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[SOLVED] Firewall noob vs. port forward

Hi there, hoping to find some help with a naive networking question.

I recently bought my first firewall appliance, installed Opnsense and am going to use it with my ISP modem in bridge mode, but while I'm learning I added it to my existing LAN with a 192.168.0.0/24 address assigned to the WAN port by my current DHCP. On the firewall's LAN port I set up a 10.0.0.0/24 network and am starting to build up my services. So far so good, but there's one thing I can't get to work: I can't port forward the firewall's WAN IP to a service on the firewall's LAN network and I can't figure out why.

To illustrate, I would like laptop with IP 192.168.0.161 to be able to reach service on 10.0.0.22:8888 by requesting firewall WAN IP 192.168.0.136:8888.

Private IPs and bogons are permitted on the WAN interface and I have followed every guide I can find for the port forwarding, but the closest I have come to this working is a "connection reset" browser error.

Hope my question is clear and isn't very dumb. Thanks for the help or any explanation why I might be struggling to get this to work. Am I missing something obvious?

UPDATE The thread is all over the place, but I have made some progress:

  • RDR rule gets triggered when requesting 192.168.0.136:8888 from 192.168.0.123
  • Apache logs show 2024-02-09T17:39:17.056208857Z 192.168.0.123 - - [09/Feb/2024:17:39:17 +0000] "GET / HTTP/1.1" 200 161
  • a tcpdump (in spoiler below) on the apache container looks inconspicuous to my untrained eye, with the exception of checksum errors in some packets from the docker container (172.20.0.2). The last five lines, after the second GET request (why is there a second GET request?) appear in tcpdump after a delay of about five seconds.
tcpdump 
    192.168.0.123.54120 > 172.20.0.2.80: Flags [S], cksum 0xfdc5 (correct), seq 4106772895, win 64240, options [mss 1460,sackOK,TS val 1485594466 ecr 0,nop,wscale 7], length 0
17:45:14.918207 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 60)
    172.20.0.2.80 > 192.168.0.123.54120: Flags [S.], cksum 0x6d68 (incorrect -> 0x2fd7), seq 3999845366, ack 4106772896, win 65160, options [mss 1460,sackOK,TS val 1469298770 ecr 1485594466,nop,wscale 7], length 0
17:45:14.924098 IP (tos 0x0, ttl 62, id 63128, offset 0, flags [DF], proto TCP (6), length 52)
    192.168.0.123.54120 > 172.20.0.2.80: Flags [.], cksum 0x5b30 (correct), ack 3999845367, win 502, options [nop,nop,TS val 1485594472 ecr 1469298770], length 0
17:45:14.924102 IP (tos 0x0, ttl 62, id 63129, offset 0, flags [DF], proto TCP (6), length 134)
    192.168.0.123.54120 > 172.20.0.2.80: Flags [P.], cksum 0x70f5 (correct), seq 4106772896:4106772978, ack 3999845367, win 502, options [nop,nop,TS val 1485594472 ecr 1469298770], length 82: HTTP, length: 82
        GET / HTTP/1.1
        Host: 192.168.0.136:8888
        User-Agent: curl/7.74.0
        Accept: */*

        <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
        <html>
         <head>
          <title>Index of /</title>
         </head>
         <body>
        <h1>Index of /</h1>
        <ul></ul>
        </body></html>

17:45:14.924119 IP (tos 0x0, ttl 64, id 34500, offset 0, flags [DF], proto TCP (6), length 52)
    172.20.0.2.80 > 192.168.0.123.54120: Flags [.], cksum 0x6d60 (incorrect -> 0x5ad1), ack 4106772978, win 509, options [nop,nop,TS val 1469298776 ecr 1485594472], length 0
17:45:14.924407 IP (tos 0x0, ttl 64, id 34501, offset 0, flags [DF], proto TCP (6), length 364)
    172.20.0.2.80 > 192.168.0.123.54120: Flags [P.], cksum 0x6e98 (incorrect -> 0x0a74), seq 3999845367:3999845679, ack 4106772978, win 509, options [nop,nop,TS val 1469298776 ecr 1485594472], length 312: HTTP, length: 312
        HTTP/1.1 200 OK
        Date: Fri, 09 Feb 2024 17:45:14 GMT
        Server: Apache/2.4.58 (Unix)
        Content-Length: 161
        Content-Type: text/html;charset=ISO-8859-1
17:45:14.929077 IP (tos 0x0, ttl 61, id 0, offset 0, flags [DF], proto TCP (6), length 40)
    192.168.0.123.54120 > 172.20.0.2.80: Flags [R], cksum 0x1833 (correct), seq 4106772978, win 0, length 0
17:45:15.138862 IP (tos 0x0, ttl 62, id 63130, offset 0, flags [DF], proto TCP (6), length 134)
    192.168.0.123.54120 > 172.20.0.2.80: Flags [P.], cksum 0x701e (correct), seq 4106772896:4106772978, ack 3999845367, win 502, options [nop,nop,TS val 1485594687 ecr 1469298770], length 82: HTTP, length: 82
        GET / HTTP/1.1
        Host: 192.168.0.136:8888
        User-Agent: curl/7.74.0
        Accept: */*

17:45:15.138872 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 40)
    172.20.0.2.80 > 192.168.0.123.54120: Flags [R], cksum 0xb48d (correct), seq 3999845367, win 0, length 0
17:45:19.995097 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 172.20.0.1 tell 172.20.0.2, length 28
17:45:19.995161 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 172.20.0.2 tell 172.20.0.1, length 28
17:45:19.995164 ARP, Ethernet (len 6), IPv4 (len 4), Reply 172.20.0.2 is-at 02:42:ac:14:00:02, length 28
17:45:19.995164 ARP, Ethernet (len 6), IPv4 (len 4), Reply 172.20.0.1 is-at 02:42:b8:07:c2:99, length 28```

UPDATE 2 I see the exact same behaviour with a second VM and apache directly installed on it instead of in a docker container.

UPDATE 3 Thank you everybody for coming up with ideas. And thank you most of all to @maxwellfire@lemmy.world: The culprit was the Filter rule association in my Port Forward settings which I had as Add associated filter rule but needs to be Pass. As soon as that is set, everything works.

The full solution is a NAT Port forwarding rule with filter rule "pass", an outbound NAT rule for hairpinning, and everything related to reflection turned off in Settings > Advanced. It's that easy! 😵‍💫

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