![]() The standby system can then take over without breaking connectivity even on established flows. ![]() Entries of an active firewall are replicated to a standby system. This is used by conntrackd for state replication. Add “ –output ktimestamp” to see the absolute start date as well. “sudo conntrack -L” then displays the seconds elapsed since the flow was first seen. ![]() “sudo sysctl _conntrack_timestamp=1” records a “start timestamp” for each connection. “sudo sysctl _conntrack_acct=1” makes “sudo conntrack -L” track byte and packet counters for each flow. Two useful extensions are conntrack accounting and timestamping. Other types of NAT rules, such as “dnat to” or “redirect to” would be shown in a similar fashion, with the reply tuples destination different from the original one. This source NAT is due to a nft masquerade rule: inet nat postrouting meta oifname "veth0" masquerade When 10.8.2.12 sends a reply, it changes the destination back to 10.0.0.10. Whenever 10.0.0.10 sends another packet, the router with this entry replaces the source address with 192.168.1.2. But unlike the previous example, the reply direction is not just the inverted original direction: the source address is changed. This allows to see which kind of NAT transformation is active on a given flow. Its possible to filter the output to only show entries with source or destination nat applied. Conntrack state table and NATĪs explained in the previous section, the reply tuple listed contains the NAT information. By default conntrack allows mid-stream pickups to not cause problems for flows that existed prior to conntrack becoming active. In the case of TCP conntrack can be configured to only add the new entry if the TCP packet has the SYN bit set. In the case of UDP this happens automatically. When a packet does not map to an existing entry, conntrack may add a new state entry for it. Changes to the first quadruple would be pointless: netfilter has no control over the initiators state, it can only influence the packet as it is received/forwarded. NAT manipulation only alters the reply (second) quadruple because that is what the receiver will see. The original (first shown) quadruple stored never changes: Its what the initiator sent. ![]() A lookup in the state table will be successful even if its a reply packet to a flow that has any form of network or port address translation applied.If a NAT rule matches, such as IP address masquerading, this is recorded in the reply part of the connection tracking entry and can then be automatically applied to all future packets that are part of the same flow.The second quadruple is what conntrack expects to see when a reply from the peer is received. The first address quadruple (source and destination address and ports) are those recorded in the original direction, i.e. You might notice that each line shows the addresses and port numbers twice and even with inverted address and port pairs! This is because each entry is inserted into the state table twice. ![]()
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