Merge pull request #7 from nberlee/docs

chore: update docs

Author: nberlee

LICENSE

@@ -1,6 +1,7 @@
 MIT License
 
 Copyright (c) 2018 Cihangir Akturk
+Copyright (c) 2023 Nico Berlee
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -1,16 +1,150 @@
-###  Netstat implementation for Talos Linux
+# go-netstat
+go-netstat is a Golang library that provides a fast and multi-threaded implementation of the netstat utility on Linux. It is designed to be feature-complete with the net-tools netstat implementation and supports network namespaces for containers. It was written with Talos Linux in mind.
 
+## Installation
+You can install go-netstat by running the following command:
+```bash
+go get github.com/nberlee/go-netstat/netstat
 ```
-Usage of ./go-netstat:
-  -4	display only IPv4 sockets
-  -6	display only IPv6 sockets
-  -U	display udplite sockets
-  -a	display all sockets (default: connected)
-  -l	display listening server sockets
-  -n	don't resolve names (default true)
-  -p	display PID/Program name for sockets
-  -t	display tcp sockets
-  -u	display udp sockets
-  -w	display raw sockets
+
+## Usage
+Here's an example of how to use go-netstat:
+
+```go
+import (
+    "context"
+    "fmt"
+
+    "github.com/nberlee/go-netstat/netstat"
+)
+
+func main() {
+    ctx := context.Background()
+
+    features := netstat.EnableFeatures{
+        TCP:           true,
+        TCP6:          true,
+        UDP:           true,
+        UDP6:          true,
+        UDPLite:       true,
+        UDPLite6:      true,
+        Raw:           true,
+        Raw6:          true,
+        PID:           true,
+        NoHostNetwork: false,
+        AllNetNs:      true,
+        NetNsName:     []string{},
+        NetNsPids:     []uint32{},
+    }
+
+    fn := netstat.NoopFilter
+
+    netstatResp, err := netstat.Netstat(ctx, features, fn)
+    if err != nil {
+        panic(err)
+    }
+
+    for _, entry := range netstatResp {
+        fmt.Println(entry)
+    }
+}
+```
+The `features` struct specifies which types of sockets and connections to include in the result, while the `fn` function can be used to filter the results further.
+
+## Contributing
+We welcome contributions to go-netstat! If you have bug fixes, feature requests, or performance improvements, feel free to submit a pull request. We are especially interested in contributions that are close to the core functionality and that benefit Talos Linux netstat.
+
+## Credits
+The initial version of go-netstat was written by Cihangir Akturk, who contributed the functions ParseAddr, ParseIpv4, and ParseIpv6. Nico Berlee later rewrote and reworked every other function to make it faster by multi-threading and to add features like network namespaces, udplite+raw support and tests.
+
+## License
+go-netstat is released under the MIT License. See [LICENSE](https://github.com/nberlee/go-netstat/blob/main/LICENSE) for details.
+
+## Types
+### EnableFeatures
+The EnableFeatures struct specifies which types of sockets and connections to include in the result.
+```go
+type EnableFeatures struct {
+    TCP           bool
+    TCP6          bool
+    UDP           bool
+    UDP6          bool
+    UDPLite       bool
+    UDPLite6      bool
+    Raw           bool
+    Raw6          bool
+    PID           bool
+    NoHostNetwork bool
+    AllNetNs      bool
+    NetNsName     []string
+    NetNsPids     []uint32
+}
+```
+
+### SockTabEntry
+The SockTabEntry struct represents each line of the /proc/net/[tcp|udp] file.
+```go
+type SockTabEntry struct {
+    Transport      string
+    LocalEndpoint  *SockEndpoint
+    RemoteEndpoint *SockEndpoint
+    State          SkState
+    TxQueue        uint64
+    RxQueue        uint64
+    Tr             TimerActive
+    TimerWhen      uint64
+    Retrnsmt       uint64
+    UID            uint32
+    Timeout        uint64
+    Inode          uint64
+    Ref            uint64
+    Process        *common.Process
+    NetNS          string
+}
 ```
 
+### SockEndpoint
+
+The `SockEndpoint` struct represents an IP address and port.
+
+```go
+type SockEndpoint struct {
+    IP   net.IP
+    Port uint16
+}
+```
+The `IP` field can be an IPv4 or IPv6 address.
+
+## Filter Function
+The fn function is used to filter the results of the Netstat function.
+```go
+type FilterFunc func(*SockTabEntry) bool
+```
+
+Here are some examples of filter functions:
+```go
+func NoopFilter(s *SockTabEntry) bool {
+    return true
+}
+
+func ConnectedFilter(s *SockTabEntry) bool {
+    return s.State != Listen
+}
+
+func ListeningFilter(s *SockTabEntry) bool {
+    return s.State == Listen
+}
+```
+
+## Design decisions
+* /proc/net/[tcp|udp|udplite|raw] are chosen over syscalls as accessing directly kernel data structures is can be faster than making syscalls with context switching overhead.
+* `go-netstat` does not enter an network namespace, instead it gets the process id and reads the /proc/`processid`/net/[tcp|udp|udplite|raw]. This is safer and faster.
+
+### Flows
+
+#### Network namespace
+Network Namespace Name -> Find in `/var/run/netns` -> Follow bindmount to /proc/`processid`/ns/net, get the symlink which is if for `net:[12345678]`. Search in /proc/`processid`/ns/net for the file descriptor with the same symlink.
+
+#### Process ID + Name
+To enrich netstat entries with the process id, the inode is matched against /proc/`processid`/fd/* symlinks.
+