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MTU bug

General pfSense Questions
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  • J
    jc1976
    last edited by Jan 26, 2023, 5:47 PM

    Hi all,

    if this is the wrong place to post a bug that i've found, i apologize.

    I changed my MTU to 1472 because i thought that was the correct mtu setting for my cable internet (comcast).

    I realized that i was wrong and i should've left it at the default 1500.

    Upon attempting to reset the mtu size back to 1500, nothing changes.

    I've attempted to reset it via the gui several times, even after reboots but the gui still reads 1472.

    M 1 Reply Last reply Jan 26, 2023, 6:02 PM Reply Quote 0
    • M
      michmoor LAYER 8 Rebel Alliance @jc1976
      last edited by Jan 26, 2023, 6:02 PM

      @jc1976 So you're saying you cleared the MTU setting under your WAN interface and when you check it still shows up as 1472

      My set up is below

      login-to-view

      login-to-view

      Firewall: NetGate,Palo Alto-VM,Juniper SRX
      Routing: Juniper, Arista, Cisco
      Switching: Juniper, Arista, Cisco
      Wireless: Unifi, Aruba IAP
      JNCIP,CCNP Enterprise

      J 1 Reply Last reply Jan 26, 2023, 6:11 PM Reply Quote 0
      • J
        jc1976 @michmoor
        last edited by Jan 26, 2023, 6:11 PM

        @michmoor i didn't clear it, i just typed 1500 in the space and hit save, and every time i went back to the page it said 1472.

        when i get home, i'll try clearing and saving as you did, to see if that returns it to 1500.

        Thanks!

        J 1 Reply Last reply Feb 1, 2023, 5:12 AM Reply Quote 1
        • J
          jc1976 @jc1976
          last edited by Feb 1, 2023, 5:12 AM

          @jc1976 Hey sorry for not getting back sooner.. was away for a while and just remembered..

          Anyway, no dice on the mtu settings. i released my wan ip, blanked out the MTU box.. still comes back as 1472.

          rebooted the box, same thing.. it's as if the "save" button at the bottom isn't working.. clicked on it several times but that didn't change anything.

          J 1 Reply Last reply Feb 1, 2023, 12:02 PM Reply Quote 0
          • J
            JKnott @jc1976
            last edited by Feb 1, 2023, 12:02 PM

            @jc1976

            If you're using DHCP, then the MTU will be set automagically to whatever the ISP sets it to.

            PfSense running on Qotom mini PC
            i5 CPU, 4 GB memory, 32 GB SSD & 4 Intel Gb Ethernet ports.
            UniFi AC-Lite access point

            I haven't lost my mind. It's around here...somewhere...

            M 1 Reply Last reply Feb 1, 2023, 12:29 PM Reply Quote 0
            • M
              michmoor LAYER 8 Rebel Alliance @JKnott
              last edited by Feb 1, 2023, 12:29 PM

              @jknott certainly possible but the OP stated he changed his MTU so I assume it was once at 1500

              Firewall: NetGate,Palo Alto-VM,Juniper SRX
              Routing: Juniper, Arista, Cisco
              Switching: Juniper, Arista, Cisco
              Wireless: Unifi, Aruba IAP
              JNCIP,CCNP Enterprise

              S J 2 Replies Last reply Feb 1, 2023, 1:15 PM Reply Quote 0
              • S
                stephenw10 Netgate Administrator @michmoor
                last edited by Feb 1, 2023, 1:15 PM

                @michmoor said in MTU bug:

                so I assume it was once at 1500

                But maybe it wasn't. 😉

                If the MTU setting has been removed entirely from pfSense I would recommend power cycling the firewall completely. Some NICs will retain settings across a reboot.

                Steve

                J 1 Reply Last reply Feb 1, 2023, 1:51 PM Reply Quote 1
                • J
                  JKnott @michmoor
                  last edited by Feb 1, 2023, 1:47 PM

                  @michmoor said in MTU bug:

                  @jknott certainly possible but the OP stated he changed his MTU so I assume it was once at 1500

                  The question becomes whether the DHCP client overrides a manual config. Normally, you don't set MTU with DHCP. Perhaps he could set the MTU before connecting and see if it retains the setting after.

                  PfSense running on Qotom mini PC
                  i5 CPU, 4 GB memory, 32 GB SSD & 4 Intel Gb Ethernet ports.
                  UniFi AC-Lite access point

                  I haven't lost my mind. It's around here...somewhere...

                  1 Reply Last reply Reply Quote 0
                  • J
                    JKnott @stephenw10
                    last edited by Feb 1, 2023, 1:51 PM

                    @stephenw10

                    Another thing he could do is run Packet Capture to see if DHCP option 26 is used and what value it is.

                    PfSense running on Qotom mini PC
                    i5 CPU, 4 GB memory, 32 GB SSD & 4 Intel Gb Ethernet ports.
                    UniFi AC-Lite access point

                    I haven't lost my mind. It's around here...somewhere...

                    1 Reply Last reply Reply Quote 0
                    • S
                      stephenw10 Netgate Administrator
                      last edited by Feb 1, 2023, 2:04 PM

                      Yeah it would be very unusual to see that set.

                      J 1 Reply Last reply Feb 1, 2023, 2:51 PM Reply Quote 0
                      • J
                        jc1976 @stephenw10
                        last edited by Feb 1, 2023, 2:51 PM

                        @stephenw10

                        My internet connection is typical cable provided by comcast/xfinity so yes, it's dhcp.

                        when i adjusted the mtu, i arrived at 1472 by plugging my laptop directly into the modem and running the commands until it stopped fragmenting, and then i used that number input the mtu size in pfsense, so i was able to adjust it.

                        i've rebooted the firewall a few times, so it's definitely locked to that number where rebooting wouldn't solve the problem.

                        seems like for me to correct the setting i'd have to manually edit a config file.

                        J 1 Reply Last reply Feb 1, 2023, 3:14 PM Reply Quote 0
                        • S
                          stephenw10 Netgate Administrator
                          last edited by Feb 1, 2023, 2:57 PM

                          But did you fully power cycle it? Like actually remove the power rather than just reboot?

                          On many devices the NICs remain powered across a reboot and will retain their state.

                          J 1 Reply Last reply Feb 13, 2023, 9:16 PM Reply Quote 0
                          • J
                            JKnott @jc1976
                            last edited by Feb 1, 2023, 3:14 PM

                            @jc1976 said in MTU bug:

                            when i adjusted the mtu, i arrived at 1472 by plugging my laptop directly into the modem and running the commands until it stopped fragmenting, and then i used that number input the mtu size in pfsense, so i was able to adjust it.

                            On the WAN interface, you have to use the MTU your ISP uses. Otherwise, some frames may be discarded. Do you know where the fragmenting is happening? It could be anywhere between you and the destination. You can determine that by looking at the source address of the ICMP messages. Fragmentation is the mechanism to get around the different MTUs and is entirely normal, though these days Path MTU Detection is often used and is mandatory with IPv6. Also, what was fragmenting? On Linux PMTUD is generally used for everything and with TCP on Windows.

                            PfSense running on Qotom mini PC
                            i5 CPU, 4 GB memory, 32 GB SSD & 4 Intel Gb Ethernet ports.
                            UniFi AC-Lite access point

                            I haven't lost my mind. It's around here...somewhere...

                            J 1 Reply Last reply Feb 13, 2023, 9:19 PM Reply Quote 0
                            • J
                              jc1976 @stephenw10
                              last edited by Feb 13, 2023, 9:16 PM

                              @stephenw10 yes, fully powered off. changed the setting, shut the pfsense box off, turned off the modem, and went away for the weekend. Still hangs onto 1472.

                              1 Reply Last reply Reply Quote 0
                              • J
                                jc1976 @JKnott
                                last edited by Feb 13, 2023, 9:19 PM

                                @jknott that i don't know.

                                I arrived at 1472 by plugging my win10 laptop directly into the modem and pinging with the flag set at whatever it was and working my way down until it stopped fragmenting. i didn't realize that the 28bits for the header were to be added onto the mtu size once the fragmentation limit was found. it's all fine, works great without any issue. just thought you'd all like to know about my experience.

                                1 Reply Last reply Reply Quote 0
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