Core files name control in Linux kernel 2.6

By default on many Linux distributions core files generation is disabled.
If you choose to enable it (because let's say you might have applications crashing and you want to see an image of the application's process memory at the time of termination) and control the name of the core file generated you need to edit /proc/sys/kernel/core_pattern. The default value in this file is "core", which means that core files will be dumped in the working directory and the filename is core. If you want to change this pattern and dump the core files in /tmp direcory and also append the process ID to the file name do the following (be aware that this is a system wide settting):

echo /tmp/core%p > /proc/sys/kernel/core_pattern

or

sysctl -w kernel.core_pattern=/tmp/core%p

If you choose to make the change persistent after reboot edit the /etc/sysctl.conf file and add:

Recovering err-disabled ports on Cisco Catalysts IOS platforms

By default, Catalyst switches detect errors that occur on switch ports.
If the errors are serious it can take the action to shut down the switch port until someone will manually enable it or until a
predefined period of time has elapsed. This happens in order to avoid for example unwanted or faulty connections on the switch port.

Serious errors causes include (but are not limited to):

bpduguard — Detects when a Spanning Tree bridge protocol data unit (BPDU) is received on a port configured for STP portfast
link-flap — Detects when the port link state is “flapping” between the up and down states
pagp-flap — Detects when an EtherChannel bundle’s ports no longer have consistent configurations
rootguard — Detects when an STP BPDU is received from the root bridge on an unexpected port

For a complete list of errdisable causes go here.
If you decide that some of the error are not serious enough, you can tune the switch to trigger a port being disabled only for some causes.
Type in global configuration mode:

Switch(config)# errdisable detect cause [all | cause-name]

To check for errdisabled ports run in global EXEC mode:

Switch# show interface status
..........
Port Name     Status   Vlan   Duplex   Speed   Type
Fa0/1     err-disabled   5    full     100   100BaseTX
..........

NAT & IP forwarding on Linux gateway

Suppose we have only one publicly routable IP address assigned by our ISP and we want to be able to connect from the computers located in our internal LAN to the internet. Using private IP addresses is a common way to access the internet and internal shared resources
For the ease of explanation/understanding we’ll add some details in our scenario.

eth0 – the network interface card (NIC) connected to the ISP net
eth1 – the NIC connected to the internal LAN

As for the gateway there are some basic requirements:
- we’ll need at least 2 network interface cards (one/more connected to the internal LAN switch/hub, one/more connected to your ISP provider net) supported by your kernel
- support for networking, iptables and NAT in the kernel (for default 2.6/ 2.4 kernels on major Linux distributions this is enabled by default)
- enable IP forwarding (disabled by default on modern Linux distribution). To enable IP forwarding there are several ways to accomplish this. The common accepted method is through sysctl

Run the following command as root:

sysctl -w net.ipv4.ip_forward = 1

To make the change permanent we can add the following line in /etc/sysctl.conf

net.ipv4.ip_forward = 1

To enable the change made to the /etc/sysctl.conf file run

sysctl -p /etc/sysctl.conf

Finally, to allow hosts connected in the internal LAN to access internet resources configure the Linux gateway as:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Duplex mismatches 100BASE-TX

I encountered a situation a few days ago where the two devices were not correctly negotiating the duplex setting. One box was a Cisco 2950 switch, the other was a Linux machine.
Although the ehternet interfaces of the 2 boxes were both capable of 100BASE-TX (full duplex) it was clear that the Linux machine's eth1 was running in half duplex mode
A tool on Linux which can display/change an ethernet card setting is ethtool.
Now for a bit of theory:

The link speed is determined by electrical signaling, so that either end of a link can determine what
the other end is trying to use. If both ends of the link are configured to autonegotiate, they will use
the highest speed that is common to them.
A link’s duplex mode, however, is negotiated through an exchange of information. This means that
for one end to successfully autonegotiate the duplex mode, the other end must also be set to autonegotiate.
Otherwise, one end will never see any duplex information from the other end and won’t
determine the correct common mode.