doc: Add a README.hurd.md file.

Author: infrastation

CHANGES

@@ -32,6 +32,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
     Documentation:
       Document a standard format for writing out BPF filter programs.
       Explain "other addresses" in pcap_findalldevs(3PCAP).
+      Add a README.hurd.md file.
     Building and testing:
       Apply GNU Hurd support patch from the Debian package.
       CI: Introduce and use LIBPCAP_CMAKE_TAINTED.

INSTALL.md

@@ -195,6 +195,7 @@ in `/usr/include/sys/dlpi.h`, and find the corresponding value.
 	doc/README.dag	    - notes on using libpcap to capture on Endace DAG devices
 	doc/README.haiku.md - notes on using libpcap on Haiku
 	doc/README.hpux	    - notes on using libpcap on HP-UX
+	doc/README.hurd.md  - notes on using libpcap on GNU/Hurd
 	doc/README.linux    - notes on using libpcap on Linux
 	doc/README.macos    - notes on using libpcap on macOS
 	doc/README.septel   - notes on using libpcap to capture on Intel/Septel devices

Makefile.in

@@ -252,6 +252,7 @@ EXTRA_DIST = \
 	doc/README.dag \
 	doc/README.haiku.md \
 	doc/README.hpux \
+	doc/README.hurd.md \
 	doc/README.linux \
 	doc/README.macos \
 	doc/README.septel \

doc/README.hurd.md

@@ -0,0 +1,22 @@
+# Compiling and using libpcap on GNU/Hurd
+
+libpcap on Hurd currently does not support packet capture on the loopback
+network interface.  BPF works in kernel only if the filter length is not
+greater that 31 BPF instructions and does not use `BPF_MOD` or `BPF_XOR`.
+Packet timestamping always occurs in userland.  Wireless monitor mode is not
+supported.  Packet capture sees only packets received on the interface.
+[**pcap_set_buffer_size**](https://www.tcpdump.org/manpages/pcap_set_buffer_size.3pcap.html)(3PCAP)
+has no effect.
+[**pcap_setdirection**](https://www.tcpdump.org/manpages/pcap_setdirection.3pcap.html)(3PCAP)
+is not supported.
+[**pcap_set_promisc**](https://www.tcpdump.org/manpages/pcap_set_promisc.3pcap.html)(3PCAP)
+has no effect.
+
+## Debian GNU/Hurd 12
+
+* flex 2.6.4 and GNU Bison 3.8.2 work.
+* CMake 3.25.1 works.
+* GCC 12.2.0 and Clang 14.0.6 work.
+
+For reference, the tests were done using a system installed from the 2023-06-08
+snapshot from [here](https://cdimage.debian.org/cdimage/ports/12.0/hurd-i386/).