如何在Wireshark Lua dissectors中处理横跨字段？

这里应该能解决大部分问题。（问题在于 y，因为你指出了最不重要的半字节在低位字节中，而不是在一般情况下可能期望的最重要的半字节中。）

local p_foo = Proto("foo", "FOO 协议")

local f_foo_a = ProtoField.uint8("foo.a", "A", base.DEC, nil, 0xf0)
local f_foo_b = ProtoField.uint8("foo.b", "B", base.DEC, nil, 0x0e)
local f_foo_c = ProtoField.uint8("foo.c", "C", base.DEC, nil, 0x01)

local f_foo_x = ProtoField.uint8("foo.x", "X", base.DEC, nil, 0xf0)
local f_foo_y = ProtoField.uint16("foo.y", "Y", base.DEC, nil, 0x0ff0)
local f_foo_z = ProtoField.uint8("foo.z", "Z", base.DEC, nil, 0x0f)

p_foo.fields = { f_foo_a, f_foo_b, f_foo_c, f_foo_x, f_foo_y, f_foo_z }

function p_foo.dissector(buf, pinfo, tree)
    local foo_tree = tree:add(p_foo, buf(0,-1))

    pinfo.cols.protocol:set("FOO")
    foo_tree:add(f_foo_a, buf(0, 1))
    foo_tree:add(f_foo_b, buf(0, 1))
    foo_tree:add(f_foo_c, buf(0, 1))

    foo_tree:add(f_foo_x, buf(1, 1))
    foo_tree:add(f_foo_y, buf(1, 2))
    foo_tree:add(f_foo_z, buf(2, 1))
end

-- 注册：TODO

如果您确实需要处理 y，则必须进行位交换。也许有一种更优雅的方法，但这是一个解决方案：

local p_foo = Proto("foo", "FOO 协议")

local f_foo_a = ProtoField.uint8("foo.a", "A", base.DEC, nil, 0xf0)
local f_foo_b = ProtoField.uint8("foo.b", "B", base.DEC, nil, 0x0e)
local f_foo_c = ProtoField.uint8("foo.c", "C", base.DEC, nil, 0x01)

local f_foo_x = ProtoField.uint8("foo.x", "X", base.DEC, nil, 0xf0)
local f_foo_y = ProtoField.uint16("foo.y", "Y", base.DEC, nil, 0x0ff0)
local f_foo_z = ProtoField.uint8("foo.z", "Z", base.DEC, nil, 0x0f)

p_foo.fields = { f_foo_a, f_foo_b, f_foo_c, f_foo_x, f_foo_y, f_foo_z }

nib2bin = {
    [0] = "0000", [1] = "0001",
    [2] = "0010", [3] = "0011",
    [4] = "0100", [5] = "0101",
    [6] = "0110", [7] = "0111",
    [8] = "1000", [9] = "1001",
    [10] = "1010", [11] = "1011",
    [12] = "1100", [13] = "1101",
    [14] = "1110", [15] = "1111"
}

function nibble2binary(n)
    return nib2bin[bit.band(n, 0x0f)]
end

function p_foo.dissector(buf, pinfo, tree)
    local foo_tree = tree:add(p_foo, buf(0,-1))
    local y_lsn = bit.band(buf(1, 1):uint(), 0x0f)
    local y_msn = bit.band(buf(2, 1):uint(), 0xf0)
    local y = bit.bor(y_lsn, y_msn)

    pinfo.cols.protocol:set("FOO")
    foo_tree:add(f_foo_a, buf(0, 1))
    foo_tree:add(f_foo_b, buf(0, 1))
    foo_tree:add(f_foo_c, buf(0, 1))

    foo_tree:add(f_foo_x, buf(1, 1))
    foo_tree:add(f_foo_y, buf(1, 2)):set_text(".... " ..
        nibble2binary(bit.rshift(y_msn, 4)) .. " " ..  nibble2binary(y_lsn) ..
        " .... = Y: " .. y)
    foo_tree:add(f_foo_z, buf(2, 1))
end

-- 注册：TODO