使用抽象名称空间UNIX套接字时连接被拒绝

Question

I have a strange problem with unix socket (US) using so-called abstract namespaces when using Python and "pure" C (Python 3.x but looks like 2.x have the same problem). "Normal" socket works like a charm. With "abstract" one US my code works only when I'm using same "code platform" (C or Python).

First I thought it has something to do with memset / str(n)cpy (see Can not connect to an abstract unix socket in python ) but imho it's not that case.

Test matrix (srv - server, cli - client):

• srv + cli @ "abstract" unix sock:
  • python + python = OK
  • c + c = OK
• srv + cli @ "normal" unix sock:
  • python + python = OK
  • c + c = OK
• srv + cli @ "normal" unix sock:
  • python + c = OK
  • c + python = OK
• srv + cli @ "abstract" unix sock:
  • python + c = FAIL [cli / strace output: ECONNREFUSED (Connection refused)]
  • c + python = FAIL [cli / raised exception: socket.error: [Errno 111] Connection refused]

/proc/net/unix / lsof or strace show nothing unusual:

• working "normal" socket C client:
  // ...
socket(PF_LOCAL, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_LOCAL, sun_path=@"/var/tmp/sock.tmp"}, 110) = 0
// ...

• misbehaving "abstract" socket C client:
  // ...
socket(PF_LOCAL, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_LOCAL, sun_path=@"/var/tmp/sock.tmp"}, 110) = -1 ECONNREFUSED (Connection refused)
// ...

Python bug or what...?

Gist with code samples for my test matrix : https://gist.github.com/soutys/ffbe2e76a86835a9cc6b

Original code / samples:

• C servers/clients - based on https://github.com/troydhanson/network/tree/master/unixdomain
• Python servers/clients - based on Python 3.X Unix Socket Example

Update 2015-11-02

More info about system and compilations:

• host system: Ubuntu 14.04.3 LTS (x64);
• all C test files compiled with gcc (example: gcc -Wall -Wextra -pedantic -o abs_cli abs_cli.c );
• all programs (both compiled and built-in python) run as non-root user;

Answer 1

When binding a UNIX domain socket to an abstract name, the addrlen argument should be sizeof(struct sockaddr_un's sun_family) + number of characters in abstract name + 1. The "+1" is for the null byte in front of the abstract name in sockaddr_un's sun_path. Let's look at an example:

• Server in C:

   int sockfd; struct sockaddr_un addr; /* create socket, set addr.sun_family, set addr.sun_path to null byte followed by abstract_name */ bind(sockfd, (struct sockaddr *)&addr, sizeof(addr.sun_family) + strlen(abstract_name) + 1); 

• Client in Python:

   client = socket.socket(socket.AF_UNIX, ...) client.connect( "\\0abstract_name" ) 

References:

• The Linux Programming Interface by Michael Kerrisk, Example 57-8.
• http://blog.eduardofleury.com/archives/2007/09/13