socket_wrapper - A library passing all socket communications
through unix sockets.
socket_wrapper aims to help client/server software development
teams willing to gain full functional test coverage. It makes possible to
run several instances of the full software stack on the same machine and
perform locally functional testing of complex network configurations.
•Redirects all network communication to happen
over Unix sockets.
•Support for IPv4 and IPv6 socket and addressing
•Ability to capture network traffic in pcap
•Passing IP sockets (up to 6) via SCM_RIGHTS is
supported, but pcap support only works reliable if the socket is used by a
single process at a time.
The user defines a directory where to put all the unix
sockets using the environment variable
"SOCKET_WRAPPER_DIR=/path/to/socket_dir". When a server opens a port
or a client wants to connect, socket_wrapper will translate IP addresses to a
special socket_wrapper name and look for the relevant Unix socket in the
By default the loopback IPv4 network
"127.0.0.0/8" and the "127.0.0.x" can be used. In order to
make more realistic testing possible it is possible to use the
"10.0.0.0/8" IPv4 network instead. But note within
"10.0.0.0/8" only "10.53.57.<ID>" can be used, but
the broadcast address is "10.255.255.255". The following two value
are allowed: SOCKET_WRAPPER_IPV4_NETWORK="127.0.0.0" (the default)
Additionally, the default interface to be used by an
application is defined with
"SOCKET_WRAPPER_DEFAULT_IFACE=<ID>" where the valid range for
<ID> starts with 1 (the default) and ends with 64. This is analogous to
use the IPv4 addresses
"127.0.0.<ID>"/"10.53.57.<ID>" or IPv6
addresses "fd00::5357:5f<IDx>" (where <IDx> is a
hexadecimal presentation of <ID>). You should always set the default
interface. If you listen on INADDR_ANY then it will use the default interface
to listen on.
When debugging, it is often interesting to investigate
the network traffic between the client and server within your application. If
you define SOCKET_WRAPPER_PCAP_FILE=/path/to/file.pcap, socket_wrapper will
dump all your network traffic to the specified file. After the test has been
finished you’re able to open the file for example with Wireshark.
With this variable you can change the MTU size. However
we do not recomment to do that as the default size of 1500 byte is best for
formatting PCAP files.
The minimum value you can set is 512 and the maximum 32768.
This variable can be used to set the maximum number of
sockets to be used by an application.
The default value is set to 65535 and the maximum 256000.
If you need to see what is going on in socket_wrapper
itself or try to find a bug, you can enable logging support in socket_wrapper
if you built it with debug symbols.
•0 = ERROR
•1 = WARNING
•2 = DEBUG
•3 = TRACE
This allows you to disable deep binding in
socket_wrapper. This is useful for running valgrind tools or sanitizers like
(address, undefined, thread).
SOCKET_WRAPPER_DIR is resolved by socket_wrapper using
. Given that Unix sockets are constructed relative to this
directory, the resulting path can sometimes be too long to allow valid socket
paths to be constructed due to length restrictions. Setting this variable (to
any value) allows socket_wrapper to fall back to the original value of
SOCKET_WRAPPER_DIR if realpath(3)
makes it too long to be usable.
# Open a console and create a directory for the unix sockets.
$ mktemp -d
# Then start nc to listen for network traffic using the temporary directory.
$ LD_PRELOAD=libsocket_wrapper.so \
SOCKET_WRAPPER_DEFAULT_IFACE=10 nc -v -l 127.0.0.10 7
# (If nc, listens on 0.0.0.0 then listener will be open on 127.0.0.10 because
# it is the default interface)
# Now open another console and start 'nc' as a client to connect to the server:
$ LD_PRELOAD=libsocket_wrapper.so \
SOCKET_WRAPPER_DEFAULT_IFACE=100 nc -v 127.0.0.10 7
# (The client will use the address 127.0.0.100 when connecting to the server)
# Now you can type 'Hello!' which will be sent to the server and should appear
# in the console output of the server.
Socket wrapper advanced helpers.
Applications with the need to alter their behaviour when socket
wrapper is active, can link use these functions.
By default it’s required for applications to use any of
these functions as libsocket_wrapper.so is injected at runtime via
Applications using these functions should link against
libsocket_wrapper_noop.so by using -lsocket_wrapper_noop, or implement their
own noop stubs.
•This returns true when socket wrapper is actively
void socket_wrapper_indicate_no_inet_fd(int fd);
•This allows socket_wrapper aware applications to
indicate that the given fd does not belong to an inet socket.
•socket_wrapper may not be able to intercept the
__close_nocancel() syscall made from within libc.so. As result it’s
possible that the in memory meta date of socket_wrapper references stale file
descriptors, which are already reused for unrelated kernel objects, e.g.
files, directories, ...
•Socket wrapper already intercepts a lot of
unrelated functions like eventfd(), timerfd_create(), ... in order to remove
stale meta data for the returned fd, but it will never be able to handle all
applications a way to do the same, explicitly without waiting for new syscalls
to be added to libsocket_wrapper.so.
•This is a no-op if socket_wrapper is not in use
or if the there is no in memory meta data for the given fd.