My configuration is apache and tomcat behind an aws elb. Apache is configured with no keepalive, and set with max clients to a low number due to each query being very cpu intensive. I'll load test the machine with queries. Then the number of available requests goes to zero as can be seen by curl -s localhost/server-status?auto not responding immediately. When I stop the loadtest I can see the scoreboard from curl -s localhost/server-status?auto still is full of R's even though from the tomcat logs it is clear nothing is happening. Does anyone have an idea on what possible causes there might be?
If your apache display 'R' in the status it means there is open TCP connections from ELB to apache (just an open TCP connection, no data sent yet).
There is no official complete documentation on this subject (how the numbers of pre-opened connections is optimized ), but the amazon documentation state (at this page: https://docs.aws.amazon.com/elasticloadbalancing/latest/userguide/how-elastic-load-balancing-works.html ) that:
Classic Load Balancers use pre-open connections but Application Load Balancers do not.
So, the answer is: it is an optimization from amazon (TCP connection cost a little bit to be openned).
Related
Assuming the following setup:
Apache server 2.4
mpm_prefork with default settings (256 workers?)
Default Timeout (300s)
High KeepAliveTimeout (100s)
reqtimeout_mod enabled with the following config: RequestReadTimeout header=62,MinRate=500 body=62,MinRate=500
Outdated mod_wsgi 3.5 using Daemon mode with 15 threads and 1 process
AWS ElasticBeanstalk's load balancer acting as a reverse proxy to apache with 60s idle connection timeout
Python/Django being the wsgi application
A simple slowloris attack like the one described here, using a "slow" request body: https://www.blackmoreops.com/2015/06/07/attack-website-using-slowhttptest-in-kali-linux/
The above attack, with just 15 requests (same as mod_wsgi threads) can easily lock the server until a timeout happens, either due to:
Load balancer timeout (60s) happens due to no data sent, this kills the apache connection and mod_wsgi can once again serve requests
Apache RequestReadTimeout happens due to data being sent, but not enough, again mod_wsgi is able to serve requests after this
However, with just 15 concurrent "slow" requests, I was able to lock the server up to 60 seconds.
Repeating the same but with a more bizarre number, like 4096 requests, pretty much locks the server permanently since there will be always a new request that needs to be served by mod_wsgi once the previous times out.
I would expect that the load balancer should handle/detect this before even sending requests to apache, which it already does for similar attacks (partial headers, or tcp syn flood attacks never hit apache which is nice)
What options are available to help against this? I know there's no failproof option since these kind of attacks are difficult to detect and protect, but it's quite silly that the server can be locked that easily.
Also, if the wsgi application never reads request body, I would expect for the issue to not happen as well since the request should return immediately, but I'm not sure about this or the internals of mod_wsgi, for example, this is true when using a local dev wsgi server (the attack files since the request body is never read) but the attack succeeds when using mod_wsgi, which leads me to think it tries to read the body even before sending it to the wsgi code.
Slowloris is a very simple Denial-of-Service attack. This is easy to detect and block.
Detecting and preventing DoS and DDos attacks are complex topics with many solutions. In your case you are making the situation worse by using outdated software and picking a low worker thread count so that the problem arises quickly.
A combination of services are available that would be used to manage Dos and DDos attacks.
The front-end of the total system would be protected by a firewall. Typically this firewall would include a Web Application Firewall to understand the nuances of HTTP protocols. In the AWS world, Amazon WAF and Shield are commonly used.
Another service that helps is a CDN. Amazon CloudFront uses Amazon Shield so it has good DDoS support.
The next step is to combine load balancers with auto scaling mechanisms. When the health checks start to fail (caused by Slowloris), the auto scaler will begin launching new instances and terminating failed instances. However, a sustained Slowloris attack will just hit the new servers. This is why the Web Application Firewall needs to detect the attack and start blocking it.
For your studies, take a look at mod_reqtimeout. This is an effective and tuneable solution for Apache for most Slowloris attacks.
[Update]
In the Amazon DDoS White Paper June 2015, Slowloris is specifically mentioned.
On AWS, you can use Amazon CloudFront and AWS WAF to defend your
application against these attacks. Amazon CloudFront allows you to
cache static content and serve it from AWS Edge Locations that can
help reduce the load on your origin. Additionally, Amazon CloudFront
can automatically close connections from slow-reading or slow-writing
attackers (e.g., Slowloris).
Amazon DDoS White Paper June 2015
In mod_wsgi daemon mode there are a bunch of options to further help to combat such attacks by recovering from it and discarding queued requests as well which have been waiting too long. Try your tests using mod_wsgi-express as it defines defaults for a lot of these options whereas when using mod_wsgi yourself directly, there are no defaults. Use mod_wsgi-express start-server --help to see what defaults are. The actual options you want to look at for mod_wsgi daemon mode are request-timeout, connect-timeout, socket-timeout and queue-timeout. There are also other options related to buffer sizes and listener backlog you can play with. Do note that ultimately the listen backlog of the main Apache worker processes can still be an issue because it usually defaults to 500, which means a lot of requests can queue up stuck before you can even tag them with a time so as to help discard the backlog by tracking queue time.
You can find the documentation at:
http://modwsgi.readthedocs.io/en/develop/configuration-directives/WSGIDaemonProcess.html
On the point of whether mod_wsgi reads the request body before sending it, no it doesn't. Apache itself because it reads in block may partially read the request body when reading the headers, but it shouldn't block on it. Once the full request headers are passed off to mod_wsgi and sent through to the daemon process, then mod_wsgi will start transferring the request body.
Soloution:
If you are getting hit, I recommend you go to a provider that protects against DDoS attacks. However your best bet would be to programatically block the IP once it has been decided that it is being malicious. If you receive two large Content-Length POST requests than you should block the IP for a few minutes for suspicious activities. Many large companies are very cheap, and some of them are free for the basic package such as Cloud Flare. I use them for my company and I am beyond happy to have them!
Edit: Their job is literally just to protect you. That is it.
We are running multiple Tomcat JVMs under a single Apache cluster. If we shut down all the JVMs except one, sometime we get 503s. If we increase the
retry interval to 180(from retry=10), problem goes away. That bring me
to this question, how does Apache detects a stopped Tomcat JVM? If I
have a cluster which contains multiple JVMs and some of them are down,
how Apache finds that one out? Somewhere I read, Apache uses a real
request to determine health of a back end JVM. In that case, will that
request failed(with 5xx) if JVM is stopped? Why higher retry value is
making the difference? Do you think introducing ping might help?
If someone can explain a bit or point me to some doc, that would be awesome.
We are using Apache 2.4.10, mod_proxy, byrequests LB algorithm, sticky session,
keepalive is on and ttl=300 for all balancer members.
Thanks!
Well let's examine a little what your configuration is actually doing in action and then move to what might help.
[docs]
retry - Here either you 've set it 10 or 180 what you specify is how much time apache will consider your backend server down and thus won't send him requests. So the higher the value, you gain the time for your backend to get up completely but you put more load to the others since you are -1 server for more time.
stickysession - Here if you lose a backend server for whatever reason all the sessions are on it get an error.
All right now that we described the relevant variables for your situation let's clear that apache mod_proxy does not have a health check mechanism embedded, it updates the status of your backend based on responses on real requests.
So your current configuration works as following:
Request arrives on apache
Apache send it to an alive backend
If request gets an error http code for response or doesn't get a response at all, apache puts that backend in ERROR state.
After retry time is passed apache sends to that backend server requests again.
So reading the above you understand that the first request that will reach a backend server which is down will get an error page.
One of the things you can do is indeed ping, according to the docs will check the backend before send any request. Consider of course the overhead that produces.
Although I would suggest you to configure mod_proxy_ajp which is offering extra functionality (and configuration ofc) to your tomcat backend failover detection.
We have a legacy cluster of servers running Apache 2.4 that run our application sitting behind an ELB. This ELB has two listeners, one HTTP, and one HTTPS which terminates at the ELB and sends regular HTTP traffic to the instances behind it. This ELB also has pre-open turned off (it was causing a busy worker buildup). Under normal load we have 1-3 busy workers per instance.
We have a new cluster of servers we are trying to migrate to behind a new ELB. The purpose of this migration is to allow for SNI – serving TLS traffic to thousands of domains. As such this cluster uses mod_proxy_protocol which has been enabled at the ELB level. For the purposes of testing we’ve been weighting traffic at the DNS (Route 53) level to send 30% of our traffic to the new load balancer. Under even this small load we see 5 – 10 busy workers and that grows as traffic does.
As a further test we took one of these new instances, disabled proxy_protocol, and moved it from the new ELB to the old ELB, the worker count drops to average levels, being 1-3 busy workers. This seems to indicate that there is an issue wither with the ELB (differences between HTTP and TCP handling?) or mod_proxy_protocol.
My question: Why is it that we have twice the busy apache workers when using proxy protocol and the new ELB? I would think that since TCP listeners are dumb and don’t do any processing on the traffic, they would be faster and as a result consume less workers time than HTTP listeners which actively ‘modify’ the traffic going thru them.
Any guidance to help us diagnose this issue is appreciated.
The difference is simple and significant:
An ELB in HTTP mode takes care of holding the idle keep-alive connections from browsers without holding open corresponding connections to the instance. There's no necessary correlation between browser connections and back-end connections -- a backend connection can be reused.
In TCP mode, it's 1:1. It has to be, because the ELB can't reuse a back-end connection for different browser connection on the front-end -- it's not interpreting what's going down the pipe. That's always true for TCP, but if the reason isn't intuitive, it should be particularly obvious with the proxy protocol enabled. The PROXY "header" is not in fact a "header" in the usual sense -- it's a preamble. It can only be sent at the very beginning of a connection, identifying the source address and port. The connection persists until the browser or server closes it, or it times out. It's 1:1.
This is not likely to be viable with Apache.
Back to HTTP mode, for a minute.
This ELB also has pre-open turned off (it was causing a busy worker buildup).
I don't know how you did that -- I've never seen it documented, so I assume this must have been through a support request.
This seems like a case of solving entirely the wrong problem. Instead of having a number of connections that seems to you to be artificially high, all you've really accomplished is keeping the number of connections artificially low -- ultimately, you're probably actually impairing your performance and ability to scale. Those spare connections are for the purpose of handling bursts of demand. If your instance is too small to handle them, then I would suggest that the real problem is just that: your instance is too small.
Another approach -- which is exactly the solution I use for my dreaded legacy Apache-based applications (one of which has a single Apache server sitting behind a total of about 15 to 20 different ELBs -- necessary because each ELB is offloading SSL using a certificate provided by one of the old platform's customers) -- is HAProxy between the ELBs and Apache. HAProxy can handle literally hundreds of connections and millions of requests per day on tiny instances (I'm talking tiny -- t2.nano and t2.micro), and it has no problem keeping the connections alive from all of the ELBs yet closing the Apache connection after each request... so it's optimizing things in both directions.
And of course, you can also use HAProxy with a TCP balancer and the proxy protocol -- the author of HAProxy was also the creator of the proxy protocol standard. You can also just run it on the instances with Apache rather than on separate instances. It's lightweight in memory and CPU and doesn't fork. I'm not affilated with the project, other than having submitted occasional bug reports during the development of the Lua integration.
tl;dr:
When a google cloud HTTPS load balancer opens a tcp stream (with a "Connection: keep-alive" header in the request), are there any guarantees around how long (at max) that stream will be kept open to the backend server?
longer:
I deployed a Go http server behind an HTTPS load balancer and quickly ran into a lot of issues because I had set an aggressive (10s) read deadline on my socket connections, which meant that my server often closed connections in the middle of reading subsequent requests. So clearly I'm doing that wrong, but at the same time I don't want to not set ANY deadlines on my sockets, because I want to guard against the possibility of these servers slowly leaking dead connections over time, eating up all my file descriptors.
As such, it would be nice if, for example, the load balancers automatically close any tcp streams that they have open after 5 minutes. That way I can set my server's read deadline to (e.g.) 6 minutes and I can be sure that I'll never interrupt any requests - the deadline will only be invoked in exceptional cases (e.g. the FIN packet from the load balancer was not received by my server).
I was unable to get an official answer on this from Google enterprise support, but from my experiments (analyzing multi-hour tcpdumps) it looks like the load balancer will close connections after ~10 minutes of idleness (meaning no tcp data packets for 10 minutes).
Per here, idle TCP connections to Compute Instances are timed out after 10 minutes, which would seem to confirm your hypothesis.
As far as I researched, the scenario when all worker threads are busy serving requests, what happens to the requests that comes next.
Do they wait?
Is this related to some configurable parameters?
Can I get the count of such requests ?
Adding to this please can you explain or give a link where I can get a clear picture of request processing strategy of Apache webserver?
Thanks for Looking at!!
When all Apache worker threads are busy, the new request is stalled (it waits) until one of those worker threads is available. If the client gives up waiting, or you surpass the maximum wait time in your configuration file; it will drop the connection.
This answer is given in 2015. So I talk about apache httpd 2.4.
They wait because the connection is queued on the TCP socket (the connection is not ACKed) Although the default length of the backlog may be set way too high on linux boxes. This may result in connections being closed due to kernel limits being in place.
ListenBacklog (with caveats. See 1.)
This is described here. With lots of interesting stuff.
Read through Apache TCP Backlog by Ryan Frantz to get the glory details about the Apache backlog.