New: connect(2) changes INADDR_ANY meaning for UDP sockets
https://bugzilla.kernel.org/show_bug.cgi?id=43249
Summary: connect(2) changes INADDR_ANY meaning for UDP sockets
Product: Documentation
Version: unspecified
Kernel Version: 3.0.0
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: man-pages
AssignedTo: documentation_man-pages-ztI5WcYan/[email protected]
ReportedBy: [email protected]
Regression: No
Created an attachment (id=73297)
--> (https://bugzilla.kernel.org/attachment.cgi?id=73297)
Testcase which demonstrates the bug scenario
The POSIX man page for connect(2) [1] specifies that:
Quoted 3 lines
If the socket has not already been bound to a local address,
connect()shall bind it to an address which, unless the socket's address family isAF_UNIX, is an unused local address.
The Linux man page for connect(2) does not provide this information, and this may be confusing, regarding the following behaviour:
After creating a socket of type SOCK_DGRAM through socket(2), and calling bind(2) on it to have it "listen" on all interfaces (through the use of INADDR_ANY as value of sockaddr_in.sin_addr.s_addr), it is possible to call connect(2) on said socket. This behaviour is correctly explained by the connect(2) Linux man page [2]:
Quoted 3 lines
If the socket sockfd is of type
SOCK_DGRAMthen addr is the address to which datagrams are sent by default, and the only address from which data‐ grams are received.
However, upon doing so, the socket is no longer "bound" to any interface (as could be assumed through the usage of INADDR_ANY), but one in particular (and it is apparently not possible to know which one, before calling connect(2)). This seems to contradict the information provided by the ip(7) man page [3]:
Quoted 5 lines
When a process wants to receive new incoming packets or connections, it should bind a socket to a local interface address using bind(2). Only one IP socket may be bound to any given local (address, port) pair. When
INADDR_ANYis specified in the bind call, the socket will be bound to all local interfaces.
I have attached a test-case which demonstrates this apparently conflicting behaviour.
I would suggest adding a paragraph or comment that would explain this subtlety, as it can cause difficult to diagnose issues when using connected UDP sockets.
[1]: http://www.unix.com/man-page/posix/3posix/connect/
[2]: http://linux.die.net/man/2/connect
[3]: http://linux.die.net/man/7/ip
Signature · 6 lines
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