I'm trying to code my own DNS server, I'm reading through RFC1035 on DNS but I have a few queries:
1) I want my server to respond with a CNAME for a particular request, but no A records - can I do this? for example, receive request for 'server1.com', response 'CNAME server2.com', and then the client queries another DNS server to get the A record for 'server2.com'.
I've currently set the header to: '\x84\x00' such to say this is the authoritive server, but recurse is not possible. Is this right?
2) I want my server to respond with no records for any other request, such that the client then queries a different DNS server for the records. I've currently set header to '\x83\x03' such to signal a NAME ERROR reply code. Is this right? Then what do I follow this with, zeros in all the other fields, or just end the packet there? I don't want to respond with 'this name doesn't exist', rather 'I don't know this name, try someone else' - how do I do this?
Many Thanks :)
Sounds about right - in fact, CNAME with A records is incorrect (RFC1034 section 3.6.2: "If a CNAME RR is present at a node, no other data should be present").
This would be very unusual behaviour from an authoritative nameserver - I'd suggest rethinking it or at least testing with some real-life resolvers to ensure they do what you want. RCODE #3 ("name error" or NXDOMAIN) is positive confirmation that the name doesn't exist. This would cause resolvers to terminate resolution and possibly cache the nonexistence of the name, which doesn't sound like what you're after. If you want the resolver to query one of the other nameservers that was delegated to for that zone, I guess SERVFAIL (RCODE #2) is the most appropriate/likely to have the desired effect.
By the way, for debugging the exact format of your DNS packets I can highly recommend Wireshark for its decoding accuracy compared with pasting hex codes into Stack Overflow ;)
In the CNAME case, your (authoritative) server should just return the CNAME in the answer section unless it is also authoritative for the domain that the CNAME points to, in which case it should also include the result of following the CNAME.
For your second case you should return RCODE 5 ("REFUSED") - this is the preferred error that an authoritative server should give when asked a question for a domain for which it is not configured.
Following that, you still need to send the four 16-bit count fields and a copy of the question from the original request. In this case the four counts would be (1, 0, 0, 0) - one question, no answer, no ns records, no additional records.
Related
Lately i have seen high time to first byte for my sites.
Most of the time it is by javascript. A test at webpagetest.org usually shows like....
URL: http://example.com/
Loaded by: http://example.com/some-kind-of-javascript.js
When i remove that javascript then anothe javascript appears in that place.
What does loaded by exactly mean??
Check example test result....
https://www.webpagetest.org/result/190729_JY_cb028989b0f44671fba830c9eaca29d7/1/details/#waterfall_view_step1
I'm not sure why you think it's Javascript that is the problem? It looks to me like it's the initial HTML that is causing the problem.
Time to First byte for a resource is the time take from after sending the request (e.g. GET /) until it receives the first byte back. It excludes the DNS lookup, TCP connection and SSL handshake time, so really is a measure of the time take to start receiving that resource. Note that the "first byte" time at the top of the waterfall is the full end to end time, including DNS/TCP/SSL and any redirects, but the TTFB for each resource this is split out more.
I don't know how your home page is created - I would guess it's not a static page so whatever is generating this (PHP?) is taking too long. Whether this is due to bad backend code, an under-resourced server, a slow database, or something else is impossible to say from the outside. I would suggest getting in touch with your hosting provider and/or reviewing your code and server.
Say we have an A REC that points to IP x of our LB for one of our services. It has a TTL of 3600s. But... what it should have been was a C NAME that points to a A REC for a VIP. It's already in production and has about 10 services that calls the new A REC comprising of ~100 machines. If the A REC is deleted and a new C NAME is created with the same name and points to a new A REC, will the consumers notice this change? Is there a chance that the callers would time out?
I'd assume with the amount of machines some are bound to be impacted. If I set the TTL to 5 hours would there be a better chance of no one noticing?
So my question is, how do I swap an A REC for a C NAME without consumers of our service noticing?
Would it matter if the record is for use inside the network only vs available to the public?
I ask because we will need to load balance across data centers soon, and we have some records that are stuck pointing to an IP.
It would be nice to have an explanation of how the DNS system would behave in this scenario. Thanks.
Let's assume that you have a name foo.example.org that has nothing except an A record with the IPv4 address 192.0.2.1 and a 3600 second TTL. Anyone who looks up foo.example.org will get that A record, and remember it for an hour before they go and ask your name server for fresher information.
Then assume you change things so that foo.example.org has a CNAME record pointing at bar.example.net, which in turn has an A record holding the address 192.0.2.1. Anyone who looks up the name foo.example.org for the first time will get the CNAME, proceed to look up bar.example.net, and get the A record from there.
The only complication is that anyone who looked up foo.example.org during the 3600 seconds immediately before you change to the CNAME chain took effect will remember the direct lookup, and thus not see the new information until the TTL expires. So for up to an hour after you do the change, some people may still see the old information. So to keep the change transparent to users, make sure that the old information (the old IP address) still works for at least one full TTL period after you make a change.
This is not in any way special for changing from A to CNAME. No matter what you change, there will be a full TTL period during which clients can legitimately get the old info. That's just how DNS works.
On top of that, of course, there are clients and caching servers that don't pay as much attention to the TTL value as they should, but that's a whole different thing.
Goal
I would like to stitch up a GNU GPL licensed Knot Resolver module either in C or in CGO that would examine the client's query and the corresponding resolved answer with the goal of querying an external API offering a knowledge base of malware infected hostnames and ip addresses (e.g. GNU AGPL v3 IntelMQ).
If there is a match with the resolved A's (AAAA's) IP address it is to be logged, likewise a match with the queried hostname should be logged or (optionally) it could result in sending the client an IP address of a sinkhole instead of the resolved one.
Means
I studied the layers and I came to the conclusion that the phase I'm interested in is consume. I don't want to affect the resolution process, I just want to step in at the last moment and check the results and possibly modify them.
I ventured to register the a consume function
with
static knot_layer_api_t _layer = {
.consume = &consume,
};
but I'm not sure it is the right place to do the deed.
Furthermore, I also looked into module hints.c, especially its query method
and module stats.c for its _to_wire function usage.
Question(s)
Phase (Layer?)
When is the right time to step in and read/write the answer to the query before it's send to the client? Am I at the right spot in consume layer?
Answer sections
If the following attempt at getting the resolved IP address gives me the Name Server's address:
char addr_str[INET6_ADDRSTRLEN];
memset(addr_str, 0, sizeof(addr_str));
const struct sockaddr *src = &(req->answer->sections);
inet_ntop(qry->ns.addr[0].ip.sa_family, kr_inaddr(src), addr_str, sizeof(addr_str));
DEBUG_MSG(NULL, "ADDR: %s\n", addr_str);
how do I get the resolved (A, AAAA) IP address for the query's hostname? I would like to iterate over A/AAAA IP addresses and CNAMEs in the answer and look at the IP addresses they were resolved to.
Modifying the answer
If the module setting demands it, I would like to be able to "ditch" the resolved answer and provide a new one comprising an A record pointed at a sinkhole.
How do I prepare the record so as it could be translated from char* to Knot's wire format and the proper structure in the right context at the right phase?
I guess it might go along functions such as knot_rrset_init and knot_rrset_add_rdata, but I wasn't able to arrive at any successful result.
THX for pointers and suggestions.
If you want to step in the last moment when the response is finalised but not yet sent to the requestor, the right place is finish. You can do it in consume as well, but you'll be overwriting responses from authoritative servers here, not the assembled response to requestor (which means DNSSEC validator is likely to stop your rewritten answers).
Disclaimer: Go interface is rough and requires a lot of CGO code to access internal structures. You'd be probably better suited by a LuaJIT module, there is another module doing something similar that you may take as an example, it also has wrappers for creating records from text etc. If you still want to do it, that's awesome and improvements to Go interface are welcome, read on.
What you need to do is roughly this (as CGO).
That will walk you through RR sets in the packet (C.knot_rrset_t),
where you can match type (rr.type) and contents (rr.rdata).
Contents is stored in DNS wire format, for address records it is the address in network byte order, e.g. {0x7f, 0, 0, 1}.
You will have to compare that to address/subnet you're looking for - example in C code.
When you find a match, you want to clear the whole packet and insert sinkhole record (you cannot selectively remove records, because the packet is append-only for performance reasons). This is relatively easy as there is a helper for that. Here's code in LuaJIT from policy module, you'd have to rewrite it in Go, using all functions mentioned above and using A/AAAA sinkhole record instead of SOA. Good luck!
I own a domain, and clearly its DNS resolution is fine, everywhere seems to point to the right server : https://dnschecker.org/#A/e-bis.fr
I created a wildcard for subdomains, and it seems like it only points to the right server in some random places in the world, changes randomly every once in a while (as in sometimes a server will say it resolves, and one hour later it won't anymore) : https://dnschecker.org/#A/whatever.e-bis.fr
At first I thought it was a propagation issue, but it's been a week now so clearly it's me messing up the config at some point.
Here's the zone file used by bind9 for this domain :
# IN SOA ns3032550.ip-91-121-79.eu. postmaster.e-bis.fr. (
2014070501 ; Serial
8H ; Refresh
30M ; Retry
4W ; Expire
8H ; Minimum TTL
)
IN NS ns3032550.ip-91-121-79.eu.
IN NS ns.kimsufi.com.
e-bis.fr. IN A 91.121.79.161
*.e-bis.fr. IN A 91.121.79.161
ownercheck IN TXT "28834a04"
I do a service bind9 reload every time I update it, so the only thing I can see is the issue being in the zone file. I'm terrible with them, so it wouldn't surprise me if it was a beginner mistake.
Thanks in advance to anyone who can help,
Éric B.
Turns out I had just forgotten to update the serial (I think?).
For anyone running into the same problem, it was this line 2014070501 ; Serial which I had not updated. Incrementing it then restarting the service is enough.
I run a list of subscribed webmasters and recently migrated to PHPMailer.
I have my phpmailer script working fine. It takes values from my mysql database and send them an email but sometimes I get the Mailer Error: You must provide at least one recipient email address.
After some tests, I found the error is displayed when the full email address is more than 82 characters.
As you know, working with email address of webmasters of seo micro niche sites can give me lot of addresses with more than 82 characters since their domains are very long.
Is there any setting to change for that? I dont know why its limited if 82 characters is less than 256 characters allowed for any email address.
Any help? I searched google and stack overflow for this reply but nothing was found.
Thank you very much.
You could try using PHPMailer's validateAddress() method before trying to send, though it's used internally anyway when you call addAddress().
The unit tests contain a long list of valid and invalid addresses that test many corner cases.
I would guess that you are not running into an overall limit, but a lower limit within the domain name. Though the overall address can be 255 chars, elements within it have lower limits, for example each label within a domain (e.g. example and com in example.com) has a limit of 63 chars. 82 chars sounds like this limit plus a few other bits, like user#<somethingover63charslong>.com. If this is like the addresses you have, that's hard luck; You can't use invalid addresses and expect them to work. It's the addresses that are the problem.