Hello everybody, and welcome again to my little nook of the Internet. I all the time take inspiration from what I’m at present engaged on in my day job when placing collectively an thought for a publish and/or video. Right now, we’re constructing a brand new information middle to host the hands-on lab environments for learners, whether or not you’re coaching in Cisco U. or taking a course along with your favourite Cisco teacher. As chances are you’ll know, A LOT goes into constructing a brand new information middle. But since I’m engaged on constructing the IPSEC VPN connections between this new information middle and the others in our community, let’s slim it down and take a technical take a look at IPSEC VPN tunnel creation.
In this weblog publish and the accompanying video, I’ll cowl the IPSEC VPN tunnel creation course of. We’ll discover “Phase 1” and “Phase 2” and try how the ACLs that determine “interesting traffic” influence the safety associations which can be constructed. We’ll even take a look at the packets concerned within the communications as tunnels are arrange. If that sounds good to you, proceed on, community adventurer!
A Technical Look at IPSEC VPN Tunnel Creation
“Technically Speaking… with Hank Preston” is a phase on The U. collection.
Available on the Cisco U. by Learning and Certifications YouTube Channel. View Playlist
If you’re new right here, I’m Hank Preston, Principal Engineer on the Labs and Systems workforce in Cisco Learning and Certifications. I’ve been constructing IPSEC VPNs for nearly my whole profession as a community engineer. In reality, one in all my first jobs as a shiny new community engineer was constructing out IPSEC VPN connections utilizing Cisco PIX firewalls for a Cisco Partner. For me, that meant taking the configuration templates constructed by the workforce’s extra senior engineers and updating them with the small print for a specific tunnel creation.
It wasn’t an issue… till there was one. You see, I didn’t actually know what all of the instructions did again then. So when issues didn’t work instantly, discovering the issue and realizing the best way to repair it was a little bit of a thriller to me. Thankfully, there have been some superb mentors and senior engineers to information me.
I needed to study the instructions to run to assist me decide the issue and the best way to repair it. It was throughout these troubleshooting classes I first realized phrases like “Phase 1,” “Phase 2,” “Main Mode,” “Quick Mode,” and “Aggressive Mode,” in addition to the protocols concerned, like ISAKMP, IKE, IPSEC. It was a whole lot of enjoyable, and it was solely the start.
Over the years, my depth of understanding grew, remodeling me right into a senior engineer, not in contrast to those that nurtured my very own curiosity. In addition to studying on the job, I needed to dive deep into IPSEC VPNs to arrange for my Cisco certification exams. Even although I used to be getting ready for now-retired certifications like CCNA Security, CCSP, and “VPN Specialist,” IPSEC data continues to be essential to at the present time.
So, ought to you study IPSEC?
IPSEC data is essential for real-world purposes and present Cisco certification exams. In reality, it’s listed on the 200-301 CCNA examination subjects, which is kind of telling for the reason that CCNA certification is the mark of somebody who has the foundational data to take their tech profession in a number of instructions. But that’s not all. IPSEC is on the CCNP Enterprise Core Exam, CCNP Security Core Exam, CCNP Security VPN Specialist, CCIE Enterprise Lab Exam, CCIE Security Lab Exam, and doubtless others. I didn’t examine.
So when honing in on a subject for this month, my first selection was IPSEC VPNs. IPSEC VPNs is a big matter, although. I knew I couldn’t cowl every thing in a single brief “Technically Speaking…” installment. In reality, I hadn’t determined precisely the place to focus till I used to be in the midst of standing up a brand new tunnel connection between two of our information facilities.
There I used to be, monitoring the tunnel standing to make sure every thing was wholesome, when I discovered myself on the CLI of one of many firewalls, operating instructions I’d run 1000’s of occasions: “show crypto isakmp sa” and “show crypto ipsec sa.” As I verified that every safety affiliation for the visitors varieties had come up and was wholesome, I mirrored on my early days of constructing VPNs on PIXs operating these similar instructions and never realizing what I used to be . And that’s when it hit me: this is able to make a wonderful addition to the library.
And right here have been are. Feel free to make use of the video above that can assist you comply with what I’ve outlined beneath. Alright, adventurers… let’s soar in.
Can’t have a VPN with out a few websites to attach collectively…
Before we begin trying on the tunnel creation, we’d like a community to work with.
So, I put collectively a reasonably simple 2-site community:
Site 1 (backside within the diagram) has two native networks; a YELLOW community and a BLUE community.
Site 2 (prime within the diagram) has a single native community, the PURPLE community.
Each web site is related to an untrusted WAN by a firewall. The firewall is configured like firewalls typically are: to carry out NAT/PAT on visitors passing from “inside” to “outside.”
Bringing the IPSEC VPN idea into this community, the objective is to create a tunnel between the 2 firewalls that can permit visitors between the websites to be securely tunneled throughout the WAN. This would then present a community path for hosts on Site 1’s YELLOW and BLUE networks to succeed in the hosts on Site 2’s PURPLE community.
Just to let … the main target of this publish is NOT on the configuration required to arrange the community or the IPSEC tunnel itself. Instead, we are going to take a look at the course of that occurs to ascertain and construct the connections when related visitors arrives on the firewall and initiates the IPSEC course of.
If you’d prefer to see the configurations on this setup, I’ve posted a CML topology file for this community within the CML Community on GitHub. If you’d prefer to dive deeper and take a look at a few of this exploration your self, obtain the file and run it in your CML server.
Saying one thing “interesting”
Just as a result of a VPN is configured on a firewall doesn’t imply the tunnel shall be established.
- Tunnels are established when they’re wanted and can ultimately be torn down if left idle (with out visitors passing by way of them) for lengthy sufficient.
- A firewall determines what sort of visitors ought to set off the constructing of a VPN based mostly on an entry record that’s related to the IPSEC crypto map that defines the VPN.
Let’s check out the entry record on Site1-FW that defines this “interesting traffic.”
Site1-FW# present access-list s2svpn_to_site2 access-list s2svpn_to_site2; 2 parts; identify hash: 0xa681e779 access-list s2svpn_to_site2 line 1 prolonged allow ip object-group SITE1 object-group SITE2 log default (hitcnt=0) 0xb520aee6 access-list s2svpn_to_site2 line 1 prolonged allow ip 192.168.200.0 255.255.255.0 172.16.10.0 255.255.255.0 log default (hitcnt=0) 0xfab888fb access-list s2svpn_to_site2 line 1 prolonged allow ip 192.168.100.0 255.255.255.0 172.16.10.0 255.255.255.0 log default (hitcnt=0) 0xb7b04209 Site1-FW# present run crypto map | inc match crypto map outside_map 1 match handle s2svpn_to_site2
In the ACL above, you’ll see there’s a line that allows visitors from the BLUE community (192.168.200.0/24) to the PURPLE community (172.16.10.0) and a second line that allows visitors from the YELLOW community (192.168.100.0/24) additionally to the PURPLE community. This ACL is used to MATCH visitors within the crypto map configuration. So when visitors passes by way of the router that matches this ACL, it’s going to provoke the tunnel bring-up course of.
The ACL on Site2-FW seems similar to this one. However, the supply and vacation spot networks are swapped, with PURPLE being the supply and BLUE and YELLOW because the locations in every line.
If we take a look at the present state of the VPN tunnel, we’ll see that there isn’t a ISAKMP or IPSEC safety affiliation constructed but.
Site1-FW# present crypto isakmp sa There aren't any IKEv1 SAs There aren't any IKEv2 SAs Site1-FW# present crypto ipsec sa There aren't any ipsec sas
…Everyone will get a Security Association!
Let’s take only a minute to speak about what a “security association” or “sa” is within the context of IPSEC VPNs.
A Security Association (SA) is a longtime relationship between units that outline the express mechanisms that can permit safe communications. An SA consists of the encryption protocols (akin to AES), hashing mechanisms (akin to SHA), and Diffie-Hellman Group (akin to group-14) used for communications. The two gateway units constructing the tunnel negotiate these particulars through the safety affiliation institution course of. Phase 2 SAs, or IPSEC SAs, may also embrace the native and distant addresses allowed to speak over the safety affiliation.
While we regularly consider IPSEC VPNs as being one tunnel, as in a single tunnel between two places. However, it’s extra correct to consider an IPSEC VPN as a assortment of tunnels between two places, with every safety affiliation as its personal distinctive encrypted tunnel. We’ll discover this concept a bit extra as we discover the institution of the VPN between the 2 websites.
Let’s convey it up already…
And now, the time has come to convey up the VPN. We’ll begin by sending some fascinating visitors from Site1-Host1 within the type of 5 100-byte ping packets.
Site1-Host1:~$ ping -s 100 -c 5 172.16.10.11
PING 172.16.10.11 (172.16.10.11): 100 information bytes
108 bytes from 172.16.10.11: seq=1 ttl=42 time=11.127 ms
108 bytes from 172.16.10.11: seq=2 ttl=42 time=11.032 ms
108 bytes from 172.16.10.11: seq=3 ttl=42 time=12.246 ms
108 bytes from 172.16.10.11: seq=4 ttl=42 time=11.046 ms
--- 172.16.10.11 ping statistics ---
5 packets transmitted, 4 packets obtained, 20% packet loss
round-trip min/avg/max = 11.032/11.362/12.246 ms
Notice within the output above that 5 packets have been despatched, however solely 4 have been obtained? This is as a result of the primary packet is misplaced whereas the tunnel is established.
Now let’s take a look at the state of the VPN tunnel on Site1-FW—however first, let’s start with the ISAKMP Security Association.
Site1-FW# present crypto isakmp sa There aren't any IKEv1 SAs IKEv2 SAs: Session-id:85, Status:UP-ACTIVE, IKE depend:1, CHILD depend:1 Tunnel-id Local Remote Status Role 188271715 10.255.1.2/500 10.255.2.2/500 READY INITIATOR Encr: AES-CBC, keysize: 256, Hash: SHA256, DH Grp:14, Auth signal: PSK, Auth confirm: PSK Life/Active Time: 86400/13 sec Child sa: native selector 192.168.100.0/0 - 192.168.100.255/65535 distant selector 172.16.10.0/0 - 172.16.10.255/65535 ESP spi in/out: 0xed866a3c/0xb89f38c9
Let’s take a second to know what this output is telling us:
- In RED and BLUE above, you see the native and distant endpoints of the tunnel. These are the skin IP addresses of every of the firewalls making up the 2 sides of this tunnel.
- In ORANGE, we are able to see the particular companies that present encryption (AES-256), hashing (SHA256), safe key era (DH Group 14), and authentication (preshared key). The lifetime and lively time for the tunnel are additionally displayed.
- In GREEN, we see the “Child SAs” of the preliminary ISAKMP SA. This refers back to the IPSEC Security Associations. We’ll discuss extra about them in only a second, however when you take a look at this output, you possibly can already see the references to the “interesting” visitors allowed by way of the tunnel.
An apart about Phase 1 and Phase 2
Now is a superb time to debate the Phase 1 and Phase 2 components of IPSEC VPN tunnels.
Phase 1 refers back to the ISAKMP Security Association institution, whereas Phase 2 is commonly thought-about the IPSEC Security Association. In reality, the command we run to discover the Phase 2 SAs is “show crypto ipsec sa.” To be a bit extra correct, Phase 2 is definitely the institution of both the Encapsulating Security Payload (ESP) or Authentication Header (AH) Security Associations. Both Phase 1 and Phase 2 should full and negotiate their related SAs earlier than visitors can circulation over the VPN connection.
I do know what you might be possible considering… 2 phases? Why not simply 1? It’s an excellent query, and the small print of the “why” are a bit out of scope for this weblog publish. But I’ll clarify what occurs in every Phase and the way they’re associated.
In Phase 1, the IKE (Identity Key Exchange) protocol and ISAKMP are used to arrange a management channel between the 2 VPN endpoints. That management channel is used to create the encryption keys and negotiate particulars essential to securely transport information between them. In our instance, a preshared key (PSK) is used on each units for preliminary identification and authentication of one another. Then, Diffie-Hellman is used to create the precise encryption keys used to safe the communications. With the Phase 1, or ISAKMP, Security Association established, the units transfer onto Phase 2.
In Phase 2, the 2 units construct both ESP or AH Security Associations utilizing keys created and communicated between the units utilizing the Phase 1 Security Association. Once established, information can now be despatched over the Phase 2 SAs between units.
The ESP and AH protocols haven’t any strategies of their very own to carry out the management steps and negotiations essential to arrange a Security Association; they depend on ISAKMP and IKE to offer that service. And ISAKMP and IKE can’t transport information payloads over their SAs. Each “phase” supplies important components of the entire IPSEC VPN tunnel creation.
Getting again to Phase 2
The output of “show crypto isakmp sa” listed the “Child SA” and a few particulars of Phase 2, however let’s take a look at all the small print of this part now.
Site1-FW# present crypto ipsec sa interface: exterior Crypto map tag: outside_map, seq num: 1, native addr: 10.255.1.2 access-list s2svpn_to_site2 prolonged allow ip 192.168.100.0 255.255.255.0 172.16.10.0 255.255.255.0 log default native ident (addr/masks/prot/port): (192.168.100.0/255.255.255.0/0/0) distant ident (addr/masks/prot/port): (172.16.10.0/255.255.255.0/0/0) current_peer: 10.255.2.2 #pkts encaps: 4, #pkts encrypt: 4, #pkts digest: 4 #pkts decaps: 4, #pkts decrypt: 4, #pkts confirm: 4 #pkts compressed: 0, #pkts decompressed: 0 #pkts not compressed: 4, #pkts comp failed: 0, #pkts decomp failed: 0 #pre-frag successes: 0, #pre-frag failures: 0, #fragments created: 0 #PMTUs despatched: 0, #PMTUs rcvd: 0, #decapsulated frgs needing reassembly: 0 #TFC rcvd: 0, #TFC despatched: 0 #Valid ICMP Errors rcvd: 0, #Invalid ICMP Errors rcvd: 0 #ship errors: 0, #recv errors: 0 native crypto endpt.: 10.255.1.2/500, distant crypto endpt.: 10.255.2.2/500 path mtu 1500, ipsec overhead 74(44), media mtu 1500 PMTU time remaining (sec): 0, DF coverage: copy-df ICMP error validation: disabled, TFC packets: disabled present outbound spi: B89F38C9 present inbound spi : ED866A3C inbound esp sas: spi: 0xED866A3C (3985009212) SA State: lively remodel: esp-aes-256 esp-sha-hmac no compression in use settings ={L2L, Tunnel, PFS Group 14, IKEv2, } slot: 0, conn_id: 165, crypto-map: outside_map sa timing: remaining key lifetime (kB/sec): (3962879/28775) IV measurement: 16 bytes replay detection assist: Y Anti replay bitmap: 0x00000000 0x0000001F outbound esp sas: spi: 0xB89F38C9 (3097442505) SA State: lively remodel: esp-aes-256 esp-sha-hmac no compression in use settings ={L2L, Tunnel, PFS Group 14, IKEv2, } slot: 0, conn_id: 165, crypto-map: outside_map sa timing: remaining key lifetime (kB/sec): (3916799/28775) IV measurement: 16 bytes replay detection assist: Y Anti replay bitmap: 0x00000000 0x00000001
This output has a whole lot of element, which may make it a bit overwhelming. Let’s break it down:
- In RED, we are able to see the particular line from the ACL that this SA (technically pair of SAs) matched. And proper beneath the ACL line, the YELLOW community is listed as “local,” and the PURPLE community is listed as “remote.”
- If this makes you assume that visitors from BLUE to PURPLE would require new SAs to be negotiated and constructed, give your self a excessive 5 from Hank. We’ll see that actual factor in somewhat bit.
- In GREEN, we are able to see some actually helpful counters and statistics about visitors by way of this SA. So far, we are able to see the 4 ICMP echo and echo-reply’s listed as “encaps” and “decaps.”
- In BLUE and BROWN, we see the 2 precise SAs that make up this pairing. A Security Association is a one-way connection, so to have bidirectional communications by way of a VPN, two SAs have to be negotiated; one for inbound and one for outbound.
- Find the “spi” traces for every of the inbound and outbound SAs. SPI is the Security Parameter Index. It is used throughout the precise ESP packets to uniquely determine the Security Association a packet belongs to. (We’ll see this in only a second.)
- Two traces beneath the SPI, you’ll see the “transform” utilized in every SA. The remodel lists the encryption and hashing algorithms used to safe these communications. The negotiation of the remodel set can also be performed throughout Phase 1.
Pretty cool, however… SHOW ME THE PACKETS!
Seeing the output of the tunnel institution on the firewall CLI is good, however I discover I perceive the method even higher by trying on the packets concerned within the communications. And this is without doubt one of the causes I like utilizing Cisco Modeling Labs (CML) when labbing and studying. With CML, you possibly can simply arrange a packet seize on any interface within the topology. And it even helps filters to restrict and goal the visitors I’m occupied with seeing.
I arrange a packet seize on the interface between Site1-FW and the WAN router, filtered to only ISAKMP (udp/500), ESP (ip/50), and ICMP (ip/1) and began capturing packets earlier than sending the visitors to convey up the tunnel. Then as soon as accomplished, I downloaded the PCAP file to discover intimately with Wireshark.
The picture above reveals the packets despatched when the 5 pings have been despatched throughout the community. You can see the 2 separate phases fairly clearly right here simply by trying on the Protocol of the communications. My tunnel is configured to make use of IKEv2, the most recent model of IKE, which requires fewer packets to convey up a tunnel than IKEv1. So right here we are able to see that solely 4 packets are transmitted between the firewalls earlier than the ESP Security Associations are constructed and in a position to ship the ICMP visitors. We can’t inform that the information within the packets is ICMP as a result of it’s encrypted (we constructed a VPN, in any case).
Also, check out the SPI values proven within the output for the ESP packets. These match the SPI values we noticed within the output from “show crypto ipsec sa.”
inbound esp sas: spi: 0xED866A3C (3985009212) . . outbound esp sas: spi: 0xB89F38C9 (3097442505) . .
We may even see the small print of the negotiation between friends by trying on the Initiator Request packet.
With the Security Association Payload of the packet, you possibly can take a look at the Phase 1 proposal particulars for the encryption, hashing, and DH group, in addition to the Transform Sets obtainable to be used within the Phase 2 SAs.
Am I the one one who’s all the time amazed after I see packets match what I configured or count on? (Networking actually is fairly superior.)
But what concerning the BLUE to PURPLE visitors?
At this level, the VPN is up, however just one set of “interesting” visitors has been despatched to date. So what occurs when a bunch on the BLUE community tries to speak with the PURPLE community?
To see this in motion, we’ll ship 5 2 hundred byte packets from Site1-Host2 to Site2-Host2.
Site1-Host2:~$ ping -c 5 -s 200 172.16.10.21
PING 172.16.10.21 (172.16.10.21): 200 information bytes
208 bytes from 172.16.10.21: seq=1 ttl=42 time=12.105 ms
208 bytes from 172.16.10.21: seq=2 ttl=42 time=10.356 ms
208 bytes from 172.16.10.21: seq=3 ttl=42 time=11.046 ms
208 bytes from 172.16.10.21: seq=4 ttl=42 time=11.158 ms
--- 172.16.10.21 ping statistics ---
5 packets transmitted, 4 packets obtained, 20% packet loss
round-trip min/avg/max = 10.356/11.166/12.105 ms
Just just like the final time, solely 4 of the 5 packets have been obtained. You is perhaps considering… But Hank, the tunnel is already up… why was a packet misplaced?”
The tunnel, or Security Association, that’s “up” is the one that permits visitors from YELLOW to PURPLE. Traffic from BLUE is completely different “interesting” visitors, which requires its personal Security Association to be created. We can see this new SA by exploring the output of the instructions on the firewall.
First up, the “show crypto isakmp sa” command.
Site1-FW# present crypto isakmp sa There aren't any IKEv1 SAs IKEv2 SAs: Session-id:85, Status:UP-ACTIVE, IKE depend:1, CHILD depend:2 Tunnel-id Local Remote Status Role 188271715 10.255.1.2/500 10.255.2.2/500 READY INITIATOR Encr: AES-CBC, keysize: 256, Hash: SHA256, DH Grp:14, Auth signal: PSK, Auth confirm: PSK Life/Active Time: 86400/66 sec Child sa: native selector 192.168.200.0/0 - 192.168.200.255/65535 distant selector 172.16.10.0/0 - 172.16.10.255/65535 ESP spi in/out: 0xc8fce690/0xf34ce0e2 Child sa: native selector 192.168.100.0/0 - 192.168.100.255/65535 distant selector 172.16.10.0/0 - 172.16.10.255/65535 ESP spi in/out: 0xed866a3c/0xb89f38c9
If you scroll up, you possibly can confirm that the Tunnel-id is similar because the final time we ran the command, exhibiting that the identical Phase 1 Security Association continues to be lively and getting used. And now we see a second “Child SA” listed. The YELLOW SA continues to be listed, and the SPI values are additionally the identical as earlier than. Only now, we have now a brand new BLUE Security Association with distinctive SPI values and “local selector” values.
We may also take a look at the small print of the BLUE ESP SA by checking the “show crypto ipsec sa” command. (The command may also present the most recent particulars concerning the YELLOW SA, however I’ve deleted that from the output to deal with the brand new one.)
Site1-FW# present crypto ipsec sa
interface: exterior
.
.
Crypto map tag: outside_map, seq num: 1, native addr: 10.255.1.2
access-list s2svpn_to_site2 prolonged allow ip 192.168.200.0 255.255.255.0 172.16.10.0 255.255.255.0 log default
native ident (addr/masks/prot/port): (192.168.200.0/255.255.255.0/0/0)
distant ident (addr/masks/prot/port): (172.16.10.0/255.255.255.0/0/0)
current_peer: 10.255.2.2
#pkts encaps: 4, #pkts encrypt: 4, #pkts digest: 4
#pkts decaps: 4, #pkts decrypt: 4, #pkts confirm: 4
#pkts compressed: 0, #pkts decompressed: 0
#pkts not compressed: 4, #pkts comp failed: 0, #pkts decomp failed: 0
#pre-frag successes: 0, #pre-frag failures: 0, #fragments created: 0
#PMTUs despatched: 0, #PMTUs rcvd: 0, #decapsulated frgs needing reassembly: 0
#TFC rcvd: 0, #TFC despatched: 0
#Valid ICMP Errors rcvd: 0, #Invalid ICMP Errors rcvd: 0
#ship errors: 0, #recv errors: 0
native crypto endpt.: 10.255.1.2/500, distant crypto endpt.: 10.255.2.2/500
path mtu 1500, ipsec overhead 74(44), media mtu 1500
PMTU time remaining (sec): 0, DF coverage: copy-df
ICMP error validation: disabled, TFC packets: disabled
present outbound spi: F34CE0E2
present inbound spi : C8FCE690
inbound esp sas:
spi: 0xC8FCE690 (3372017296)
SA State: lively
remodel: esp-aes-256 esp-sha-hmac no compression
in use settings ={L2L, Tunnel, PFS Group 14, IKEv2, }
slot: 0, conn_id: 165, crypto-map: outside_map
sa timing: remaining key lifetime (kB/sec): (4239359/28783)
IV measurement: 16 bytes
replay detection assist: Y
Anti replay bitmap:
0x00000000 0x0000001F
outbound esp sas:
spi: 0xF34CE0E2 (4081901794)
SA State: lively
remodel: esp-aes-256 esp-sha-hmac no compression
in use settings ={L2L, Tunnel, PFS Group 14, IKEv2, }
slot: 0, conn_id: 165, crypto-map: outside_map
sa timing: remaining key lifetime (kB/sec): (4008959/28782)
IV measurement: 16 bytes
replay detection assist: Y
Anti replay bitmap:
0x00000000 0x00000001
We’ll finish this take a look at IPSEC tunnel creation with another take a look at how the packets behave when an extra set of “interesting traffic” triggers the creation of a brand new Security Association between units that have already got a relationship constructed.
This packet seize reveals that the Phase 1 course of differs when including an extra “child security association.” The ISAKMP message “CREATE_CHILD_SA” is used to make use of to barter the small print for the brand new ESP Security Association. That occurs with a single pair of packets, after which the Phase 2 ESP Security Association is accessible to transmit the ICMP visitors.
That brings us to the top of this take a look at IPSEC VPN tunnel creation. So let’s replace the community diagram we began with to be somewhat extra “accurate” with what we’ve realized.
I hope this take a look at IPSEC has helped you perceive this core community expertise somewhat higher. Whether you might be actively finding out for a certification or working with IPSEC VPNs as a part of your “day job,” a deeper understanding of what occurs when a tunnel is being constructed is commonly very important. (Particularly when a tunnel isn’t developing if you count on it to.)
If you’d prefer to dive deeper into IPSEC VPNs, listed here are a number of useful hyperlinks that may be helpful:
Got a query on one thing from this publish? Or an thought for an additional “Technically Speaking…” installment? Let me know within the feedback!
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