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NETDUMP(4) FreeBSD Kernel Interfaces Manual NETDUMP(4)

netdump
protocol for transmitting kernel dumps to a remote server

To compile netdump client support into the kernel, place the following line in your kernel configuration file:
options NETDUMP

netdump is a UDP-based protocol for transmitting kernel dumps to a remote host. A netdump client is a panicking kernel, and a netdump server is a host running the netdump daemon, available in ports as ports/ftp/netdumpd. netdump clients are configured using the dumpon(8) utility.
netdump client messages consist of a fixed-size header followed by a variable-sized payload. The header contains the message type, a sequence number, the offset of the payload data in the kernel dump, and the length of the payload data (not including the header). The message types are HERALD, FINISHED, KDH, VMCORE, and EKCD_KEY. netdump server messages have a fixed size and contain only the sequence number of the client message. These messages indicate that the server has successfully processed the client message with the corresponding sequence number. All client messages are acknowledged this way. Server messages are always sent to port 20024 of the client.
To initiate a netdump, the client sends a HERALD message to the server at port 20023. The client may include a relative path in its payload, in which case the netdump server should attempt to save the dump at that path relative to its configured dump directory. The server will acknowledge the HERALD using a random source port, and the client must send all subsequent messages to that port.
The KDH, VMCORE, and EKCD_KEY message payloads contain the kernel dump header, dump contents, and dump encryption key respectively. The offset in the message header should be treated as a seek offset in the corresponding file. There are no ordering requirements for these messages.
A netdump is completed by sending the FINISHED message to the server.
The following network drivers support netdump: alc(4), bge(4), bnxt(4), bxe(4), cxgb(4), em(4), igb(4), ix(4), re(4), vtnet(4).

The following variables are available as both sysctl(8) variables and loader(8) variables:
net.netdump.debug
Control debug message verbosity. Debug messages are disabled by default, but are useful when troubleshooting or when developing driver support.
net.netdump.path
Specify a path relative to the server's dump directory in which to store the dump. For example, if the netdump server is configured to store dumps in /var/crash, a path of “foo” will cause the server to attempt to store dumps from the client in /var/crash/foo. The server will not automatically create the relative directory.

decryptcore(8), dumpon(8), savecore(8)

netdump client support first appeared in FreeBSD 12.0.

Only IPv4 is supported.
netdump may only be used after the kernel has panicked.
June 8, 2018 FreeBSD 12.0-RELEASE

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