Neighbour table overflow linux
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Linux: Neighbour Table Overflow Error and Solution
I setup a CentOS Linux based Linux server running as a gateway and firewall server. However, I’m getting the following messages in the /var/log/messages log file:
Dec 20 00:41:01 fw01 kernel: Neighbour table overflow.
Dec 20 00:41:01 fw01 last message repeated 20 times
Dec 20 00:41:01 fw03 kernel: [ 8987.821184] Neighbour table overflow.
Dec 20 00:41:01 fw03 kernel: [ 8987.860465] printk: 100 messages suppressed.
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Why does kernel throw “Neighbour table overflow” messages in syslog? How do I fix this problem under Debian / CentOS / RHEL / Fedora / Ubuntu Linux?
For busy networks (or gateway / firewall Linux server) it is mandatory to increase the kernel’s internal ARP cache size. The following kernel variables are used:
To see current values, type:
# sysctl net.ipv4.neigh.default.gc_thresh1
Sample outputs:
Type the following command:
# sysctl net.ipv4.neigh.default.gc_thresh2
Sample outputs:
Type the following command:
# sysctl net.ipv4.neigh.default.gc_thresh3
Sample outputs:
So you need to make sure that the arp table to become bigger than the above defaults. The above limitations are good for small network or a single server. This will also affect your DNS traffic.
How Do I Fix “Neighbour Table Overflow” Error?
Edit /etc/sysctl.conf file, enter:
# vi /etc/sysctl.conf
Append the following values (this is taken from server that protects over 200 desktops running MS-Windows, Linux, and Apple OS X):
To load new changes type the following command:
# sysctl -p
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That’s great, but WHY? What are the three different levels of thresholds, when does each one become relevant? Are there any adverse affects to increasing these? Why wouldn’t I want to set these extremely high? Why are they set to what they are in the first place?
I just found this while googling hope it helps.
These 3 parameters are defined in the Linux Kernel Code in the header file “/include/net/neighbour.h” as integer, which suggests that maximal accepted value is (232 – 1).
Gaia Portal accepts maximal value of 16384.
gc_thresh1
The minimum number of entries to keep in the ARP cache.
The garbage collector will not run if there are fewer than this number of entries in the cache.
gc_thresh2
The soft maximum number of entries to keep in the ARP cache.
The garbage collector will allow the number of entries to exceed this for 5 seconds before collection will be performed.
The hard maximum number of entries to keep in the ARP cache.
The garbage collector will always run if there are more than this number of entries in the cache.
In order for the garbage collector to work properly, and not to overload the machine with garbage collections, when changing the ‘gc_thresh3’ parameter, user should (note: does not have to) change the ‘gc_thresh2’ and ‘gc_thresh1’ parameters accordingly.
Thanks a TON! Quick fix to an issue I was having on Debian 7.
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If you have a big network with the hundreds of hosts you can expect “Neighbour table overflow” error which occurs in large networks when there are two many ARP requests which the server is not able to reply. For example you’re using server as a DHCP server, cable modems provisioning, etc.
Nov 10 03:18:17 myhost Neighbour table overflow. Nov 10 03:18:23 myhost printk: 12 messages suppressed.
Of curse, this can be fixed. The solution is to increase the threshhold values in /etc/sysctl.conf. Add following lines to /etc/sysctl.conf (RH based distros)
net.ipv4.neigh.default.gc_thresh1 = 4096
net.ipv4.neigh.default.gc_thresh2 = 8192
net.ipv4.neigh.default.gc_thresh3 = 8192
net.ipv4.neigh.default.base_reachable_time = 86400
net.ipv4.neigh.default.gc_stale_time = 86400
Save sysctl.conf and exec sysctl -p. You can also reboot but it isn’t necessary.
The default sysctl.conf file
net.ipv4.ip_forward=0 kernel.shmmax=68719476736 kernel.msgmax=65536 kernel.msgmnb=65536 net.ipv4.conf.default.rp_filter=1 kernel.sysrq=0 net.ipv4.conf.default.accept_source_route=0 kernel.shmall=4294967296 kernel.core_uses_pid=1 net.ipv4.tcp_syncookies=1
net.ipv4.ip_forward=0 kernel.shmmax=4294967295 kernel.msgmax=65536 kernel.msgmnb=65536 net.ipv4.conf.default.rp_filter=1 kernel.sysrq=0 net.ipv4.conf.default.accept_source_route=0 kernel.shmall=268435456 kernel.core_uses_pid=1 net.ipv4.tcp_syncookies=1 net.ipv4.neigh.default.gc_thresh1 = 4096 net.ipv4.neigh.default.gc_thresh2 = 8192 net.ipv4.neigh.default.gc_thresh3 = 8192 net.ipv4.neigh.default.base_reachable_time = 86400 net.ipv4.neigh.default.gc_stale_time = 86400
The neighbour table is generally known as ARP table and the default value for gc_thresh1 is 128 (Adjust where the gc will leave arp table alone)
]# cat /proc/sys/net/ipv4/neigh/default/gc_thresh1 128
which is not enough for large networks (more than 128 hosts). Thats why we need to tune this value. The gc_thresh2 is a soft limit (Tell the gc when to become aggressive with arp table cleaning.) and the gc_thresh3 is a hard limit (Don’t allow the arp table to become bigger than this).
To enlarge the ARP cache table on the live system run:
# sysctl -w net.ipv4.neigh.default.gc_thresh3=8192 # sysctl -w net.ipv4.neigh.default.gc_thresh2=8192 # sysctl -w net.ipv4.neigh.default.gc_thresh1=4096
It is possible that after distro update your systctl.conf will be replaced with the default values. Check this file periodically..
6 thoughts on “Neighbour table overflow – sysctl.conf tunning”
It helped me to tune production server
Thanks for this post
The other solution is to enable the reverse path filter (rp_filter)
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
Thanks for solution! It was very helpful.
I had the same problem even though the arp cache contained roughly a hundred entries and net.ipv4.neigh.default.gc_thresh1 was set to 1024 and so on.
net.ipv6.neigh.default.gc_thresh1 (ipv6!!) was still set to 128…
Don’t forget to set the ipv6 values if your system is configured with both ipv4 and ipv6
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