One of the important features built on top of the Netfilter framework is connection tracking. Connection tracking allows the kernel to keep track of all logical network connections or sessions, and thereby relate all of the packets which may make up that connection. NAT relies on this information to translate all related packets in the same way, and iptables can use this information to act as a stateful firewall.
The connection state however is completely independent of any upper-level state, such as TCP’s or SCTP’s state. Part of the reason for this is that when merely forwarding packets, i.e. no local delivery, the TCP engine may not necessarily be invoked at all. Even connectionless-mode transmissions such as UDP, IPsec (AH/ESP), GRE and other tunneling protocols have, at least, a pseudo connection state. The heuristic for such protocols is often based upon a preset timeout value for inactivity, after whose expiration a Netfilter connection is dropped.
Each Netfilter connection is uniquely identified by a (layer-3 protocol, source address, destination address, layer-4 protocol, layer-4 key) tuple. The layer-4 key depends on the transport protocol; for TCP/UDP it is the port numbers, for tunnels it can be their tunnel ID, but otherwise is just zero, as if it were not part of the tuple. To be able to inspect the TCP port in all cases, packets will be mandatorily defragmented.
It is possible to use either Multicast or Unicast to sync conntrack traffic. Most examples below show Multicast, but unicast can be specified by using the “peer” keywork after the specificed interface, as in the following example:
set service conntrack-sync interface eth0 peer 192.168.0.250
Accept only certain protocols: You may want to replicate the state of flows depending on their layer 4 protocol.
Protocols are: tcp, sctp, dccp, udp, icmp and ipv6-icmp.
The daemon doubles the size of the netlink event socket buffer size if it detects netlink event message dropping. This clause sets the maximum buffer size growth that can be reached.
Queue size for listening to local conntrack events in MB.
Protocol for which expect entries need to be synchronized.
Failover mechanism to use for conntrack-sync.
Only VRRP is supported. Required option.
IP addresses or networks for which local conntrack entries will not be synced
Local IPv4 addresses for service to listen on.
Multicast group to use for syncing conntrack entries.
Defaults to 126.96.36.199.
Peer to send unicast UDP conntrack sync entires to, if not using Multicast configuration from above above.
Make sure conntrack is enabled by running and show connection tracking table.
[email protected]:~$ show conntrack table ipv4 TCP state codes: SS - SYN SENT, SR - SYN RECEIVED, ES - ESTABLISHED, FW - FIN WAIT, CW - CLOSE WAIT, LA - LAST ACK, TW - TIME WAIT, CL - CLOSE, LI - LISTEN CONN ID Source Destination Protocol TIMEOUT 1015736576 10.35.100.87:58172 172.31.20.12:22 tcp  ES 430279 1006235648 10.35.101.221:57483 172.31.120.21:22 tcp  ES 413310 1006237088 10.100.68.100 172.31.120.21 icmp  29 1015734848 10.35.100.87:56282 172.31.20.12:22 tcp  ES 300 1015734272 172.31.20.12:60286 188.8.131.52:694 udp  29 1006239392 10.35.101.221 172.31.120.21 icmp  29
If the table is empty and you have a warning message, it means
conntrack is not enabled. To enable conntrack, just create a NAT or a firewall
set firewall state-policy established action accept
Retrieve current statistics of connection tracking subsystem.
[email protected]:~$ show conntrack-sync statistics Main Table Statistics: cache internal: current active connections: 19606 connections created: 6298470 failed: 0 connections updated: 3786793 failed: 0 connections destroyed: 6278864 failed: 0 cache external: current active connections: 15771 connections created: 1660193 failed: 0 connections updated: 77204 failed: 0 connections destroyed: 1644422 failed: 0 traffic processed: 0 Bytes 0 Pckts multicast traffic (active device=eth0.5): 976826240 Bytes sent 212898000 Bytes recv 8302333 Pckts sent 2009929 Pckts recv 0 Error send 0 Error recv message tracking: 0 Malformed msgs 263 Lost msgs
Retrieve current status of connection tracking subsystem.
[email protected]:~$ show conntrack-sync status sync-interface : eth0.5 failover-mechanism : vrrp [sync-group GEFOEKOM] last state transition : no transition yet! ExpectationSync : disabled
The next example is a simple configuration of conntrack-sync.
Now configure conntrack-sync service on
set high-availablilty vrrp group internal virtual-address ... etc ... set high-availability vrrp sync-group syncgrp member 'internal' set service conntrack-sync accept-protocol 'tcp' set service conntrack-sync accept-protocol 'udp' set service conntrack-sync accept-protocol 'icmp' set service conntrack-sync failover-mechanism vrrp sync-group 'syncgrp' set service conntrack-sync interface 'eth0' set service conntrack-sync mcast-group '184.108.40.206'
On the active router, you should have information in the internal-cache of conntrack-sync. The same current active connections number should be shown in the external-cache of the standby router
On active router run:
$ show conntrack-sync statistics Main Table Statistics: cache internal: current active connections: 10 connections created: 8517 failed: 0 connections updated: 127 failed: 0 connections destroyed: 8507 failed: 0 cache external: current active connections: 0 connections created: 0 failed: 0 connections updated: 0 failed: 0 connections destroyed: 0 failed: 0 traffic processed: 0 Bytes 0 Pckts multicast traffic (active device=eth0): 868780 Bytes sent 224136 Bytes recv 20595 Pckts sent 14034 Pckts recv 0 Error send 0 Error recv message tracking: 0 Malformed msgs 0 Lost msgs
On standby router run:
$ show conntrack-sync statistics Main Table Statistics: cache internal: current active connections: 0 connections created: 0 failed: 0 connections updated: 0 failed: 0 connections destroyed: 0 failed: 0 cache external: current active connections: 10 connections created: 888 failed: 0 connections updated: 134 failed: 0 connections destroyed: 878 failed: 0 traffic processed: 0 Bytes 0 Pckts multicast traffic (active device=eth0): 234184 Bytes sent 907504 Bytes recv 14663 Pckts sent 21495 Pckts recv 0 Error send 0 Error recv message tracking: 0 Malformed msgs 0 Lost msgs