sandbox: eth: Add support for using the 'lo' interface

The 'lo' interface on Linux doesn't support thinks like ARP or
link-layer access like we use to talk to a normal network interface.
A higher-level network API must be used to access localhost.

As written, this interface is limited to not supporting ICMP since the
API doesn't allow the socket to be opened for all IP traffic and be able
to receive at the same time. UDP is far more useful to test with, so it
was selected over ICMP. Ping won't work, but things like TFTP should
work.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 9f5d3f71d5f..08489e38807 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -241,6 +241,28 @@ dhcp
 set serverip WWW.XXX.YYY.ZZZ
 tftpboot u-boot.bin
 
+The bridge also support (to a lesser extent) the localhost inderface, 'lo'.
+
+The 'lo' interface cannot use the RAW AF_PACKET API because the lo interface
+doesn't support Ethernet-level traffic. It is a higher-level interface that is
+expected only to be used at the AF_INET level of the API. As such, the most raw
+we can get on that interface is the RAW AF_INET API on UDP. This allows us to
+set the IP_HDRINCL option to include everything except the Ethernet header in
+the packets we send and receive.
+
+Because only UDP is supported, ICMP traffic will not work, so expect that ping
+commands will time out.
+
+The default device tree for sandbox includes an entry for lo on the sandbox
+host machine whose alias is "eth5". The following is an example of a network
+operation being tested on the lo interface.
+
+TFTP
+....
+
+set ethact eth5
+tftpboot u-boot.bin
+
 
 SPI Emulation
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