Briefly Inspecting HTTPS (TLS Communication) with curl -vvv

12 min

httpsunixtls
language: ja bn en es hi pt ru zh-cn zh-tw

Hello, this is Incompetent.

Introduction

HTTPS itself is TLS communication, and as HTTP over TLS suggests, it's HTTP communication wrapped in TLS communication. If we focus only on TLS communication, it becomes different from a request to a domain that supports HTTPS communication, so by detailed debugging with curl -vvv, you can check the communication from the client hello. That being said, I stumbled upon this by chance.

Let's Take a Look

Now, let's peek in as follows.

curl -vvv -sl
-vvv # Detailed debug option
-s # Silent mode, optional
-I # Output headers only to stdout

Now, let's see.

$ curl -vvv https://soulminingrig.com/ -sI
19:30:07.019276 [0-x] == Info: [READ] client_reset, clear readers
19:30:07.064770 [0-0] == Info: Host soulminingrig.com:443 was resolved.
19:30:07.065031 [0-0] == Info: IPv6: (none)
19:30:07.065180 [0-0] == Info: IPv4: 167.179.75.206
19:30:07.065413 [0-0] == Info: [HTTPS-CONNECT] adding wanted h2
19:30:07.065587 [0-0] == Info: [HTTPS-CONNECT] added
19:30:07.065718 [0-0] == Info: [HTTPS-CONNECT] connect, init
19:30:07.065881 [0-0] == Info:   Trying 167.179.75.206:443...
19:30:07.066065 [0-0] == Info: [HTTPS-CONNECT] connect -> 0, done=0
19:30:07.066197 [0-0] == Info: [HTTPS-CONNECT] Curl_conn_connect(block=0) -> 0, done=0
19:30:07.066400 [0-0] == Info: [HTTPS-CONNECT] adjust_pollset -> 1 socks
19:30:07.083945 [0-0] == Info: [HTTPS-CONNECT] connect -> 0, done=0
19:30:07.084308 [0-0] == Info: [HTTPS-CONNECT] Curl_conn_connect(block=0) -> 0, done=0
19:30:07.084750 [0-0] == Info: [HTTPS-CONNECT] adjust_pollset -> 1 socks
19:30:07.095330 [0-0] == Info: [SSL] cf_connect()
19:30:07.095574 [0-0] == Info: [SSL] ossl_connect, step1
19:30:07.098502 [0-0] == Info: ALPN: curl offers h2,http/1.1
19:30:07.098614 [0-0] == Info: [SSL] ossl_connect, step2
19:30:07.099069 [0-0] => Send SSL data, 5 bytes (0x5)
0000: .....
19:30:07.099205 [0-0] == Info: TLSv1.3 (OUT), TLS handshake, Client hello (1):
19:30:07.099360 [0-0] => Send SSL data, 1563 bytes (0x61b)
~omitted~

And a huge amount of text appeared...

Communication Confirmation

Here, DNS name resolution on port 443 was successful.

19:30:07.064770 [0-0] == Info: Host soulminingrig.com:443 was resolved.

Here, we requested and successfully connected with h2, HTTP/2.

19:30:07.065413 [0-0] == Info: [HTTPS-CONNECT] adding wanted h2
19:30:07.065587 [0-0] == Info: [HTTPS-CONNECT] added
19:30:07.065718 [0-0] == Info: [HTTPS-CONNECT] connect, init
19:30:07.065881 [0-0] == Info:   Trying 167.179.75.206:443...
19:30:07.066065 [0-0] == Info: [HTTPS-CONNECT] connect -> 0, done=0

And finally, SSL/TLS communication has begun.

19:30:07.095330 [0-0] == Info: [SSL] cf_connect()
19:30:07.095574 [0-0] == Info: [SSL] ossl_connect, step1
19:30:07.098502 [0-0] == Info: ALPN: curl offers h2,http/1.1
19:30:07.098614 [0-0] == Info: [SSL] ossl_connect, step2
19:30:07.099069 [0-0] => Send SSL data, 5 bytes (0x5)

And then, the TLS handshake of client hello on the protocol finally began with TLS 1.3!

19:30:07.099205 [0-0] == Info: TLSv1.3 (OUT), TLS handshake, Client hello (1):

Packet transmission begins as a client transmission.

19:30:07.099205 [0-0] == Info: TLSv1.3 (OUT), TLS handshake, Client hello (1):
19:30:07.099360 [0-0] => Send SSL data, 1563 bytes (0x61b)
0000: .........r.q.....GC...._4....k..C.P... ..?...x...HT..09..z4b....
0040: ..,9....<.......,.0.........+./...$.(.k.#.'.g.....9.....3.....=.
0080: <.5./..................soulminingrig.com........................
00c0: .............h2.http/1.1.........1.....6.4......................
0100: ...............................+........-.....3...........RIw..7

At this point, the domain soulminingrig.com is sent in plain text within the request itself, and if the server supports SNI, consistency can be maintained.

19:30:07.105068 [0-0] == Info: [SSL] ossl_bio_cf_out_write(len=1568) -> 0, 1568
19:30:07.105244 [0-0] == Info: [SSL] ossl_bio_cf_in_read(len=5) -> 81, 0
19:30:07.105355 [0-0] == Info: [SSL] ossl_populate_x509_store, path=/etc/ssl/certs/ca-certificates.crt, blob=0
19:30:07.114557 [0-0] == Info:  CAfile: /etc/ssl/certs/ca-certificates.crt
19:30:07.114669 [0-0] == Info:  CApath: none
19:30:07.114768 [0-0] == Info: [SSL] SSL_connect() -> err=-1, detail=2
19:30:07.114903 [0-0] == Info: [SSL] SSL_connect() -> want recv
19:30:07.115050 [0-0] == Info: [SSL] cf_connect() -> 0, done=0

At this point, the client is preparing its possessed Root certificate.
The connection has not been established yet.
In the case of Let’s Encrypt, it seems to correspond to ISRG Root X1.
Chain of Trust - Let’s Encrypt

$ grep -A 3 "ISRG Root X1" /etc/ssl/certs/ca-certificates.crt
# ISRG Root X1
-----BEGIN CERTIFICATE-----
MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw
TzELMAkGA1UEBhMCVVMxKTAnBgNVBAoTIEludGVybmV0IFNlY3VyaXR5IFJlc2Vh

And then, Server Hello returned!

19:30:07.178149 [0-0] == Info: [SSL] ossl_bio_cf_in_read(len=122) -> 0, 122
19:30:07.178363 [0-0] == Info: TLSv1.3 (IN), TLS handshake, Server hello (2):
19:30:07.178583 [0-0] <= Recv SSL data, 122 bytes (0x7a)
0000: ...v....y'...>........&.....,5O....... ..?...x...HT..09..z4b....
0040: ..,9.........+.....3.$... F........`~..l[..uhE..F.P?..V..6
19:30:07.179600 [0-0] == Info: [SSL] ossl_bio_cf_in_read(len=5) -> 0, 5
19:30:07.179730 [0-0] <= Recv SSL data, 5 bytes (0x5)

And then, the certificate returned from the server.

19:30:07.183139 [0-0] <= Recv SSL data, 2049 bytes (0x801)
0000: ...........0...0.................g...4...C..0...*.H.=...021.0...
0040: U....US1.0...U....Let's Encrypt1.0...U....E50...250714140356Z..2
0080: 51012140355Z0.1.0...U....soulminingrig.com0Y0...*.H.=....*.H.=..
00c0: ..B...V...$.....}.hN.f......n@F&...GR.....-.....?z]6d.=..<..eu..

Using the private key held by the server, the signature was sent.

0000: .
19:30:07.199683 [0-0] == Info: TLSv1.3 (IN), TLS handshake, CERT verify (15):
19:30:07.199941 [0-0] <= Recv SSL data, 79 bytes (0x4f)

Here, the TLS handshake finally completed and was established.

19:30:07.202057 [0-0] == Info: TLSv1.3 (IN), TLS handshake, Finished (20):

Rarely Seen TLS Handshake

Initially, when I asked ChatGPT and others, they told me to use Wireshark, but Wireshark often has a lot of noise, so I was wondering if there was an easier way to check. Then I remembered something and tried it, and it worked.
It might also be interesting to check the patterns where TLS handshake fails using the following:
badssl.com

$ curl -vvv -sl https://wrong.host.badssl.com/
19:50:53.161049 [0-x] == Info: [READ] client_reset, clear readers
19:50:53.202041 [0-0] == Info: Host wrong.host.badssl.com:443 was resolved.
19:50:53.202233 [0-0] == Info: IPv6: (none)
19:50:53.202326 [0-0] == Info: IPv4: 104.154.89.105
19:50:53.202496 [0-0] == Info: [HTTPS-CONNECT] adding wanted h2
~~~
19:50:53.759424 [0-0] == Info: SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256 / secp256r1 / rsaEncryption
19:50:53.759894 [0-0] == Info: ALPN: server accepted http/1.1
19:50:53.760124 [0-0] == Info: [SSL] ossl_connect, step3
19:50:53.760337 [0-0] == Info: Server certificate:
19:50:53.760541 [0-0] == Info:  subject: CN=*.badssl.com
19:50:53.760763 [0-0] == Info:  start date: Jul 15 20:02:58 2025 GMT
19:50:53.761029 [0-0] == Info:  expire date: Oct 13 20:02:57 2025 GMT
19:50:53.761406 [0-0] == Info:  subjectAltName does not match hostname wrong.host.badssl.com
19:50:53.761853 [0-0] == Info: SSL: no alternative certificate subject name matches target hostname 'wrong.host.badssl.com'
19:50:53.762409 [0-0] == Info: [SSL] cf_connect() -> 60, done=0

In this case, the certificate was obtained for *.badssl.com, but a wildcard certificate should have been obtained for *.host.badssl.com, so it's complaining about the mistake.

Until next time. Best regards.

Related Posts