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Fixing ip_conntrack Bottlenecks: The Tale Of The DNS Server With Many Tiny Connections

I manage a server which has a sole purpose: serving DNS requests. We use PowerDNS, which has been great. It is a DNS server whose backend is SQL, making administration of large numbers of records very easy. It is also fast, easy to use, open source and did I mention it is free?

The server has been humming along for years now. The traffic graphs don’t show a lot of data moving through it because it only serves DNS requests (plus MySQL replication) in the form of tiny UDP packets.

Read on to follow my story of how I fixed this tricky problem. No kittens were harmed in the writing of this post.


It is very easy to create a random file using the linux command line. Much like the command to fill a file with all zeros, for example a 1 Meg file: dd if=/dev/zero of=zero.filename bs=1024 count=1000 You do the same using /dev/urandom: dd if=/dev/urandom of=random.filename bs=1024 count=1000 Resulting in a 1MB file: 1000+0 records in […]


We recently did some speed testing on a few of the servers on our network, and we were not receiving the speeds expected considering they were sitting on a physical 100mbps ethernet port. The servers were indeed on physical 100mbps connection, however wget (TCP/IP, HTTP Port 80) download tests showed only a max of about 1.5MB/sec (note the 8bit/byte conversion, so this translates to about 12mbits).


Did you ever have a situation where you needed to access a website that had an IP restriction in place? I recently had a situation where I needed to access the web via my university connection (due to IP restrictions placed on accessing databases of research papers). They do not have a VPN setup so […]



I recently received this error while mounting an iso image in a CentOS 5.3 install: [root@host ~]# mount -t iso9660 -o loop /mnt/glusterfs/ecp-spe-4867.iso /mnt/iso/ ioctl: LOOP_SET_FD: Invalid argument For more detail: [root@host ~]# mount -v -t iso9660 -o loop /mnt/glusterfs/ecp-spe-4867.iso /mnt/iso/ mount: going to use the loop device /dev/loop0 ioctl: LOOP_SET_FD: Invalid argument mount: failed […]




In case you were wondering, I just invented the phrase PVPN – Personal Virtual Private Network. I use Hamachi to connect my work, home and laptop PCs and I’ve found it invaluable over the past few years for a number of reasons. Music over VPN, Remote Desktop/VNC over VPN, and more. So now, I’d like […]



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