chmod 755

Running a bastion host/jumpbox on Fargate

February 07, 2020

Where I work we strive to keep long-term maintenance burden as small as possible. We’re a small team that sells SaaS to enterprise-grade companies. Some of these very large brand names only sign a SaaS contract with us after extensive data security questionnaire and vetting process. We learned that it not only increase our chances, but also make it easier to answer these questions with “It’s AWS responsibility”.

Bastion / Jumpbox

When using AWS VPC it’s common to have your RDS, among other resources, behind a private subnet with a NAT Gateway to the internet. That means the outside world, our office included, cannot initiate a request to these resources. A bastion host tries to mitigate this issue by creating a point of entry inside AWS VPC with a public IP. We can then connect to the RDS by tunneling all of the network communication through the bastion.

A bastion host is a machine running an OpenSSH service that can act as a tunnel between your requests and the desired resource. For instance, a service such as Elasticache or RDS running inside a VPC with a network address of 10.0.0.8 can only be accessed by compute resources running inside the VPC itself. The bastion will receive all of our requests that should be forwarded to Elasticache or RDS and deliver us the response back.

Why Fargate?

With the increasing popularity of serverless and the context in which I work, having an EC2 running on our AWS account means we need to patch the operating system for security as well as provide legal documents as proof that we have this process in place. It costs more money to handle these processes than the small fee that Fargate incur above EC2 pricing. By using Fargate and getting rid of all of our EC2, our security footprint is reduced to Docker images and the actual software running in the machine. It also helps the sales pitch because these large enterprises asking the questions know how AWS works and it helps to establish trust.

What about drawbacks?

I’ve been working in the software industry long enough to know that no solution comes without it’s downsides. Currently there is no way to assign an ENI to a Fargate instance, nor I think it would be possible. Fargate is a service designed for scaling containers and we could not have 2 containers taking the same IP address. Besides that, if the container ever goes down and ECS starts a new one, it will have a new IP address. To mitigate that problem, we would like to have a DNS associated with the container so that we don’t have to worry about container fluctuation. But that requirement bring it’s own limitations: Network Load Balancer.

We already pay for an Application Load Balancer, but it cannot handle TCP connections that are not HTTP/HTTPS. We would need to pay for an additional Network Load Balancer just so that we can have a DNS associated with the NLB so that the NLB can establish a connection with the Fargate container for us. This seemed rather wasteful as we’d be using 0.01% of an NLB without any other use case for it.

After almost giving up on this, I had a nice little idea to try and overcome this limitation. We can have a sidecar on ECS that updates a Route 53 entry everytime a new container comes up. This would allow us to have a reliable-ish DNS without the need for a NLB.

The implementation

Alpine Linux has become a big part of our organization. After running security scanners on standard Docker images (PHP, Python, NodeJS, etc), we noticed that most of them comes with vulnerability out of the box. By designing our own Alpine images, we keep things as slim down as possible that can pass security scanners in it’s sleep.

FROM alpine:3.11

RUN apk add openssh

RUN ssh-keygen -f /etc/ssh/ssh_host_rsa_key -N '' -t rsa

RUN echo "root:$(head /dev/urandom | tr -dc A-Za-z0-9 | head -c 36 ; echo '')" | chpasswd

COPY authorized_keys /root/.ssh/authorized_keys

COPY sshd_config /etc/ssh/sshd_config

RUN chmod 0700 ~/.ssh \
&&  chmod 0600 ~/.ssh/authorized_keys

EXPOSE 22

CMD ["/usr/sbin/sshd", "-D"]

This is a very small and simple Docker image that will run the OpenSSH Daemon service on port 22. We generate a key and put a random password on root so that OpenSSH can properly work. The important aspect is the authorized_keys that will have public keys allowed to tunnel through this container. For the SSHD configuration we can strengthen a few things.

PasswordAuthentication no
PermitEmptyPasswords no

Match User root
  AllowTcpForwarding yes
  X11Forwarding no
  AllowAgentForwarding no
  ForceCommand /bin/false

These settings will configure the service to only allow private key authentication and will disable terminal access altogether. It can only be used as a tunnelling mechanism for other resources inside the VPC.

Finally, the cherry of the cake is the sidecar.

FROM alpine:3.11

RUN apk add --no-cache curl jq python groff less py-pip \
&&  pip --no-cache-dir install awscli \
&&  apk del py-pip

COPY dns.json /root/dns.json

COPY init.sh /root/init.sh

RUN chmod +x /root/init.sh

WORKDIR /root

CMD ["/root/init.sh"]
#!/bin/sh

sed -i "s/!NEW_IP!/$(curl ipinfo.io | jq $data.ip)/g" /root/dns.json \
&& sed -i "s/!REGION!/${REGION}/g" /root/dns.json \
&& aws route53 change-resource-record-sets --hosted-zone-id ${HOSTED_ZONE} --change-batch file:///root/dns.json
{
  "Comment": "Update Route 53 Bastion DNS",
  "Changes": [{
    "Action": "UPSERT",
    "ResourceRecordSet": {
      "Name": "my-service-name.!REGION!.mydomain.com",
      "Type": "A",
      "TTL": 600,
      "ResourceRecords": [{ "Value": !NEW_IP!}]
    }}]
}

These 3 files configure all that’s necessary for the sidecar to work. We have !REGION variable because we run the same setup in multiple AWS regions and we use curl to grab our own ip address on start-up. The AWS CLI allows for updating Route 53.

It’s important to note that the lack of an NLB makes it so that we cannot scale this solution. However, given the fact that this is about a jumpbox used by developers to access VPC-related AWS resources, we do not need to scale this service. 1 container per region is more than enough. One may also argue that when the container is being replaced, DNS caching might become an issue, but the truth is we expect to replace this container once every 6 months with the release of a new Alpine Linux and if it crashes, we can get back to it soon enough.

Conclusion

I strive to keep things as simple as possible while providing security, stability and costs optimized. It’s very likely that my company would be willing to pay for the NLB anyway to improve our security footprint, but I wanted to provide a simple and robust solution for the problem without any wasteful cost.

AWS ECS has integration with Route 53 for Service Discovery, but that is limited to private IP addresses only. Until they provide the same for public IP, this solution will do just fine. It also doesn’t hurt to have an extremely slim down version of a jumpbox available for tunneling only.

Hope you enjoyed the reading. If you have any questions, send them my way on Twitter.

Cheers.


Marco Aurélio Deleu

Marco Aurélio Deleu
Writing bad code for 10 years. Passionate about Laravel and AWS.

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