Search Results: "charon"

12 June 2022

Russ Allbery: Review: The Shattered Sphere

Review: The Shattered Sphere, by Roger MacBride Allen
Series: Hunted Earth #2
Publisher: Tor
Copyright: July 1994
Printing: September 1995
ISBN: 0-8125-3016-0
Format: Mass market
Pages: 491
The Shattered Sphere is a direct sequel to The Ring of Charon and spoils everything about the plot of the first book. You don't want to start here. Also be aware that essentially everything you can read about this book will spoil the major plot driver of The Ring of Charon in the first sentence. I'm going to review the book without doing that, but it's unlikely anyone else will try. The end of the previous book stabilized matters, but in no way resolved the plot. The Shattered Sphere opens five years later. Most of the characters from the first novel are joined by some new additions, and all of them are trying to make sense of a drastically changed and far more dangerous understanding of the universe. Humanity has a new enemy, one that's largely unaware of humanity's existence and able to operate on a scale that dwarfs human endeavors. The good news is that humans aren't being actively attacked. The bad news is that they may be little more than raw resources, stashed in a safe spot for future use. That is reason enough to worry. Worse are the hints of a far greater danger, one that may be capable of destruction on a scale nearly beyond human comprehension. Humanity may be trapped between a sophisticated enemy to whom human activity is barely more noticeable than ants, and a mysterious power that sends that enemy into an anxious panic. This series is an easily-recognized example of an in-between style of science fiction. It shares the conceptual bones of an earlier era of short engineer-with-a-wrench stories that are full of set pieces and giant constructs, but Allen attempts to add the characterization that those books lacked. But the technique isn't there; he's trying, and the basics of characterization are present, but with none of the emotional and descriptive sophistication of more recent SF. The result isn't bad, exactly, but it's bloated and belabored. Most of the characterization comes through repetition and ham-handed attempts at inner dialogue. Slow plotting doesn't help. Allen spends half of a nearly 500 page novel on setup in two primary threads. One is mostly people explaining detailed scientific theories to each other, mixed with an attempt at creating reader empathy that's more forceful than effective. The other is a sort of big dumb object exploration that failed to hold my attention and that turned out to be mostly irrelevant. Key revelations from that thread are revealed less by the actions of the characters than by dumping them on the reader in an extended monologue. The reading goes quickly, but only because the writing is predictable and light on interesting information, not because the plot is pulling the reader through the book. I found myself wishing for an earlier era that would have cut about 300 pages out of this book without losing any of the major events. Once things finally start happening, the book improves considerably. I grew up reading large-scale scientific puzzle stories, and I still have a soft spot for a last-minute scientific fix and dramatic set piece even if the descriptive detail leaves something to be desired. The last fifty pages are fast-moving and satisfying, only marred by their failure to convince me that the humans were required for the plot. The process of understanding alien technology well enough to use it the right way kept me entertained, but I don't understand why the aliens didn't use it themselves. I think this book falls between two stools. The scientific mysteries and set pieces would have filled a tight, fast-moving 200 page book with a minimum of characterization. It would have been a throwback to an earlier era of science fiction, but not a bad one. Allen instead wanted to provide a large cast of sympathetic and complex characters, and while I appreciate the continued lack of villains, the writing quality is not sufficient to the task. This isn't an awful book, but the quality bar in the genre is so much higher now. There are better investments of your reading time available today. Like The Ring of Charon, The Shattered Sphere reaches a satisfying conclusion but does not resolve the series plot. No sequel has been published, and at this point one seems unlikely to materialize. Rating: 5 out of 10

3 January 2015

Russ Allbery: Review: The Ring of Charon

Review: The Ring of Charon, by Roger MacBride Allen
Series: Hunted Earth #1
Publisher: Tor
Copyright: December 1990
ISBN: 0-8125-3014-4
Format: Mass market
Pages: 500
Larry Chao is a junior scientist at a gravity research on Pluto, at the very outer limits of human reach in the solar system. The facility is for researching artificial gravity, which is one reason it's in the middle of nowhere. Another is that their experimental generator is built in a ring around Charon, and the close proximity of the two bodies is useful for scientific observation. Unfortunately, there hasn't been much to observe. They can create very short-lived gravity fields in very small areas, but nothing like the artificial gravity that was the original promise of the facility. As a result, the government is shutting the facility down. The authoritarian director, Simon Raphael, is... not exactly happy with that decision, but resigned to it and running the facility to completion with a sullen anger. When Larry makes a startling breakthrough at nearly the last minute for the society, Simon is uninterested and hostile. This leads Larry and his fellow scientist Sondra Berghoff to attempt a more radical demonstration of Larry's success and prove to the rest of the solar system that the facility should be kept open. That decision has a far deeper impact on humanity and the solar system than they could have possibly imagined. The Ring of Charon and its sequel, Shattered Sphere, were recommended to me as good harder science fiction. It took me a while to track down copies in fact, it took an in-person trip to Powell's. Once I found them, a relatively straightforward, old-school science fiction novel seemed like just the thing to read during my commute. Allen delivers there. I'm not spoiling the main plot driver of the book even though it's given away on the back cover, since it's some time in coming in the novel. But The Ring of Charon turns into a multi-viewpoint cross between a disaster novel and a scientific investigation. Larry and Sondra stay central to the plot, of course, but Allen adds a variety of other characters who are attempting to figure out what happened to the solar system and then deal with the consequences: everyone from scientists to pilots to communications officers in weird surrealistic stations. The science is mostly believable, apart from the scientific breakthrough that motivates the plot. Characterization isn't absent completely, but it's simple and unsubtle; dialogue is a bit wooden, characters aren't entirely multidimensional, but Allen does a reasonably good job with both pacing and the sense of mystery and investigation, and a surprisingly good job portraying organizational politics. As you might guess from the tone of my review, this is not the book to reach for if you want something ground-breaking. It's a very conventional, multi-viewpoint SF novel full of scientists and investigation of unknown and possibly hostile phenomena. If you've read much science fiction, you've read books like this before. But one thing that Allen does surprisingly well, which makes The Ring of Charon stand a bit above the pack, is that he doesn't write villains. Even Simon, who goes out of his way to make the reader hate him at the start of the book, becomes a surprisingly sympathetic character. The characters who are usually villains or at least foils in books like this the smooth PR person, the religious man, the blindly-focused scientist who isn't interested in anyone else's theories never turn into caricatures, play important roles in the plot, and turn out to be competent in their fields of expertise. There are actual villains, sort of, but I found myself feeling sympathetic even towards them, at least in places. Allen takes a rather old SF dodge to achieve conflict without an evil enemy, and, because of that, the end of the book felt like a bit of an anticlimax. But I did like the feel of the book where there isn't a good versus evil fight, just a wide variety of people (and others) trying to understand and control the universe in the best ways they know how. I'm not sure I can quite recommend this book. The quality of the writing is not particularly high, and I'm not generally a fan of the disaster novel style of storytelling. But despite not being very original, there's just something likable about it. It moves along reasonably well for a 500 page book, and it's refreshingly light on irritating stereotypes. I think one has to be in the right mood when reading it and set expectations accordingly, but it fit what I was looking for when I picked it up. One warning, though: although The Ring of Charon reaches a climax, the major plot conflict is not resolved at the end of this book, so you may want to have the sequel around. Followed by Shattered Sphere. Rating: 6 out of 10

6 September 2014

Russ Allbery: Accumulated hauls

I haven't made one of these in a long time, so I have some catching from random purchases to do, which includes a (repurposed) nice parting gift from my previous employer and a trip to Powell's since I was in the area for DebConf14. This also includes the contents of the Hugo voter's packet, which contained a wide variety of random stuff even if some of the novels were represented only by excerpts. John Joseph Adams (ed.) The Mad Scientist's Guide to World Domination (sff anthology)
Roger McBride Allen The Ring of Charon (sff)
Roger McBride Allen The Shattered Sphere (sff)
Iain M. Banks The Hydrogen Sonata (sff)
Julian Barnes The Sense of an Ending (mainstream)
M. David Blake (ed.) 2014 Campbellian Anthology (sff anthology)
Algis Budrys Benchmarks Continued (non-fiction)
Algis Budrys Benchmarks Revisited (non-fiction)
Algis Budrys Benchmarks Concluded (non-fiction)
Edgar Rice Burroughs Carson of Venus (sff)
Wesley Chu The Lives of Tao (sff)
Ernest Cline Ready Player One (sff)
Larry Correia Hard Magic (sff)
Larry Correia Spellbound (sff)
Larry Correia Warbound (sff)
Sigrid Ellis & Michael Damien Thomas (ed.) Queer Chicks Dig Time Lords (non-fiction)
Neil Gaiman The Ocean at the End of the Lane (sff)
Max Gladstone Three Parts Dead (sff)
Max Gladstone Two Serpents Rise (sff)
S.L. Huang Zero Sum Game (sff)
Robert Jordan & Brandon Sanderson The Wheel of Time (sff)
Drew Karpyshyn Mass Effect: Revelation (sff)
Justin Landon & Jared Shurin (ed.) Speculative Fiction 2012 (non-fiction)
John J. Lumpkin Through Struggle, the Stars (sff)
L. David Marquet Turn the Ship Around! (non-fiction)
George R.R. Martin & Raya Golden Meathouse Man (graphic novel)
Ramez Naam Nexus (sff)
Eiichiro Oda One Piece Volume 1 (manga)
Eiichiro Oda One Piece Volume 2 (manga)
Eiichiro Oda One Piece Volume 3 (manga)
Eiichiro Oda One Piece Volume 4 (manga)
Alexei Panshin New Celebrations (sff)
K.J. Parker Devices and Desires (sff)
K.J. Parker Evil for Evil (sff)
Sofia Samatar A Stranger in Olondria (sff)
John Scalzi The Human Division (sff)
Jonathan Straham (ed.) Fearsome Journeys (sff anthology)
Vernor Vinge The Children of the Sky (sff)
Brian Wood & Becky Cloonan Demo (graphic novel)
Charles Yu How to Live Safely in a Science Fictional Universe (sff) A whole bunch of this is from the Hugo voter's packet, and since the Hugos are over, much of that probably won't get prioritized. (I was very happy with the results of the voting, though.) Other than that, it's a very random collection of stuff, including a few things that I picked up based on James Nicoll's reviews. Now that I have a daily train commute, I should pick up the pace of reading, and as long as I can find enough time in my schedule to also write reviews, hopefully there will be more content in this blog shortly.

12 July 2013

Daniel Pocock: Practical VPNs with strongSwan, Shorewall, Linux firewalls and OpenWRT routers

There is intense interest in communications privacy at the moment thanks to the Snowden scandal. Open source software has offered credible solutions for privacy and encryption for many years. Sadly, making these solutions work together is not always plug-and-play. In fact, secure networking, like VoIP, has been plagued by problems with interoperability and firewall/filtering issues although now the solutions are starting to become apparent. Here I will look at some of them, such as the use of firewall zones to simplify management or the use of ECDSA certificates to avoid UDP fragmentation problems. I've drawn together a lot of essential tips from different documents and mailing list discussions to demonstrate how to solve a real-world networking problem. A typical network scenario and requirements Here is a diagram of the network that will be used to help us examine the capabilities of these open source solutions. Some comments about the diagram:
  • The names in square brackets are the zones for Shorewall, they are explained later.
  • The dotted lines are IPsec tunnels over the untrusted Internet.
  • The road-warrior users (mobiles and laptops) get virtual IP addresses from the VPN gateway. The branch offices/home routers do not use virtual IPs.
  • The mobile phones are largely untrusted: easily lost or stolen, many apps have malware, they can only tunnel to the central VPN and only access a very limited range of services.
  • The laptops are managed devices so they are trusted with access to more services. For efficiency they can connect directly to branch office/home VPNs as well as the central server.
  • Smart-phone user browsing habits are systematically monitored by mobile companies with insidious links into mass-media and advertising. Road-warriors sometimes plug-in at client sites or hotels where IT staff may monitor their browsing. Therefore, all these users want to tunnel all their browsing through the VPN.
  • The central VPN gateway/firewall is running strongSwan VPN and Shorewall firewall on Linux. It could be Debian, Fedora or Ubuntu. Other open source platforms such as OpenBSD are also very well respected for building firewall and VPN solutions, but Shorewall, which is one of the key ingredients in this recipe, only works on Linux at present.
  • The branch office/home network could be another strongSwan/Shorewall server or it could also be an OpenWRT router
  • The default configuration for most wifi routers creates a bridge joining wifi users with wired LAN users. Not here, it has been deliberately configured as an independent network. Road-warriors who attach to the wifi must use VPN tunnelling to access the local wired network. In OpenWRT, it is relatively easy to make the Wifi network an independent subnet. This is an essential security precaution because wifi passwords should not be considered secure: they are often transmitted to third parties, for example, by the cloud backup service in many newer smart phones.
Package mayhem The major components are packaged on all the major Linux distributions. Nonetheless, in every case, I found that it is necessary to re-compile fresh strongSwan packages from sources. It is not so hard to do but it is necessary and worth the effort. Here are related blog entries where I provide the details about how to re-build fresh versions of the official packages with all necessary features enabled: Using X.509 certificates as a standard feature of the VPN For convenience, many people building a point-to-point VPN start with passwords (sometimes referred to as pre-shared keys (PSK)) as a security mechanism. As the VPN grows, passwords become unmanageable. In this solution, we only look at how to build a VPN secured by X.509 certificates. The certificate concept is not hard. In this scenario, there are a few things that make it particularly easy to work with certificates:
  • strongSwan comes with a convenient command line tool, ipsec pki. Many of it's functions are equivalent to things you can do with OpenSSL or GnuTLS. However, the ipsec pki syntax is much more lightweight and simply provides a convenient way to do the things you need to do when maintaining a VPN.
  • Many of the routine activities involved in certificate maintainence can be scripted. My recent blog about using Android clients with strongSwan gives a sample script demonstrating how ipsec pki commands can be used to prepare a PKCS#12 (.p12) file that can be easily loaded into an Android device using the SD-card.
  • For building a VPN, there is no need to use a public certificate authority such as Verisign. Consequently, there is no need to fill in all their forms or make any payments for each device/certificate. Some larger organisations do choose to outsource their certificate management to such firms. For smaller organisations, an effective and sometimes better solution can be achieved by maintaining the process in-house with a private root CA.
UDP fragmentation during IPsec IKEv2 key exchange and ECDSA A common problem for IPsec VPNs using X.509 certificates is the fragmentation of key exchange datagrams during session setup. Sometimes it works, sometimes it doesn't. Various workarounds exist, such as keeping copies of all certificates from potential peers on every host. As the network grows, this becomes inconvenient to maintain and to some extent it eliminates the benefits of using PKI. Fortunately, there is a solution: Elliptic Curve Cryptography (ECC). Many people currently use RSA key-pairs. Best practice suggests using RSA keys of at least 2048 bits and often 4096 bits. Using ECC with a smaller 384-bit key is considered to be equivalent to a 7680 bit RSA key pair. Consequently, ECDSA certificates are much smaller than RSA certificates. Furthermore, at these key sizes, the key exchange packets are almost always smaller than the typical 1500 byte MTU. A further demand for ECDSA is arising due to the use of ECC within smart cards. Many smartcards don't support any RSA key larger than 2048 bits. The highly secure 384-bit ECC key is implemented in quite a few common smart cards. Smart card vendors have shown a preference for the ECC keys due to the US Government's preference for ECC and the lower computational overheads make them more suitable for constrained execution environments. Anyone who wants to use smart cards as part of their VPN or general IT security now, or in the future, needs to consider ECC/ECDSA. Making the network simple with Shorewall zones For this example, we are not just taking some easy point-to-point example. We have a real-world, multi-site, multi-device network with road warriors. Simplifying this architecture is important to help us understand and secure it. The solution? Each of these is abstracted to a "zone" in Shorewall. In the diagram above, the zone names are in square brackets. The purpose of each zone is described below:
Zone name Description
loc This is the private LAN and contains servers like databases, private source code respositories and NFS file servers
dmz This is the DMZ and contains web servers that are accessible from the public internet. Some of these servers talk to databases or message queues in the LAN network loc
vpn_a These are road warriors that are not very trustworthy, such as mobile devices. They are occasionally stolen and usually full of spyware (referred to by users as "apps"). They have limited access to ports on some DMZ servers, e.g. for sending and receiving mail using SMTP and IMAP (those ports are not exposed to the public Internet at large). They use the VPN tunnel for general internet access/browsing, to avoid surveillance by their mobile carrier.
vpn_b These are managed laptops that have a low probability of malware infection. They may well be using smart cards for access control. Consequently, they are more trusted than the vpn_a users and have access to some extra intranet pages and file servers. Like the smart-phone users, they use the VPN tunnel for general internet access/browsing, to avoid surveillance by third-party wifi hotspot operators.
vpn_c This firewall zone represents remote sites with managed hardware, such as branch offices or home networks with IPsec routers running OpenWRT.
cust These are servers hosted for third-parties or collaborative testing/development purposes. They have their own firewall arrangements if necessary.
net This zone represents traffic from the public Internet.
A practical Shorewall configuration Shorewall is chosen to manage the iptables and ip6tables firewall rules. Shorewall provides a level of abstraction that makes netfilter much more manageable than manual iptables scripting. The Shorewall concept of zones is very similar to the zones implemented in OpenWRT and this is an extremely useful paradigm for firewall management. Practical configuration of Shorewall is very well explained in the Shorewall quick start. The one thing that is not immediately obvious is a strategy for planning the contents of the /etc/shorewall/policy and /etc/shorewall/rules files. The exact details for making it work effectively with a modern IPsec VPN are not explained in a single document, so I've gathered those details below as well. An effective way to plan the Shorewall zone configuration is with a table like this:
Destination zone
loc dmz vpn_a vpn_b vpn_c cust net
Source zone loc \
dmz ? \ X X X
vpn_a ? ? \ X X
vpn_b ? ? X \ X
vpn_c X \
cust X ? X X X \
net X ? X X X \
The symbols in the table are defined:
Symbol Meaning
ACCEPT in policy file
X REJECT or DROP in policy file
? REJECT or DROP in policy file, but ACCEPT some specific ports in the rules file
Naturally, this modelling technique is valid for both IPv4 and IPv6 firewalling (with Shorewall6) Looking at the diagram in two dimensions, it is easy to spot patterns. Each pattern can be condensed into a single entry in the rules file. For example, it is clear from the first row that the loc zone can access all other zones. That can be expressed very concisely with a single line in the policy file:
loc all ACCEPT
Specific Shorewall tips for use with IPsec VPNs and strongSwan Shorewall has several web pages dedicated to VPNs, including the IPsec specific documentation.. Personally, I found that I had to gather a few details from several of these pages to make an optimal solution. Here are those tips:
  • Ignore everything about the /etc/shorewall/tunnels file. It is not needed and not used any more
  • Name the VPN zones (we call them vpn_a, vpn_b and vpn_c) in the zones file but there is no need to put them in the /etc/shorewall/interfaces file.
  • The /etc/shorewall/hosts file is not just for hosts and can be used to specify network ranges, such as those associated with the VPN virtual IP addresses. The ranges you put in this file should match the rightsourceip pool assignments in strongSwan's /etc/ipsec.conf
  • One of the examples suggests using mss=1400 in the /etc/shorewall/zones file. I found that is too big and leads to packets being dropped in some situations. To start with, try a small value such as 1024 and then try larger values later after you prove everything works. Setting mss for IPsec appears to be essential.
  • Do not use the routefilter feature in the /etc/shorewall/interfaces file as it is not compatible with IPsec
Otherwise, just follow the typical examples from the Shorewall quick start guide and configure it to work the way you want. Here is an example /etc/shorewall/zones file:
fw firewall
net ipv4
dmz ipv4
loc ipv4
cust ipv4
vpn_a ipsec mode=tunnel mss=1024
vpn_b ipsec mode=tunnel mss=1024
vpn_c ipsec mode=tunnel mss=1024
Here is an example /etc/shorewall/hosts file describing the VPN ranges from the diagram:
vpn_a eth0: ipsec
vpn_b eth0: ipsec
vpn_c eth0: ipsec
Here is an example /etc/shorewall/policy file based on the table above:
loc all ACCEPT
vpn_c all ACCEPT
cust net ACCEPT
net cust ACCEPT
all all REJECT
Here is an example /etc/shorewall/rules file based on the network:
# allow connections to the firewall itself to start VPNs:
# Rule  source    dest    protocol/port details
ACCEPT   all       fw                ah
ACCEPT   all       fw                esp
ACCEPT   all       fw                udp 500
ACCEPT   all       fw                udp 4500
# allow access to HTTP servers in DMZ:
ACCEPT   all       dmz               tcp 80
# allow connections from HTTP servers to MySQL database in private LAN:
ACCEPT   dmz       loc:     tcp 3306
# allow connections from all VPN users to IMAPS server in private LAN:
ACCEPT vpn_a,vpn_b,vpn_c loc: tcp 993
# allow VPN users (but not the smartphones in vpn_a) to the
# PostgresQL database for PostBooks accounting system:
ACCEPT vpn_b,vpn_c loc:      tcp 5432
ACCEPT   all       all
ACCEPT   all       all
Once the files are created, Shorewall can be easily activated with:
# shorewall compile && shorewall restart
strongSwan IPsec VPN setup Like Shorewall, strongSwan is also very well documented and I'm just going to focus on those specific areas that are relevant to this type of VPN project.
  • Allow the road-warriors to send all browsing traffic over the VPN means including leftsubnet= in the VPN server's /etc/ipsec.conf file. Be wary though: sometimes the road-warriors start sending the DHCP renewal over the tunnel instead of to their local DHCP server.
  • As we are using Shorewall zones for firewalling, you must set the options leftfirewall=no and lefthostaccess=no in ipsec.conf. Shorewall already knows about the remote networks as they are defined in the /etc/shorewall/hosts file and so firewall rules don't need to be manipulated each time a tunnel goes up or down.
  • As discussed above, X.509 certificates are used for peer authentication. In the certificate Distinguished Name (DN), store the zone name in the Organizational Unit (OU) component, for example, OU=vpn_c,
  • In the ipsec.conf file, match the users to connections using wildcard specifications such as rightid="OU=vpn_a, CN=*"
  • Put a subjectAltName with hostname in every certificate. The --san option to the ipsec pki commands adds the subjectAltName.
  • Keep the certificate distinguished names (DN) short, this makes the certificate smaller and reduces the risk of fragmented authentication packets. Many examples show a long and verbose DN such as C=GB, O=Acme World Wide Widget Corporation, OU=Engineering, On a private VPN, it is rarely necessary to have all that in the DN, just include OU for the zone name and CN for the host name.
  • As discussed above, use the ECDSA scheme for keypairs (not RSA) to ensure that the key exchange datagrams don't suffer from fragmentation problems. For example, generate a keypair with the command ipsec pki --gen --type ecdsa --size 384 > user1Key.der
  • Road warriors should have leftid=%fromcert in their ipsec.conf file. This forces them to use the Distinguished Name and not the subjectAltName (SAN) to identify themselves.
  • For road warriors who require virtual tunnel IPs, configure them to request both IPv4 and IPv6 addresses (dual stack) with leftsourceip=%config4,%config6 and on the VPN gateway, configure the ranges as arguments to rightsourceip
  • To ensure that roadwarriors query the LAN DNS, add the DNS settings to strongswan.conf and make sure road warriors are using a more recent strongSwan version that can dynamically update /etc/resolv.conf. Protecting DNS traffic is important for privacy reasons. It also makes sure that the road-warriors can discover servers that are not advertised in public DNS records.
Central firewall/VPN gateway configuration Although they are not present in the diagram, IPv6 networks are also configured in these strongSwan examples. It is very easy to combine IPv4 and IPv6 into a single /etc/ipsec.conf file. As long as the road-warriors have leftsourceip=%config4,%config6 in their own configurations, they will operate dual-stack IPv4/IPv6 whenver they connect to the VPN. Here is an example /etc/ipsec.conf for the central VPN gateway:
config setup
conn %default
conn branch1
conn rw_vpn_a
        rightid="OU=vpn_a, CN=*"
conn rw_vpn_b
        rightid="OU=vpn_b, CN=*"
Sample branch office or home router VPN configuration Here is an example /etc/ipsec.conf for the Linux server or OpenWRT VPN at the branch office or home:
conn head_office
# notice we only allow vpn_b users, not vpn_a
# these users are given virtual IPs from our own
# subnet
conn rw_vpn_b
        rightid="OU=vpn_b, CN=*"
Further topics Shorewall and Shorewall6 don't currently support a unified configuration. This can make it slightly tedious to duplicate rules between the two IP variations. However, the syntax for IPv4 and IPv6 configuration is virtually identical. Shorewall only currently supports Linux netfilter rules. In theory it could be extended to support other types of firewall API, such as pf used by OpenBSD and the related BSD family of systems. A more advanced architecture would split the single firewall into multiple firewall hosts, like the inner and outer walls of a large castle. The VPN gateway would also become a standalone host in the DMZ. This would require more complex routing table entries. Smart cards with PIN numbers provide an effective form of two-factor authentication that can protect certificates for remote users. Smart cards are documented well by strongSwan already so I haven't repeated any of that material in this article. Managing a private X.509 certificate authority in practice may require slightly more effort than I've described, especially when an organisation grows. Small networks and home users don't need to worry about these details too much, but for most deployments it is necessary to consider things like certificate revocation lists and special schemes to protect the root certificate's private key. EJBCA is one open source project that might help. Some users may want to consider ways to prevent the road-warriors from accidentally browsing the Internet when the IPsec tunnel is not active. Such policies could be implemented with some basic iptables firewalls rules in the road-warrior devices. Summary Using these strategies and configuration tips, planning and building a VPN will hopefully be much simpler. Please feel free to ask questions on the mailing lists for any of the projects discussed in this blog.