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Technical Note: Video/Telephony traffic issue giving 'No socket found. Drop' message

April 16, 2026 at 12:42 PM
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Description

This article addresses a Video/Telephony traffic issue when using IPPool NAT with a VIP different IP than FortiGate public (external) IP address.

If one hit one-way traffic issue (internet to private). Running "diag debug flow" command, one observes: "No socket found. Drop".

Solution

Set 'config firewall ippool' with 'type one-to-one' instead of default 'overload'.

Problem

diag debug flow filter add <VIP_ip>
diag debug flow trace start

trace_id=127 func=print_pkt_detail line=4471 msg="vd-root received a packet(proto=17, VIP_ip:port) from port11. "
trace_id=127 func=resolve_ip_tuple_fast line=4532 msg="Find an existing session, id-19e1f6d5, original direction"
trace_id=127 func=udp_rcv line=980 msg="No socket found. Drop."

Extract of a typical default configuration with all TCP/UDP ports opened for NAT

config firewall ippool
        edit "pool_VIP"
        set startip VIP_ip
        set endip VIP_ip
------------------------------------------------
config firewall vip
     edit "VIP_10.16_tcp"
        set comment "videoconf_TCP"
        set extip <VIP_ip >
        set extintf "port11"
        set portforward enable
        set mappedip "10.16.2.101"
        set extport 1-65535
        set mappedport 1-65535
    next
    edit "VIP_10.16_UDP"
       set comment "videoconf_UDP"
        set extip <VIP_ip >
        set extintf "port11"
        set portforward enable
        set mappedip "10.16.2.101"
        set protocol udp
        set extport 1-65535
        set mappedport 1-65535