Apr 11, 2020

# iptables -L -vn Chain INPUT (policy ACCEPT 9827 packets, 6479K bytes) pkts bytes target prot opt in out source destination Chain FORWARD (policy ACCEPT 196 packets, 17202 bytes) pkts bytes target prot opt in out source destination 1620 183K ACCEPT tcp -- * * 0.0.0.0/0 192.168.1.40 tcp dpt:1080 state NEW,RELATED,ESTABLISHED 107 21217 ACCEPT nf_conntrack: automatic helper assignment is deprecated Dec 09, 2012 iptables rules to forward tftp via NAT - Unix & Linux iptables -A PREROUTING -t raw -p udp --dport 69 -s 192.168.11.0/24 -d 172.16.0.0/16 -j CT --helper tftp This rule alone should now have the helper be activated when adequate, thus triggering the mangling of TFTP data and ports, since TFTP is a complex protocol where server replies can come back from unrelated source ports to the dynamic/ephemeral client source port, as seen in this Wikipedia entry for …

NAT helper modules do some application specific NAT handling. Usually this includes on-the-fly manipulation of data: think about the PORT command in FTP, where the client tells the server which IP/port to connect to. Therefor an FTP helper module must replace the IP/port after the PORT command in the FTP control connection.

networking - IPTABLES port forwarding to SIP UDP not SIP is a complex (multi ports) protocol and requires a protocol helper (aka ALG). netfilter/conntrack provides a (n in-kernel ALG) SIP helper that you must use. More informations there: Secure use of iptables and connection tracking helpers. Also, port 5061 is usually TLS so can't be snooped by the helper so traffic there still won't work. 1369489 – Kernel 4.7 and net.netfilter.nf_conntrack_helper i would call that a regression within a stable release and this default should be changed in the fedora kernel! looks like iwth kernel 4.7 you need "net.netfilter.nf_conntrack_helper = 1" in sysctl.conf to continue things like PASV FTP or Hylafax (which uses FTP as procotocol) working like before there are warnings over years now at boot but nobody was able to tell until today how you are

Automatic Helper Assignment | firewalld

iptables -A INPUT -i eth0 -p tcp --dport 80 -m state --state INVALID -j DROP TCPMSS This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface's MTU minus 40 for IPv4 or 60 for IPv6, respectively). IPtables Configure | ZP Helper Jul 29, 2017 A Deep Dive into Iptables and Netfilter Architecture