A limit of one packet per second or per minute is a good choice. Consider adding a rate limit to keep the number of trace events at a manageable level. Be as specific as possible to get a good signal-to-noise ratio. This enables tracing events for all packets that match the rule. The only important part, however, is the last fragment of the third rule: “ meta nftrace set 1″. The second rule creates a base hook before routing decisions have been made (prerouting) and with a negative priority value to make sure it will be evaluated before connection tracking and the NAT rules. A single “nft delete table inet trace_debug” will be enough to undo all rules and chains added to the temporary table during debugging. The first rule adds a new table This allows easier removal of the trace and debug rules later. # nft 'insert rule inet trace_debug trace_pre ip saddr $C ip daddr $S tcp dport $P tcp flags syn limit rate 1/second meta nftrace set 1' # nft 'add chain inet trace_debug trace_pre ' A change to the destination address often results in packets to get forwarded rather than being handled by the host itself. Prerouting means that the kernel has not yet made a decision on where the packet will be sent to. We want to know which NAT transformation is picked up, which rules get checked and if the packet gets dropped somewhere.īecause we are dealing with incoming connections, add a rule to the prerouting hook point. Lets assume that a host coming from IP address C is trying to reach the service on address S and port P. These rules select the packet(s) that should be traced. To use this special feature “trace rules” are inserted at a suitable location. The utility nftables (and, to a lesser extent, iptables) allow for examining how a packet is evaluated and which rules in the ruleset were matched by it. Reverse translation is done automatically. Each connection has its own NAT state attached to it. A netfilter system called “conntrack” recognizes packets that are replies to an existing connection. For instance, when mapping address X to Y, there is no need to add a rule to do the reverse translation. NAT involves more than just changing the ip addresses or port numbers. One way to debug such problems is to check that the incoming request matches the expected or configured translation. The wrong service is exposed, connections end up in the wrong container, request time out, and so on. The same technique can be used for load-balancing where incoming connections get distributed among a pool of machines.Ĭonnection requests fail when network address translation is not working as expected. Packets are then routed to a container or virtual machine instead. Incoming connection requests have their destination address rewritten to a different one.

Network address translation is one way to expose containers or virtual machines to the wider internet. Part 1 shows how to use the iptables/nftables packet tracing feature to find the source of NAT related connectivity problems.

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