Signed-off-by: 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>

…196)

Motivation:
-----------
Enable routing DNS-over-HTTPS traffic through different network paths per instance. Primary use case: running multiple `https_dns_proxy` instances on a router where each WiFi LAN gateway routes through a different WireGuard tunnel to different geographic locations. This allows DNS traffic from different WiFi networks to exit via different VPN endpoints.

Implementation:
---------------
- Added `source_addr` field to `struct Options`
- New `-S` command-line flag to specify source IPv4/v6 address
- Uses `CURLOPT_INTERFACE` to bind outbound HTTPS connections
- Backward compatible: without -S, uses system default routing
- Logs `Using source address: X` at `debug` level when configured

Example Usage:
--------------
### Instance 1: WiFi LAN 1 gateway (routes via WireGuard to US)
```shell
https_dns_proxy -a 192.168.1.1 -p 53 -S 192.168.1.1 \
  -r https://security.cloudflare-dns.com/dns-query \
  -b 1.1.1.2,1.0.0.2
```
### Instance 2: WiFi LAN 2 gateway (routes via WireGuard to EU)
```shell
https_dns_proxy -a 192.168.2.1 -p 53 -S 192.168.2.1 \
  -r https://security.cloudflare-dns.com/dns-query \
  -b 1.1.1.2,1.0.0.2
```

Each instance binds to its WiFi interface address for both listening and outbound HTTPS, ensuring traffic routes through the correct WireGuard tunnel configured for that interface.

Verification:
-------------
With `-S` flag, CURL binds to specified source address:

```
  [D] https_client.c:260 F0C1: Requesting HTTP/2
  [D] https_client.c:324 F0C1: Using source address: 192.168.1.1
  [D] https_client.c:218 F0C1: * Added security.cloudflare-dns.com:443:1.0.0.2,1.1.1.2,... to DNS cache
  [D] https_client.c:218 F0C1: * Hostname security.cloudflare-dns.com was found in DNS cache
  [D] https_client.c:94 curl opened socket: 9
  [D] https_client.c:218 F0C1: *   Trying 1.0.0.2:443...
  [D] https_client.c:218 F0C1: * Name '192.168.1.1' family 2 resolved to '192.168.1.1' family 2
  [D] https_client.c:218 F0C1: * Local port: 0
  [D] https_client.c:639 Reserved new io event: 0xffffc0ed3568
  [D] https_client.c:218 F0C1: * Connected to security.cloudflare-dns.com (1.0.0.2) port 443 (#0)
```
Without `-S` flag, no source binding (backward compatible):

```
  [D] https_client.c:260 39BF: Requesting HTTP/2
  [D] https_client.c:218 39BF: * Added security.cloudflare-dns.com:443:1.1.1.2,1.0.0.2,... to DNS cache
  [D] https_client.c:218 39BF: * Hostname security.cloudflare-dns.com was found in DNS cache
  [D] https_client.c:94 curl opened socket: 9
  [D] https_client.c:218 39BF: *   Trying 1.1.1.2:443...
  [D] https_client.c:639 Reserved new io event: 0xffffe69a0f18
  [D] https_client.c:218 39BF: * Connected to security.cloudflare-dns.com (1.1.1.2) port 443 (#0)
```
Note the presence of `Using source address` and `Name '192.168.1.1' ... resolved` lines only when `-S` is specified.

Files Modified:
---------------
- `src/options.h`: Added source_addr field
- `src/options.c`: Added -S flag parsing and help text
- `src/https_client.c`: Implemented CURLOPT_INTERFACE binding
- `tests/robot/functional_tests.robot`: Added test case
- `README.md`: Updated documentation

- main.c: fix NULL pointer dereference in https_resp_cb() — req was
  used before NULL check, and FLOG also dereferenced NULL req->tx_id
- main.c: fix potential crash in get_host_from_uri() — strlen(host)
  was called before checking curl_url_get() return code, host could
  be NULL
- dns_server_tcp.c: fix data corruption on EAGAIN — sent += len was
  executed with len == -1 when EAGAIN/EWOULDBLOCK occurred, causing
  negative offset; added continue to skip the addition
- dns_server_tcp.c: fix format string mismatches in FLOG() calls —
  format had 2 specifiers but 4 arguments were passed (ipstr, port,
  strerror, errno); changed format to include all 4 arguments
- dns_server_tcp.c: fix typo 'listaning' -> 'listening'
- https_client.c: remove deprecated CURLPIPE_HTTP1 (since libcurl
  7.62.0), keep only CURLPIPE_MULTIPLEX

Co-authored-by: afanasyev <aafanasev@rtk-soft.ru>

Change max response size from UINT16_MAX to UINT16_MAX - 2 to account
for the 2-byte length prefix in TCP DNS framing.

Add comment explaining why no race condition exists: single-threaded
event loop ensures no callbacks can execute during this function.

Add bounds check to prevent overflow when calculating needed_space.
Reject TCP requests exceeding UINT16_MAX bytes.

Store client->next pointer before freeing to prevent potential UAF.

Reduce max retries from 255 to 50 iterations. Fix bug where sent
counter was incremented on EAGAIN error.

Always free new_resp when non-NULL, regardless of size comparison.

Move NULL check before accessing req->tx_id. Include buflen in
error message for debugging.

Add 8KB size limit to prevent unbounded memory growth. Add NULL
check after malloc to prevent crash on allocation failure.

Move NULL check before accessing req->tx_id. Include buflen in
error message for debugging. Add missing return statement to
prevent dereferencing NULL after logging.

Defer flight recorder dump from SIGUSR2 handler to main loop via
ev_async to ensure fprintf() is called outside signal context.

- ring_buffer.c: Add NULL check in ring_buffer_dump() to prevent segfault
- logging.c: Add fdopen() failure check and NULL logfile after fclose
- main.c: Remove unnecessary curl_global_init() in version display
- main.c: Add bounds check before strncpy to prevent stack overflow

- dns_server.c: Close socket on bind failure to prevent FD leak
- dns_server_tcp.c: Limit buffer growth to prevent memory exhaustion
- https_client.c: Remove global state modification in HTTP version fallback
- dns_poller.c: Add bounds check before writing comma separator

- https_client.c: Set CURLOPT_NOSIGNAL to 1 to disable curl's internal
  signal handling and prevent interference with libev's signal watchers.

- main.c: Add hostname_len > 0 check to prevent strncpy underflow
- dns_server.c: Set new_resp = NULL after ares_free_string to prevent UAF
- ring_buffer.c: Set *rbp = NULL on allocation failure paths
- dns_poller.c: Fix type mismatch between size_t and ares_socklen_t
- logging.c: Add fdopen failure check in _log()

Motivation:
-----------
PR #196 added the `-S` flag to bind HTTPS connections to a source address, enabling policy-based routing. However, bootstrap DNS queries (used to resolve DoH server hostnames like "dns.google") were not bound to the source address.

This caused two issues:
1. **Privacy leak**: Bootstrap DNS queries go via default route (local ISP), exposing which DoH server you're using
2. **Routing mismatch**: HTTPS connection routes via VPN but may fail if resolved IP is unreachable from VPN

Implementation:
---------------
- Bind bootstrap DNS queries using `ares_set_local_ip4()` and `ares_set_local_ip6()` from c-ares
- Validate address family matches proxy mode (`-4`/`-6`), warn on mismatch
- Warn on invalid address literals
- Robot Framework tests for source binding and validation warnings
- Docker-based test infrastructure for CI/CD and macOS development

Example Usage:
--------------
```bash
https_dns_proxy -S 192.168.12.1 -b 1.1.1.1,8.8.8.8 -r https://dns.google/dns-query
```

With PBR rules routing traffic from source 192.168.12.1 via VPN:

```text
# Route DoH HTTPS (port 443) via VPN
config policy
	option name 'DoH WA via wg_wa'
	option interface 'wg_wa'
	option chain 'output'
	option proto 'tcp'
	option src_addr '192.168.12.1'
	option dest_port '443'

# Route bootstrap DNS (port 53) via VPN
config policy
	option name 'Bootstrap DNS WA via wg_wa'
	option interface 'wg_wa'
	option chain 'output'
	option proto 'udp'
	option src_addr '192.168.12.1'
	option dest_port '53'
	option dest_addr '1.1.1.1 8.8.8.8'
```

Both rules now match because `-S` binds both HTTPS and bootstrap DNS to the same source address.

Verification:
-------------
Bootstrap DNS bound to source address:
```
[I] dns_poller.c:163 Using source address: 192.168.12.1
[I] dns_poller.c:208 Received new DNS server IP: 142.250.80.110 for dns.google
```

Warning on address family mismatch:
```
[W] dns_poller.c:133 Bootstrap source address '::1' is IPv6, but IPv4-only mode is set
```

Warning on invalid address:
```
[W] dns_poller.c:141 Bootstrap source address 'not-an-ip' is not a valid IP literal
```

Files Modified:
---------------
- `src/dns_poller.c`: Added `set_bootstrap_source_addr()` function
- `src/dns_poller.h`: Added source_addr parameter to poller init
- `src/main.c`: Pass source_addr to dns_poller
- `src/options.c`: Fix format string type
- `tests/robot/functional_tests.robot`: Source binding and validation tests
- `tests/docker/Dockerfile`: Test image with valgrind and ctest integration
- `tests/docker/run_all_tests.sh`: Simplified test runner using Dockerfile CMD
- `CMakeLists.txt`: Fix robot test WORKING_DIRECTORY, add distclean target
- `README.md`: Update Docker test documentation
- `.gitignore`: Add build/ directory