Hello! I’ve been working on writing a zine about debugging for a while now (we’re getting close to finishing it!!!!), and one of the pages is about analyzing logs. I asked for some tips on Mastodon and got WAY more tips than could fit on the page, so I thought I’d write a quick blog post.

I’m going to talk about log analysis in the context of distributed systems debugging (you have a bunch of servers with different log files and you need to work out what happened) since that’s what I’m most familiar with.

search for the request’s ID

Often log lines will include a request ID. So searching for the request ID of a failed reques will show all the log lines for that request.

This is a GREAT way to cut things down, and it’s one of the first helpful tips I got about distributed systems debugging – I was staring at a bunch of graphs on a dashboard fruitlessly trying to find patterns, and a coworker gave me the advice (“julia, try looking at the logs for a failed request instead!”). That turned out to be WAY more effective in that case.

correlate between different systems

Sometimes one set of logs doesn’t have the information you need, but you can get that information from a different service’s logs about the same request.

If you’re lucky, they’ll both share a request ID.

More often, you’ll need to manually piece together context from clues and the timestamps of the request.

This is really annoying but I’ve found that often it’s worth it and gets me a key piece of information.

beware of time issues

If you’re trying to correlate events based on time, there are a couple of things to be aware of:

• sometimes the time in a logging system is based on the time the log was ingested, not the time that the event actually happened. Sometimes you have to write a date parser to get the actual time the event happened.
• different machines can have slightly skewed clocks
  

log lines for the same request can be very far apart

Especially if a request takes a long time (maybe it took 5 minutes because of a long timeout!), the log lines for the request might be much more spread out than you expected. You can accumulate many thousands of log lines in 5 minutes!

Searching for the request ID really helps with this – it makes it harder to accidentally miss a log entry with an important clue.

Also, log lines can occasionally get completely lost if a server dies.

build a timeline

Keeping all of the information straight in your head can get VERY confusing, so I find it helpful to keep a debugging document where I copy and paste bits of information.

This might include:

• key error messages
• links to relevant dashboards / log system searches
• pager alerts
• graphs
• human actions that were taken (“right before this message, we restarted the load balancer…”)
• my interpretation of various messages (“I think this was caused by…”)

reformat them into a table

Sometimes I’ll reformat the log lines to just print out the information I’m interested in, to make it easier to scan. I’ve done this on the command line with a simple awk command:

cat ... | awk '{print $5 - $8}'

but also with fancy log analysis tools (like Splunk) that let you make a table on the web

check that a “suspicious” error is actually new

Sometimes I’ll notice a suspicious error in the logs and think “OH THERE’S THE CULPRIT!!!“. But when I search for that message to make sure that it’s actually new, I’ll find out that this error actually happens constantly during normal operation, and that it’s completely unrelated to the (new) situation that I’m dealing with.

use the logs to make a graph

Some log analysis tools will let you turn your log lines into a graph to detect patterns.

You can also make a quick histogram with grep and sort. For example I’ve often done something like:

grep -o (some regex) | sort | uniq -c | sort -n

to count how many of each line matching my regular expression there are

filter out irrelevant lines

You can remove irrelevant lines with grep like this:

cat file | grep -v THING1 | grep -v THING2 | grep -v THING3 | grep -v THING4

for the reply guys: yes, we all know you don’t need to use cat here :)

Or if your log system has some kind of query language, you can search for NOT THING1 AND NOT THING2 ...

find the first error

Often an error causes a huge cascade of related errors. Digging into the later errors can waste a lot of your time – you need to start by finding the first thing that triggered the error. Often you don’t need to understand the exact deals of why the 15th thing in the error cascade failed, you can just fix the original problem and move on.

scroll through the log really fast

If you already have an intuition for what log lines for this service should normally look like, sometimes scrolling through them really fast will reveal something that looks off.

turn the log level up (or down)

Sometimes turning up the log level will give you a key error message that explains everything.

But other times, you’ll get overwhelmed by a million irrelevant messages because the log level is set to INFO, and you need to turn the log level down.

put it in a spreadsheet/database

I’ve never tried this myself, but a couple of people suggested copying parts of the logs into a spreadsheet (with the timestamp in a different column) to make it easier to filter / sort.

You could also put the data into SQLite or something (maybe with sqlite-utils?) if you want to be able to run SQL queries on your logs.

on generating good logs

A bunch of people also had thoughts on how to output easier-to-analyze logs. This is a bigger topic than a few bullet points but here are a few quick things:

• use a standard schema/format to make them easier to parse
• include a transaction ID/request ID, to make it easier to filter for all lines related to a single transaction/request
• include relevant information. For example, “ERROR: Invalid msg size” is less helpful than “ERROR: Invalid msg size. Msg-id 234, expected size 54, received size 0”.
• avoid logging personally identifiable information
• use a logging framework instead of using print statements (this helps you have things like log levels and a standard structure)

that’s all!

Let me know on Twitter/Mastodon if there’s anything I missed! I might edit this to add a couple more things.

Hello!

I’ve been doing Advent of Code in Rust for the past couple of days, because I’ve never really gotten comfortable with the language and I thought doing some Advent of Code problems might help.

My solutions aren’t anything special, but because I’m trying to learn, I’ve been trying to take a slightly more rigorous approach than usual to compiler errors. Instead of just fixing the error and moving on, I’m trying to make sure that I actually understand what the error message means and what it’s telling me about how the language works.

My steps to do that are:

1. fix the bug
2. make a tiny standalone program reproducing the same compiler error
3. think about it and try to explain it to myself to make sure I actually understand why that error happened
4. ask for help if I still don’t understand

So here are a couple of compiler errors and my explanations to myself of why the error is happening.

Both of them are pretty basic Rust errors, but I had fun thinking about them today. I wrote this for an imagined audience of “people who know some Rust basics but are still pretty bad at Rust”, if there are any of you out there.

error 1: a borrowing error

Here’s some code (rust playground link):

fn inputs() -> Vec<(i32, i32)> {
    return vec![(0, 0)];
}
fn main() {
    let scores = inputs().iter().map(|(a, b)| {
        a + b
    });
    println!("{}", scores.sum::<i32>());
    
}

And here’s the compiler error:

5 |     let scores = inputs().iter().map(|(a, b)| {
  |                  ^^^^^^^^ creates a temporary which is freed while still in use
6 |         a + b
7 |     });
  |       - temporary value is freed at the end of this statement
8 |     println!("{}", scores.sum::<i32>());
  |                    ------ borrow later used here
  help: consider using a `let` binding to create a longer lived value
  |
5 ~     let binding = inputs();
6 ~     let scores = binding.iter().map(|(a, b)| {
  |

For more information about this error, try `rustc --explain E0716`.

This is a pretty basic Rust error message about borrowing, but I’ve forgotten everything about Rust so I didn’t understand it.

There are 2 things I didn’t know / wasn’t thinking about here:

thing 1: Variables are semantically meaningful in Rust.

What I mean by that is that this code:

let scores = inputs().iter().map(|(a, b)| { ... };

does not do the same thing as if we factor out inputs() into a variable, in this code:

let input = inputs();
let scores = input.iter().map(|(a, b)| { ... };

If some memory is allocated and it isn’t in its own variable, then it’s freed at the end of the expression (though there are some exceptions to this apparently, see rustc --explain E0716 for more). But it does have its own variable, then it’s kept around until the end of the block.

In the error message the Rust compiler actually suggests an explanation to read (rustc --explain E0716), which explains all of this and more. I didn’t notice it right away, but once I read it (and Googled a little), it really helped me.

thing 2:. Computations with iter() don’t happen right away.

This is something that I theoretically knew, but wasn’t thinking about how it might relate to compiler errors.

When I call .map(...), that doesn’t actually do the map right away – it just sets up an iterator that can do actual calculation later, when we call .sum().

This means that I need to keep around the memory from inputs(), because none of the calculation has even happened yet!

error 2: summing an Iterator<()>

Here’s some code (rust playground link) (This isn’t the actual code I was debugging, but it’s the fastest way to demonstrate the error message)

fn main() {
    vec![(), ()].iter().sum::<i32>();
}

This has a pretty obvious bug: You can’t sum a bunch of () (the empty type) and get an i32 as a result. Here’s the compiler error, though:

2 |     vec![(), ()].iter().sum::<i32>();
  |     ^^^^^^^^^^^^^^^^^^^ --- required by a bound introduced by this call
  |     |
  |     the trait `Sum<&()>` is not implemented for `i32`

This was very confusing to me – I’d expect to see an error saying something like Sum is not implemented for Iterator<()>. But instead it says that Sum is not implemented for i32. But I’m not trying to sum i32s! What’s going on?

What’s actually going on here is (thanks to some lovely people who helped me out!):

• i32 has a static method called sum(iter: Iterator<i32>), that comes from the Sum trait. (defined here for integers)
• Iterator has a sum() method that calls this static method on i32 (defined here)
• when I run my_iter.sum(), it tries to call i32::sum(my_iter)
• But i32::sum isn’t defined for Iterator<&()>!
• The type parameter in Sum (eg) Sum<&()> refers to the type of the iterator that’s being passed to i32::sum()
• as a result, we get the error message the trait Sum<&()> is not implemented for i32

I might not have gotten all the types/terms exactly right here, but I think that’s the gist of it.

This was a good reminder that sometimes methods (like sum() on Iterator are defined in slightly indirect/counterintuitive ways and that you have to hunt down the details of how it’s implemented to understand the compiler errors.

(my actual bug here was actually that I’d accidentally added an extra ; in my code, which meant that I accidentally created an Iterator<()> instead of an Iterator<i32>, and the confusing error message made it harder to figure out that out)

Rust error messages are cool

I found these error messages pretty helpful, I especially really appreciated the --explain output on the borrowing error.

Hello! A while back I wrote a post about how to write a toy DNS resolver in Go.

In that post I left out “how to generate and parse DNS queries” because I thought it was boring, but a few people pointed out that they did not know how to parse and generate DNS queries and they were interested in how to do it.

This made me curious – how much work is it do the DNS parsing? It turns out we can do it in a pretty nice 120-line Ruby program, which is not that bad.

So here’s a quick post on how to generate DNS queries and parse DNS responses! We’re going to do it in Ruby because I’m giving a talk at a Ruby conference soon, and this blog post is partly prep for that talk :). I’ve tried to keep it readable for folks who don’t know Ruby though, I’ve only used pretty basic Ruby code.

At the end we’re going to have a very simple toy Ruby version of dig that can look up domain names like this:

$ ruby dig.rb example.com
example.com	   20314    A    93.184.216.34

The whole thing is about 120 lines of code, so it’s not that much. (The final program is dig.rb if you want to skip the explanations and just read some code.) We won’t implement the “how a DNS resolver works” from the previous post because, well, we already did that. Let’s get into it!

Along the way I’m going to try to explain how you could figure out some of this stuff yourself if you were trying to figure out how DNS queries are formatted from scratch. Mostly that’s “poke around in Wireshark” and “read RFC 1035, the DNS RFC”.

step 1: open a UDP socket

We need to actually send our queries, so to do that we need to open a UDP socket. We’ll send our queries to 8.8.8.8, Google’s DNS server.

Here’s the code to set up a UDP connection to 8.8.8.8, port 53 (the DNS port).

require 'socket'
sock = UDPSocket.new

sock.bind('0.0.0.0', 12345)
sock.connect('8.8.8.8', 53)

a quick note on UDP

I’m not going to say too much about UDP here, but I will say that the basic unit of computer networking is the “packet” (a packet is a string of bytes), and in this program we’re going to do the simplest possible thing you can do with a computer network – send 1 packet and receive 1 packet in response.

So UDP is a way to send packets in the simplest possible way.

It’s the most common way to send DNS queries, though you can also use TCP or DNS-over-HTTPS instead.

step 2: copy a DNS query from Wireshark

Next: let’s say we have no idea how DNS works but we want to send a working query as fast as possible. The easiest way to get a DNS query to play with and make sure our UDP connection is working is to just copy one that already works!

So that’s what we’re going to do, using Wireshark (an incredible packet analysis tool)

The steps I used to this are roughly:

1. Open Wireshark and click ‘capture’
2. Enter udp.port == 53 as a filter (in the search bar)
3. Run ping example.com in my terminal (to generate a DNS query)
4. Click on the DNS query (“Standard query A example.com”)
5. Right click on “Domain Name System (query”) in the bottom left pane
6. Click ‘Copy’ -> ‘as a hex stream’
7. Now I have “b96201000001000000000000076578616d706c6503636f6d0000010001” on my clipboard, to use in my Ruby program. Hooray!

step 3: decode the hex stream and send the DNS query

Now we can send our DNS query to 8.8.8.8! Here’s what that looks like: we just need to add 5 lines of code

hex_string = "b96201000001000000000000076578616d706c6503636f6d0000010001"
bytes = [hex_string].pack('H*')
sock.send(bytes, 0)

# get the reply
reply, _ = sock.recvfrom(1024)
puts reply.unpack('H*')

[hex_string].pack('H*') is translating our hex string into a byte string. At this point we don’t really know what this data means but we’ll get there in a second.

We can also take this opportunity to make sure our program is working and is sending valid data, using tcpdump. How I did that:

1. Run sudo tcpdump -ni any port 53 and host 8.8.8.8 in a terminal tab
2. In a different terminal tab, run this Ruby program (ruby dns-1.rb)

Here’s what the output looks like:

$ sudo tcpdump -ni any port 53 and host 8.8.8.8
08:50:28.287440 IP 192.168.1.174.12345 > 8.8.8.8.53: 47458+ A? example.com. (29)
08:50:28.312043 IP 8.8.8.8.53 > 192.168.1.174.12345: 47458 1/0/0 A 93.184.216.34 (45)

This is really good - we can see the DNS request (“what’s the IP for example.com”) and the response (“it’s 93.184.216.34”). So everything is working. Now we just need to, you know, figure out how to generate and decode this data ourselves.

step 4: learn a little about how DNS queries are formatted

Now that we have a DNS query for example.com, let’s learn about what it means.

Here’s our query, formatted as hex.

b96201000001000000000000076578616d706c6503636f6d0000010001

If you poke around in Wireshark, you’ll see that this query has 2 parts:

• The header (b96201000001000000000000)
• The question (076578616d706c6503636f6d0000010001)

step 5: make the header

Our goal in this step is to generate the byte string b96201000001000000000000, but with a Ruby function instead of hardcoding it.

So: the header is 12 bytes. What do those 12 bytes mean? If you look at Wireshark (or read RFC 1035), you’ll see that it’s 6 2-byte numbers concatenated together.

The 6 numbers correspond to the query ID, the flags, and then the number of questions, answer records, authoritative records, and additional records in the packet.

We don’t need to worry about what all those things are yet though – we just need to put in 6 numbers.

And luckily we know exactly which 6 numbers to put because our goal is to literally generate the string b96201000001000000000000.

So here’s a function to make the header. (note: there’s no return because you don’t need to write return in Ruby if it’s the last line of the function)

def make_question_header(query_id)
  # id, flags, num questions, num answers, num auth, num additional
  [query_id, 0x0100, 0x0001, 0x0000, 0x0000, 0x0000].pack('nnnnnn')
end

This is very short because we’ve hardcoded everything except the query ID.

what’s nnnnnn?

You might be wondering what nnnnnn is in .pack('nnnnnn'). That’s a format string telling .pack() how to convert that array of 6 numbers into a byte string.

The documentation for .pack is here, and it says that n means “represent it as “16-bit unsigned, network (big-endian) byte order”.

16 bits is the same as 2 bytes, and we need to use network byte order because this is computer networking. I’m not going to explain byte order right now (though I do have a comic attempting to explain it)

test the header code

Let’s quickly test that our make_question_header function works.

puts make_question_header(0xb962) == ["b96201000001000000000000"].pack("H*")

This prints out “true”, so we win and we can move on.

step 5: encode the domain name

Next we need to generate the question (“what’s the IP for example.com?“). This has 3 parts:

• the domain name (for example “example.com”)
• the query type (for example “A” is for “IPv4 Address”
• the query class (which is always the same, 1 is for IN is for INternet)

The hardest part of this is the domain name so let’s write a function to do that.

example.com is encoded in a DNS query, in hex, as 076578616d706c6503636f6d00. What does that mean?

Well, if we translate the bytes into ASCII, it looks like this:

076578616d706c6503636f6d00
 7 e x a m p l e 3 c o m 0

So each segment (like example) has its length (like 7) in front of it.

Here’s the Ruby code to translate example.com into 7 e x a m p l e 3 c o m 0:

def encode_domain_name(domain)
  domain
    .split(".")
    .map { |x| x.length.chr + x }
    .join + "\0"
end

Other than that, to finish generating the question section we just need to append the type and class onto the end of the domain name.

step 6: write make_dns_query

Here’s the final function to make a DNS query:

def make_dns_query(domain, type)
  query_id = rand(65535)
  header = make_question_header(query_id)
  question =  encode_domain_name(domain) + [type, 1].pack('nn')
  header + question
end

Here’s all the code we’ve written before in dns-2.rb – it’s still only 29 lines.

now for the parsing

Now that we’ve managed to generate a DNS query, we get into the hard part: the parsing. Again, we’ll split this into a bunch of different

• parse a DNS header
• parse a DNS name
• parse a DNS record

The hardest part of this (maybe surprisingly) is going to be “parse a DNS name”.

step 7: parse the DNS header

Let’s start with the easiest part: the DNS header. We already talked about how it’s 6 numbers concatenated together.

So all we need to do is

• read the first 12 bytes
• convert that into an array of 6 numbers
• put those numbers in a class for convenience

Here’s the Ruby code to do that.

class DNSHeader
  attr_reader :id, :flags, :num_questions, :num_answers, :num_auth, :num_additional
  def initialize(buf)
    hdr = buf.read(12)
    @id, @flags, @num_questions, @num_answers, @num_auth, @num_additional = hdr.unpack('nnnnnn')
  end
end

Quick Ruby note: attr_reader is a Ruby thing that means “make these instance variables accessible as methods”. So you can call header.flags to look at the @flags variable.

We can call this with DNSHeader(buf). Not so bad.

Let’s move on to the hardest part: parsing a domain name.

step 8: parse a domain name

First, let’s write a partial version.

def read_domain_name_wrong(buf)
  domain = []
  loop do
    len = buf.read(1).unpack('C')[0]
    break if len == 0
    domain << buf.read(len)
  end
  domain.join('.')
end

This repeatedly reads 1 byte and then reads that length into a string until the length is 0.

This works great, for the first time we see a domain name (example.com) in our DNS response.

trouble with domain names: compression!

But the second time example.com appears, we run into trouble – in Wireshark, it says that the domain is represented cryptically as just the 2 bytes c00c.

This is something called DNS compression and if we want to parse any DNS responses we’re going to have to implement it.

This is luckily not that hard. All c00c is saying is:

• The first 2 bits (0b11.....) mean “DNS compression ahead!”
• The remaining 14 bits are an integer. In this case that integer is 12 (0x0c), so that means “go back to the 12th byte in the packet and use the domain name you find there”

If you want to read more about DNS compression, I found the explanation in the DNS RFC relatively readable.

step 9: implement DNS compression

So we need a more complicated version of our read_domain_name function

Here it is.

  domain = []
  loop do
    len = buf.read(1).unpack('C')[0]
    break if len == 0
    if len & 0b11000000 == 0b11000000
      # weird case: DNS compression!
      second_byte = buf.read(1).unpack('C')[0]
      offset = ((len & 0x3f) << 8) + second_byte
      old_pos = buf.pos
      buf.pos = offset
      domain << read_domain_name(buf)
      buf.pos = old_pos
      break
    else
      # normal case
      domain << buf.read(len)
    end
  end
  domain.join('.')

Basically what’s happening is:

• if the first 2 bits are 0b11, we need to do DNS compression. Then:
  • read the second byte and do a little bit arithmetic to convert that into the offset
  • save the current position in the buffer
  • read the domain name at the offset we calculated
  • restore our position in the buffer

This is kind of messy but it’s the most complicated part of parsing the DNS response, so we’re almost done!

a DNS compression exploit

Someone pointed out that a malicious actor could exploit this code by sending a DNS response with a DNS compression entry that points to itself, so that read_domain_name would end up in an infinite loop. I won’t update it (the code is already complicated enough!) but a real DNS parser would be smarter and deal with that. For example here’s the code that avoids infinite loops in miekg/dns

There are also probably other edge cases that would be problematic if this were a real DNS parser.

step 10: parse a DNS query

You might think “why do we need to parse a DNS query? This is the response!”. But every DNS response has the original query in it, so we need to parse it.

Here’s the code for parsing the DNS query.

class DNSQuery
  attr_reader :domain, :type, :cls
  def initialize(buf)
    @domain = read_domain_name(buf)
    @type, @cls = buf.read(4).unpack('nn')
  end
end

There’s not very much to it: the type and class are 2 bytes each.

step 11: parse a DNS record

This is the exciting part – the DNS record is where our query data lives! The “rdata field” (“record data”) is where the IP address we’re going to get in response to our DNS query lives.

Here’s the code:

class DNSRecord 
  attr_reader :name, :type, :class, :ttl, :rdlength, :rdata
  def initialize(buf)
    @name = read_domain_name(buf)
    @type, @class, @ttl, @rdlength = buf.read(10).unpack('nnNn')
    @rdata = buf.read(@rdlength)
  end

We also need to do a little work to make the rdata field human readable. The meaning of the record data depends on the record type – for example for an “A” record it’s a 4-byte IP address, for but a “CNAME” record it’s a domain name.

So here’s some code to make the request data human readable:

  def read_rdata(buf, length)
    @type_name = TYPES[@type] || @type
    if @type_name == "CNAME" or @type_name == "NS"
      read_domain_name(buf)
    elsif @type_name == "A"
      buf.read(length).unpack('C*').join('.')
    else
      buf.read(length)
    end
  end

This function uses this TYPES hash to map the record type to a human-readable name:

TYPES = {
  1 => "A",
  2 => "NS",
  5 => "CNAME",
  # there are a lot more but we don't need them for this example
}

The most interesting part of read_rdata is probably the line buf.read(length).unpack('C*').join('.') – it’s saying “hey, an IP address is 4 bytes, so convert it into an array of 4 numbers and then join those with “.“s”.

step 12: finish parsing the DNS response

Now we’re ready to parse the DNS response!

Here’s some code to do that:

class DNSResponse
  attr_reader :header, :queries, :answers, :authorities, :additionals
  def initialize(bytes)
    buf = StringIO.new(bytes)
    @header = DNSHeader.new(buf)
    @queries = (1..@header.num_questions).map { DNSQuery.new(buf) }
    @answers = (1..@header.num_answers).map { DNSRecord.new(buf) }
    @authorities = (1..@header.num_auth).map { DNSRecord.new(buf) }
    @additionals = (1..@header.num_additional).map { DNSRecord.new(buf) }
  end
end

This mostly just calls the other functions we’ve written to parse the DNS response.

It uses this cute (1..@header.num_answers).map construction to create an array of 2 DNS records if @header.num_answers is 2. (which is maybe a little bit of Ruby magic but I think it’s kind of fun and hopefully isn’t too hard to read)

We can integrate this code into our main function like this:

sock.send(make_dns_query("example.com", 1), 0) # 1 is "A", for IP address
reply, _ = sock.recvfrom(1024)
response = DNSResponse.new(reply) # parse the response!!!
puts response.answers[0]

Printing out the records looks awful though (it says something like #<DNSRecord:0x00000001368e3118>). So we need to write some pretty printing code to make it human readable.

step 13: pretty print our DNS records

We need to add a .to_s field to DNS records to make them have a nice string representation. This is just a 1-line method in DNSRecord:

  def to_s
    "#{@name}\t\t#{@ttl}\t#{@type_name}\t#{@parsed_rdata}"
  end

You also might notice that I left out the class field of the DNS record. That’s because it’s always the same (IN for “internet”) so I felt it was redundant. Most DNS tools (like real dig) will print out the class though.

and we’re done!

Here’s our final main function:

def main
  # connect to google dns
  sock = UDPSocket.new
  sock.bind('0.0.0.0', 0)
  sock.connect('8.8.8.8', 53)

  # send query
  domain = ARGV[0]
  sock.send(make_dns_query(domain, 1), 0)

  # receive & parse response
  reply, _ = sock.recvfrom(1024)
  response = DNSResponse.new(reply)
  response.answers.each do |record|
    puts record
  end

I don’t think there’s too much to say about this – we connect, send a query, print out each of the answers, and exit. Success!

$ ruby dig.rb example.com
example.com   18608   A   93.184.216.34

You can see the final program as a gist here: dig.rb. You could add more features to it if you want, like

• pretty printing for other query types
• options to print out the “authority” and “additional” sections of the DNS response
• retries
• making sure that the DNS response we see is actually a response to the query we sent (the query ID has to match!

Also you can let me know on Twitter if I’ve made a mistake in this post somewhere – I wrote this pretty quickly so I probably got something wrong.

Hello! When I was writing the zine How DNS Works earlier this year, someone asked me – why do people sometimes put a dot at the end of a domain name? For example, if you look up the IP for example.com by running dig example.com, you’ll see this:

$ dig example.com
example.com.		5678	IN	A	93.184.216.34

dig has put a . to the end of example.com – now it’s example.com.! What’s up with that?

Also, some DNS tools require domains to have a "." at the end: if you try to pass example.com to miekg/dns, like this, it’ll fail:

// trying to send this message will return an error
m := new(dns.Msg)
m.SetQuestion("example.com", dns.TypeA)

Originally I thought I knew the answer to this (“uh, the dot at the end means the domain is fully qualified?“). And that’s true – a fully qualified domain name is a domain with a “.” at the end!

But that doesn’t explain why dots at the end are useful or important.

in a DNS request/response, domain names don’t have a trailing “.”

I once (incorrectly) thought the answer to “why is there a dot at the end?” might be “In a DNS request/response, domain names have a “.” at the end, so we put it in to match what actually gets sent/received by your computer”. But that’s not true at all!

When a computer sends a DNS request or response, the domain names in it don’t have a trailing dot. Actually, the domain names don’t have any dots.

Instead, they’re encoded as a series of length/string pairs. For example, the domain example.com is encoded as these 13 bytes:

7example3com0

So there are no dots at all. Instead, an ASCII domain name (like “example.com”) gets translated into the format used in a DNS request / response by various DNS software.

So let’s talk about one place where domain names are translated into DNS responses: zone files.

the trailing “.” in zone files

One way that some people manage DNS records for a domain is to create a text file called a “zone file” and then configure some DNS server software (like nsd or bind) to serve the DNS records specified in that zone file.

Here’s an imaginary zone file for example.com:

orange  300   IN    A     1.2.3.4
fruit   300   IN    CNAME orange
grape   3000  IN    CNAME example.com.

In this zone file, anything that doesn’t end in a "." (like "orange") gets .example.com added to it. So "orange" is shorthand for "orange.example.com". The DNS server knows from its configuration that this is a zone file for example.com, so it knows to automatically append example.com at the end of any name that doesn’t end with a dot.

I assume the idea here is just to save typing – you could imagine writing this zone file by fully typing out all of the domain names:

orange.example.com.  300   IN    A     1.2.3.4
fruit.example.com.   300   IN    CNAME orange.example.com.
grape.example.com.   3000  IN    CNAME example.com.

But that’s a lot of typing.

you don’t need zone files to use DNS

Even though the zone file format is defined in the official DNS RFC (RFC 1035), you don’t have to use zone files at all to use DNS. For example, AWS Route 53 doesn’t use zone files to store DNS records! Instead you create records through the web interface or API, and I assume they store records in some kind of database and not a bunch of text files.

Route 53 (like many other DNS tools) does support importing and exporting zone files though and it can be a good way to migrate records from one DNS provider to another.

the trailing “.” in dig

Now, let’s talk about dig’s output:

$ dig example.com
; <<>> DiG 9.18.1-1ubuntu1.1-Ubuntu <<>> +all example.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10712
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;example.com.			IN	A

;; ANSWER SECTION:
example.com.		81239	IN	A	93.184.216.34

One weird thing about this is that almost every line starts with a ;;. What’s up with that? Well ; is the comment character in zone files!

So I think the reason that dig prints out its output in this weird way is so that if you wanted, you could just paste this into a zone file and have it work without any changes.

This also explains why there’s a . at the end of example.com. – zone files require a trailing dot at the end of a domain name (because otherwise they’re interpreted as being relative to the zone). So dig does too.

I really wish dig had a +human flag that printed out all of this information in a more human readable way, but for now I’m too lazy to put in the work to actually contribute code to do that (and I’m a pretty bad C programmer) so I’ll just complain about it on my blog instead :)

the trailing "." in curl

Let’s talk about another case where the trailing "." shows up: curl!

One of the computers in my house is called “grapefruit”, and it’s running a webserver. Here’s what happens if I run curl grapefruit:

$ curl grapefruit
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

<html>
<head>

It works! Cool. But what happens if I add a . at the end? Suddenly it doesn’t work:

$ curl grapefruit.
curl: (6) Could not resolve host: grapefruit.

What’s going on? To understand, we need to learn about search domains:

meet search domains

When I run curl grapefrult, how does that get translated into a DNS request? You might think that my computer would send a request for the domain grapefruit, right? But that’s not true.

Let’s use tcpdump to see what domain is actually being looked up:

$ sudo tcpdump -i any port 53
[...] A? grapefruit.lan. (32)

It’s actually sending a request for grapefruit.lan. What’s up with that?

Well, what’s going on is that:

1. To look up grapefruit, curl calls a function called getaddrinfo
2. getaddrinfo looks in a file on my computer called /etc/resolv.conf
3. /etc/resolv.conf contains these 2 lines:

   nameserver 127.0.0.53
   search lan
   

4. Because it sees search lan, getaddrinfo adds a lan at the end of grapefruit and looks up grapefruit.lan instead

when are search domains used?

Now we know something weird: that when we look up a domain, sometimes an extra thing (like lan) will be added to the end. But when does that happen?

1. If we put a "." at the end of the domain (like curl grapefruit., then search domains aren’t used
2. If the domain has an "." inside it (like example.com has a dot in it), then by default search domains aren’t used either. But this can be changed with configuration (see this blog post about ndots that talks about this more)

So now we know why curl grapefruit. has different results than curl grapefruit – it’s because one looks up the domain grapefruit. and the other one looks up grapefruit.lan.

how does my computer know what search domain to use?

When I connect to my router, it tells me that its search domain is lan with DHCP – it’s the same way that my computer gets assigned an IP address.

so why do people put a dot at the end of domain names?

Now that we know about zone files and search domains, here’s why I think people like to put dots at the end of a domain name.

There are two contexts where domain names are modified and get something else added to the end:

• in a zone file for example.com, grapefruit get translated to grapefruit.example.com
• on my local network (with my computer configured to use the search domain lan), grapefruit gets translated to grapefruit.lan

So because domain names can actually be translated to something else in some cases, people like to put a "." at the end to communicate “THIS IS THE DOMAIN NAME, NOTHING GETS ADDED AT THE END, THIS IS THE WHOLE THING”. Because otherwise it can get confusing.

The technical term for “THIS IS THE WHOLE THING” is “fully qualified domain name” or “FQDN”. So google.com. is a fully qualified domain name, and google.com isn’t.

I always have to remind myself for the reasons for this because I rarely use zone files or search domains, so I often feel like – “of course I mean google.com and not google.com.something.else! Why would I mean anything else?? That’s silly!”

But some people do use zone files and search domains (search domains are used in Kubernetes, for example!), so the “.” at the end is useful to make it 100% clear that nothing else should be added.

when to put a “.” at the end?

Here are a couple of quick notes about when to put a “.” at the end of your domain names:

Yes: when configuring DNS

It’s never bad to use fully qualified domain names when configuring DNS. You don’t always have to: a non-fully-qualified domain name will often work just fine as well, but I’ve never met a piece of DNS software that wouldn’t accept a fully qualified domain name.

And some DNS software requires it: right now the DNS server I use for jvns.ca makes me put a "." at the end of domains names (for example in CNAME records) and warns me otherwise it’ll append .jvns.ca to whatever I typed in. I don’t agree with this design decision but it’s not a big deal, I just put a “.” at the end.

No: in a browser

Confusingly, it often doesn’t work to put a "." at the end of a domain name in a browser! For example, if I type https://twitter.com. into my browser, it doesn’t work! It gives me a 404.

I think what’s going on here is that it’s setting the HTTP Host header to Host: twitter.com. and the web server on the other end is expecting Host: twitter.com.

Similarly, https://jvns.ca. gives me an SSL error for some reason.

I think relative domain names used to be more common

One last thing: I think that “relative” domain names (like me using grapefruit to refer to the other computer in my house, grapefruit.lan) used to be more commonly used, because DNS was developed in the context of universities or other big institutions which have big internal networks.

On the internet today, it seems like it’s more common to use “absolute” domain names (like example.com).

Hello!

Recently I’ve been working on a project where I implement a bunch of tiny toy working versions of computer networking protocols in Python without using any libraries, as a way to explain how computer networking works.

I’m still working on writing up that project, but today I wanted to talk about how to do the very first step: sending network packets in Python.

In this post we’re going to send a SYN packet (the first packet in a TCP connection) from a tiny Python program, and get a reply from example.com. All the code from this post is in this gist.

what’s a network packet?

A network packet is a byte string. For example, here’s the first packet in a TCP connection:

b'E\x00\x00,\x00\x01\x00\x00@\x06\x00\xc4\xc0\x00\x02\x02"\xc2\x95Cx\x0c\x00P\xf4p\x98\x8b\x00\x00\x00\x00`\x02\xff\xff\x18\xc6\x00\x00\x02\x04\x05\xb4'

I’m not going to talk about the structure of this byte string in this post (though I’ll say that this particular byte string has two parts: the first 20 bytes are the IP address part and the rest is the TCP part)

The point is that to send network packets, we need to be able to send and receive strings of bytes.

why tun/tap?

The problem with writing your own TCP implementation on Linux (or any operating system) is – the Linux kernel already has a TCP implementation!

So if you send out a SYN packet on your normal network interface to a host like example.com, here’s what will happen:

1. you send a SYN packet to example.com
2. example.com replies with a SYN ACK (so far so good!)
3. the Linux kernel on your machine gets the SYN ACK, thinks “wtf?? I didn’t make this connection??”, and closes the connection
4. you’re sad. no TCP connection for you.

I was talking to a friend about this problem a few years ago and he said “you should use tun/tap!“. It took quite a few hours to figure out how to do that though, which is why I’m writing this blog post :)

tun/tap gives you a “virtual network device”

The way I like to think of tun/tap is – imagine I have a tiny computer in my network which is sending and receiving network packets. But instead of it being a real computer, it’s just a Python program I wrote.

That explanation is honestly worse than I would like. I wish I understood exactly how tun/tap devices interfaced with the real Linux network stack but unfortunately I do not, so “virtual network device” is what you’re getting. Hopefully the code examples below will make all it a bit more clear.

tun vs tap

The system called “tun/tap” lets you create two kinds of network interfaces:

• “tun”, which lets you set IP-layer packets
• “tap”, which lets you set Ethernet-layer packets

We’re going to be using tun, because that’s what I could figure out how to get to work. It’s possible that tap would work too.

how to create a tun interface

Here’s how I created a tun interface with IP address 192.0.2.2.

sudo ip tuntap add name tun0 mode tun user $USER
sudo ip link set tun0 up
sudo ip addr add 192.0.2.1 peer 192.0.2.2 dev tun0

sudo iptables -t nat -A POSTROUTING -s 192.0.2.2 -j MASQUERADE
sudo iptables -A FORWARD -i tun0 -s 192.0.2.2 -j ACCEPT
sudo iptables -A FORWARD -o tun0 -d 192.0.2.2 -j ACCEPT

These commands do two things:

1. Create the tun device with the IP 192.0.2.2 (and give your user access to write to it)
2. set up iptables to proxy packets from that tun device to the internet using NAT

The iptables part is very important because otherwise the packets would only exist inside my computer and wouldn’t be sent to the internet, and what fun would that be?

I’m not going to explain this ip addr add command because I don’t understand it, I find ip to be very inscrutable and for now I’m resigned to just copying and pasting ip commands without fully understanding them. It does work though.

how to connect to the tun interface in Python

Here’s a function to open a tun interface, you call it like openTun('tun0'). I figured out how to write it by searching through the scapy source code for “tun”.

import struct
from fcntl import ioctl

def openTun(tunName):
    tun = open("/dev/net/tun", "r+b", buffering=0)
    LINUX_IFF_TUN = 0x0001
    LINUX_IFF_NO_PI = 0x1000
    LINUX_TUNSETIFF = 0x400454CA
    flags = LINUX_IFF_TUN | LINUX_IFF_NO_PI
    ifs = struct.pack("16sH22s", tunName, flags, b"")
    ioctl(tun, LINUX_TUNSETIFF, ifs)
    return tun

All this is doing is

1. opening /dev/net/tun in binary mode
2. calling an ioctl to tell Linux that we want a tun device, and that the one we want is called tun0 (or whatever tunName we’ve passed to the function).

Once it’s open, we can read from and write to it like any other file in Python.

let’s send a SYN packet!

Now that we have the openTun function, we can send a SYN packet!

Here’s what the Python code looks like, using the openTun function.

syn = b'E\x00\x00,\x00\x01\x00\x00@\x06\x00\xc4\xc0\x00\x02\x02"\xc2\x95Cx\x0c\x00P\xf4p\x98\x8b\x00\x00\x00\x00`\x02\xff\xff\x18\xc6\x00\x00\x02\x04\x05\xb4'
tun = openTun(b"tun0")
tun.write(syn)
reply = tun.read(1024)
print(repr(reply))

If I run this as sudo python3 syn.py, it prints out the reply from example.com:

b'E\x00\x00,\x00\x00@\x00&\x06\xda\xc4"\xc2\x95C\xc0\x00\x02\x02\x00Px\x0cyvL\x84\xf4p\x98\x8c`\x12\xfb\xe0W\xb5\x00\x00\x02\x04\x04\xd8'

Obviously this is a pretty silly way to send a SYN packet – a real implementation would have actual code to generate that byte string instead of hardcoding it, and we would parse the reply instead of just printing out the raw byte string. But I didn’t want to go into the structure of TCP in this post so that’s what we’re doing.

looking at these packets with tcpdump

If we run tcpdump on the tun0 interface, we can see the packet we sent and the answer from example.com:

$ sudo tcpdump -ni tun0
12:51:01.905933 IP 192.0.2.2.30732 > 34.194.149.67.80: Flags [S], seq 4101019787, win 65535, options [mss 1460], length 0
12:51:01.932178 IP 34.194.149.67.80 > 192.0.2.2.30732: Flags [S.], seq 3300937416, ack 4101019788, win 64480, options [mss 1240], length 0

Flags [S] is the SYN we sent, and Flags [S.] is the SYN ACK packet in response! We successfully communicated! And the Linux network stack didn’t interfere at all!

tcpdump also shows us how NAT is working

We can also run tcpdump on my real network interface (wlp3so, my wireless card), to see the packets being sent and received. We’ll pass -i wlp3s0 instead of -i tun0.

$ sudo tcpdump -ni wlp3s0 host 34.194.149.67
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on wlp3s0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
12:56:01.204382 IP 192.168.1.181.30732 > 34.194.149.67.80: Flags [S], seq 4101019787, win 65535, options [mss 1460], length 0
12:56:01.228239 IP 34.194.149.67.80 > 192.168.1.181.30732: Flags [S.], seq 144769955, ack 4101019788, win 64480, options [mss 1240], length 0
12:56:05.334427 IP 34.194.149.67.80 > 192.168.1.181.30732: Flags [S.], seq 144769955, ack 4101019788, win 64480, options [mss 1240], length 0
12:56:13.524973 IP 34.194.149.67.80 > 192.168.1.181.30732: Flags [S.], seq 144769955, ack 4101019788, win 64480, options [mss 1240], length 0
12:56:29.705007 IP 34.194.149.67.80 > 192.168.1.181.30732: Flags [S.], seq 144769955, ack 4101019788, win 64480, options [mss 1240], length 0

A couple of things to notice here:

• The IP addresses are different – that IPtables rule from above has rewritten them from 192.0.2.2 to 192.168.1.181. This rewriting is called “network address translation”, or “NAT”.
• We’re getting a bunch of replies from example.com – it’s doing an exponential backoff where it retries after 4 seconds, then 8 seconds, then 16 seconds. This is because we didn’t finish the TCP handshake – we just sent a SYN and left it hanging! There’s actually a type of DDOS attack like this called SYN flooding, but just sending one or two SYN packets isn’t a big deal.
• I had to add host 34.194.149.67 because there are a lot of TCP packets being sent on my real wifi connection so I needed to ignore those

I’m not totally sure why we see more SYN replies on wlp3s0 than on tun0, my guess is that it’s because we only read 1 reply in our Python program.

this is pretty easy and really reliable

The last time I tried to implement TCP in Python I did it with something called “ARP spoofing”. I won’t talk about that here (there are some posts about it on this blog back in 2013), but this way is a lot more reliable.

And ARP spoofing is kind of a sketchy thing to do on a network you don’t own.

here’s the code

I put all the code from this blog post in this gist, if you want to try it yourself, you can run

bash setup.sh # needs to run as root, has lots of `sudo` commands
python3 syn.py # runs as a regular user

It only works on Linux, but I think there’s a way to set up tun/tap on Mac too.

a plug for scapy

I’ll close with a plug for scapy here: it’s a really great Python networking library for doing this kind of experimentation without writing all the code yourself.

This post is about writing all the code yourself though so I won’t say more about it than that.