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authorLennart Poettering <lennart@poettering.net>2016-06-21 00:58:47 +0200
committerLennart Poettering <lennart@poettering.net>2016-06-21 14:15:23 +0200
commitb30bf55d5c9942f15f27a641c2c34bbb646ec981 (patch)
treeb631fe686f058165a3c1289a8e25c3e85061565b /src/resolve/resolved-conf.c
parent2a3900d7e484faefb14fd868d5c17ae2e7b2f21f (diff)
resolved: respond to local resolver requests on 127.0.0.53:53
In order to improve compatibility with local clients that speak DNS directly (and do not use NSS or our bus API) listen locally on 127.0.0.53:53 and process any queries made that way. Note that resolved does not implement a full DNS server on this port, but simply enough to allow normal, local clients to resolve RRs through resolved. Specifically it does not implement queries without the RD bit set (these are requests where recursive lookups are explicitly disabled), and neither queries with DNSSEC DO set in combination with DNSSEC CD (i.e. DNSSEC lookups with validation turned off). It also refuses zone transfers and obsolete RR types. All lookups done this way will be rejected with a clean error code, so that the client side can repeat the query with a reduced feature set. The code will set the DNSSEC AD flag however, depending on whether the data resolved has been validated (or comes from a local, trusted source). Lookups made via this mechanisms are propagated to LLMNR and mDNS as necessary, but this is only partially useful as DNS packets cannot carry IP scope data (i.e. the ifindex), and hence link-local addresses returned cannot be used properly (and given that LLMNR/mDNS are mostly about link-local communication this is quite a limitation). Also, given that DNS tends to use IDNA for non-ASCII names, while LLMNR/mDNS uses UTF-8 lookups cannot be mapped 1:1. In general this should improve compatibility with clients bypassing NSS but it is highly recommended for clients to instead use NSS or our native bus API. This patch also beefs up the DnsStream logic, as it reuses the code for local TCP listening. DnsStream now provides proper reference counting for its objects. In order to avoid feedback loops resolved will no silently ignore 127.0.0.53 specified as DNS server when reading configuration. resolved listens on 127.0.0.53:53 instead of 127.0.0.1:53 in order to leave the latter free for local, external DNS servers or forwarders. This also changes the "etc.conf" tmpfiles snippet to create a symlink from /etc/resolv.conf to /usr/lib/systemd/resolv.conf by default, thus making this stub the default mode of operation if /etc is not populated.
Diffstat (limited to 'src/resolve/resolved-conf.c')
-rw-r--r--src/resolve/resolved-conf.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index fecf7ecccf..dd233e7c4a 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -37,6 +37,10 @@ int manager_add_dns_server_by_string(Manager *m, DnsServerType type, const char
if (r < 0)
return r;
+ /* Silently filter out 0.0.0.0 and 127.0.0.53 (our own stub DNS listener) */
+ if (!dns_server_address_valid(family, &address))
+ return 0;
+
/* Filter out duplicates */
s = dns_server_find(manager_get_first_dns_server(m, type), family, &address, ifindex);
if (s) {