Re: [PATCH] DNS: Fall back to permissive mode if recursor mode is unavailable

[Michael Tremer <michael.tremer@ipfire.org>]

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Hello,

so I wanted to highlight this patch a little which has been merged into
next.

It will change fallback behaviour of DNS again which before switched to
recursor mode if no usable forwarder could be found. Now IPFire will
test if any of the root servers is available and if so, fall back to
recursor mode. If not, it will change DNSSEC into permissive mode and
will use all given forwarders.

The idea behind this is to always be able to provide at least *some*
DNS, although DNSSEC will be practically deactivated.

It is still missing that we show a big warning where necessary, but at
least for some people who were forced by their providers to use their
own name servers which do not support DNSSEC at all.

So, for the people who have been affected by this issue I can only
recommend to test this and give us feedback within about one week. I
would like to close the merge window for the next core update around
then.

Best,
-Michael

On Wed, 2017-03-01 at 16:11 +0000, Michael Tremer wrote:
> The tests when assigning DNS name servers has been extended so that
> if no working forwarder can be found, we will test if the local
> recursor
> mode is an option.
> 
> If not, we will configure unbound's validator module into permissive
> mode so that at least some DNS functionality is available.
> 
> Signed-off-by: Michael Tremer <michael.tremer(a)ipfire.org>
> ---
>  config/rootfiles/core/110/filelists/files          |  1 +
>  lfs/unbound                                        |  1 +
>  src/initscripts/init.d/unbound                     | 67
> ++++++++++++++++++++--
>  ...ting-validator-permissive-mode-at-runtime.patch | 43
> ++++++++++++++
>  4 files changed, 107 insertions(+), 5 deletions(-)
>  create mode 100644 src/patches/unbound-allow-setting-validator-
> permissive-mode-at-runtime.patch
> 
> diff --git a/config/rootfiles/core/110/filelists/files
> b/config/rootfiles/core/110/filelists/files
> index 670b9ae..f4ce989 100644
> --- a/config/rootfiles/core/110/filelists/files
> +++ b/config/rootfiles/core/110/filelists/files
> @@ -1,5 +1,6 @@
>  etc/system-release
>  etc/issue
> +etc/rc.d/init.d/unbound
>  srv/web/ipfire/cgi-bin/index.cgi
>  srv/web/ipfire/cgi-bin/vpnmain.cgi
>  usr/lib/libssp.so.0
> diff --git a/lfs/unbound b/lfs/unbound
> index 2b7745c..f361f24 100644
> --- a/lfs/unbound
> +++ b/lfs/unbound
> @@ -70,6 +70,7 @@ $(subst %,%_MD5,$(objects)) :
>  $(TARGET) : $(patsubst %,$(DIR_DL)/%,$(objects))
>  	@$(PREBUILD)
>  	@rm -rf $(DIR_APP) && cd $(DIR_SRC) && tar axf
> $(DIR_DL)/$(DL_FILE)
> +	cd $(DIR_APP) && patch -Np1 <
> $(DIR_SRC)/src/patches/unbound-allow-setting-validator-permissive-
> mode-at-runtime.patch
>  	cd $(DIR_APP) && \
>  		./configure \
>  			--prefix=/usr \
> diff --git a/src/initscripts/init.d/unbound
> b/src/initscripts/init.d/unbound
> index 8802781..bbf9c00 100644
> --- a/src/initscripts/init.d/unbound
> +++ b/src/initscripts/init.d/unbound
> @@ -114,17 +114,38 @@ update_forwarders() {
>  			echo_warning
>  		fi
>  
> -		if [ -n "${broken_forwarders}" -a -z "${forwarders}"
> ]; then
> -			boot_mesg "Falling back to recursor mode"
> ${WARNING}
> -			echo_warning
> -
> -		elif [ -n "${forwarders}" ]; then
> +		if [ -n "${forwarders}" ]; then
>  			boot_mesg "Configuring upstream name
> server(s): ${forwarders:1}" ${INFO}
>  			echo_ok
>  
> +			# Make sure DNSSEC is activated
> +			enable_dnssec
> +
>  			echo "${forwarders}" > /var/ipfire/red/dns
>  			unbound-control -q forward ${forwarders}
>  			return 0
> +
> +		# In case we have found no working forwarders
> +		else
> +			# Test if the recursor mode is available
> +			if can_resolve_root
> +bufsize=${new_edns_buffer_size}; then
> +				# Make sure DNSSEC is activated
> +				enable_dnssec
> +
> +				boot_mesg "Falling back to recursor
> mode" ${WARNING}
> +				echo_warning
> +
> +			# If not, we set DNSSEC in permissive mode
> and allow using all recursors
> +			elif [ -n "${broken_forwarders}" ]; then
> +				disable_dnssec
> +
> +				boot_mesg "DNSSEC has been set to
> permissive mode" ${FAILURE}
> +				echo_failure
> +
> +				echo "${broken_forwarders}" >
> /var/ipfire/red/dns
> +				unbound-control -q forward
> ${broken_forwarders}
> +				return 0
> +			fi
>  		fi
>  	fi
>  
> @@ -370,6 +391,42 @@ ns_determine_edns_buffer_size() {
>  	return 1
>  }
>  
> +get_root_nameservers() {
> +	while read -r hostname ttl record address; do
> +		# Searching for A records
> +		[ "${record}" = "A" ] || continue
> +
> +		echo "${address}"
> +	done < /etc/unbound/root.hints
> +}
> +
> +can_resolve_root() {
> +	local ns
> +	for ns in $(get_root_nameservers); do
> +		if dig @${ns} +dnssec SOA . $@ >/dev/null; then
> +			return 0
> +		fi
> +	done
> +
> +	# none of the servers was reachable
> +	return 1
> +}
> +
> +enable_dnssec() {
> +	local status=$(unbound-control get_option val-permissive-
> mode)
> +
> +	# Don't do anything if DNSSEC is already activated
> +	[ "${status}" = "no" ] && return 0
> +
> +	# Activate DNSSEC and flush cache with any stale and
> unvalidated data
> +	unbound-control -q set_option val-permissive-mode: no
> +	unbound-control -q flush_zone .
> +}
> +
> +disable_dnssec() {
> +	unbound-control -q set_option val-permissive-mode: yes
> +}
> +
>  case "$1" in
>  	start)
>  		# Print a nicer messagen when unbound is already
> running
> diff --git a/src/patches/unbound-allow-setting-validator-permissive-
> mode-at-runtime.patch b/src/patches/unbound-allow-setting-validator-
> permissive-mode-at-runtime.patch
> new file mode 100644
> index 0000000..f476d08
> --- /dev/null
> +++ b/src/patches/unbound-allow-setting-validator-permissive-mode-at-
> runtime.patch
> @@ -0,0 +1,43 @@
> +diff --git a/validator/validator.c b/validator/validator.c
> +index 676dcdf..7c19f3d 100644
> +--- a/validator/validator.c
> ++++ b/validator/validator.c
> +@@ -113,7 +113,7 @@ val_apply_cfg(struct module_env* env, struct
> val_env* val_env,
> + 	int c;
> + 	val_env->bogus_ttl = (uint32_t)cfg->bogus_ttl;
> + 	val_env->clean_additional = cfg->val_clean_additional;
> +-	val_env->permissive_mode = cfg->val_permissive_mode;
> ++	val_env->permissive_mode = &cfg->val_permissive_mode;
> + 	if(!env->anchors)
> + 		env->anchors = anchors_create();
> + 	if(!env->anchors) {
> +@@ -170,7 +170,6 @@ val_init(struct module_env* env, int id)
> + 	}
> + 	env->modinfo[id] = (void*)val_env;
> + 	env->need_to_validate = 1;
> +-	val_env->permissive_mode = 0;
> + 	lock_basic_init(&val_env->bogus_lock);
> + 	lock_protect(&val_env->bogus_lock, &val_env-
> >num_rrset_bogus,
> + 		sizeof(val_env->num_rrset_bogus));
> +@@ -2084,7 +2083,7 @@ processFinished(struct module_qstate* qstate,
> struct val_qstate* vq,
> + 			}
> + 		}
> + 		/* If we are in permissive mode, bogus gets
> indeterminate */
> +-		if(ve->permissive_mode)
> ++		if(*ve->permissive_mode)
> + 			vq->orig_msg->rep->security =
> sec_status_indeterminate;
> + 	}
> + 
> +diff --git a/validator/validator.h b/validator/validator.h
> +index 23d3072..f8464b8 100644
> +--- a/validator/validator.h
> ++++ b/validator/validator.h
> +@@ -104,7 +104,7 @@ struct val_env {
> + 	 * This allows an operator to run validation 'shadow'
> without
> + 	 * hurting responses to clients.
> + 	 */
> +-	int permissive_mode;
> ++	int* permissive_mode;
> + 
> + 	/**
> + 	 * Number of entries in the NSEC3 maximum iteration count
> table.

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