4 ms·

And this is news somehow? Color me unimpressed. First time we'd been subjected to this kind of attack was, what, a year ago? Maybe two. It's a trivial next step

by jsn 17y ago

And this is news somehow? Color me unimpressed. First time we'd been subjected to this kind of attack was, what, a year ago? Maybe two. It's a trivial next step after the old boring flood of http get requests.

Locke1689 17y ago
It's also not that difficult to counter. The easiest way is to just do packet inspection - if something looks wrong (repeated bad/incomplete headers), just blacklist the IP for however many minutes. Repeat ad nauseam. Any IDS worth its salt shouldn't even hiccup at this.
- jsn 17y ago
  It's trivial to counter indeed; you don't need any packet inspection for that. The main anomaly is not in the header structure, it's in the number of sessions opened simultaneously by one host. This particular tool uses broken headers for some reasons; other ones don't. But they all open sessions excessively. So you just use iptables connlimit match block the clients with more than e.g. 10 connections.
  - rbanffy 17y ago
    Actually, just blocking originating IPs would not stop a DDoS based on this. The simplest approach would be to timeout the connection if a credible set of headers is not received within a certain time. This attack is based on the idea the attacker commands more resources than the target server. It doesn't fly very well.
    - jsn 17y ago
      Sorry, most of that is wrong :) 1. No, it's not the simplest approach. The attacker can trivially send perfectly credible set of headers, then start receiving the response, acknowledging 1 byte per 30 minutes. It's routinely seen in the wild, too. Header-based approach completely misses the point of anomaly. 2. Blocking originating IPs is perfectly effective as soon as all existing connections from the blocked IPs time out. Which is fairly soon. 3. connlimit approach doesn't just block originating IPs, it prevents anyone from opening a lot of connections. 4. In any real DDOS scenario attacker does command an order of magnitude more resources than the target servers. [edit: formatting]
      - rbanffy 17y ago
        1. I don't think the client can acknowledge one byte per minute - I think the minimum would be one network packet. Anyway, the whole response would timeout. Apache has a config setting for that, does it (I have been using mod_proxy a lot and mod_proxy does timeout)? Perhaps, there should be a minimum bandwidth threshold that forces the connection to abort. I see your point, however. Even sending a real complete header should not prevent the request from being treated as hostile. 2. I am not sure I got what you say. In a real DDoS attack, there should be a lot of different IP addresses and blocking thousands of them (many of whom would reset connection and come back with a different IP) would make the life of the firewall very uncomfortable. 3. I know this has problems with proxies and firewalls with large number of computers behind them. I don't remember proposing this idea. 4. It all depends on what is being attacked and how. I don't think any serious DDoS attacker would use this approach.
        jsn 17y ago
        1. You're wrong, see rfc793. TCP flow control acknowledges streamed bytes, not packets. 2. I just tell you what works in real life. Blocking 500 supernodes (out of 5000 machines botnet) is basically a non-issue for linux iptables on usual modern processes. Other 4500 are usually pretty tame in terms of firewall processing power. 3. You didn't propose that, it's what we do (and what works best for these attacks). Its rate of false positives is sufficiently low. Folks behind NATs don't use sustained 4 connections per user, so it's usually a non-issue either. 4. Well, you're wrong if you don't think so. Mitigating DDOS-es is one of the things i do for living [i'm in charge of web infrastructure of certain political opposition websites in Russia, if it rings any bells]. The scenarios i described is what actually happens.
        rbanffy 17y ago
        Interesting. Acknowledging receival on two layers (packets and butes) makes little sense. Wonder why such decision was made. About 4, are the DDoS aimed at your servers or at your office infrastructure?
        17y ago
  - e4m 17y ago
    Clients? You mean IP. NAT anyone?

by jsn 17y ago

Locke1689 17y ago

It's also not that difficult to counter. The easiest way is to just do packet inspection - if something looks wrong (repeated bad/incomplete headers), just blacklist the IP for however many minutes. Repeat ad nauseam. Any IDS worth its salt shouldn't even hiccup at this.

jsn 17y ago
It's trivial to counter indeed; you don't need any packet inspection for that. The main anomaly is not in the header structure, it's in the number of sessions opened simultaneously by one host. This particular tool uses broken headers for some reasons; other ones don't. But they all open sessions excessively. So you just use iptables connlimit match block the clients with more than e.g. 10 connections.
- rbanffy 17y ago
  Actually, just blocking originating IPs would not stop a DDoS based on this. The simplest approach would be to timeout the connection if a credible set of headers is not received within a certain time. This attack is based on the idea the attacker commands more resources than the target server. It doesn't fly very well.
  - jsn 17y ago
    Sorry, most of that is wrong :) 1. No, it's not the simplest approach. The attacker can trivially send perfectly credible set of headers, then start receiving the response, acknowledging 1 byte per 30 minutes. It's routinely seen in the wild, too. Header-based approach completely misses the point of anomaly. 2. Blocking originating IPs is perfectly effective as soon as all existing connections from the blocked IPs time out. Which is fairly soon. 3. connlimit approach doesn't just block originating IPs, it prevents anyone from opening a lot of connections. 4. In any real DDOS scenario attacker does command an order of magnitude more resources than the target servers. [edit: formatting]
    - rbanffy 17y ago
      1. I don't think the client can acknowledge one byte per minute - I think the minimum would be one network packet. Anyway, the whole response would timeout. Apache has a config setting for that, does it (I have been using mod_proxy a lot and mod_proxy does timeout)? Perhaps, there should be a minimum bandwidth threshold that forces the connection to abort. I see your point, however. Even sending a real complete header should not prevent the request from being treated as hostile. 2. I am not sure I got what you say. In a real DDoS attack, there should be a lot of different IP addresses and blocking thousands of them (many of whom would reset connection and come back with a different IP) would make the life of the firewall very uncomfortable. 3. I know this has problems with proxies and firewalls with large number of computers behind them. I don't remember proposing this idea. 4. It all depends on what is being attacked and how. I don't think any serious DDoS attacker would use this approach.
      - jsn 17y ago
        1. You're wrong, see rfc793. TCP flow control acknowledges streamed bytes, not packets. 2. I just tell you what works in real life. Blocking 500 supernodes (out of 5000 machines botnet) is basically a non-issue for linux iptables on usual modern processes. Other 4500 are usually pretty tame in terms of firewall processing power. 3. You didn't propose that, it's what we do (and what works best for these attacks). Its rate of false positives is sufficiently low. Folks behind NATs don't use sustained 4 connections per user, so it's usually a non-issue either. 4. Well, you're wrong if you don't think so. Mitigating DDOS-es is one of the things i do for living [i'm in charge of web infrastructure of certain political opposition websites in Russia, if it rings any bells]. The scenarios i described is what actually happens.
        rbanffy 17y ago
        Interesting. Acknowledging receival on two layers (packets and butes) makes little sense. Wonder why such decision was made. About 4, are the DDoS aimed at your servers or at your office infrastructure?
        17y ago
- e4m 17y ago
  Clients? You mean IP. NAT anyone?