em(4) cannot maintain link in 12.0-STABLE

[R. Tyler Croy]

I have two NICs in a machine which acts as a gateway, and with the upgrade from
11-STABLE to the latest 12.0-STABLE tree (r346473), neither em(4) based NIC
will maintain a connection.

The behavior is similar to what is described in the last couple comments of
this ticket: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219428 

The behavior is identical with both devices. Link is established, dhclient
starts securing a DHCP lease, and then once traffic starts to flow, the link
switches to DOWN. Then back to UP, and back and forth.

A syslog snippet:

    Apr 21 20:03:42 strawberry kernel: em0: Link is up 100 Mbps Full Duplex
    Apr 21 20:03:42 strawberry kernel: em0: link state changed to UP
    Apr 21 20:03:46 strawberry dhclient[15534]: New IP Address (em0): 173.228.83.96
    Apr 21 20:03:46 strawberry dhclient[16069]: New Subnet Mask (em0): 255.255.255.0
    Apr 21 20:03:46 strawberry dhclient[18914]: New Broadcast Address (em0): 173.228.83.255
    Apr 21 20:03:46 strawberry dhclient[21483]: New Routers (em0): 173.228.83.1
    Apr 21 20:03:48 strawberry kernel: em0: link state changed to DOWN
    Apr 21 20:03:50 strawberry kernel: em0: Link is up 100 Mbps Full Duplex
    Apr 21 20:03:50 strawberry kernel: em0: link state changed to UP
    Apr 21 20:03:57 strawberry kernel: em0: link state changed to DOWN
    Apr 21 20:03:57 strawberry ntpd[75785]: error resolving pool 0.freebsd.pool.ntp.org: Name does not resolve (8)
    Apr 21 20:03:58 strawberry dhclient[38043]: send_packet: Network is down
    Apr 21 20:03:59 strawberry kernel: em0: Link is up 100 Mbps Full Duplex
    Apr 21 20:03:59 strawberry kernel: em0: link state changed to UP
    Apr 21 20:04:05 strawberry kernel: em0: link state changed to DOWN
    Apr 21 20:04:06 strawberry dhclient[38043]: send_packet: Network is down
    Apr 21 20:04:07 strawberry kernel: em0: Link is up 100 Mbps Full Duplex
    Apr 21 20:04:07 strawberry kernel: em0: link state changed to UP
    Apr 21 20:04:15 strawberry kernel: em0: link state changed to DOWN
    Apr 21 20:04:15 strawberry dhclient[38043]: send_packet: Network is down
    Apr 21 20:04:16 strawberry kernel: em0: Link is up 100 Mbps Full Duplex

THe version/hardware details:

    FreeBSD strawberry 12.0-STABLE FreeBSD 12.0-STABLE r346473 GENERIC  amd64


    em0 at pci0:7:0:0:	class=0x020000 card=0xa01f8086 chip=0x10d38086 rev=0x00 hdr=0x00
        vendor     = 'Intel Corporation'
        device     = '82574L Gigabit Network Connection'
        class      = network
        subclass   = ethernet
    em1 at pci0:8:0:0:	class=0x020000 card=0xa01f8086 chip=0x10d38086 rev=0x00 hdr=0x00
        vendor     = 'Intel Corporation'
        device     = '82574L Gigabit Network Connection'
        class      = network
        subclass   = ethernet


My temporary workaround has been to use a USB-based NIC, but I am wondering if
there are any tunables I could tweak to get this misbehaving driver to mind its
manners?



Cheers

--
GitHub:  https://github.com/rtyler

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