linux/fs/cifs/dns_resolve.c
Shyam Prasad N 506c1da44f cifs: use the expiry output of dns_query to schedule next resolution
We recently fixed DNS resolution of the server hostname during reconnect.
However, server IP address may change, even when the old one continues
to server (although sub-optimally).

We should schedule the next DNS resolution based on the TTL of
the DNS record used for the last resolution. This way, we resolve the
server hostname again when a DNS record expires.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Cc: <stable@vger.kernel.org> # v5.11+
Signed-off-by: Steve French <stfrench@microsoft.com>
2021-07-14 10:06:03 -05:00

91 lines
2.3 KiB
C

// SPDX-License-Identifier: LGPL-2.1
/*
* fs/cifs/dns_resolve.c
*
* Copyright (c) 2007 Igor Mammedov
* Author(s): Igor Mammedov (niallain@gmail.com)
* Steve French (sfrench@us.ibm.com)
* Wang Lei (wang840925@gmail.com)
* David Howells (dhowells@redhat.com)
*
* Contains the CIFS DFS upcall routines used for hostname to
* IP address translation.
*
*/
#include <linux/slab.h>
#include <linux/dns_resolver.h>
#include "dns_resolve.h"
#include "cifsglob.h"
#include "cifsproto.h"
#include "cifs_debug.h"
/**
* dns_resolve_server_name_to_ip - Resolve UNC server name to ip address.
* @unc: UNC path specifying the server (with '/' as delimiter)
* @ip_addr: Where to return the IP address.
* @expiry: Where to return the expiry time for the dns record.
*
* The IP address will be returned in string form, and the caller is
* responsible for freeing it.
*
* Returns length of result on success, -ve on error.
*/
int
dns_resolve_server_name_to_ip(const char *unc, char **ip_addr, time64_t *expiry)
{
struct sockaddr_storage ss;
const char *hostname, *sep;
char *name;
int len, rc;
if (!ip_addr || !unc)
return -EINVAL;
len = strlen(unc);
if (len < 3) {
cifs_dbg(FYI, "%s: unc is too short: %s\n", __func__, unc);
return -EINVAL;
}
/* Discount leading slashes for cifs */
len -= 2;
hostname = unc + 2;
/* Search for server name delimiter */
sep = memchr(hostname, '/', len);
if (sep)
len = sep - hostname;
else
cifs_dbg(FYI, "%s: probably server name is whole unc: %s\n",
__func__, unc);
/* Try to interpret hostname as an IPv4 or IPv6 address */
rc = cifs_convert_address((struct sockaddr *)&ss, hostname, len);
if (rc > 0)
goto name_is_IP_address;
/* Perform the upcall */
rc = dns_query(current->nsproxy->net_ns, NULL, hostname, len,
NULL, ip_addr, expiry, false);
if (rc < 0)
cifs_dbg(FYI, "%s: unable to resolve: %*.*s\n",
__func__, len, len, hostname);
else
cifs_dbg(FYI, "%s: resolved: %*.*s to %s expiry %llu\n",
__func__, len, len, hostname, *ip_addr,
expiry ? (*expiry) : 0);
return rc;
name_is_IP_address:
name = kmalloc(len + 1, GFP_KERNEL);
if (!name)
return -ENOMEM;
memcpy(name, hostname, len);
name[len] = 0;
cifs_dbg(FYI, "%s: unc is IP, skipping dns upcall: %s\n",
__func__, name);
*ip_addr = name;
return 0;
}