What version of Go are you using (go version)?

go version go1.13.8 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13.8/libexec"
GOSUMDB="sum.golang.org"
GOTOOLDIR="/usr/local/Cellar/go/1.13.8/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Set the ipv4 header total length field

// ...

iph := &ipv4.Header{
	Version:  ipv4.Version,
	Len:      ipv4.HeaderLen,
	TOS:      0,
	TotalLen: ipv4.HeaderLen + len(msg),
	TTL:      64,
	Protocol: protocol,
	Dst:      dst,
	Src:      src,
}

ip, _ := iph.Marshal()

// ... packet has incorrectly set total length - ip[2:4] bytes are reversed

What did you expect to see?

for example if packet total length is 37 I expect to see [0 37] set on byte index 2 and 3 (ip[2:4])

What did you see instead?

for example if packet total length is 37 I see [37 0] set on byte index 2 and 3 (ip[2:4])