwise the reply to your output IRC/HTTP etc request will be dropped.

Also, if it's a simple VPS, why are you bothering with IPTABLES rules at all? Just for the sake of it?
Dropping packets to ports that aren't listening for connections anyway doesn't get you anything at all for a single box. If you were a firewall in front of a /24 network I'd say it was worthwhile.

If you want to make it more secure I'd be building your own linux kernel with loadable modules disabled (hard to modprobe in a backdoor then). I'd actually be patching it with the grsecurity.net kernel patch, but that might be a too complex if you've not built your own kernels before.

Also look at mod_security for your (I assume) apache server. With the core-rules for it, it can block a lot of SQL and other nasty exploits in the wild.

Audiophiles are such twits! They buy such pointless stuff: Gold plated cables, $2000 power cords. Idiots.

OOOHHHH HYPERFIBRE!

1080p

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#474575 26-May-2011 21:38

Thanks for the reply. I have the established connections under the IPv6 blocking. Probably not in the right order but I was just mind farting out a basic plan.

I am still learning about Linux so am just playing with iptables for the sake of it. I realise that it won't do much but wanna do it anyway :)

I can't think of any services I might have missed out allowing through so I'll take a crack after finishing reading some more documentation.

Building kernels will probably be my next project but I'm not sure how much flexibility I have with a VPS. I'll probably do it at home first.

The website is, at the moment, just hosting static content so I was looking at nginx or perhaps lighttpd as slightly lighter options to apache.

kyhwana2

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#474578 26-May-2011 21:47

Why are you blocking IPv6?

marpada

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#474599 26-May-2011 22:20

Personally I find the tutorial in linuxhomenetworking.com quite comprenhensive and practical, for example

http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables

Building kernels will probably be my next project but I'm not sure how much flexibility I have with a VPS. I'll probably do it at home first.

Probably you won't be able. Most VPS (specially the cheaper ones) are based in Virtuozzo/OpenVZ, where the guests (nodes or VPS) use the same kernel that the physical host.

1080p

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#474667 27-May-2011 08:54

kyhwana2: Why are you blocking IPv6?

I don't use it. Playing with IPv6 is a few projects away. One thing at a time ;-)

1080p

1332 posts

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#474669 27-May-2011 08:56

Cheers for the LHN link. I think I have it bookmarked for reading :)

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philelvey

104 posts

Master Geek

#474841 27-May-2011 15:12

Also this is a great, easy to follow iptables tutorial

Cheers

https://help.ubuntu.com/community/IptablesHowTo

Reluctant geek

1080p

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#475170 28-May-2011 16:42

Nice find, was just thinking that the Ubuntu forums/documentation would have something simple to start on.

Would I be correct in thinking configuring iptables for each server is a good idea from a layered security perspective?

A border firewall is a good first line but if something was able to infect or access your machine, configuring iptables to drop traffic on all outbound ports except the ones you will be using and monitoring could be a good idea.

heydonms

27 posts

Geek

#475668 30-May-2011 09:29

Configuring iptables on each machine (as long as it is done correctly) isn't a bad idea. It might be overkill depending on what exactly the machine is doing, but there is no harm in it.

I've got it set up on my VPS and my home server (actually this one is openbsd running pf, but the principle is the same), but not my desktop or laptop. If I was going to set it up on my other machines, I wouldn't bother filtering outbound connections.

One thing I would probably add to your list is rate limiting/blacklisting on the ssh and possibly irc ports. Running on non-standard ports is a start but identifying those services based on their output isn't particularly difficult and then the brute forcing attempts will start. On my machines more than 3 connections in 1 minute or more than 6 connections in 5 minutes to ssh will get your address put in a blacklist for 24 hours.

If you have multiple users on your machine you may also want to limit who can logon through ssh (although that isn't really related to iptables).

1080p

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#475685 30-May-2011 10:24

Cheers for the input, I've already got fail2ban & exclusive public/private key validation. Combined with a non-standard port and I am all set SSH-wise.

I am glad to know I am not the only crazy setting up iptables on a regular VPS :P

Were you able to enable connection tracking on your VPS? I think I might have a crippled one as adding:

iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

gave me an error, but:

iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

worked fine.

heydonms

27 posts

Geek

#475700 30-May-2011 11:29

It might be slightly unnecessary on some servers, but really for the amount of effort involved in setting it up it is cheap insurance in my mind.

I think my current VPS Just Worked but with one or two of the other providers I have used I had to log a support call asking for certain modules to be loaded.