PCAP:

U-boot supports live Ethernet packet capture in PCAP(2.4) format.
This is enabled by CONFIG_CMD_PCAP.

The capture is stored on physical memory, and should be copied to
a machine capable of parsing and displaying PCAP files (IE. wireshark)
If networking works properly one can copy the capture file from physical memory
using tftpput, or save it to local storage with (sf write, mmc write, fatwrite, etc)

the pcap capturing requires maximum buffer size.
when the buffer is full an error message will be displayed and then packets
will silently drop.
the actual capture file size is populate in the environment variable "pcapsize".

Usage example:

# Initialize pcap capture to physical address (0x100000) with maximum size of
# 100000 bytes.

# Start capture
pcap start

# Initialize network activity
env set ipaddr 10.0.2.15; env set serverip 10.0.2.2; tftp uImage64

# Stop capture
pcap stop

# pcap init 0x100000 100000
PCAP capture initialized: addr: 0xffffffff80100000 max length: 100000

# pcap start
# env set ipaddr 10.0.2.15; env set serverip 10.0.2.2; tftp uImage64
eth0@10000000: PHY present at 0
eth0@10000000: link up, 1000Mbps full-duplex (lpa: 0x7c00)
Using eth0@10000000 device
TFTP from server 10.0.2.2; our IP address is 10.0.2.15
Filename 'uImage64'.
Load address: 0xffffffff88000000
Loading: #################################################################
         #################################################################
         #################################################################
         #################################################################
!!! Buffer is full, consider increasing buffer size !!!
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #
         18.2 MiB/s
done
Bytes transferred = 8359376 (7f8dd0 hex)
PCAP status:
        Initialized addr: 0xffffffff80100000    max length: 100000
        Status: Active.  file size: 99991
        Incoming packets: 66 Outgoing packets: 67

# pcap stop
# tftpput 0xffffffff80100000 $pcapsize 10.0.2.2:capture.pcap