HOWTO :: Increase size of ARP tables (linux)

If your network card gives you "No Buffer Space Available" like the following

Code:

# ping www.google.com.sg
connect: No buffer space available

Do a tail of the logs and you should see the following

Code:

# tail -f /var/log/messages
Apr 14 03:02:11 svrXXX kernel: __ratelimit: 195 callbacks suppressed
Apr 14 03:02:11 svrXXX kernel: Neighbour table overflow.
Apr 14 03:02:12 svrXXX kernel: Neighbour table overflow.
Apr 14 03:02:12 svrXXX kernel: Neighbour table overflow.
Apr 14 03:02:12 svrXXX kernel: Neighbour table overflow.

In this case, it might be advisable to increase the size of your ARP tables.

First, let's see what values have been set on our system

Code:

# cat /proc/sys/net/ipv4/neigh/default/gc_thresh1
          128
# cat /proc/sys/net/ipv4/neigh/default/gc_thresh2
          512
# cat /proc/sys/net/ipv4/neigh/default/gc_thresh3
          1024

Next, increase these values

Code:

# echo 1024 > /proc/sys/net/ipv4/neigh/default/gc_thresh1
# echo 4096 > /proc/sys/net/ipv4/neigh/default/gc_thresh2
# echo 8192 > /proc/sys/net/ipv4/neigh/default/gc_thresh3

Restart the network interface

Code:

# ifdown eth0
# ifup eth0

Test it out. Your problem should have been solved.

Do note that these settings are NOT PERSISTENT. To make these settings persistent, please run the following for a RHEL/CENTOS machine

Code:

# echo "net.ipv4.neigh.default.gc_thresh1=1024" >> /etc/sysctl.conf
# echo "net.ipv4.neigh.default.gc_thresh2=4096" >> /etc/sysctl.conf
# echo "net.ipv4.neigh.default.gc_thresh3=8192" >> /etc/sysctl.conf
# sysctl -p

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