Resolving “nf_conntrack: table full, dropping packet.” flood message in dmesg Linux kernel log

On many busy servers, you might encounter in /var/log/syslog or dmesg kernel log messages like

nf_conntrack: table full, dropping packet

to appear repeatingly:

[1737157.057528] nf_conntrack: table full, dropping packet.
[1737157.160357] nf_conntrack: table full, dropping packet.
[1737157.260534] nf_conntrack: table full, dropping packet.
[1737157.361837] nf_conntrack: table full, dropping packet.
[1737157.462305] nf_conntrack: table full, dropping packet.
[1737157.564270] nf_conntrack: table full, dropping packet.
[1737157.666836] nf_conntrack: table full, dropping packet.
[1737157.767348] nf_conntrack: table full, dropping packet.
[1737157.868338] nf_conntrack: table full, dropping packet.
[1737157.969828] nf_conntrack: table full, dropping packet.
[1737157.969928] nf_conntrack: table full, dropping packet
[1737157.989828] nf_conntrack: table full, dropping packet
[1737162.214084] __ratelimit: 83 callbacks suppressed

There are two type of servers, I've encountered this message on:

1. Xen OpenVZ / VPS (Virtual Private Servers)
2. ISPs – Internet Providers with heavy traffic NAT network routers

I. What is the meaning of nf_conntrack: table full dropping packet error message

In short, this message is received because the nf_conntrack kernel maximum number assigned value gets reached.
The common reason for that is a heavy traffic passing by the server or very often a DoS or DDoS (Distributed Denial of Service) attack. Sometimes encountering the err is a result of a bad server planning (incorrect data about expected traffic load by a company/companeis) or simply a sys admin error…

– Checking the current maximum nf_conntrack value assigned on host:

linux:~# cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max

– Alternative way to check the current kernel values for nf_conntrack is through:

linux:~# /sbin/sysctl -a|grep -i nf_conntrack_max
error: permission denied on key 'net.ipv4.route.flush'
net.netfilter.nf_conntrack_max = 65536
error: permission denied on key 'net.ipv6.route.flush'
net.nf_conntrack_max = 65536

– Check the current sysctl nf_conntrack active connections

To check present connection tracking opened on a system:


linux:~# /sbin/sysctl net.netfilter.nf_conntrack_count
net.netfilter.nf_conntrack_count = 12742

The shown connections are assigned dynamicly on each new succesful TCP / IP NAT-ted connection. Btw, on a systems that work normally without the dmesg log being flooded with the message, the output of lsmod is:

linux:~# /sbin/lsmod | egrep 'ip_tables|conntrack'
ip_tables 9899 1 iptable_filter
x_tables 14175 1 ip_tables

On servers which are encountering nf_conntrack: table full, dropping packet error, you can see, when issuing lsmod, extra modules related to nf_conntrack are shown as loaded:

linux:~# /sbin/lsmod | egrep 'ip_tables|conntrack'
nf_conntrack_ipv4 10346 3 iptable_nat,nf_nat
nf_conntrack 60975 4 ipt_MASQUERADE,iptable_nat,nf_nat,nf_conntrack_ipv4
nf_defrag_ipv4 1073 1 nf_conntrack_ipv4
ip_tables 9899 2 iptable_nat,iptable_filter
x_tables 14175 3 ipt_MASQUERADE,iptable_nat,ip_tables


II. Remove completely nf_conntrack support if it is not really necessery

It is a good practice to limit or try to omit completely use of any iptables NAT rules to prevent yourself from ending with flooding your kernel log with the messages and respectively stop your system from dropping connections.

Another option is to completely remove any modules related to nf_conntrack, iptables_nat and nf_nat.
To remove nf_conntrack support from the Linux kernel, if for instance the system is not used for Network Address Translation use:

/sbin/rmmod iptable_nat
/sbin/rmmod ipt_MASQUERADE
/sbin/rmmod rmmod nf_nat
/sbin/rmmod rmmod nf_conntrack_ipv4
/sbin/rmmod nf_conntrack
/sbin/rmmod nf_defrag_ipv4

Once the modules are removed, be sure to not use iptables -t nat .. rules. Even attempt to list, if there are any NAT related rules with iptables -t nat -L -n will force the kernel to load the nf_conntrack modules again.

Btw nf_conntrack: table full, dropping packet. message is observable across all GNU / Linux distributions, so this is not some kind of local distribution bug or Linux kernel (distro) customization.

III. Fixing the nf_conntrack … dropping packets error

– One temporary, fix if you need to keep your iptables NAT rules is:

linux:~# sysctl -w net.netfilter.nf_conntrack_max=131072

I say temporary, because raising the nf_conntrack_max doesn't guarantee, things will get smoothly from now on.
However on many not so heavily traffic loaded servers just raising the net.netfilter.nf_conntrack_max=131072 to a high enough value will be enough to resolve the hassle.

– Increasing the size of nf_conntrack hash-table

The Hash table hashsize value, which stores lists of conntrack-entries should be increased propertionally, whenever net.netfilter.nf_conntrack_max is raised.

linux:~# echo 32768 > /sys/module/nf_conntrack/parameters/hashsize
The rule to calculate the right value to set is:
hashsize = nf_conntrack_max / 4

– To permanently store the made changes ;a) put into /etc/sysctl.conf:

linux:~# echo 'net.netfilter.nf_conntrack_count = 131072' >> /etc/sysctl.conf
linux:~# /sbin/sysct -p

b) put in /etc/rc.local (before the exit 0 line):

echo 32768 > /sys/module/nf_conntrack/parameters/hashsize

Note: Be careful with this variable, according to my experience raising it to too high value (especially on XEN patched kernels) could freeze the system.
Also raising the value to a too high number can freeze a regular Linux server running on old hardware.

– For the diagnosis of nf_conntrack stuff there is ;

/proc/sys/net/netfilter kernel memory stored directory. There you can find some values dynamically stored which gives info concerning nf_conntrack operations in "real time":

linux:~# cd /proc/sys/net/netfilter
linux:/proc/sys/net/netfilter# ls -al nf_log/

total 0
dr-xr-xr-x 0 root root 0 Mar 23 23:02 ./
dr-xr-xr-x 0 root root 0 Mar 23 23:02 ../
-rw-r--r-- 1 root root 0 Mar 23 23:02 0
-rw-r--r-- 1 root root 0 Mar 23 23:02 1
-rw-r--r-- 1 root root 0 Mar 23 23:02 10
-rw-r--r-- 1 root root 0 Mar 23 23:02 11
-rw-r--r-- 1 root root 0 Mar 23 23:02 12
-rw-r--r-- 1 root root 0 Mar 23 23:02 2
-rw-r--r-- 1 root root 0 Mar 23 23:02 3
-rw-r--r-- 1 root root 0 Mar 23 23:02 4
-rw-r--r-- 1 root root 0 Mar 23 23:02 5
-rw-r--r-- 1 root root 0 Mar 23 23:02 6
-rw-r--r-- 1 root root 0 Mar 23 23:02 7
-rw-r--r-- 1 root root 0 Mar 23 23:02 8
-rw-r--r-- 1 root root 0 Mar 23 23:02 9


IV. Decreasing other nf_conntrack NAT time-out values to prevent server against DoS attacks

Generally, the default value for nf_conntrack_* time-outs are (unnecessery) large.
Therefore, for large flows of traffic even if you increase nf_conntrack_max, still shorty you can get a nf_conntrack overflow table resulting in dropping server connections. To make this not happen, check and decrease the other nf_conntrack timeout connection tracking values:

linux:~# sysctl -a | grep conntrack | grep timeout
net.netfilter.nf_conntrack_generic_timeout = 600
net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 120
net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 60
net.netfilter.nf_conntrack_tcp_timeout_established = 432000
net.netfilter.nf_conntrack_tcp_timeout_fin_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close_wait = 60
net.netfilter.nf_conntrack_tcp_timeout_last_ack = 30
net.netfilter.nf_conntrack_tcp_timeout_time_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close = 10
net.netfilter.nf_conntrack_tcp_timeout_max_retrans = 300
net.netfilter.nf_conntrack_tcp_timeout_unacknowledged = 300
net.netfilter.nf_conntrack_udp_timeout = 30
net.netfilter.nf_conntrack_udp_timeout_stream = 180
net.netfilter.nf_conntrack_icmp_timeout = 30
net.netfilter.nf_conntrack_events_retry_timeout = 15
net.ipv4.netfilter.ip_conntrack_generic_timeout = 600
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_sent = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_sent2 = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_recv = 60
net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 432000
net.ipv4.netfilter.ip_conntrack_tcp_timeout_fin_wait = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close_wait = 60
net.ipv4.netfilter.ip_conntrack_tcp_timeout_last_ack = 30
net.ipv4.netfilter.ip_conntrack_tcp_timeout_time_wait = 120
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close = 10
net.ipv4.netfilter.ip_conntrack_tcp_timeout_max_retrans = 300
net.ipv4.netfilter.ip_conntrack_udp_timeout = 30
net.ipv4.netfilter.ip_conntrack_udp_timeout_stream = 180
net.ipv4.netfilter.ip_conntrack_icmp_timeout = 30

All the timeouts are in seconds. net.netfilter.nf_conntrack_generic_timeout as you see is quite high – 600 secs = (10 minutes).
This kind of value means any NAT-ted connection not responding can stay hanging for 10 minutes!

The value net.netfilter.nf_conntrack_tcp_timeout_established = 432000 is quite high too (5 days!)
If this values, are not lowered the server will be an easy target for anyone who would like to flood it with excessive connections, once this happens the server will quick reach even the raised up value for net.nf_conntrack_max and the initial connection dropping will re-occur again …

With all said, to prevent the server from malicious users, situated behind the NAT plaguing you with Denial of Service attacks:

Lower net.ipv4.netfilter.ip_conntrack_generic_timeout to 60 – 120 seconds and net.ipv4.netfilter.ip_conntrack_tcp_timeout_established to stmh. like 54000

linux:~# sysctl -w net.ipv4.netfilter.ip_conntrack_generic_timeout = 120
linux:~# sysctl -w net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 54000

This timeout should work fine on the router without creating interruptions for regular NAT users. After changing the values and monitoring for at least few days make the changes permanent by adding them to /etc/sysctl.conf

linux:~# echo 'net.ipv4.netfilter.ip_conntrack_generic_timeout = 120' >> /etc/sysctl.conf
linux:~# echo 'net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 54000' >> /etc/sysctl.conf

Share this on

Download PDFDownload PDF

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

13 Responses to “Resolving “nf_conntrack: table full, dropping packet.” flood message in dmesg Linux kernel log”

  1. designeru says:
    Google Chrome 21.0.1180.83 Google Chrome 21.0.1180.83 Windows 7 x64 Edition Windows 7 x64 Edition
    Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.83 Safari/537.1

    very helpful… saved me from a lot of googling… thanks!

    View CommentView Comment
  2. scragg says:
    Google Chrome 24.0.1312.52 Google Chrome 24.0.1312.52 Windows 8 x64 Edition Windows 8 x64 Edition
    Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.17 (KHTML, like Gecko) Chrome/24.0.1312.52 Safari/537.17

    Thanks for the post. It was very helpful. When doing the rmmod, is that just unloading the modules or permanently deleting them? You mention don’t do the “iptables -t nat -L -n” because they will load again, so I assume the former. If I accidentally loaded them, would I just need to rmmod the modules again. I use iptables to close off all ports and poke holes in it for services and making blacklist/whitelists for certain IPs. Is the conntrack needed for this? I assume some of the modules you suggested to remove are required.

    View CommentView Comment
  3. Cant start more than 10 nodejs processes - node.js Solutions - Developers Q & A says:
    WordPress 3.5.2 WordPress 3.5.2
    The Incutio XML-RPC PHP Library -- WordPress/3.5.2

    […] More info can be found here… […]

    View CommentView Comment
  4. Felipe says:
    Firefox 31.0 Firefox 31.0 GNU/Linux x64 GNU/Linux x64
    Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Firefox/31.0

    There is a little error:

    linux:~# echo 'net.netfilter.nf_conntrack_count = 131072' >> /etc/sysctl.conf

    The correct should be _max

    linux:~# echo 'net.netfilter.nf_conntrack_max = 131072' >> /etc/sysctl.conf


    Thanks for the article!

    View CommentView Comment
  5. Maradona says:
    Internet Explorer 6.0 Internet Explorer 6.0 Windows 2000 Windows 2000
    Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; KTXN)

    Thank you for your assistance

    View CommentView Comment
  6. Christian says:
    Firefox 37.0 Firefox 37.0 Mac OS X  10.10 Mac OS X 10.10
    Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:37.0) Gecko/20100101 Firefox/37.0


    thank you very much for this article. I had many packet drop messages on my router and raising values has helped me a lot. Do you know, where hashsize and nf_conntrack_max has its limits?

    My router is a cluster that synchronizes connection tracking tables. So I guess I still need the conntrack modules even the system is pure routing, right? I sync the tables so connection can still continue even on cluster node switch.

    What I don’t know is, at which point raising the max and hash sizes become a problem.



    View CommentView Comment
    • admin says:
      Firefox 37.0 Firefox 37.0 Windows 7 x64 Edition Windows 7 x64 Edition
      Mozilla/5.0 (Windows NT 6.1; WOW64; rv:37.0) Gecko/20100101 Firefox/37.0

      Hi Christian,

      It depends on your Router Hardware / CPU and installed kernel. Check the kernel with uname -a and according to Kernel google for maximum settings you can set for conntrack max and hash sizes values. Also try to experiment as usual 🙂

      Hope this helps,


      View CommentView Comment
  7. DanAm says:
    Google Chrome 44.0.2403.125 Google Chrome 44.0.2403.125 Windows 7 Windows 7
    Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.125 Safari/537.36

    Spot On ! You got it.

    View CommentView Comment
  8. M says:
    Google Chrome 49.0.2623.110 Google Chrome 49.0.2623.110 Windows 10 x64 Edition Windows 10 x64 Edition
    Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.110 Safari/537.36

    in your commands:
    /sbin/rmmod rmmod nf_nat
    /sbin/rmmod rmmod nf_conntrack_ipv4
    there seems to be one “rmmod” too many

    View CommentView Comment
  9. Richard says:
    Firefox 50.0 Firefox 50.0 Ubuntu x64 Ubuntu x64
    Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0

    Thanks for the article, I ran across this dropping packets issue while building a web crawler. One small typo in the article still, missing an 'l' here 'linux:~# /sbin/sysct -p'

    View CommentView Comment
  10. 遇到nf_conntrack: table full, dropping packet的问题 – Blog4fun says:
    WordPress 4.5.3 WordPress 4.5.3
    The Incutio XML-RPC PHP Library -- WordPress/4.5.3

    […] 内核参数的问题,因为使用ss-client需要使用防火墙转发数据包,然后server ip地址一直被qiang的情况下就会抱各种网络资源耗尽的情况,比如这个,好在找到了参考链接,才知道是因为系统参数过低耗尽导致的,在不需要nat转发的情况下可以把内核的nat模块儿卸载掉,即可解决,如果说需要使用nat功能,那就通过修改修改系统内核参数结果:) […]

    View CommentView Comment

Leave a Reply

CommentLuv badge