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Posts in Networking
Using netstat and dropwatch to observe packet loss on Linux servers
Anyone that is running a modern Operating System is most likely utilizing TCP/IP to send and receive data. Modern TCP/IP stacks are somewhat complex and have a slew of tunables to control their behavior. The choice of when and when not to tune is not always super clear cut, since documentation and the advice of various network "experts" doesn't always jive. When I'm looking into performance problems that are network related one of the first things I review is the netstat "-s" output: The netstat output contains a slew of data you can be used to see how much data your host is processing, if it's accepting and processing data efficiently and if the buffers that link the various layers (Ethernet -> IP -> TCP -> APP) are working optimally…
$ read more →Configuring NSCD to cache DNS host lookups
I haven't really spent that much time configuring nscd, so I thought I would take a crack at it this morning while sipping my cup of joe. Looking at one of my production hosts, I queried for the "host" cache statistics. This is the nscd cache which keeps DNS lookups. With the nscd daemon running, you can query the size / performance of the caches with the -g flag…
$ read more →Using the Linux arping utility to send out gratuitious ARPs
I managed a number of Redhat and Heartbeat clusters. On a couple of occassions the services that manage the virtual IPs have misbehaved, and the storage has ended up on one node and the virtual IP on another. To fix this I need to manually move the virtual IP to the host it belongs on, and then issue a gratuitous ARP so other hosts on the network clear their ARP cache and use the MAC address associated with the device the virtual IP now resides on. The Linux arping utility can be used to send out a gratuitious ARP (an "ARP Request" or "ARP Response" is the actual item sent) to update hosts on your network …
$ read more →Stopping your RHEL virtual interfaces from starting at boot. ONPARENT you say?
I recently debugged a pretty interesting problem with one of my clusters. When I rebooted one of the nodes, I noticed that a virtual interface that had ONBOOT set to no was started when the network interfaces were initialized. For those not familiar with RHEL systems, the ONBOOT directive tells the network initialization scripts not to start a given interface. This was rather confusing, and after some experimenting with a virtual machine I saw the EXACT same behavior…
$ read more →A great presentation on IPv6
I was fortunate enough to attend Michael Warfield's talk on DNS security this past Monday. If you don't know Mike, he is a crazy smart dude who gives spectacular presentations. Can't recommend him enough! Mike has been at the forefront of the IPv6 movement, and has given a number of IPv6 presentations in the past few months…
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