IPtables SNAT eats packets

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http://serverfault.com – I am trying to translate outgoing UDP packets with a source port of X to a source port of Y. I have done this using the following iptables rule: iptables -t nat -A POSTROUTING -s -p udp --sport X -j SNAT --to-source The counters for this rule increase when packets with a source port of X are generated, but completely vanish after that. I cannot find them in the counters of any other chain or table, and cannot see them on any interface using tcpdump. If I remove that rule, then the packets are received fine with the source port of X. But as soon as I put the rule back, (HowTos)