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> Is the attack on its webmail/website or on their smtp servers? It is an attack on the httpd and smtpd daemons. imap has been unaffected as far as I can tell

> Is the attack on its webmail/website or on their smtp servers? It is an attack on the httpd and smtpd daemons. imap has been unaffected as far as I can tell. > how does one protect smtp servers from distributed attacks? By design, MX can be as distributed and in large number as you can afford or as willing to spend. This can be a combination of load balancer virtual IP's distributing load to many MX servers behind it and many MX DNS records with the same or different priorities. This of course won't help much if the people attacking are paying ddos-as-a-service farms to bring on massive volume and packet rates that overload all your servers. There are DDoS scrubbing services you can pay for that will advertise your AS number or use GRE tunnels or VPN's to clean the attack data for you. These scrubbing solutions are no guarantee of mitigation. > Do tools like fail2ban help? No. That would be pointless whack-a-mole. If an individual person is mad at you and launching a tool from their own PC or a handful of VM's, then yes fail2ban will help. Blocking individual IP's on your MX servers under a real distributed DDoS attack would be futile. Scrubbing centers are about the only solution once the attack is big enough. Or if you had unlimited funds you could deploy many datacenters or point-of-pressence destinations and build your own scrubbing networks but that is very expensive.

rgrmrts 5y ago
Thanks for your response! So using specific examples here for smtp, I get a 1gbps guaranteed network from Hetzner so in theory I’d need to distribute over 50 servers to withstand this attack? It’s not clear to me why fail2ban wouldn’t at least help, if the botnet is a thousand machines wouldn’t I (eventually) have them all blocked? And therefore reduce the overall duration of the attack? Or is the problem that it’s hard to differentiate between good clients and bad clients because no single client is sending enough traffic to be suspicious? Also, do you have any specific examples of ddos scrubbing services? Would like to take a look specifically at affordability for individuals.
- deadbunny 5y ago
  You're thinking too small. DDoSaaS use hundreds of thousands to millions of compromised machines* to attack. You might only see a handful of connections from each IP. Whch makes it fruitless to use something like f2b. * Machines here could be anything from a lightbulb to a server.
- mirashii 5y ago
  The problem is that fail2ban or any sort of dropping of packets at the host is too late, the volume has overloaded the link.
- LinuxBender 5y ago
  In regards to fail2ban, assuming the attack was purely SMTP specific which it won't be even then blocking IP's would be futile. The DDoS-as-a-service farms have hundreds of thousands to millions of IP's under their control. Block one IP and ten more show up. One IP is not one attacker. Those farms have probes that can tell how effective their attack is. Some of them even have "proven work" that is reported back to the buyer to validate the effectiveness of their attack. You may want to research this one as it is a very big topic. In reality however, a large portion of the attack won't even show up in logs. The attack will also contain depending on how much the attacker is willing to spend tens of millions or hundreds of millions of packets per second of TCP, UDP packets on random ports, no ports, random protocols, random sizes, random TTL's, random headers. That is a volumetric attack. Fail2ban and most network IDS/IDP's would not even see this attack. It would saturate the uplinks to the ISP before you even see anything. Your ISP will most likely null route you and encourage you to stop advertising your AS number. In reference to scrubbing centers for individuals, that is not a thing unless you have unlimited funds. There are some VPS providers that can scrub tiny attacks. Linode, Vultr, OVH to name a few but they can only deal with tiny attacks. If you want to research this for your business, my suggestion would be to get on the NANOG mailing lists and discuss it with all the network engineers to find out which scrubbing services are currently most effective. This is a moving target and a very big investment. I am not a fan of any of the companies that provide these services, but that is only based on my limited experiences with them.
  - rgrmrts 5y ago
    Word, thanks for all the details! Yeah, I don't have any actual business use-cases outside of a personal interest in making my dedicated server reasonably protected.
    - LinuxBender 5y ago
      You're welcome! For your personal MX servers, the most cost effective solution I know of would be to have multiple domains and each domain have their own MX records and corresponding VM/server on its own unique provider to isolate them. Then ensure that people/businesses that are important to you know to contact you at 2+ email addresses/domains. If someone attacks one of your domains or MX servers, you can safely ignore the attack. These separate MX servers should be on different server/VPS providers in the event that the attack causes one of them to suspend your account. Your imap client can poll each of the servers/domains so that you get your emails.
tgsovlerkhgsel 5y ago
I agree that you do need some form of first stage that can take the traffic volume (usually a third party service, but could also be 100+ Gbps of bandwidth you get somewhere and an appliance that can do IP filtering), no good way around that. But once you have that, you should be able to tell them "forward me all traffic except <list of IPs>" and then list the IPs that are sending you the most (remaining) traffic, or even cost (e.g. if the IP is sending little traffic, but performing many TLS handshakes). That's where a fail2ban like tool would come in, no? The benefit of this approach is that it works completely independently of the protocol you're running. TCP, UDP, doesn't matter, as long as the attacker cannot spoof IP addresses at scale.
- LinuxBender 5y ago
  Agreed adding bandwidth can help against some attacks. as long as the attacker cannot spoof IP addresses at scale That is kindof the rub. Until a majority of tier-1 backbone providers implement bcp38 [1] or some derivative of it, spoofing from the ddos farms is trivial. There has been talk of implementing this for many years but very little action. Perhaps when DDoS attacks cost enough tax revenue or impact investors, perhaps there may be push for legislation in some countries to implement but in an ideal world most of the providers would work through this as one big team. I just make some network engineers laugh, or smirk, or other [1] - https://en.wikipedia.org/wiki/Ingress_filtering#Networks https://en.wikipedia.org/wiki/Ingress_filtering#Networks
  - elaus 5y ago
    Curious as you speak of "very little action", yet the Wikipedia article you linked says: > […] contrary to general opinion about the lack of BCP 38 deployment, some 80% of the Internet (by various measures) were already applying anti-spoofing packet filtering I know very little about global routing, so I'm wondering if it's an ineffective way of filtering against these attacks we see today or is 80% not enough to be effective?
    - LinuxBender 5y ago
      It really isn't enough. Everyone needs to do their part [1] not just ISP's but also large businesses, cloud providers, others. And this has to occur in all regions. I admittedly over-simplified the problem statement. [1] - https://archive.nanog.org/sites/default/files/1_Kaeo_Ddos_Trends.pdf https://archive.nanog.org/sites/default/files/1_Kaeo_Ddos_Tr... [PDF]

rgrmrts 5y ago

Thanks for your response! So using specific examples here for smtp, I get a 1gbps guaranteed network from Hetzner so in theory I’d need to distribute over 50 servers to withstand this attack? It’s not clear to me why fail2ban wouldn’t at least help, if the botnet is a thousand machines wouldn’t I (eventually) have them all blocked? And therefore reduce the overall duration of the attack? Or is the problem that it’s hard to differentiate between good clients and bad clients because no single client is sending enough traffic to be suspicious? Also, do you have any specific examples of ddos scrubbing services? Would like to take a look specifically at affordability for individuals.

deadbunny 5y ago
You're thinking too small. DDoSaaS use hundreds of thousands to millions of compromised machines* to attack. You might only see a handful of connections from each IP. Whch makes it fruitless to use something like f2b. * Machines here could be anything from a lightbulb to a server.
mirashii 5y ago
The problem is that fail2ban or any sort of dropping of packets at the host is too late, the volume has overloaded the link.
LinuxBender 5y ago
In regards to fail2ban, assuming the attack was purely SMTP specific which it won't be even then blocking IP's would be futile. The DDoS-as-a-service farms have hundreds of thousands to millions of IP's under their control. Block one IP and ten more show up. One IP is not one attacker. Those farms have probes that can tell how effective their attack is. Some of them even have "proven work" that is reported back to the buyer to validate the effectiveness of their attack. You may want to research this one as it is a very big topic. In reality however, a large portion of the attack won't even show up in logs. The attack will also contain depending on how much the attacker is willing to spend tens of millions or hundreds of millions of packets per second of TCP, UDP packets on random ports, no ports, random protocols, random sizes, random TTL's, random headers. That is a volumetric attack. Fail2ban and most network IDS/IDP's would not even see this attack. It would saturate the uplinks to the ISP before you even see anything. Your ISP will most likely null route you and encourage you to stop advertising your AS number. In reference to scrubbing centers for individuals, that is not a thing unless you have unlimited funds. There are some VPS providers that can scrub tiny attacks. Linode, Vultr, OVH to name a few but they can only deal with tiny attacks. If you want to research this for your business, my suggestion would be to get on the NANOG mailing lists and discuss it with all the network engineers to find out which scrubbing services are currently most effective. This is a moving target and a very big investment. I am not a fan of any of the companies that provide these services, but that is only based on my limited experiences with them.
- rgrmrts 5y ago
  Word, thanks for all the details! Yeah, I don't have any actual business use-cases outside of a personal interest in making my dedicated server reasonably protected.
  - LinuxBender 5y ago
    You're welcome! For your personal MX servers, the most cost effective solution I know of would be to have multiple domains and each domain have their own MX records and corresponding VM/server on its own unique provider to isolate them. Then ensure that people/businesses that are important to you know to contact you at 2+ email addresses/domains. If someone attacks one of your domains or MX servers, you can safely ignore the attack. These separate MX servers should be on different server/VPS providers in the event that the attack causes one of them to suspend your account. Your imap client can poll each of the servers/domains so that you get your emails.

tgsovlerkhgsel 5y ago

I agree that you do need some form of first stage that can take the traffic volume (usually a third party service, but could also be 100+ Gbps of bandwidth you get somewhere and an appliance that can do IP filtering), no good way around that. But once you have that, you should be able to tell them "forward me all traffic except <list of IPs>" and then list the IPs that are sending you the most (remaining) traffic, or even cost (e.g. if the IP is sending little traffic, but performing many TLS handshakes). That's where a fail2ban like tool would come in, no? The benefit of this approach is that it works completely independently of the protocol you're running. TCP, UDP, doesn't matter, as long as the attacker cannot spoof IP addresses at scale.

LinuxBender 5y ago
Agreed adding bandwidth can help against some attacks. as long as the attacker cannot spoof IP addresses at scale That is kindof the rub. Until a majority of tier-1 backbone providers implement bcp38 [1] or some derivative of it, spoofing from the ddos farms is trivial. There has been talk of implementing this for many years but very little action. Perhaps when DDoS attacks cost enough tax revenue or impact investors, perhaps there may be push for legislation in some countries to implement but in an ideal world most of the providers would work through this as one big team. I just make some network engineers laugh, or smirk, or other [1] - https://en.wikipedia.org/wiki/Ingress_filtering#Networks https://en.wikipedia.org/wiki/Ingress_filtering#Networks
- elaus 5y ago
  Curious as you speak of "very little action", yet the Wikipedia article you linked says: > […] contrary to general opinion about the lack of BCP 38 deployment, some 80% of the Internet (by various measures) were already applying anti-spoofing packet filtering I know very little about global routing, so I'm wondering if it's an ineffective way of filtering against these attacks we see today or is 80% not enough to be effective?
  - LinuxBender 5y ago
    It really isn't enough. Everyone needs to do their part [1] not just ISP's but also large businesses, cloud providers, others. And this has to occur in all regions. I admittedly over-simplified the problem statement. [1] - https://archive.nanog.org/sites/default/files/1_Kaeo_Ddos_Trends.pdf https://archive.nanog.org/sites/default/files/1_Kaeo_Ddos_Tr... [PDF]