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# HOW TO : Use curl to check the impact of DNS changes
- URL: https://kudithipudi.org/how-to-use-curl-to-check-the-impact-of-dns-changes/
- Published: 2012-11-12T22:48:16.000Z
- Updated: 2012-11-12T22:48:16.000Z
- Description: Ran into an interesting scenario at work today. We had to check the impact of a DNS change on a certain hostname. Normally, you would edit...
- Author: admin
- Tags: HOWTO, Linux, security, Technology, Uncategorized, Web, #wp, #wp-post, #Import 2026-08-23 02:32

Ran into an interesting scenario at work today. We had to check the impact of a DNS change on a certain hostname. Normally, you would edit your host file entry to reflect the DNS change and do your testing. Here is another way you can do it using [cURL](http://curl.haxx.se/?ref=kudithipudi.org). In this particular example, I am checking the SSL certificate details of the hostname .

```
curl –insecure –trace-ascii debug.txt https://HOSTNAME:PORT –resolve HOSTNAME:PORT:IP_ADDRESS
```

That’s a pretty convoluted command :). Let’s try to break it down

```
–insecure
```

: tells cURL to ignore certificate warnings. This is helpful if you are using self signed certs

```
–trace-ascii
```

: tells cURL to save the SSL connection details (in debug mode) to a file called debug.txt

```
–resolve
```

: tells cURL to use the options mentioned after it to resolve the hostname, rather than using DNS. The format for resolve is <host:port:address>

**NOTE:** You need to have [version 7.21.3](http://curl.haxx.se/changes.html?ref=kudithipudi.org) or higher of cURL to use this option

Here’s a real world example. Say, I want to see how the IP address 72.30.38.140 would reacts if www.google.com requests are routed to it

```

samurai@samurai:~$ curl –insecure –trace-ascii debug.txt https://www.google.com –resolve www.google.com:443:72.30.38.140
The document has moved <A HREF="http://www.google.com/?s=https">here</A>.<P>
<!– ir2.fp.sp2.yahoo.com uncompressed/chunked Mon Nov 12 22:44:41 UTC 2012 –>
samurai@samurai:~$ more debug.txt
== Info: Added www.google.com:443:72.30.38.140 to DNS cache
== Info: About to connect() to www.google.com port 443 (#0)
== Info: Trying 72.30.38.140… == Info: connected
== Info: Connected to www.google.com (72.30.38.140) port 443 (#0)
== Info: successfully set certificate verify locations:
== Info: CAfile: none
 CApath: /etc/ssl/certs
== Info: SSLv3, TLS handshake, Client hello (1):
=> Send SSL data, 223 bytes (0xdf)
0000: ……P.|v..1..kA…….=J.xr.=ft.3.|…Z…..9.8………5…..
0040: …………….3.2…..E.D…../…A………………………
0080: …….W………www.google.com………..4.2……………….
00c0: ………………………….
== Info: SSLv3, TLS handshake, Server hello (2):
<= Recv SSL data, 42 bytes (0x2a)
0000: …&..P.{.I"L….3x..N…9…./<n….A..5.
== Info: SSLv3, TLS handshake, CERT (11):
<= Recv SSL data, 1272 bytes (0x4f8)
0000: ……….0…0..S……….0…*.H……..0N1.0…U….US1.0…
0040: U….Equifax1-0+..U…$Equifax Secure Certificate Authority0…1
0080: 00401230014Z..150703045000Z0..1)0′..U… 2g8aO5wI1bKJ2ZD588UsLvD
00c0: e3gTbg8DU1.0…U….US1.0…U….California1.0…U….Sunnyvale1
0100: .0…U….Yahoo Inc.1.0…U….www.yahoo.com0.."0…*.H……..
0140: …..0……….5.p./……..O…k.C…9E+.J..H.s….Bm.T.E.-..<
0180: ^…m…r.v<\…&Qq..l………. @'(q.m..ZJ.*kt…!.AWU…….M.
01c0: …n…O….0.._…H….4……>.m..K…….Z…:.Df%.lR.!…(!.
0200: .FV.dQ…f.V….P,.J9.c..dM.s>C=….Y..#…47#2…..cP.{….g.rU
0240: .d…P……………..0…0…U………..0…U………….t5.
0280:……U..0:..U…3010/.-.+.)http://crl.geotrust.com/crls/secure
02c0: ca.crl0..[..U…..R0..N..www.yahoo.com..yahoo.com..us.yahoo.com.
0300: .kr.yahoo.com..uk.yahoo.com..ie.yahoo.com..fr.yahoo.com..in.yaho
0340: o.com..ca.yahoo.com..br.yahoo.com..de.yahoo.com..es.yahoo.com..m
0380: x.yahoo.com..it.yahoo.com..sg.yahoo.com..id.yahoo.com..ph.yahoo.
03c0: com..qc.yahoo.com..tw.yahoo.com..hk.yahoo.com..cn.yahoo.com..au.
0400: yahoo.com..ar.yahoo.com..vn.yahoo.com0…U.#..0…H.h.+….G.# .
0440: O3….0…U.%..0…+………+…….0…*.H……………2..0.
0480: S.’.y….GD.Q…=…K+..q..kv…….<h…….ZLE.h$..M2^.C..IT..
04c0: ".5j….Vc7.4……1.Wu.[.a>+………9..{.a:………
== Info: SSLv3, TLS handshake, Server finished (14):
<= Recv SSL data, 4 bytes (0x4)
0000: ….
== Info: SSLv3, TLS handshake, Client key exchange (16):
=> Send SSL data, 262 bytes (0x106)
0000: …….R…..b.,.&.. s.Ob;.E_.EnSw../D…’…..(aB<<……F..]..
0040: o………~…*..r?.C..%..22…J.bu&.x(j|…….>A5..OF.G…C.$.
0080: .9u9n.z…K…..u…..~:W.{Sii.{2..6……..<…..i…8y$y…..6
00c0: …1.(M…fx….#k..r….47..t.q…..A.?.0. .D…..~…G+.,….~
0100: ..=.#y
== Info: SSLv3, TLS change cipher, Client hello (1):
=> Send SSL data, 1 bytes (0x1)
0000: .
== Info: SSLv3, TLS handshake, Finished (20):
=> Send SSL data, 16 bytes (0x10)
0000: ….!9)…6…+.
== Info: SSLv3, TLS change cipher, Client hello (1):
<= Recv SSL data, 1 bytes (0x1)
0000: .
== Info: SSLv3, TLS handshake, Finished (20):
<= Recv SSL data, 16 bytes (0x10)
0000: …..(qN..l.]…
== Info: SSL connection using AES256-SHA
== Info: Server certificate:
== Info: subject: serialNumber=2g8aO5wI1bKJ2ZD588UsLvDe3gTbg8DU; C=US; ST=California; L=Sunnyvale; O=Yahoo Inc.; CN=www.yahoo.com
== Info: start date: 2010-04-01 23:00:14 GMT
== Info: expire date: 2015-07-03 04:50:00 GMT
== Info: subjectAltName does not match www.google.com
=> Send header, 167 bytes (0xa7)
0000: GET / HTTP/1.1
0010: User-Agent: curl/7.21.6 (x86_64-pc-linux-gnu) libcurl/7.21.6 Ope
0050: nSSL/1.0.0e zlib/1.2.3.4 libidn/1.22 librtmp/2.3
0082: Host: www.google.com
0098: Accept: */*
00a5:
<= Recv header, 32 bytes (0x20)
0000: HTTP/1.1 301 Moved Permanently
<= Recv header, 37 bytes (0x25)
0000: Date: Mon, 12 Nov 2012 22:44:41 GMT
<= Recv header, 42 bytes (0x2a)
0000: Location: http://www.google.com/?s=https
<= Recv header, 23 bytes (0x17)
0000: Vary: Accept-Encoding
<= Recv header, 19 bytes (0x13)
0000: Connection: close
<= Recv header, 28 bytes (0x1c)
0000: Transfer-Encoding: chunked
<= Recv header, 40 bytes (0x28)
0000: Content-Type: text/html; charset=utf-8
<= Recv header, 24 bytes (0x18)
0000: Cache-Control: private
<= Recv header, 2 bytes (0x2)
0000:
<= Recv data, 173 bytes (0xad)
0000: 000009d
0009: The document has moved <A HREF="http://www.google.com/?s=https">
0049: here</A>.<P>.<!– ir2.fp.sp2.yahoo.com uncompressed/chunked Mon
0089: Nov 12 22:44:41 UTC 2012 –>.
00a8: 0
00ab:
== Info: Closing connection #0
== Info: SSLv3, TLS alert, Client hello (1):
=> Send SSL data, 2 bytes (0x2)
0000: ..

```