问题

用 Net::DNS 测试 nameserver，客户端发 EDNS 查询，服务端的响应包一直不带 EDNS OPT。

rt上的bug单 #115558: Net::DNS::Nameserver does not allow EDNS replies

补丁

--- Net-DNS-1.06_05/lib/Net/DNS/Packet.pm	2016-11-12 11:23:47.000000000 +0800
+++ Net-DNS-1.06_05.fix/lib/Net/DNS/Packet.pm	2016-12-28 16:13:02.577195268 +0800
@@ -237,10 +237,12 @@
 =cut
 
 sub edns {
-	my $self = shift;
-	my $link = \$self->{xedns};
-	($$link) = grep $_->isa(qw(Net::DNS::RR::OPT)), @{$self->{additional}} unless $$link;
-	$$link = new Net::DNS::RR( type => 'OPT' ) unless $$link;
+    my $self = shift;
+    my $link ;
+    $link = \$self->{xedns} if(exists $self->{xedns});
+    ($$link) = grep $_->isa(qw(Net::DNS::RR::OPT)), @{$self->{additional}} unless ($link and $$link);
+    $$link = new Net::DNS::RR( type => 'OPT' ) unless ($link and $$link);
+    return $$link;
 }

bug分析

Nameserver.pm

服务端 tcp_connection / udp_connection 函数先调用 make_reply 生成应答包，再调用 $reply->data 发送给客户端服务端

    my $reply = $self->make_reply( $query, $sock->peerhost, $conn );

    $reply->data

$self->make_reply 制作应答包

$query->reply() 初始化应答包

    sub make_reply() 

          my $reply = $query->reply();

Packet.pm : sub reply() 如果收到的是edns查询，则初始化应答包时预设edns size

     $reply->edns->size($UDPmax) if $query->edns->_specified;

Packet.pm : sub edns() 取出edns opt对象，bug发生地

sub edns {
     my $self = shift;
     my $link = \$self->{xedns};
     ($$link) = grep $_->isa(qw(Net::DNS::RR::OPT)), @{$self->{additional}} unless $$link;
     $$link = new Net::DNS::RR( type => 'OPT' ) unless $$link;
}

如果收到带edns的查询，make_reply初始化时，依次调用

my $reply = $query->reply();  

$reply->edns->size($UDPmax),

由于 $self->{xedns} 为空，$link指向 $self->{xedns}　的地址

且由于$self->{additional}　为空，$$link 将被置为一个新的空OPT

也就是说，$self->{xedns}　被置为该空OPT

后续如果再次调用 $reply->edns，

$link = $self->{xedns} 一直指向该空OPT的地址，$$link一直为true，后面两行操作不会生效：

     ($$link) = grep $_->isa(qw(Net::DNS::RR::OPT)), @{$self->{additional}} unless $$link;
     $$link = new Net::DNS::RR( type => 'OPT' ) unless $$link;

$self->edns 总是返回空OPT

$reply->data 发送给客户端

由于 $self->edns 为空OPT，$edns->_specified将一直为0，导致edns OPT不会被unshift到@addl

响应包丢失edns OPT

#Packet.pm

sub data {
    &encode;
}

sub encode {
    my ( $self, $size ) = @_;               # uncoverable pod


    my $edns = $self->edns;                 # EDNS support

    my @addl = grep !$_->isa('Net::DNS::RR::OPT'), @{$self->{additional}};
    unshift( @addl, $edns ) if $edns->_specified;
    $self->{additional} = \@addl;

    # ...

}