Re: [Bugme-new] [Bug 6646] New: UDP socket doesn't return to bound state after association is dissolved by connect(..AF_UNSPEC)

Re: [Bugme-new] [Bug 6646] New: UDP socket doesn't return to bound state after association is dissolved by connect(..AF_UNSPEC)

[Andrew Morton]

On Sun, 4 Jun 2006 23:58:19 -0700
bugme-daemon@bugzilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=6646
> 
>            Summary: UDP socket doesn't return to bound state after
>                     association is dissolved by connect(..AF_UNSPEC)
>     Kernel Version: 2.6.12
>             Status: NEW
>           Severity: high
>              Owner: shemminger@osdl.org
>          Submitter: yujiang.wang@sun.com
> 
> 
> Most recent kernel where this bug did not occur:
> Distribution:
> Hardware Environment:
> Software Environment:
> Problem Description:
> When disconnect a UDP socket, Linux kernel set local port to zero if the port
> number comes from a implicit bind. Linux connect(2) man page reads:
> "Generally, connection-based protocol sockets may successfully *connect* only
> once; connectionless protocol sockets may use *connect* multiple times to change
> their association. Connectionless sockets may dissolve the association by
> connecting to an address with the /sa_family/ member of *sockaddr* set to
> *AF_UNSPEC*."
> But dissolve the association should not impact the local binding, while
> currently it does. In contrast, Unix variants like Solaris don't alter local
> binding when disconnecting a UDP socket. 
> 
> Steps to reproduce:
> Compile attached c file, and run it. You'll see local port number changed after
> a UDP socket disconnected.
> 
> #include <errno.h>
> #include <string.h>
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <netinet/in.h>
> #include <net/if.h>
> #include <netdb.h>
> #include <arpa/inet.h>
> #include <stdio.h>
> #include <stdlib.h>
> 
> #define SERV_PORT 12345
> 
> void print_local_addr(int s);
> 
> int main(int argc, char** argv)
> {
>     int sockfd;
>     struct sockaddr_in servaddr, cliaddr;
> 
>     if (argc != 2) {
>         printf("Usage: disconnect_udp <ipaddress>");
>         exit(0);
>     }
>     
>     // creat a UDP socket which binds to a local address
>     sockfd = socket(AF_INET, SOCK_DGRAM, 0);
>     bzero(&cliaddr, sizeof(cliaddr));
>     cliaddr.sin_family = AF_INET;
>     if (inet_pton(AF_INET, argv[1], &cliaddr.sin_addr) != 1) {
>         perror("inet_pton failed");
>     }
>     bind(sockfd, (struct sockaddr *)&cliaddr, sizeof(cliaddr));
>     
>     // connect this UDP socket
>     bzero(&servaddr, sizeof(servaddr));
>     servaddr.sin_family = AF_INET;
>     servaddr.sin_port = htons(SERV_PORT);
>     if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) != 1) {
>         perror("inet_pton failed");
>     }
>     if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) != 0) {
>         perror("connect failed");
>     }
>     print_local_addr(sockfd);
>     
>     // disconnect it
>     servaddr.sin_family = AF_UNSPEC;
>     if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) != 0) {
>         perror("connect failed");
>     }
>     print_local_addr(sockfd);
>     
>     close(sockfd);
> }
> 
> void print_local_addr(int s)
> {
>     struct sockaddr_in localaddr;
>     socklen_t len = 0;
>     char temp[INET_ADDRSTRLEN];
>     
>     len = sizeof(localaddr);
>     if (getsockname(s, (struct sockaddr *)&localaddr, &len) != 0) {
>         perror("getsockname failed");
>     }
>     
>     inet_ntop(AF_INET, &localaddr.sin_addr, temp, INET_ADDRSTRLEN);
>     printf("Local binding: address=%s, port=%d\n",
>            temp, ntohs(localaddr.sin_port));
> }
> 
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Re: [Bugme-new] [Bug 6646] New: UDP socket doesn't return to bound state after association is dissolved by connect(..AF_UNSPEC)

[David Miller]

From: Andrew Morton <akpm@osdl.org>
Date: Mon, 5 Jun 2006 00:07:51 -0700

> > When disconnect a UDP socket, Linux kernel set local port to zero if the port
> > number comes from a implicit bind. Linux connect(2) man page reads:
> > "Generally, connection-based protocol sockets may successfully *connect* only
> > once; connectionless protocol sockets may use *connect* multiple times to change
> > their association. Connectionless sockets may dissolve the association by
> > connecting to an address with the /sa_family/ member of *sockaddr* set to
> > *AF_UNSPEC*."
> > But dissolve the association should not impact the local binding, while
> > currently it does. In contrast, Unix variants like Solaris don't alter local
> > binding when disconnecting a UDP socket. 

You can only preserve the parts of a local binding which are
explicitly specified.  Since you make an explicit bind to a source
address, that will be preserved by the disconnect.

However, since you use a zero anonymous port during the bind,
the one choosen by the kernel will not be preserved.  If you
had choosen an explicit port during bind() it would be preserved
by the disconnect.

This behavior is intentional and will not change.