AWS net.ipv4.tcp_tw_recycle follow-up

Yesterday I wrote a post on AWS EC2 instance networking problem that I was pretty surprised to find out. And while yesterday I was focusing on fixing the problem, today my first task was to find out what actually sets the flag, and quick grep on /etc of the instance revealed that the settings were applied by /etc/sysctl.d/net.ipv4.tcp_tw_recycle.

Very strange to find it there along with net.ipv4.tcp_tw_reuse which is also something that you should not touch. Anyhow, the problem identified, fixed and is about to be added to monitoring…

Amazon AWS, WTF?

Spend the whole day troubleshooting a problem of some pretty random, but stable tcp connection timeouts to one of the Amazon AWS EC2 instance. The problem was that some PCs/laptops/servers would face long term connection timeout to the instance, while others were working fine. The ones with timeouts would experience problems only on TCP level, while ICMP ping would pass normally. The other strange thing is that rebooting client to different kernel would fix the problem for that particular client for a while.

After checking and googling with no luck and getting completely pissed off, I gave the problem another thought and this time I felt that something is wrong with AWS NATting. That clearly brought the memories of troubleshooting TCP fine tuning. So I checked the article, found out the values to make sure are present and went to check the actual instance. Quick look into /proc/sys/net/ipv4/tcp_tw_recycle revealed the problem with its value being 1, so changing it back to 0 with cat to apply immediately fixed the connectivity issues, but then, when I looked into /etc/sysctl.conf, I saw that the value there was already 0!!! How come is it possible if we didn’t change it manually via proc, nor have we touched sysctl.conf for ages and the last server reboot was only few days ago done by Amazon due to their planned maintenance?


Amazon AWS Subnet Custom Gateway

While Amazon provides different ways to route traffic within and out of your subnets by means of internet gateways and NAT gateways, it’s not always the case that you it will suite your needs. If you want full control with lots of possibilities for customisation, you might consider building your own firewall instance and push all traffic via it.

Amazon provides NAT instances, but they also have some limitations, so to gain full features, its is possible to build a custom EC2 instance with whatever AMI and settings you like, attach two network interfaces to it where one is in a private and one in a public subnet and do classic iptables NAT on it.

For all instances in the private subnet to be routed via your custom firewall, you need to adjust the routing table for that subnet and point default route to the network interface of the firewall instance that is in that same private subnet.

All of the above works pretty good, but looks a bit weird: lets assume the following:

  • We have a VPC
  • We have a private subnet within our VPC
  • We have a public subnet within our VPC
  • We have a firewall instance with:
    • public subnet IP:
    • private subnet IP:
  • We have a host in private subnet with IP

Now, the first question is why we don’t use on firewall instance in a private subnet? Easy: it is used by amazon gateway and even though we have created routing table to through all to, on an actual host in a private subnet, the routing table will be:

default via dev eth0 dev eth0 proto kernel scope link src 

This means that host will send traffic to AWS gateway, and that one will pass it over to our firewall. Probably the idea behind such configuration is that AWS still needs to check security groups and so on, before it handles traffic to us.

Cool, and this work fine, but there is a small issue: if you will try to ping or access any services at firewalls public IP ( from the host in your private subnet – you will fail! Moreover, if you fire up tcpdump on a firewall server listening for any packets from host in private subnet via interface of private subnet ( and try to ping from the private host – you will see completely nothing related to this. Nor you will see any other activity from your private host towards public IP address of your firewall.

Wanna go even more weird? If you will try to access any other hosts in the public subnet from your private host (for a sake of example assume you have another host with IP and you try to ping it from this will work as expected and traffic will flow via firewall as configured.

Not sure why and how, but Amazon seems to block access on from any host in network to, because that IP belongs to a firewall that is a default gateway for any host on (even though the IP is another subnet).

The solution for this problem (if that’s a problem for your case) is either to put a direct route to via on the private host to make sure private host avoids using amazon for this route:

default via dev eth0 via dev eth0 dev eth0 proto kernel scope link src

or even completely ignore and set default gateway via

default via dev eth0 dev eth0 proto kernel scope link src

In either way you won’t be able to do it via AWS routing tables, but will have to configure routing right on the private host via ip route tool or route-ifcfgN file for persistence.

Keep in mind that if you will divert all traffic via, you might lose some security group checks within a subnet, so make sure to implement whatever security you need on the actual firewall.