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Overflowing ARP table in VM with network in routed mode

APPLIES TO:
  • Parallels Cloud Server
  • Parallels Server

Symptoms

Virtual Machine with CentOS/Redhat can experience ARP (neighbour) table overflow.

This can happen when Parallels Tools are installed in VM, the network interface is configured in the routed mode:

~# prlctl list VM_Name -i | egrep '^Guest|^  net[0-9]'
GuestTools: state=installed version=7.0.13392.803029
  net0 (+) dev='vme58a0a9f1.0' type=routed mac=001C42xxxxxx card=rtl ips='192.168.86.124/255.255.255.255 ' gw='192.168.86.1' 
~#

The log file /var/log/messages in VM contains records like these:

Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:38 collector01 kernel: Neighbour table overflow.
Aug  7 15:49:43 collector01 kernel: __ratelimit: 3769 callbacks suppressed

Cause

The issue occurs because the default gateway in VM is configured with scope host instead of scope link.

~# ip route
169.255.30.1 dev eth0  scope host 
169.254.0.0/16 dev eth0  scope link  metric 1002 
default via 169.255.30.1 dev eth0 
~# ip route get 8.8.8.8
8.8.8.8 dev eth0  src 192.168.86.124 
    cache  mtu 1500 advmss 1460 hoplimit 64
~# 

Since eth0 is the Ethernet interface, ARP-capable - this means that ARP resolution should be used for communication with remote hosts and the kernel stores ARP record for external IPs with the MAC address of the host server.

Whereas the expected output should be:

~# ip route get 8.8.8.8
8.8.8.8 via 169.255.30.1 dev eth0  src 192.168.86.124 
    cache  mtu 1500 advmss 1460 hoplimit 64
~# 

Resolution

To correct the behaviour, the proper scope link should be configured. The configuration is set by the scripts installed in VM with Parallels Tools:

/usr/lib/parallels-tools/tools/scripts/redhat-set_gateway.sh
/usr/lib/parallels-tools/tools/scripts/redhat-set_route.sh

So, to resolve this issue please follow the steps:

  1. Download attached patch on your server to /root/ directory.

  2. Implement it:

    ~# (cd /usr/lib/parallels-tools/tools/scripts/; patch -p1 -i /root/scope.patch)
    
  3. restart VM or suspend/resume it or reapply network settings.

Attachments

scope.patch

Search words:

Neighbour table overflow




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