Tag Archives: iptables

It’s all about the pcaps baby

So my android phone as an interesting problem, granted it’s an S4, running not the latest build so I don’t know if that problem still exists. Apparently the way the default mail application is set up, it can’t sync the mailboxes unless the Sync button is turned on. But that doesn’t stop that the mail application from trying to sync on a schedule.

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knowing what your tools do

When I changed my firewall rule policy, part the reason for doing it was because I was getting tired of seeing dovecot:auth failures in the logs. People around the world were brute forcing my mail server, and the rules were 100 lines long of just blocking. I had thought that they were coming from people hitting port 993 (IMAPS), and to a point there were. You can see below where it is dropping port 993 access attempts.

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All the ip addresses that DenyHosts blocked

One of the things I did after getting iptables tweaked, was to clear my /etc/hosts.deny file. About 99% of these were put there by DenyHosts. Which is a great little background daemon that looks for failed log in attempts over ssh and then blocks the attacker by adding them to the /etc/hosts.deny file.

Since I know some people are looking for these types of addresses for different reasons, and there are over 2300+ unique ones in the list, I shared it publicly. You can get the list at pastebin.



Firewalls and the default deny rule, does that make us bad?

Over the last few weeks I’ve been thinking of redoing my iptables, but by putting in a default deny rule, does that make us bad netizens? By dropping everything that isn’t allowed, it actually makes it harder to fix the original problem. The fact that there are attacks to begin with, and the fact that boxes are compromised.

Over the last few months, since fixing the IMAPS part of my mail server, I noticed people hitting the server and failing to log in. These were not targeted attacks; they were automated bots, using the same users and possible passwords. I’d block them at the firewall but every day, there would be at least 2 new networks. Not all of them from overseas.

For historical reasons I take a stance of contacting the abuse email for North American companies, and some European ones. I found that the large Cable Company ISPs, usually turn out to be black holes. The smaller ones though actually reply back.

Last week I went through a week’s worth of logs, and out of the 47 ip addresses, there were about 30 networks. I actually sent 17 emails, and blocked only 13 networks. From that, 9 replied. Granted they were mostly the auto-replys, but I did have 2 that were interesting. One was from an ISP in England and they thanked me for letting them know. The other was an educational ISP in California. They replied back where they found the problem, and the IR procedures, with a thank you.

That worked because I wasn’t blocking the inbound connections, and letting other tools protect the server’s processes. However the iptable rules had become larger than I wanted to maintain (over 100) for inbound access.  So I re-wrote them using a default deny. I now state what on my network can be talked to, and in some cases by what networks. Everything else gets dropped and logged.

The down side to this is that while I can see the dropped connection attempts, I can’t see what the people are trying to do. Is it just a null hit, a failed loggin attempt, something else. I also can’t report it back to other interested parties. While I feel better about their failures, I realize that I’m a bad Netizen because I can’t contact their upstream, with logs showing the problem.

Yes, I know I could set up a honey pot system and fight for both the user and the internet that way, but it feels more like just being the complaint department than it does trying to solve things.

While I am going to keep using the default deny, because it’s easier to handle the rules on the firewall, I still don’t like that I’m walling myself off from the real problem, and not trying to fix underlying issue.

Over Thinking Problems

I think one of the problems we may have in this industry is over thinking the problem, and doing more than is needed for the problem. For example, I upgraded my personal VPS server recently, the one that runs this site and Rats and Rogues. It required a reboot, but because I rarely reboot this box, I keep forgetting that iptables isn’t persistent. I usually remember and restart it fairly quickly when it reboots.

The night of the upgrade wasn’t much different. However I messed up the command, being a lazy admin I use the built in tools to do work for me. I love control-r and how it scrolls through your history based on a few characters you type. Well instead of iptables-restore < firewall.rulz I typed iptables-save > firewall.rulz. Yes, I overwrote my rules with nothing.

My very first thought was WOOHOO I get to do forensics on my live system. I went to twitter to brag, though I’m not sure if people realized that was the point. @secbuff asked why not restore from backup. He was right. The majority of the rules I have are for blocking ssh brute force attempts (the ones that make it past denyhosts), blocking mail relay attempts, and blocking user account enumeration. While playing forensics would be cool, this is a live host on the internet with services that do get attacked. It would have left the box exposed way to long to the internet, and was a case of over thinking the problem.

So I went grabbed a back up file. Instead of  uploading it though, I opened it in a text editor, hand sorted the rules by network number, and then pasted them in to the terminal window. I also finally dealt with that persistence issue too, we’ll see if iptables-persistence.dpkg worked right on the next reboot. Oh and since I add networks on a regular basis (when reviewing my logs) I wrote a small shell script to make two copies of the rules in different locations, with a spare backup.