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README.md

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+# pcap-web-tools
+
+A small web application with tools for the pcap packet capture format.
+Plain Go: the standard library, `html/template` and embedded static files. There is no JavaScript frontend.
+
+## Running
+
+```sh
+make run            # or: go run . -addr localhost:8080
+```
+
+Then open http://localhost:8080.
+
+## Tools
+
+### Hex → pcap (`/hex2pcap`)
+
+Builds a single-packet pcap file from pasted hex.
+
+- Three fields: **layer 2**, **layer 3** and **payload**. Each one is hex-decoded and the three are concatenated. Any field can be empty, so you can also paste a whole packet into one field.
+- The hex input tolerates whitespace, `:`, `-`, `,`, `.`, `0x` and `\x`.
+- **Recalculate IP lengths and checksums** (optional) rewrites:
+  - IPv4 total length and header checksum, or the IPv6 payload length
+  - the UDP length
+  - the TCP, UDP, ICMP, ICMPv6 or SCTP (CRC32c) checksum
+
+  It walks IPv6 extension headers and uses the final destination from a routing header in the pseudo-header. Fragments only get their IP header fixed.
+
+  The IP header is taken to start at the layer 3 field. If that field is empty, the tool parses the link-layer header to find it: Ethernet (including VLAN and QinQ), Linux SLL or SLL2.
+- **Link type**: *Auto* picks Raw IP when layer 2 is empty and layer 3 is set, and Ethernet otherwise. You can also pick a link type from the list or enter any numeric LINKTYPE.
+- **Preview** shows the fixups that were applied and a hex dump coloured by layer, with changed bytes underlined. **Download pcap** returns the file.
+
+## Layout
+
+```
+main.go                 entry point, HTTP server
+internal/web/           handlers, tool registry, embedded templates and CSS
+internal/pcap/          pcap file writer
+internal/inet/          IPv4/IPv6 length and checksum fixups, IP header locator
+internal/hexutil/       lenient hex decoding
+```
+
+To add a tool, add an entry to `tools` in `internal/web/server.go`, a template in `internal/web/templates/`, and register its routes in `web.New`.