DDoS (Distributed Denial of Service Attacks) are generally blocked on a server level right?
Is there a way to block it on a PHP level, or at least reduce it?
If not, what is the fastest and most common way to stop DDoS attacks?
DDOS is a family of attacks which overwhelm key systems in the datacenter including:
Before you start on building your DDOS defence, consider what the worst-case value-at-risk is. For a non-critical, free-to-use service for a small community, the total value at risk might be peanuts. For a paid-for, public-facing, mission-critical system for an established multi-billion dollar business, the value might be the worth of the company. In this latter case, you shouldn't be using StackExchange :) Anyway, to defend against DDOS, you need a defence in-depth approach:
Keep all your systems and software packages updated with the latest security patches - and I mean all of them:
Ensure that you have a good firewall or security appliance set up and regularly reviewed by a qualified security expert. Strong rules on the firewall are a good defence against many simple attacks. It's also useful to be able to manage bandwidth available for each open service.
Have good network monitoring tools in place - this can help you understand:
The attack might simply be heavy use of legitimate web site services (eg hitting 'legal' URIs running queries or inserting/updating/deleting data) - thousands or millions of requests coming from tens to millions of different IP addresses will bring a site to its knees. Alternatively, some services might be so expensive to run that only a few requests cause a DOS - think a really expensive report. So you need good application level monitoring of what is going on:
Sensible constraints and limits in your application. For example, you might:
Last, but not least, write a DOS Response Plan document and get this internally reviewed by all relevant parties: Business, Management, the SW dev team, the IT team and a security expert. The process of writing the document will cause you and your team to think through the issues and help you to be prepared if the worst should happen at 3am on your day off. The document should cover (among other things):
So, preamble aside, here are some specific answers:
DDOS are generally blocked on a server level, right?
Not really - most of the worst DDOS attacks are low-level (at the IP packet level) and are handled by routing rules, firewalls, and security devices developed to handle DDOS attacks.
Is there a way to block it on a PHP level, or at least reduce it?
Some DDOS attacks are aimed at the application itself, sending valid URIs and HTTP requests. When the rate of requests goes up, your server(s) begin to struggle and you will have an SLA outage. In this case, there are things you can do at the PHP level:
Application level monitoring: Ensure each service/page logs requests in a way that you can see what is going on (so you can take actions to mitigate the attack). Some ideas:
Have a log format that you can easily load into a log tool (or Excel or similar), and parse with command-line tools (grep, sed, awk). Remember that a DDOS will generate millions of lines of log. You will likely need to slice'n'dice your logs (especially with respect to URI, time, IP and user) to work out what is going on, and need to generate data such as:
Log the IP address of each request. DON'T reverse DNS this - ironically the cost of doing this makes a DDOS easier for the attackers
Sensible rate limits: You might implement limits on how many requests a given IP or User can make in a given time period. Could a legitimate customer make more than 10 requests per second? Can anonymous users access expensive reports at all?
CAPTCHA for anonymous access: Implement a CAPTCHA for all anonymous requests to verify that the user is a person, not a DDOS bot.
What's the fastest and most common way to stop DDOS attacks?
The fastest is probably to give in to the blackmail, although this might not be desirable.
Otherwise, the first thing you to do is contact your hosting and/or CDN provider and work with them (if they haven't contacted you already asking what the hell is going on...). When a DDOS occurs, it will likely collaterally affect other customers of the hosting provider, and the provider may be under considerable pressure to shut down your site simply to protect their resources. Be prepared to share your logs (any and all information) with the provider; these logs, combined with their network monitors, may together provide enough information to block/mitigate the attack.
If you are expecting a DDOS, it's a very good idea to qualify your hosting provider on the level of protection they can provide. They should have DDOS experience and tools to mitigate it - understand their tools, processes and escalation procedures. Also ask about what support the hosting provider has from their upstream providers. These services might mean more up-front or monthly cost, but treat this as an insurance policy.
While under attack, you will need to grab your logs and mine them - try and work out the pattern of the attack. You should consider switching off anonymous access and throttling the services under attack (i.e. decrease the application's rate limit for the service).
If lucky and you have a small, fixed customer-base, you might be able to determine your valid customers IP addresses. If this is the case, you might switch to a white-list approach for a short while. Make sure all your customers know this is going on so they can call if they need to access from a new IP :)
Doug McClean has some great advice at: https://stackoverflow.com/a/1029613/1395668