A "connection filter" is a piece of code that is responsible for handling a range of operations
of curl's connections: reading, writing, waiting on external events, connecting and closing down - to name the most important ones.
The most important feat of connection filters is that they can be stacked on top of each other (or "chained" if you prefer that metaphor). In the common scenario that you want to retrieve a `https:` url with curl, you need 2 basic things to send the request and get the response: a TCP connection, represented by a `socket` and a SSL instance en- and decrypt over that socket. You write your request to the SSL instance, which encrypts and writes that data to the socket, which then sends the bytes over the network.
With connection filters, curl's internal setup will look something like this (cf for connection filter):
While connection filters all do different things, they look the same from the "outside". The code in `data` and `conn` does not really know **which** filters are installed. `conn` just writes into the first filter, whatever that is.
Same is true for filters. Each filter has a pointer to the `next` filter. When SSL has encrypted the data, it does not write to a socket, it writes to the next filter. If that is indeed a socket, or a file, or an HTTP/2 connection is of no concern to the SSL filter.
Before `Curl_easy` can send the request, the connection needs to be established. This means that all connection filters have done, whatever they need to do: waiting for the socket to be connected, doing the TLS handshake, performing the HTTP tunnel request, etc. This has to be done in reverse order: the last filter has to do its connect first, then the one above can start, etc.
The filter type `cft` is a singleton, one static struct for each type of filter. The `ctx` is where a filter will hold its specific data. That varies by filter type. An http-proxy filter will keep the ongoing state of the CONNECT here, but free it after its has been established. The SSL filter will keep the `SSL*` (if OpenSSL is used) here until the connection is closed. So, this varies.
Several things, that before were kept in `struct connectdata`, will now go into the `filter->ctx`*when needed*. So, the memory footprint for connections that do *not* use an http proxy, or socks, or https will be lower.
As to transfer efficiency, writing and reading through a filter comes at near zero cost *if the filter does not transform the data*. An http proxy or socks filter, once it is connected, will just pass the calls through. Those filters implementations will look like this:
*`TCP`, `UDP`, `UNIX`: filters that operate on a socket, providing raw I/O.
*`SOCKET-ACCEPT`: special TCP socket that has a socket that has been `accept()`ed in a `listen()`
*`SSL`: filter that applies TLS en-/decryption and handshake. Manages the underlying TLS backend implementation.
*`HTTP-PROXY`, `H1-PROXY`, `H2-PROXY`: the first manages the connection to a HTTP proxy server and uses the other depending on which ALPN protocol has been negotiated.
*`SOCKS-PROXY`: filter for the various SOCKS proxy protocol variations
*`HAPROXY`: filter for the protocol of the same name, providing client IP information to a server.
*`HTTP/2`: filter for handling multiplexed transfers over a HTTP/2 connection
*`HTTP/3`: filter for handling multiplexed transfers over a HTTP/3+QUIC connection
*`HAPPY-EYEBALLS`: meta filter that implements IPv4/IPv6 "happy eyeballing". It creates up to 2 sub-filters that race each other for a connection.
*`SETUP`: meta filter that manages the creation of sub-filter chains for a specific transport (e.g. TCP or QUIC).
*`HTTPS-CONNECT`: meta filter that races a TCP+TLS and a QUIC connection against each other to determine if HTTP/1.1, HTTP/2 or HTTP/3 shall be used for a transfer.
Meta filters are combining other filters for a specific purpose, mostly during connection establishment. Other filters like `TCP`, `UDP` and `UNIX` are only to be found at the end of filter chains. SSL filters provide encryption, of course. Protocol filters change the bytes sent and received.
Filter types carry flags that inform what they do. These are (for now):
*`CF_TYPE_IP_CONNECT`: this filter type talks directly to a server. This does not have to be the server the transfer wants to talk to. For example when a proxy server is used.
*`CF_TYPE_SSL`: this filter type provides encryption.
*`CF_TYPE_MULTIPLEX`: this filter type can manage multiple transfers in parallel.
Filter types can combine these flags. For example, the HTTP/3 filter types have `CF_TYPE_IP_CONNECT`, `CF_TYPE_SSL` and `CF_TYPE_MULTIPLEX` set.
Flags are useful to extrapolate properties of a connection. To check if a connection is encrypted, libcurl inspect the filter chain in place, top down, for `CF_TYPE_SSL`. If it finds `CF_TYPE_IP_CONNECT` before any `CF_TYPE_SSL`, the connection is not encrypted.
For example, `conn1` is for a `http:` request using a tunnel through a HTTP/2 `https:` proxy. `conn2` is a `https:` HTTP/2 connection to the same proxy. `conn3` uses HTTP/3 without proxy. The filter chains would look like this (simplified):
Inspecting the filter chains, `conn1` is seen as unencrypted, since it contains an `IP_CONNECT` filter before any `SSL`. `conn2` is clearly encrypted as an `SSL` flagged filter is seen first. `conn3` is also encrypted as the `SSL` flag is checked before the presence of `IP_CONNECT`.
Similar checks can determine if a connection is multiplexed or not.
## Filter Tracing
Filters may make use of special trace macros like `CURL_TRC_CF(data, cf, msg, ...)`. With `data` being the transfer and `cf` being the filter instance. These traces are normally not active and their execution is guarded so that they are cheap to ignore.
Users of `curl` may activate them by adding the name of the filter type to the `--trace-config` argument. For example, in order to get more detailed tracing of a HTTP/2 request, invoke curl with:
Which will give you trace output with time information, transfer+connection ids and details from the `HTTP/2` filter. Filter type names in the trace config are case insensitive. You may use `all` to enable tracing for all filter types. When using `libcurl` you may call `curl_global_trace(config_string)` at the start of your application to enable filter details.
## Meta Filters
Meta filters is a catch-all name for filter types that do not change the transfer data in any way but provide other important services to curl. In general, it is possible to do all sorts of silly things with them. One of the commonly used, important things is "eyeballing".
The `HAPPY-EYEBALLS` filter is involved in the connect phase. It's job is to try the various IPv4 and IPv6 addresses that are known for a server. If only one address family is known (or configured), it tries the addresses one after the other with timeouts calculated from the amount of addresses and the overall connect timeout.
When more than one address family is to be tried, it splits the address list into IPv4 and IPv6 and makes parallel attempts. The connection filter chain will look like this:
The modular design of connection filters and that we can plug them into each other is used to control the parallel attempts. When a `TCP` filter does not connect (in time), it is torn down and another one is created for the next address. This keeps the `TCP` filter simple.
The `HAPPY-EYEBALLS` on the other hand stays focused on its side of the problem. We can use it also to make other type of connection by just giving it another filter type to try and have happy eyeballing for QUIC:
```
* create connection for --http3-only https://curl.se
And when we plug these two variants together, we get the `HTTPS-CONNECT` filter type that is used for `--http3` when **both** HTTP/3 and HTTP/2 or HTTP/1.1 shall be attempted:
