- How it works
- Differences from Camo
- Installing pre-built binaries
- Running on Heroku
- Securing an installation
- Additional tools
- Alternative Implementations
Go version of Camo server.
Camo is a special type of image proxy that proxies non-secure images over SSL/TLS. This prevents mixed content warnings on secure pages.
It works in conjunction with back-end code that rewrites image URLs and signs them with an HMAC.
How it works
The general steps are as follows:
- A client requests a page from the web app.
- The original URL in the content is parsed.
- An HMAC signature of the url is generated.
- The url and hmac are encoded.
- The encoded url and hmac are placed into the expected format, creating the signed url.
- The signed url replaces the original image URL.
- The web app returns the content to the client.
- The client requets the signed url from Go-Camo.
- Go-Camo validates the HMAC, decodes the URL, then requests the content from the origin server and streams it to the client.
+----------+ request +-------------+ | |----------------------------->| | | | | | | | | web-app | | | img src=https://go-camo/url | | | |<-----------------------------| | | | +-------------+ | client | | | https://go-camo/url +-------------+ http://some/img | |----------------------------->| |---------------> | | | | | | | go-camo | | | img data | | img data | |<-----------------------------| |<--------------- | | +-------------+ +----------+
Go-Camo supports both hex and base64 encoded urls at the same time.
|hex||longer, case insensitive, slightly faster|
|base64||shorter, case sensitive, slightly slower|
Benchmark results with go1.8:
BenchmarkHexEncoder-2 500000 2505 ns/op BenchmarkB64Encoder-2 500000 2576 ns/op BenchmarkHexDecoder-2 500000 2542 ns/op BenchmarkB64Decoder-2 500000 2687 ns/op
For examples of url generation, see the examples directory.
While Go-Camo will support proxying HTTPS images as well, for performance reasons you may choose to filter HTTPS requests out from proxying, and let the client simply fetch those as they are. The linked code examples do this.
Note that it is recommended to front Go-Camo with a CDN when possible.
Differences from Camo
- Go-Camo supports 'Path Format' url format only. Camo's "Query String Format" is not supported.
- Go-Camo supports "allow regex host filters".
- Go-Camo supports client http keep-alives.
- Go-Camo provides native SSL support.
- Go-Camo provides native HTTP/2 support (if built using >=go1.6).
- Go-Camo supports using more than one os thread (via GOMAXPROCS) without the need of multiple instances or additional proxying.
- Go-Camo builds to a static binary. This makes deploying to large numbers of servers a snap.
- Go-Camo supports both Hex and Base64 urls. Base64 urls are smaller, but case sensitive.
- Go-Camo supports HTTP HEAD requests.
- Go-Camo allows custom default headers to be added -- useful for things like adding HSTS headers.
Installing pre-built binaries
Download the tarball appropriate for your OS/ARCH from releases. Extract, and copy files to desired locations.
- go (version 1.8 recommended)
# show make targets $ make Available targets: help this help clean clean up all build binaries and man pages test run tests cover run tests with cover output build-setup fetch dependencies build build all man build all man pages tar build release tarball cross-tar cross compile and build release tarballs # fetch vendor dependencies $ make build-setup # build all binaries and man pages # strips debug symbols by default $ make all # do not strip debug symbols $ make all GOBUILD_LDFLAGS=""
By default, Go-Camo builds with
-tags netgo. However, for Go versions
older than 1.5, this may not result in Go-Camo using the netgo resolver unless
your Go stdlib is also compiled with
-tags netgo. For this reason, it is
required to build with at least go-1.5. Building with the latest Go version is
$ go-camo -k "somekey"
Go-Camo does not daemonize on its own. For production usage, it is recommended to launch in a process supervisor, and drop privileges as appropriate.
For the reasoning behind lack of daemonization, see daemontools/why. In addition, the code is much simpler because of it.
Running on Heroku
In order to use this on Heroku with the provided Procfile, you need to:
- Create an app specifying the https://github.com/kr/heroku-buildpack-go buildpack
GOCAMO_HMACto the key you are using
Securing an installation
go-camo will generally do what you tell it to with regard to fetching signed urls. There is some limited support for trying to preventing dns rebinding attacks.
go-camo will attempt to reject any address matching an rfc1918 network block, or a private scope ipv6 address, be it in the url or via resulting hostname resolution. Do note, however, that this does not provide protecton for a network that uses public address space (ipv4 or ipv6), or some of the more exotic ipv6 addresses.
The list of networks rejected include...
||ipv4 link local|
||ipv6 link local|
||deprecated ipv6 site-local|
||IPv4-mapped IPv6 address|
More generally, it is recommended to either:
- Run go-camo on an isolated instance (physical, vlans, firewall rules, etc).
- Run a local resolver for go-camo that returns NXDOMAIN responses for
addresses in blacklisted ranges (for example unbound's
private-addressfunctionality). This is also useful to help prevent dns rebinding in general.
GOCAMO_HMAC- HMAC key to use.
Command line flags
$ go-camo -h Usage: go-camo [OPTIONS] Application Options: -k, --key= HMAC key -H, --header= Extra header to return for each response. This option can be used multiple times to add multiple headers --stats Enable Stats --no-log-ts Do not add a timestamp to logging --allow-list= Text file of hostname allow regexes (one per line) --max-size= Max response image size (KB) (5120) --timeout= Upstream request timeout (4s) --max-redirects= Maximum number of redirects to follow (3) --no-fk Disable frontend http keep-alive support --no-bk Disable backend http keep-alive support --listen= Address:Port to bind to for HTTP (0.0.0.0:8080) --ssl-listen= Address:Port to bind to for HTTPS/SSL/TLS --ssl-key= ssl private key (key.pem) path --ssl-cert= ssl cert (cert.pem) path -v, --verbose Show verbose (debug) log level output -V, --version print version and exit Help Options: -h, --help Show this help message
If an allow-list file is defined, that file is read and each line converted into a hostname regex. If a request does not match one of the listed host regex, then the request is denied.
If stats flag is provided, then the service will track bytes and clients
served, and offer them up at an http endpoint
/status via HTTP GET request.
If the HMAC key is provided on the command line, it will override (if present), an HMAC key set in the environment var.
Additional default headers (sent on every response) can also be set. The
-H, --header argument may be specified many times.
The list of default headers sent are:
X-Content-Type-Options: nosniff X-XSS-Protection: 1; mode=block Content-Security-Policy: default-src 'none'; img-src data:; style-src 'unsafe-inline'
As an example, if you wanted to return a
by default, you could add this to the command line:
-H "Strict-Transport-Security: max-age=16070400"
Go-Camo includes a couple of additional tools.
url-tool utility provides a simple way to generate signed URLs from the command line.
$ url-tool -h Usage: url-tool [OPTIONS] <decode | encode> Application Options: -k, --key= HMAC key -p, --prefix= Optional url prefix used by encode output Help Options: -h, --help Show this help message Available commands: decode Decode a url and print result encode Encode a url and print result
# hex $ url-tool -k "test" encode -p "https://img.example.org" "http://golang.org/doc/gopher/frontpage.png" https://img.example.org/0f6def1cb147b0e84f39cbddc5ea10c80253a6f3/687474703a2f2f676f6c616e672e6f72672f646f632f676f706865722f66726f6e74706167652e706e67 $ url-tool -k "test" decode "https://img.example.org/0f6def1cb147b0e84f39cbddc5ea10c80253a6f3/687474703a2f2f676f6c616e672e6f72672f646f632f676f706865722f66726f6e74706167652e706e67" http://golang.org/doc/gopher/frontpage.png # base64 $ url-tool -k "test" encode -b base64 -p "https://img.example.org" "http://golang.org/doc/gopher/frontpage.png" https://img.example.org/D23vHLFHsOhPOcvdxeoQyAJTpvM/aHR0cDovL2dvbGFuZy5vcmcvZG9jL2dvcGhlci9mcm9udHBhZ2UucG5n $ url-tool -k "test" decode "https://img.example.org/D23vHLFHsOhPOcvdxeoQyAJTpvM/aHR0cDovL2dvbGFuZy5vcmcvZG9jL2dvcGhlci9mcm9udHBhZ2UucG5n" http://golang.org/doc/gopher/frontpage.png
simple-server utility has moved to its own repo.
Released under the MIT
file for details.