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List of sysctl tunables

This list is not complete and many of these settings might not always be appropriate, however I will start going through these at a later time to explain what they do.

echo 'net.core.wmem_max=12582912' >> /etc/sysctl.conf
echo 'net.core.rmem_max=12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_rmem= 10240 87380 12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_wmem= 10240 87380 12582912' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_sack = 1' >> /etc/sysctl.conf
echo 'net.core.netdev_max_backlog = 5000' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_tw_reuse=1' >> /etc/sysctl.conf
echo 'net.ipv4.tcp_tw_recycle=1' >> /etc/sysctl.conf
echo 'net.ipv4.ip_local_port_range=1024 65535' >> /etc/sysctl.conf
sysctl -p

Virtual IP Address

You can add several virtual IP addresses to a physical network interface. A good starting point is to copy the interface definition file of the device you want to add a virtual IP address to.

cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0:0 
  • You may have to modify ifcfg-eth0 to match your physical interface name.
  • Now you can edit the new file ifcfg-eth0:0 and specify the network settings of the virtual interface.
  • If you want the virtual IP address come up on boot, you need to remove

and make it


This makes the interface only come up when the parent interface comes up, while ONBOOT=YES would pull up the parent interface even if that is configured to not come up on boot.

  • To make the changes take effect please restart the network services using:
service network restart


Find the current limit

cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max

Find the current amount of connections

cat /proc/sys/net/ipv4/netfilter/ip_conntrack_count

Raise this limit

sysctl net.ipv4.netfilter.ip_conntrack_max net.ipv4.netfilter.ip_conntrack_max =


Get network statistics


New server missing some IPs it was supposed to come with?

 service ipaliases restart

Networking commands

Show route table:

route -n

Ifconfig commands:

ifconfig ethX ip
ifconfig ethx netmask (netmask)
ifconfig ethx broadcast (broadcast)

Ubuntu Networking

Where all the nics are actually configured:

vim /etc/udev/rules.d/70-persistent-net.rules

Bonding info

cat /proc/net/bonding/bond0

Interface config

vim /etc/network/interfaces

IP Forwarding

To enable IP packet forwarding please edit /etc/sysctl.conf with your editor of choice and set:

# Controls IP packet forwarding
net.ipv4.ip_forward = 1

You can then verify your settings with:

/sbin/sysctl -p