Hacking

while working with image files, I needed a simple way to analyze content of a picture; so I wrote this tool that “walks” inside a PNG file and reports all the chunks seen; this is intended to be expanded with more features in a future. package main import ( "encoding/binary" "fmt" "io" "os" ) type chunk struct { Length uint32 ChunkType [4]byte } func main() { if len(os.Args) != 2 { fmt.Printf("Usage: %s filename.png\n", os.Args[0]) os.Exit(1) } f, err := os.Open(os.Args[1]) if err != nil { panic(err) } defer f.Close() header := make([]byte, 8) _, err = f.Read(header) fmt.Printf("header: %v\n", header) if err != nil { panic(err) } var data chunk var offset int64 offset = 8 for { err = binary.Read(f, binary.BigEndian, &data) if err != nil { if err == io.EOF { break } panic(err) } fmt.Printf("Offset: %d chunk len=%d, type: %s\n", offset, data.Length, string(data.ChunkType[:4])) f.Seek(int64(data.Length+4), io.SeekCurrent) offset += int64(data.Length) + 4 } } usage: ...

This is an example of using goproxy, a fast and robust multithread proxy engine to develop an HTTP proxy that rewrites content on the fly, with multiple search and substitutions. It can be useful for debugging and other less noble (but useful) purposes … // rewriting_proxy project main.go package main import ( "bytes" "flag" "io/ioutil" "log" "net/http" "github.com/elazarl/goproxy" ) var replacements = []struct { from []byte to []byte }{ {[]byte("#e8ecec"), []byte("Red")}, // ugly colors!! {[]byte("Comic Sans MS"), []byte("Lucida Sans Unicode")}, // for eyes sanity {[]byte("Java "), []byte("Golang ")}, // just joking } func myHandler(resp *http.Response, ctx *goproxy.ProxyCtx) *http.Response { readBody, err := ioutil.ReadAll(resp.Body) if err != nil { //TODO handle read error gracefully return resp } resp.Body.Close() for _, elem := range replacements { readBody = bytes.Replace(readBody, elem.from, elem.to, -1) } resp.Body = ioutil.NopCloser(bytes.NewReader(readBody)) return resp } func main() { verbose := flag.Bool("v", true, "should every proxy request be logged to stdout") proxy := goproxy.NewProxyHttpServer() proxy.Verbose = *verbose proxy.OnResponse().DoFunc(myHandler) log.Fatal(http.ListenAndServe(":8081", proxy)) }